At what level the 100base scrambler is applied. Description of Fast Ethernet technology. Ethernet and Fast Ethernet Adapters

Ethernet, but also to the equipment of other, less popular networks.

Ethernet and Fast Ethernet Adapters

Adapter characteristics

Network adapters (NIC, Network Interface Card) Ethernet and Fast Ethernet can interface with a computer through one of the standard interfaces:

ISA bus (Industry Standard Architecture);
PCI bus (Peripheral Component Interconnect);
PC Card bus (aka PCMCIA);

Adapters designed for the ISA system bus (backbone) were not so long ago the main type of adapters. The number of companies producing such adapters was great, which is why devices of this type were the cheapest. ISA adapters are available in 8-bit and 16-bit. 8-bit adapters are cheaper, while 16-bit adapters are faster. True, the exchange of information via the ISA bus cannot be too fast (in the limit - 16 MB / s, in reality - no more than 8 MB / s, and for 8-bit adapters - up to 2 MB / s). Therefore, Fast Ethernet adapters, which require high baud rates for efficient operation, are practically not available for this system bus. The ISA bus is a thing of the past.

The PCI bus has now practically supplanted the ISA bus and is becoming the main expansion bus for computers. It provides 32- and 64-bit data exchange and has a high throughput (theoretically up to 264 MB / s), which fully meets the requirements of not only Fast Ethernet, but also faster Gigabit Ethernet. It is also important that the PCI bus is used not only in IBM PCs, but also in PowerMac computers. In addition, it supports Plug-and-Play automatic hardware configuration. Apparently, in the near future, the majority of network adapters... The disadvantage of PCI in comparison with the ISA bus is that the number of its expansion slots in a computer is usually small (usually 3 slots). But it is precisely network adapters connect to PCI first.

The PC Card bus (formerly PCMCIA) is currently only used in notebook computers. In these computers, the internal PCI bus is usually not routed out. The PC Card interface provides a simple connection to a computer of miniature expansion cards, and the exchange rate with these cards is quite high. However, more and more laptops are equipped with built-in network adapters, as the ability to access the network becomes an integral part of the standard set of functions. These on-board adapters are again connected to the internal PCI bus of the computer.

When choosing network adapter oriented to a particular bus, you must first of all make sure that there are free expansion slots for this bus in the computer connected to the network. It is also necessary to evaluate the laboriousness of installing the purchased adapter and the prospects for the release of boards of this type. The latter may be needed in the event of an adapter failure.

Finally, there are more network adapters connecting to the computer via the parallel (printer) LPT port. The main advantage of this approach is that you do not need to open the computer case to connect the adapters. In addition, in this case, the adapters do not occupy the system resources of the computer, such as interrupt channels and DMAs, as well as the addresses of memory and I / O devices. However, the speed of information exchange between them and the computer in this case is much lower than when using the system bus. In addition, they require more processor time to communicate with the network, thereby slowing down the computer.

Recently, more and more computers are found in which network adapters embedded in system board... The advantages of this approach are obvious: the user does not have to buy a network adapter and install it in a computer. You just need to connect network cable to an external connector on your computer. However, the disadvantage is that the user cannot select the adapter with the best performance.

To others essential characteristics network adapters can be attributed:

way to configure the adapter;
the size of the buffer memory installed on the board and the modes of exchange with it;
the ability to install a read-only memory chip on the board for remote boot (BootROM).
the ability to connect the adapter to different types of transmission media (twisted pair, thin and thick coaxial cable, fiber optic cable);
used by the adapter baud rate over the network and the availability of the function of its switching;
the possibility of using the adapter of the full-duplex exchange mode;
compatibility of the adapter (more precisely, the adapter driver) with the network software used.

User configuration of the adapter was mainly used for adapters designed for the ISA bus. Configuration implies tuning to the use of computer system resources (I / O addresses, interrupt channels and direct memory access, buffer memory and remote boot memory). Configuration can be carried out by setting the switches (jumpers) to the desired position or using the DOS configuration program supplied with the adapter (Jumperless, Software configuration). When launching such a program, the user is prompted to set the hardware configuration using a simple menu: select adapter parameters. The same program allows you to make self-test adapter. The selected parameters are stored in the adapter's non-volatile memory. In any case, when choosing parameters, you must avoid conflicts with system devices computer and with other expansion cards.

The adapter can also be configured automatically in Plug-and-Play mode when the computer is powered on. Modern adapters usually support this very mode, so they can be easily installed by the user.

In the simplest adapters, exchange with the adapter's internal buffer memory (Adapter RAM) is carried out through the address space of the I / O devices. In this case, no additional configuration of memory addresses is required. The base address of the shared memory buffer must be specified. It is assigned to the area of the computer's upper memory (

Fast Ethernet - the IEEE 802.3 u specification officially adopted on October 26, 1995, defines a data link protocol standard for networks operating using both copper and fiber-optic cables at a speed of 100 Mb / s. The new specification is the successor to the Ethernet IEEE 802.3 standard, using the same frame format, CSMA / CD media access mechanism and star topology. Several physical layer configuration elements have evolved to increase throughput, including cable types, segment lengths, and number of hubs.

Physical layer

The Fast Ethernet standard defines three types of 100 Mbps Ethernet signaling media.

· 100Base-TX - two twisted pairs of wires. Transmission is carried out in accordance with the standard for data transmission in a twisted physical medium, developed by ANSI (American National Standards Institute - American National Standards Institute). Coiled data cable can be shielded or unshielded. Uses 4B / 5B data coding algorithm and MLT-3 physical coding method.

· 100Base-FX - two cores, fiber optic cable. Transmission is also carried out in accordance with the ANSI standard for data transmission in fiber optic media. Uses 4B / 5B data coding algorithm and NRZI physical coding method.

· 100Base-T4 is a special specification developed by the IEEE 802.3u committee. According to this specification, data transmission is carried out over four twisted pairs. telephone cable which is called Category 3 UTP cable. Uses 8B / 6T data coding algorithm and NRZI physical coding method.

Multimode cable

This type of fiber optic cable uses a fiber with a 50 or 62.5 micrometer core and a 125 micrometer outer sheath. Such a cable is called a 50/125 (62.5 / 125) micrometer multimode fiber optic cable. An LED transceiver with a wavelength of 850 (820) nanometers is used to transmit a light signal over a multimode cable. If a multimode cable connects two ports of switches operating in full duplex mode, then it can be up to 2000 meters long.

Single mode cable

Singlemode fiber has a smaller core diameter of 10 micrometers than multimode fiber and uses a laser transceiver for transmission over singlemode cable, which together provide efficient transmission over long distances. The wavelength of the transmitted light signal is close to the core diameter, which is 1300 nanometers. This number is known as the zero dispersion wavelength. In a single-mode cable, dispersion and signal loss are very low, which allows light signals to be transmitted over long distances than in the case of multimode fiber.

38. Gigabit Ethernet technology, general characteristics, specification of the physical environment, basic concepts.
3.7.1. general characteristics standard

Soon after the Fast Ethernet products hit the market, network integrators and administrators began to feel certain limitations when building corporate networks. In many cases, servers connected over a 100 Mbps channel overloaded network backbones that also operate at 100 Mbps - FDDI and Fast Ethernet backbones. There was a need for the next level of the speed hierarchy. In 1995, only ATM switches could provide a higher level of speed, and in the absence at that time of convenient means of migrating this technology to local networks (although the LAN Emulation - LANE specification was adopted at the beginning of 1995, its practical implementation was still ahead), they were to be implemented in almost no one dared to the local network. In addition, ATM technology was distinguished by a very high level of cost.

So the next step, taken by the IEEE, seemed logical - 5 months after the final adoption of the Fast Ethernet standard in June 1995, the IEEE High Speed Technology Research Group was ordered to look into the possibility of developing an Ethernet standard with an even higher bit rate.

In the summer of 1996, an 802.3z group was announced to develop a protocol similar to Ethernet as much as possible, but with a bit rate of 1000 Mbps. As with Fast Ethernet, the message was received with great enthusiasm by Ethernet proponents.

The main reason for the enthusiasm was the prospect of the same smooth migration of network backbones to Gigabit Ethernet, similar to the migration of congested Ethernet segments located at the lower levels of the network hierarchy to Fast Ethernet. In addition, the experience of transferring data at gigabit speeds was already available, both in territorial networks (SDH technology) and in local networks - Fiber Channel technology, which is mainly used to connect high-speed peripherals to large computers and transmit data over fiber-optic cable from speed close to gigabit by means of redundancy code 8B / 10B.

The first version of the standard was reviewed in January 1997, and the 802.3z standard was finally adopted on June 29, 1998 at a meeting of the IEEE 802.3 committee. Work on the implementation of Gigabit Ethernet on twisted pair category 5 was transferred to a special committee 802.3ab, which has already considered several versions of the draft of this standard, and since July 1998 the project has become quite stable. The final adoption of the 802.3ab standard is expected in September 1999.

Without waiting for the standard to be adopted, some companies released the first Gigabit Ethernet equipment on fiber optic cable by the summer of 1997.

The main idea of the developers of the Gigabit Ethernet standard is to preserve the ideas of classical Ethernet technology as much as possible while reaching a bit rate of 1000 Mbps.

Since when developing a new technology, it is natural to expect some technical innovations that follow the general course of the development of network technologies, it is important to note that Gigabit Ethernet, like its slower counterparts, at the protocol level will not be support:

quality of service;
redundant connections;
testing the operability of nodes and equipment (in the latter case - with the exception of testing port-to-port communication, as is done for Ethernet 10Base-T and 10Base-F and Fast Ethernet).

All three named properties are considered very promising and useful in modern networks, and especially in networks of the near future. Why are the authors of Gigabit Ethernet abandoning them?

The main idea of the developers of Gigabit Ethernet technology is that there are and will continue to exist quite a few networks in which the high speed of the backbone and the ability to assign priority packets in the switches will be sufficient to ensure the quality of transport services for all network clients. And only in those rare cases, when the backbone is sufficiently loaded, and the requirements for the quality of service are very strict, it is necessary to use ATM technology, which, due to its high technical complexity, guarantees the quality of service for all major types of traffic.

39. Structural cabling system used in network technologies.
Structured Cabling System (SCS) is a set of switching elements (cables, connectors, connectors, cross-over panels and cabinets), as well as a methodology for them sharing, which allows you to create regular, easily expandable communication structures in computer networks.

The structured cabling system is a kind of "constructor", with the help of which the network designer builds the required configuration from standard cables connected by standard connectors and switched on standard cross-over panels. If necessary, the configuration of connections can be easily changed - add a computer, segment, switch, remove unnecessary equipment, and also change the connections between computers and hubs.

When building a structured cabling system, it is assumed that each workplace in the enterprise should be equipped with sockets for connecting a telephone and a computer, even if this is not required at the moment. That is, a good structured cabling system is redundant. This can save money in the future, as changes to the connection of new devices can be made by re-connecting existing cables.

A typical hierarchical structure of a structured cabling system includes:

horizontal subsystems (within a floor);
vertical subsystems (inside the building);
a campus subsystem (within one territory with several buildings).

Horizontal subsystem connects the floor marshalling cabinet to the users' outlets. Subsystems of this type correspond to the floors of a building. Vertical subsystem connects the marshalling cabinets of each floor to the central control room of the building. The next step in the hierarchy is campus subsystem, which connects several buildings to the main control room of the entire campus. This part of the cabling system is commonly referred to as the backbone.

There are many advantages to using structured cabling instead of chaotic cables.

· Versatility. A structured cabling system with a well-thought-out organization can become a unified medium for transferring computer data in a local computer network, organizing a local telephone network, transmitting video information and even transmitting signals from fire safety sensors or security systems. This allows you to automate many processes of control, monitoring and management of economic services and life support systems of the enterprise.

· Increased service life. The obsolescence of a well-structured cabling system can be 10-15 years.

· Reducing the cost of adding new users and changing their placements. It is known that the cost of a cable system is significant and is determined mainly not by the cost of the cable, but by the cost of laying it. Therefore, it is more profitable to carry out a one-time work on laying the cable, possibly with a large margin in length, than to carry out the laying several times, increasing the length of the cable. With this approach, all work on adding or moving a user is reduced to connecting the computer to an existing outlet.

· Possibility of easy network expansion. The structured cabling system is modular and therefore easy to expand. For example, a new subnet can be added to a trunk without affecting the existing subnets. You can change the cable type on a separate subnet independently of the rest of the network. The structured cabling system is the basis for dividing the network into easily manageable logical segments, since it is itself already divided into physical segments.

· Providing more efficient service. The structured cabling system is easier to service and troubleshoot than bus cabling. In the case of bus cabling, the failure of one of the devices or connecting elements leads to a difficult-to-locate failure of the entire network. In structured cabling systems, the failure of one segment does not affect others, since the aggregation of segments is carried out using hubs. Concentrators diagnose and localize the faulty area.

· Reliability. A structured cabling system has increased reliability, since the manufacturer of such a system guarantees not only the quality of its individual components, but also their compatibility.

40. Hubs and network adapters, principles, use, basic concepts.
Hubs, along with network adapters and cabling, represent the minimum amount of equipment that can be used to create a local area network. Such a network will represent a common shared environment.

Network Adapter (Network Interface Card, NIC) together with its driver implements the second, channel level of the model open systems at the end node of the network - a computer. More precisely, in a network operating system, a pair of adapter and driver performs only the functions of the physical and MAC layers, while the LLC layer is usually implemented by an operating system module that is the same for all drivers and network adapters. Actually, this is how it should be in accordance with the model of the IEEE 802 protocol stack. For example, in Windows NT, the LLC level is implemented in the NDIS module, which is common to all network adapter drivers, regardless of which technology the driver supports.

The network adapter together with the driver perform two operations: frame transmission and reception.

In adapters for client computers, much of the work is shifted to the driver, making the adapter simpler and cheaper. The disadvantage of this approach is the high degree of loading the computer's central processor with routine work on transferring frames from random access memory computer to the network. The central processor is forced to do this work instead of performing user application tasks.

The network adapter must be configured before being installed in a computer. Configuring an adapter typically specifies the IRQ used by the adapter, the DMA channel (if the adapter supports DMA mode), and the base address of the I / O ports.

In almost all modern technologies local area networks a device is defined that has several peer names - hub(concentrator), hub (hub), repeater (repeater). Depending on the field of application of this device, the composition of its functions and design changes significantly. Only the main function remains unchanged - it is frame repetition either on all ports (as defined in the Ethernet standard), or only on some ports, according to the algorithm defined by the corresponding standard.

A hub usually has several ports to which the end nodes of the network - computers - are connected using separate physical segments of the cable. The concentrator combines individual physical network segments into a single shared environment, access to which is carried out in accordance with one of the considered LAN protocols - Ethernet, Token Ring, etc. technologies produced their own hubs - Ethernet; Token Ring; FDDI and 100VG-AnyLAN. For a specific protocol, sometimes its own, highly specialized name of this device is used, which more accurately reflects its functions or is used by virtue of traditions, for example, the name MSAU is characteristic of Token Ring concentrators.

Each hub performs some basic function defined in the corresponding protocol of the technology it supports. Although this feature is defined in some detail in the technology standard, when implemented, hubs from different manufacturers may differ in details such as the number of ports, support for several types of cables, etc.

In addition to the main function, the hub can perform a number of additional functions, which are either not defined in the standard at all, or are optional. For example, a Token Ring hub can perform the function of shutting down malfunctioning ports and switching to a backup ring, although such capabilities are not described in the standard. The hub turned out to be a convenient device for performing additional functions that facilitate the monitoring and operation of the network.

41. The use of bridges and switches, principles, features, examples, limitations
Structuring with bridges and switches

the network can be divided into logical segments using two types of devices - bridges and / or switches (switch, switching hub).

Bridge and switch are functional twins. Both of these devices advance frames based on the same algorithms. Bridges and switches use two types of algorithms: an algorithm transparent bridge, described in the IEEE 802.1D standard, or the algorithm source routing bridge from IBM for Token Ring networks. These standards were developed long before the first switch was introduced, so they use the term "bridge". When the first industrial switch model for Ethernet technology was born, it performed the same IEEE 802.ID frame forwarding algorithm, which had been worked out by bridges of local and global networks for a dozen years.

The main difference between a switch and a bridge is that the bridge processes frames sequentially, while the switch processes frames in parallel. This circumstance is due to the fact that bridges appeared in those days when the network was divided into a small number of segments, and intersegment traffic was small (it obeyed the 80 by 20% rule).

Today bridges still work on networks, but only on fairly slow global links between two remote LANs. These bridges are called remote bridges, and they work in the same way as 802.1D or Source Routing.

Transparent bridges can, in addition to transmitting frames within the same technology, translate LAN protocols, such as Ethernet to Token Ring, FDDI to Ethernet, etc. This property of transparent bridges is described in the IEEE 802.1H standard.

In what follows, we will call a device that advances frames using the bridge algorithm and works in a local network, the modern term "switch". When describing the 802.1D and Source Routing algorithms themselves in the next section, we will traditionally call the device a bridge, as it is actually called in these standards.

42. Switches for local networks, protocols, modes of operation, examples.
Each of the 8 10Base-T ports is served by one Ethernet Packet Processor (EPP). In addition, the switch has a system module that coordinates the work of all EPP processors. The system module maintains the general address table of the switch and provides SNMP management of the switch. To transfer frames between ports, a switching fabric is used, similar to those found in telephone switches or multiprocessor computers, connecting multiple processors with multiple memory modules.

Switching matrix works on the principle of switching channels. For 8 ports, the matrix can provide 8 simultaneous internal channels at half duplex port operation and 16 at full duplex, when the transmitter and receiver of each port operate independently of each other.

When a frame arrives at a port, the EPP processor buffers the first few bytes of the frame to read the destination address. After receiving the destination address, the processor immediately decides to transfer the packet, without waiting for the remaining bytes of the frame to arrive.

If the frame needs to be transmitted to another port, the processor turns to the switching matrix and tries to establish a path in it that connects its port with the port through which the route to the destination address goes. The switching fabric can only do this if the destination port is free at that moment, that is, not connected to another port; if the port is busy, then, as in any circuit switched device, the matrix fails the connection. In this case, the frame is fully buffered by the input port processor, after which the processor waits for the output port to be released and the switching matrix forms the desired path. Once the desired path is established, the buffered frame bytes are sent to it, which are received by the output port processor. As soon as the downstream processor accesses the attached Ethernet segment using the CSMA / CD algorithm, the frame bytes are immediately transferred to the network. The described method of transmitting a frame without its full buffering is called “on-the-fly” or “cut-through” switching. The main reason improving network performance when using the switch is parallel processing multiple frames This effect is illustrated in Fig. 4.26. The figure depicts an ideal situation in terms of improving performance, when four out of eight ports transmit data at a maximum speed of 10 Mb / s for the Ethernet protocol, and they transmit this data to the other four ports of the switch without conflict - data flows between network nodes are distributed so that each frame receiving port has its own output port. If the switch manages to process the input traffic even at the maximum rate of incoming frames to the input ports, then the total switch performance in the given example will be 4x10 = 40 Mbps, and when generalizing the example for N ports - (N / 2) xlO Mbps. It is said that the switch provides each station or segment connected to its ports with dedicated protocol bandwidth. Naturally, the situation in the network does not always develop as shown in Fig. 4.26. If two stations, for example stations connected to ports 3 and 4, at the same time you need to write data to the same server connected to the port 8, then the switch will not be able to allocate a 10 Mbps data stream to each station, since port 5 cannot transmit data at 20 Mbps. Station frames will wait in internal queues of input ports 3 and 4, when the port becomes free 8 to transmit the next frame. Obviously, a good solution for such a distribution of data streams would be to connect the server to a higher-speed port, for example Fast Ethernet. Since the main advantage of the switch, thanks to which it has won a very good position in local networks, is its high performance, the developers of switches are trying to release it in this way called non-blocking switch models.

43. Algorithm of the transparent bridge.
Transparent bridges are invisible to network adapters of end nodes, since they independently build a special address table, on the basis of which you can decide whether you need to transfer the incoming frame to some other segment or not. When transparent bridges are used, network adapters work in the same way as in the absence of them, that is, they do not take any additional action to get the frame through the bridge. The transparent bridging algorithm is independent of the LAN technology in which the bridge is being installed, so transparent Ethernet bridges work just like transparent FDDI bridges.

A transparent bridge builds its address table based on passive monitoring of traffic circulating in segments connected to its ports. In this case, the bridge takes into account the addresses of the sources of data frames arriving on the bridge ports. Based on the frame source address, the bridge concludes that this node belongs to one or another network segment.

Consider the process of automatically creating a bridge address table and using it using the example of a simple network shown in Fig. 4.18.

Rice. 4.18. How a transparent bridge works

The bridge connects two logical segments. Segment 1 consists of computers connected with one length of coaxial cable to port 1 of the bridge, and segment 2 consists of computers connected with another length of coaxial cable to port 2 of the bridge.

Each bridge port acts as an endpoint on its segment with one exception - a bridge port does not have its own MAC address. The port of the bridge operates in the so-called promisquous packet capture mode, when all packets arriving on the port are stored in the buffer memory. With the help of this mode, the bridge monitors all traffic transmitted in the segments attached to it, and uses the packets passing through it to study the composition of the network. Since all packets are written to the buffer, the bridge does not need a port address.

In the initial state, the bridge does not know anything about the computers with which MAC addresses are connected to each of its ports. Therefore, in this case, the bridge simply transmits any captured and buffered frame on all its ports except for the one from which this frame was received. In our example, the bridge has only two ports, so it transmits frames from port 1 to port 2, and vice versa. When a bridge is about to transmit a frame from segment to segment, for example from segment 1 to segment 2, it tries again to access segment 2 as an end node according to the rules of the access algorithm, in this example, according to the rules of the CSMA / CD algorithm.

Simultaneously with the transmission of the frame to all ports, the bridge learns the source address of the frame and makes a new entry about its belonging in its address table, which is also called the filtering or routing table.

After the bridge has passed the learning phase, it can operate more efficiently. When receiving a frame directed, for example, from computer 1 to computer 3, it scans the address table for the coincidence of its addresses with the destination address 3. Since there is such an entry, the bridge performs the second stage of table analysis - it checks whether computers with source addresses ( in our case, this is address 1) and the destination address (address 3) in one segment. Since in our example they are in different segments, the bridge performs the operation forwarding frame - transmits a frame to another port, having previously received access to another segment.

If the destination address is unknown, then the bridge transmits the frame to all its ports, except for the port - the source of the frame, as in the initial stage of the learning process.

44. Bridges with routing from the source.
Source-routed bridging is used to connect Token Ring and FDDI rings, although transparent bridging can also be used for the same purpose. Source Routing (SR) is based on the fact that the sending station puts in a frame sent to another ring all the address information about intermediate bridges and rings that the frame must pass before entering the ring to which the station is connected. recipient.

Let's consider the principles of operation of Source Routing bridges (hereinafter, SR-bridges) using the example of the network shown in Fig. 4.21. The network consists of three rings connected by three bridges. Rings and bridges have identifiers to define the route. SR-bridges do not build an address table, but when advancing frames, they use the information available in the corresponding fields of the data frame.

Fig. 4.21.Source Routing Bridges

Upon receipt of each packet, the SR-bridge only needs to look at the Routing Information Field (RIF, in a Token Ring or FDDI frame) for its own identifier. And if it is present there and accompanied by the identifier of the ring that is connected to this bridge, then in this case the bridge copies the incoming frame to the specified ring. Otherwise, the frame is not copied to the other ring. In any case, the original copy of the frame is returned over the original ring of the sending station, and if it was transmitted to another ring, then the A (address recognized) and C (frame copied) bits of the frame status fields are set to 1 to inform the sending station, that the frame was received by the destination station (in this case, it was transmitted by the bridge to another ring).

Since routing information in a frame is not always needed, but only for frame transmission between stations connected to different rings, the presence of the RIF field in the frame is indicated by setting the individual / group address (I / G) to 1 bit (in this case, this bit is not used as intended, since the source address is always individual).

The RIF has a three-part control subfield.

Frame type defines the type of the RIF field. Exists different types RIF fields used to find a route and to send a frame along a known route.
Maximum frame length field used by the bridge to connect rings that have a different MTU value. Using this field, the bridge notifies the station of the maximum possible frame length (that is, the minimum MTU value along the entire multipart route).
RIF field length is necessary, since the number of route descriptors that specify the identifiers of the crossed rings and bridges is unknown in advance.

For the source routing algorithm to work, two additional frame types are used - a single-route broadcast frame (SRBF) and an all-route broadcast frame (ARBF).

All SR bridges must be manually configured by the administrator to send ARBF frames to all ports except the source port of the frame, and for SRBF frames, some bridge ports must be blocked to avoid network loops.

Advantages and Disadvantages of Source Routing Bridges

45. Switches: technical implementation, functions, characteristics that affect their work.
Features of the technical implementation of switches. Many first-generation switches were similar to routers, that is, they were based on the central processor general purpose connected to the interface ports via the internal high-speed bus. The main disadvantage of these switches was their low speed. The general-purpose processor could in no way cope with the large volume of specialized operations for transferring frames between interface modules. In addition to the processor chips for successful non-blocking operation, the switch also needs a high-speed node to transfer frames between the port processor chips. Currently, switches use one of three schemes as a base on which such an exchange node is built:

switching matrix;
shared multi-input memory;
common bus.

The most widespread among standard networks is the Ethernet network. It appeared in 1972 and became the international standard in 1985. It was adopted by the largest international standards organizations: IEEE 802 Committee (Institute of Electrical and Electronic Engineers) and ECMA (European Computer Manufacturers Association).

The standard was named IEEE 802.3 (in English it reads as "eight oh two dot three"). It defines multiple access to a mono-channel of the bus type with collision detection and transmission control, that is, with the already mentioned CSMA / CD access method.

Key features of the original IEEE 802.3 standard:

· Topology - bus;

· Transmission medium - coaxial cable;

· Transmission speed - 10 Mbit / s;

· Maximum network length - 5 km;

· The maximum number of subscribers - up to 1024;

· The length of the network segment - up to 500 m;

· The number of subscribers on one segment - up to 100;

· Access method - CSMA / CD;

· Transmission is narrowband, that is, without modulation (mono channel).

Strictly speaking, there are minor differences between the IEEE 802.3 and Ethernet standards, but they usually prefer not to be remembered.

Ethernet is now the most popular in the world (over 90% of the market), and it is expected to remain so in the coming years. This was largely due to the fact that from the very beginning the characteristics, parameters, protocols of the network were open, as a result of which a huge number of manufacturers around the world began to produce Ethernet equipment that is fully compatible with each other.

A classic Ethernet network used a 50-ohm coaxial cable of two types (thick and thin). However, in recent years (since the beginning of the 90s), the most widespread version of the Ethernet is using twisted pairs as a transmission medium. A standard has also been defined for the use of fiber optic cable in a network. Additions have been made to the original IEEE 802.3 standard to accommodate these changes. In 1995, an additional standard appeared for a faster version of Ethernet, operating at a speed of 100 Mbit / s (the so-called Fast Ethernet, IEEE 802.3u standard), using twisted pair or fiber-optic cable as the transmission medium. In 1997, a version with a speed of 1000 Mbit / s appeared (Gigabit Ethernet, IEEE 802.3z standard).

In addition to the standard bus topology, passive star and passive tree topologies are increasingly being used. This assumes the use of repeaters and repeater hubs connecting different parts (segments) of the network. As a result, a tree-like structure on the segments can be formed. different types(Figure 7.1).

A segment (part of a network) can be a classic bus or a single subscriber. The bus segments use coaxial cable, and the passive star beams (for connecting single computers to the hub) use twisted pair and fiber optic cables. The main requirement for the resulting topology is that there are no closed paths (loops) in it. In fact, it turns out that all subscribers are connected to a physical bus, since the signal from each of them propagates in all directions at once and does not return back (as in a ring).

The maximum cable length of the network as a whole (maximum signal path) can theoretically reach 6.5 kilometers, but practically does not exceed 3.5 kilometers.

Rice. 7.1. Classic Ethernet network topology.

Fast Ethernet does not have a physical bus topology, only a passive star or passive tree is used. In addition, Fast Ethernet has much more stringent requirements for the maximum network length. Indeed, if the transmission speed is increased by 10 times and the format of the packet is preserved, its minimum length becomes ten times shorter. Thus, the permissible value of the double signal transit time through the network is reduced by 10 times (5.12 μs versus 51.2 μs in Ethernet).

The standard Manchester code is used to transmit information on an Ethernet network.

Access to the Ethernet network is carried out using a random CSMA / CD method, which ensures the equality of subscribers. The network uses packets of variable length.

For an Ethernet network operating at a speed of 10 Mbit / s, the standard defines four main types of network segments, focused on different media:

10BASE5 (thick coaxial cable);

10BASE2 (thin coaxial cable);

10BASE-T (twisted pair);

10BASE-FL (fiber optic cable).

The segment name includes three elements: the number "10" means the transmission rate of 10 Mbit / s, the BASE word - transmission in the base frequency band (that is, without modulation of the high-frequency signal), and the last element - the allowable segment length: "5" - 500 meters, "2" - 200 meters (more precisely, 185 meters) or the type of communication line: "T" - twisted pair (from English "twisted-pair"), "F" - fiber optic cable (from English "fiber optic").

Likewise, for an Ethernet network operating at a speed of 100 Mbps (Fast Ethernet), the standard defines three types of segments, differing in the types of transmission media:

· 100BASE-T4 (twisted pair);

· 100BASE-TX (double twisted pair);

· 100BASE-FX (fiber optic cable).

Here, the number "100" stands for a transmission rate of 100 Mbps, the letter "T" stands for a twisted pair, and the letter "F" stands for a fiber-optic cable. The types 100BASE-TX and 100BASE-FX are sometimes combined under the name 100BASE-X, and 100BASE-T4 and 100BASE-TX under the name 100BASE-T.

Token-Ring network

The Token-Ring (token ring) network was proposed by IBM in 1985 (the first option appeared in 1980). It was designed to network all types of computers made by IBM. The very fact that it is supported by IBM, the largest manufacturer of computer technology, suggests that it needs special attention. But no less important is the fact that Token-Ring is currently the international standard IEEE 802.5 (although there are minor differences between Token-Ring and IEEE 802.5). This puts this network on the same level as Ethernet in status.

Developed by Token-Ring as a reliable alternative to Ethernet. Although Ethernet is now superseding all other networks, Token-Ring is not hopelessly obsolete. More than 10 million computers worldwide are connected by this network.

The Token-Ring network has a ring topology, although it looks more like a star in appearance. This is due to the fact that individual subscribers (computers) are not connected to the network directly, but through special hubs or multi-station access devices (MSAU or MAU - Multistation Access Unit). Physically, the network forms a star-ring topology (Figure 7.3). In reality, the subscribers are nevertheless united in a ring, that is, each of them transmits information to one neighboring subscriber, and receives information from another.

Rice. 7.3. Star-ring token-ring network topology.

At first, twisted pair, both unshielded (UTP) and shielded (STP), were used as a transmission medium in the IBM Token-Ring network, but then there were options for equipment for coaxial cable, as well as for fiber optic cable in the FDDI standard.

The main specifications classic version of the Token-Ring network:

· The maximum number of concentrators such as IBM 8228 MAU - 12;

· The maximum number of subscribers in the network - 96;

· Maximum cable length between the subscriber and the hub - 45 meters;

· Maximum cable length between hubs - 45 meters;

· The maximum length of the cable connecting all the hubs is 120 meters;

· Data transfer rate - 4 Mbit / s and 16 Mbit / s.

All specifications are based on the use of an unshielded twisted pair cable. If a different transmission medium is used, the characteristics of the network may differ. For example, when using shielded twisted pair (STP), the number of subscribers can be increased to 260 (instead of 96), the cable length - up to 100 meters (instead of 45), the number of hubs - up to 33, and the total length of the ring connecting the hubs - up to 200 meters ... Fiber optic cable allows to extend the cable length up to two kilometers.

To transfer information in Token-Ring, a biphase code is used (more precisely, its version with a mandatory transition in the center of the bit interval). As with any star topology, no additional electrical termination or external grounding is required. Negotiation is performed by the hardware of the network adapters and hubs.

Token-Ring cables use RJ-45 (unshielded twisted pair) connectors, MIC and DB9P connectors. The wires in the cable connect the same pins of the connectors (that is, the so-called "straight" cables are used).

The Token-Ring network in the classic version is inferior to the Ethernet network both in the allowable size and in the maximum number of subscribers. In terms of transmission speed, there are currently 100 Mbps (High Speed Token-Ring, HSTR) and 1000 Mbps (Gigabit Token-Ring) versions of Token-Ring. Companies that support Token-Ring (including IBM, Olicom, Madge) do not intend to abandon their network, seeing it as a worthy competitor to Ethernet.

Compared to Ethernet hardware, Token-Ring hardware is noticeably more expensive, since it uses a more complex method of exchange control, so the Token-Ring network is not so widespread.

However, unlike Ethernet, Token-Ring network maintains a high load level much better (more than 30-40%) and provides guaranteed access time. This is necessary, for example, in industrial networks, where a delay in reaction to an external event can lead to serious accidents.

The Token-Ring network uses the classic token access method, that is, a token constantly circulates around the ring, to which subscribers can attach their data packets (see Fig. 4.15). This implies such an important advantage of this network as the absence of conflicts, but there are also disadvantages, in particular, the need to control the integrity of the token and the dependence of the functioning of the network on each subscriber (in the event of a malfunction, the subscriber must be excluded from the ring).

The maximum packet transfer time in Token-Ring is 10 ms. With a maximum number of 260 subscribers, the full cycle of the ring will be 260 x 10 ms = 2.6 s. During this time, all 260 subscribers will be able to transfer their packages (if, of course, they have something to transfer). During this time, a free marker will surely reach every subscriber. This interval is also the upper limit of the Token-Ring access time.

Arcnet network

Arcnet (or ARCnet from Attached Resource Computer Net) is one of the oldest networks. It was developed by the Datapoint Corporation back in 1977. There are no international standards for this network, although it is she who is considered the ancestor of the token access method. Despite the lack of standards, the Arcnet network until recently (in 1980 - 1990) was popular, even seriously competing with Ethernet. A large number of companies have produced equipment for this type of network. But now the production of Arcnet equipment is practically discontinued.

Among the main advantages of the Arcnet network in comparison with Ethernet are the limited amount of access time, high reliability of communication, ease of diagnostics, as well as the relatively low cost of adapters. The most significant disadvantages of the network include low data transfer rate (2.5 Mbit / s), addressing system and packet format.

To transfer information in the Arcnet network, a rather rare code is used, in which a logical one corresponds to two pulses during a bit interval, and a logical zero corresponds to one pulse. Obviously this is self-timing code that requires even more cable bandwidth than even Manchester's.

As a transmission medium in the network, a coaxial cable with a characteristic impedance of 93 Ohm is used, for example, of the RG-62A / U brand. Twisted pair options (shielded and unshielded) are not widely used. Fiber optic options have been proposed, but they haven't saved Arcnet either.

As a topology, the Arcnet network uses the classic bus (Arcnet-BUS) as well as a passive star (Arcnet-STAR). Hubs are used in the star. It is possible to combine bus and star segments into a tree topology using hubs (as with Ethernet). The main limitation is that there should be no closed paths (loops) in the topology. Another limitation is that the number of daisy chained segments using hubs must not exceed three.

Thus, the topology of the Arcnet network is as follows (Figure 7.15).

Rice. 7.15. Arcnet network topology of bus type (B - adapters for working in the bus, S - adapters for working in a star).

The main technical characteristics of the Arcnet network are as follows.

· Transmission medium - coaxial cable, twisted pair.

· The maximum length of the network is 6 kilometers.

· The maximum cable length from the subscriber to the passive concentrator is 30 meters.

· The maximum cable length from the subscriber to the active concentrator is 600 meters.

· The maximum cable length between active and passive hubs is 30 meters.

· The maximum cable length between active hubs is 600 meters.

· The maximum number of subscribers in the network is 255.

· The maximum number of subscribers on the bus segment is 8.

· The minimum distance between subscribers in the bus is 1 meter.

· The maximum length of a bus segment is 300 meters.

· Data transfer rate - 2.5 Mbit / s.

When creating complex topologies, it is necessary to ensure that the delay in the propagation of signals in the network between subscribers does not exceed 30 μs. The maximum attenuation of the signal in the cable at a frequency of 5 MHz should not exceed 11 dB.

Arcnet uses token access (pass-through), but is slightly different from Token-Ring. This method is closest to the one provided in the IEEE 802.4 standard.

As with Token-Ring, conflicts are completely eliminated in Arcnet. Like any token network, Arcnet holds the load well and guarantees the amount of network access time (as opposed to Ethernet). The total round trip time of all subscribers by the marker is 840 ms. Accordingly, the same interval determines the upper limit of the network access time.

The token is generated by a special subscriber - the network controller. It is the subscriber with the minimum (zero) address.

FDDI network

The FDDI network (from the English Fiber Distributed Data Interface, fiber-optic distributed data interface) is one of the latest developments local area network standards. The FDDI standard was proposed by the American National Standards Institute ANSI (ANSI X3T9.5 specification). Then the ISO 9314 standard was adopted, corresponding to the ANSI specifications. The level of network standardization is quite high.

Unlike other standard local area networks, the FDDI standard was initially focused on high transmission rates (100 Mbit / s) and on the use of the most promising fiber-optic cable. Therefore, in this case, the developers were not constrained by the framework of the old standards, focused on low speeds and electrical cable.

The choice of fiber as a transmission medium determined such advantages of the new network as high noise immunity, maximum secrecy of information transmission and excellent galvanic isolation of subscribers. The high transmission speed, which is much easier to achieve in the case of fiber-optic cable, allows you to solve many problems that are not available in lower-speed networks, for example, the transmission of images in real time. In addition, fiber optic cable easily solves the problem of transmitting data over a distance of several kilometers without retransmission, which makes it possible to build large networks, covering even entire cities and having all the advantages of local networks (in particular, a low error rate). All this determined the popularity of the FDDI network, although it is not yet as widespread as Ethernet and Token-Ring.

The FDDI standard was based on the token access method provided by the international standard IEEE 802.5 (Token-Ring). Insignificant differences from this standard are determined by the need to provide a high speed of information transmission over long distances. FDDI network topology is a ring, the most suitable topology for fiber optic cable. The network uses two multidirectional fiber-optic cables, one of which is usually in reserve, but this solution also allows the use of full-duplex information transmission (simultaneously in two directions) with twice the effective speed of 200 Mbit / s (with each of the two channels operating at a speed 100 Mbps). A star-ring topology with hubs included in the ring (as in Token-Ring) is also used.

Basic technical characteristics of the FDDI network.

· The maximum number of network subscribers is 1000.

· The maximum length of the network ring is 20 kilometers.

· The maximum distance between network subscribers is 2 kilometers.

· Transmission medium - multimode fiber-optic cable (electrical twisted pair can be used).

· Access method - marker.

· Information transfer rate - 100 Mbit / s (200 Mbit / s for duplex transmission mode).

The FDDI standard has significant advantages over all previously discussed networks. For example, a Fast Ethernet network with the same bandwidth of 100 Mbps cannot match FDDI in terms of network size. In addition, the FDDI token access method provides, unlike CSMA / CD, guaranteed access time and the absence of conflicts at any load level.

The limitation on the total network length of 20 km is associated not with the attenuation of signals in the cable, but with the need to limit the time for the complete passage of the signal along the ring to ensure the maximum allowable access time. But the maximum distance between subscribers (2 km with a multimode cable) is determined precisely by the attenuation of signals in the cable (it should not exceed 11 dB). The possibility of using a single-mode cable is also provided, in which case the distance between subscribers can reach 45 kilometers, and the total length of the ring is 200 kilometers.

There is also an implementation of FDDI on an electrical cable (CDDI - Copper Distributed Data Interface or TPDDI - Twisted Pair Distributed Data Interface). This uses a Category 5 cable with RJ-45 connectors. The maximum distance between subscribers in this case should be no more than 100 meters. The cost of network equipment on an electric cable is several times less. But this version of the network no longer has such obvious advantages over competitors as the original fiber-optic FDDI. The electrical versions of FDDI are much less standardized than fiber optic versions, so the compatibility of equipment from different manufacturers is not guaranteed.

For data transmission in FDDI, a 4V / 5V code is used, specially developed for this standard.

To achieve high network flexibility, the FDDI standard provides for the inclusion of two types of subscribers in the ring:

Class A subscribers (stations) (subscribers double connection, DAS - Dual-Attachment Stations) connect to both (inner and outer) rings of the network. At the same time, the possibility of exchange at a speed of up to 200 Mbit / s or redundancy of the network cable is realized (if the main cable is damaged, the reserve cable is used). Equipment of this class is used in the most critical parts of the network from the point of view of performance.

· Class B subscribers (stations) (subscribers of a single connection, SAS - Single-Attachment Stations) are connected to only one (external) ring of the network. They are simpler and cheaper than class A adapters, but lack their capabilities. They can be connected to the network only through a hub or bypass switch, which turns them off in case of an emergency.

In addition to the actual subscribers (computers, terminals, etc.), the network uses Wiring Concentrators, the inclusion of which allows you to collect all connection points in one place in order to monitor the operation of the network, diagnose faults and simplify reconfiguration. When using different types of cables (for example, fiber-optic cable and twisted pair), the hub also performs the function of converting electrical signals into optical ones and vice versa. Hubs are also available in Dual-Attachment Concentrator (DAC) and Single-Attachment Concentrator (SAC).

An example of an FDDI network configuration is shown in Fig. 8.1. The principle of combining network devices is illustrated in Figure 8.2.

Rice. 8.1. An example of an FDDI network configuration.

Unlike the access method offered by the IEEE 802.5 standard, FDDI uses what is known as multiple token passing. If, in the case of Token-Ring network, a new (free) token is transmitted by the subscriber only after returning his packet to him, then in FDDI a new token is transmitted by the subscriber immediately after the end of the packet transmission to him (similar to how it is done with the ETR method in the Token- Ring).

In conclusion, it should be noted that, despite the obvious advantages of FDDI, this network has not become widespread, which is mainly due to the high cost of its equipment (on the order of several hundred and even thousands of dollars). The main area of application of FDDI today is backbone networks that connect multiple networks. FDDI is also used to connect powerful workstations or servers that require high-speed communication. It is assumed that the Fast Ethernet network may overtake FDDI, but the advantages of fiber optic cable, token control method and record allowable network size currently put FDDI out of competition. And where hardware cost is critical, the twisted-pair version of FDDI (TPDDI) can be used in non-critical areas. In addition, the cost of FDDI equipment can greatly decrease with an increase in its production volume.

100VG-AnyLAN network

100VG-AnyLAN is one of the latest high-speed local area networks that has recently entered the market. It complies with the international standard IEEE 802.12, so the level of its standardization is quite high.

Its main advantages are a high exchange rate, a relatively low cost of equipment (about twice as expensive as the equipment of the most popular Ethernet 10BASE-T network), a centralized method of managing exchange without conflicts, as well as compatibility at the level of packet formats with Ethernet and Token-Ring networks.

In the name of the 100VG-AnyLAN network, the number 100 corresponds to a speed of 100 Mbit / s, the letters VG denote a cheap unshielded twisted pair cable of category 3 (Voice Grade), and AnyLAN (any network) denotes that the network is compatible with the two most common networks.

The main technical characteristics of the 100VG-AnyLAN network:

· Transfer rate - 100 Mbps.

· Topology - a star with the possibility of extension (tree). The number of cascading levels of concentrators (hubs) is up to 5.

· Access method - centralized, conflict-free (Demand Priority - with a priority request).

· Transmission media - quad unshielded twisted pair (UTP category 3, 4, or 5 cables), double twisted pair (UTP category 5 cable), double shielded twisted pair (STP), and fiber optic cable. Nowadays, quad twisted pair is mainly widespread.

· The maximum cable length between the hub and the subscriber and between the hubs is 100 meters (for UTP category 3 cable), 200 meters (for UTP category 5 cable and shielded cable), 2 kilometers (for fiber optic cable). The maximum possible network size is 2 kilometers (determined by the allowable delays).

· Maximum number of subscribers - 1024, recommended - up to 250.

Thus, the parameters of the 100VG-AnyLAN network are quite close to those of the Fast Ethernet network. However, the main advantage of Fast Ethernet is full compatibility with the most common Ethernet network (in the case of 100VG-AnyLAN, this requires a bridge). At the same time, the centralized management of 100VG-AnyLAN, which eliminates conflicts and guarantees the maximum amount of access time (which is not provided for in the Ethernet network), also cannot be discounted.

An example of the structure of a 100VG-AnyLAN network is shown in Fig. 8.8.

The 100VG-AnyLAN network consists of a central (main, root) level 1 concentrator, to which both individual subscribers and level 2 concentrators can be connected, to which subscribers and level 3 concentrators, etc., can be connected, etc. Moreover, the network can have no more than five such levels (in the original version there were no more than three). Maximum size the network can be 1000 meters for an unshielded twisted pair.

Rice. 8.8. Network structure 100VG-AnyLAN.

Unlike non-intelligent hubs on other networks (eg Ethernet, Token-Ring, FDDI), 100VG-AnyLAN hubs are intelligent controllers that control network access. To do this, they continuously monitor requests coming to all ports. Hubs accept incoming packets and send them only to the subscribers to whom they are addressed. However, they do not perform any information processing, that is, in this case, it is still not an active, but not a passive star. Hubs cannot be called full-fledged subscribers.

Each of the hubs can be configured to accept Ethernet or Token-Ring packet formats. In this case, the hubs of the entire network must work with packets of only one format. Bridging is required to communicate with Ethernet and Token-Ring networks, but bridging is fairly simple.

Hubs have one upper-level port (for connecting it to a higher-level hub) and several lower-level ports (for connecting subscribers). A computer (workstation), server, bridge, router, switch can act as a subscriber. Another hub can also be connected to the lower port.

Each port of the hub can be set to one of two possible modes of operation:

· Normal mode assumes forwarding to the subscriber connected to the port only packets addressed to him personally.

· Monitor mode assumes forwarding to the subscriber connected to the port of all packets arriving at the hub. This mode allows one of the subscribers to control the operation of the entire network as a whole (to perform the monitoring function).

The 100VG-AnyLAN access method is typical for star networks.

When using a quad twisted pair, each of the four twisted pairs is transmitted at a speed of 30 Mbps. The total transfer rate is 120 Mbps. However, useful information due to the use of the 5B / 6B code is transmitted only at a speed of 100 Mbps. Thus, the bandwidth of the cable must be at least 15 MHz. Category 3 twisted pair cable (bandwidth - 16 MHz) meets this requirement.

Thus, the 100VG-AnyLAN network is an affordable solution for increasing the transmission speed up to 100 Mbps. However, it does not have full compatibility with any of the standard networks, so its further fate is problematic. In addition, unlike the FDDI network, it does not have any record parameters. Most likely, 100VG-AnyLAN, despite the support of reputable companies and a high level of standardization, will remain just an example of interesting technical solutions.

If we talk about the most common 100-megabit Fast Ethernet network, then 100VG-AnyLAN provides twice the length of UTP category 5 cable (up to 200 meters), as well as a conflict-free method of exchange control.

Ethernet despite
for all his success, has never been elegant.
NICs only have rudimentary
the concept of intelligence. They really
first send the packet, and only then
see if anyone else has transmitted data
simultaneously with them. Someone compared Ethernet to
a society in which people can communicate
with each other only when everyone screams
simultaneously.

Like him
predecessor, Fast Ethernet uses the method
CSMACD (Carrier Sense Multiple Access with
Collision Detection - Multiple access to environment with
carrier sense and collision detection).
Behind this long and incomprehensible acronym
hiding a very simple technology. When
the Ethernet board should send a message, then
first she waits for silence, then
sends a packet and listens at the same time, not
did anyone send a message
simultaneously with him. If this happened then
both packages do not reach the addressee. If
there was no collision, but the board should continue
transmit data, it still waits
a few microseconds before again
will try to send a new batch. This
made to ensure that other boards also
could work and no one was able to capture
the channel is monopoly. In case of collision, both
devices fall silent for a small
time span generated
randomly and then take
a new attempt to transfer data.

Due to collisions, neither
Ethernet, nor Fast Ethernet will ever be able to achieve
its maximum performance 10
or 100 Mbps. As soon as it starts
increase network traffic, temporary
delays between sending individual packets
are reduced, and the number of collisions
increases. Real
Ethernet performance cannot exceed
70% of its potential bandwidth
ability, and maybe even lower if the line
seriously overwhelmed.

Ethernet uses
the packet size is 1516 bytes, which is fine
fit when it was first created.
Today this is considered a disadvantage when
Ethernet is used for communication
servers as servers and communication lines
tend to exchange large
the number of small packages that
overloads the network. In addition, Fast Ethernet
imposes a limit on the distance between
connected devices - no more than 100
meters and it forces to show
extra caution when
designing such networks.

Ethernet was first
designed on the basis of bus topology,
when all devices were connected to a common
cable, thin or thick. Application
twisted pair has only partially changed the protocol.
When using a coaxial cable
the collision was determined at once by all
stations. In the case of twisted pair
use the "jam" signal as soon as
the station detects a collision, then it
sends a signal to the hub, the latter in
in turn sends "jam" to everyone
devices connected to it.

In order to
reduce congestion, Ethernet networks
split into segments that
unite with bridges and
routers. This allows you to transfer
only necessary traffic between segments.
A message passed between two
stations in one segment will not
transferred to another and will not be able to call in it
overload.

Today at
building a central highway,
unifying servers use
switched Ethernet. Ethernet switches can
regarded as high-speed
multiport bridges capable of
independently determine in which of its
ports the packet is addressed to. Switch
looks at packet headers and so
compiles a table defining
where is this or that subscriber with such
physical address. This allows
limit the scope of the package
and reduce the likelihood of overflow,
sending it only to the correct port. Only
broadcast packets are sent by
all ports.

100BaseT
- big brother 10BaseT

Technology idea
Fast Ethernet was born in 1992. In August
next year a group of producers
merged into the Fast Ethernet Alliance (FEA).
The FEA's goal was to obtain
Fast Ethernet formal approval from committee
802.3 Institute of Electrical Engineers and
radioelectronics (Institute of Electrical and Electronic
Engineers, IEEE), since this committee
deals with standards for Ethernet. Luck
accompanied by new technology and
supporting alliance: in June 1995
all formal procedures have been completed, and
Fast Ethernet technology was named
802.3u.

With a light hand IEEE
Fast Ethernet is referred to as 100BaseT. This is explained
simple: 100BaseT is an extension
10BaseT standard with bandwidth from
10M bps to 100 Mbps. The 100BaseT standard includes
into a protocol for processing multiple
carrier-sense access and
CSMA / CD collision detection (Carrier Sense Multiple
Access with Collision Detection), which is also used in
10BaseT. In addition, Fast Ethernet can operate on
cables of several types, including
twisted pair. Both of these properties are new
standards are very important to potential
buyers, and thanks to them 100BaseT
turns out to be a good way to migrate networks
based on 10BaseT.

The main
a selling point for 100BaseT
is that Fast Ethernet is based on
inherited technology. Since Fast Ethernet
the same transfer protocol is used
messages as in older Ethernet versions, and
cable systems of these standards
compatible, to go to 100BaseT from 10BaseT
required

smaller
capital investment than for installation
other types of high-speed networks. Besides
addition, since 100BaseT is
continuation of the old Ethernet standard, all
tools and procedures
network analysis, as well as all
software running on
older Ethernet networks must, in this standard,
keep working capacity.
Hence the 100BaseT environment will be familiar
network administrators with experience
with Ethernet. This means that staff training will take
less time and will cost significantly
cheaper.

PRESERVATION
Of the PROTOCOL

Perhaps,
the greatest practical use of the new
technology brought the decision to leave
message transfer protocol unchanged.
The message transfer protocol, in our case
CSMA / CD, defines the way in which data
transmitted over the network from one node to another
through the cable system. In the ISO / OSI model
CSMA / CD protocol is part of the layer
media access control (MAC).
At this level, the format is defined, in
where information is transmitted over the network, and
the way the network device gets
network access (or network management) for
data transmission.

CSMA / CD name
can be broken down into two parts: Carrier Sense Multiple Access
and Collision Detection. From the first part of the name you can
conclude how a node with a network
the adapter determines the moment when it
a message should be sent. In accordance with
CSMA protocol, the network node first "listens"
network to determine if it is being transmitted to
any other message at the moment.
If you hear a carrier tone,
it means that the network is currently busy with another
message - the network node goes into the mode
waiting and dwells in it until the network
will be released. When the network comes
silence, the node starts transmitting.
In fact, the data is sent to all nodes
network or segment, but are accepted only by
the node to which they are addressed.

Collision Detection -
the second part of the name is used to resolve
situations where two or more nodes are trying
send messages at the same time.
According to the CSMA protocol, everyone is ready for
transmission, the node must first listen to the network,
to determine if she is free. But,
if two nodes are listening at the same time,
they both decide the network is free and start
transmit your packages at the same time. In this
situations transmitted data
overlap each other (network
engineers call it a conflict), and not a single
from messages does not reach the point
destination. Collision Detection requires the node
listened to the network also after the transmission
package. If a conflict is found, then
node repeats transmission through random
the chosen period of time and
checks again if a conflict has occurred.

THREE KINDS OF FAST ETHERNET

As well as
preservation of the CSMA / CD protocol, other important
the solution was to design 100BaseT like this
so that it can be applied
cables of different types - like those that
are used in older Ethernet versions and
newer models. The standard defines three
modifications to work with
different types of Fast Ethernet cables: 100BaseTX, 100BaseT4
and 100BaseFX. Modifications 100BaseTX and 100BaseT4 are calculated
twisted pair, and 100BaseFX was designed for
optical cable.

100BaseTX standard
requires two pairs of UTP or STP. One
a pair is used for transmission, the other for
reception. These requirements are met by two
major cable standard: EIA / TIA-568 UTP
Category 5 and STP Type 1 from IBM. In 100BaseTX
attractive provision
full duplex mode when working with
network servers, as well as the use
only two out of four pairs of an eight-core
cable - the other two pairs remain
free and can be used in
further to empower
networks.

However, if you
going to work with 100BaseTX, using for
of this Category 5 wiring, then you should
be aware of its shortcomings. This cable
more expensive than other eight-core cables (for example
Category 3). Also, to work with it
the use of punchdown blocks is required (punchdown
blocks), connectors and patch panels,
meeting the requirements of Category 5.
It should be added that for support
full duplex mode should be
install full duplex switches.

100BaseT4 standard
differs in softer requirements for
the cable you are using. The reason for this is
the fact that 100BaseT4 uses
all four pairs of an eight-core cable: one
for transmission, another for reception, and
the remaining two work as a transmission,
and at the reception. Thus, in 100BaseT4 and reception,
and data transmission can be carried out by
three pairs. By decomposing 100 Mbps into three pairs,
100BaseT4 decreases the frequency of the signal, so
enough and less
high quality cable. For implementation
For 100BaseT4 networks, Category 3 and UTP cables are suitable.
5, as well as UTP Category 5 and STP Type 1.

Advantage
100BaseT4 is less rigid
wiring requirements. Category 3 and
4 are more common, and in addition, they
significantly cheaper than cables
Category 5 things to keep in mind before
the beginning of installation work. The disadvantages are
are that 100BaseT4 requires all four
pairs and that full duplex is this
not supported by the protocol.

Fast Ethernet includes
also a standard for working with multimode
optical fiber with 62.5-micron core and 125-micron
shell. The 100BaseFX standard is focused on
mainly on the trunk - for connection
Fast Ethernet repeaters within one
building. Traditional benefits
optical cable are inherent in the standard
100BaseFX: immunity to electromagnetic
noise, improved data protection and large
distance between network devices.

RUNNER
SHORT DISTANCES

Although Fast Ethernet and
is a continuation of the Ethernet standard,
no migration from 10BaseT to 100BaseT
be regarded as a mechanical substitute
equipment - for this they can
changes in network topology are required.

Theoretical
Fast Ethernet segment diameter limit
is 250 meters; it's only 10
percent theoretical size limit
Ethernet network (2500 meters). This limitation
stems from the nature of the CSMA / CD protocol and
transmission speed 100Mbit / s.

What already
noted earlier transmitting data
the workstation must listen to the network in
the passage of time to make sure
that the data has reached the destination station.
On an Ethernet network with a bandwidth of 10
Mbps (for example 10Base5) time span,
required workstation for
listening to the network for a conflict,
is determined by the distance, which is 512-bit
frame (frame size is specified in the Ethernet standard)
will pass during the processing of this frame by
workstation. For Ethernet with bandwidth
with a capacity of 10 Mbps, this distance is
2500 meters.

On the other side,
the same 512-bit frame (802.3u standard
specifies a frame the same size as 802.3, then
is in 512 bits), transmitted by the working
station in the Fast Ethernet network, only 250 m will pass,
before the workstation completes it
processing. If the receiving station were
removed from the transmitting station by
distance over 250 m, then the frame could
come into conflict with another frame on
lines somewhere further, and the transmitting
the station, having completed the transmission, is no longer
would accept this conflict. So
the maximum diameter of a 100BaseT network is
250 meters.

To
use the allowed distance,
you need two repeaters to connect
all nodes. According to the standard,
maximum distance between node and
repeater is 100 meters; in Fast Ethernet,
as in 10BaseT, the distance between
hub and workstation not
must exceed 100 meters. Insofar as
connecting devices (repeaters)
introduce additional delays, real
working distance between nodes can
be even smaller. So
it seems reasonable to take all
distances with some margin.

To work on
long distances will have to be purchased
optical cable. For example, equipment
100BaseFX in half duplex mode allows
connect a switch to another switch
or a terminal station located on
distance up to 450 meters from each other.
With 100BaseFX full duplex installed, you can
connect two network devices on
distance up to two kilometers.

HOW
INSTALL 100BASET

In addition to cables,
which we have already discussed for installing Fast
Ethernet network adapters are required to
workstations and servers, hubs
100BaseT and possibly some
100BaseT switches.

Adapters,
necessary for organizing a 100BaseT network,
are called 10/100 Mbps Ethernet adapters.
These adapters are capable of (this requirement
standard 100BaseT) independently distinguish 10
Mbps from 100 Mbps. To serve the group
servers and workstations transferred to
100BaseT, you also need a 100BaseT hub.

When turned on
server or personal computer from
with a 10/100 adapter, the latter issues a signal,
announcing what he can provide
bandwidth 100Mbps. If
receiving station (most likely, this
there will be a hub) is also designed for
work with 100BaseT, it will give a signal in response,
to which both the hub and the PC or the server
automatically switch to 100BaseT mode. If
the hub only works with 10BaseT, it does not
returns a signal and the PC or server
will automatically switch to 10BaseT mode.

When
small-scale 100BaseT configurations can be
use a 10/100 bridge or switch that
will provide communication of the part of the network working with
100BaseT, with pre-existing network
10BaseT.

Deceiving
RAPIDITY

Summing it all up
the above, we note that, as it seems to us,
Fast Ethernet is best for problem solving
high peak loads. For example, if
some user is working with CAD or
image processing programs and
needs an increase in throughput
ability, then Fast Ethernet may be
a good way out. However, if
problems caused by excess
users on the network, then 100BaseT starts
slow down the exchange of information at about 50%
network load - in other words, on the same
level as 10BaseT. But in the end it is
after all, nothing more than an extension.

The most widespread among standard networks is the Ethernet network. It first appeared in 1972 (developed by the well-known company Xerox). The network turned out to be quite successful, and as a result of this, in 1980 it was supported by such major companies as DEC and Intel (the merger of these companies was named DIX after the first letters of their names). Through their efforts in 1985, the Ethernet network became an international standard, it was adopted by the largest international standards organizations: the 802 IEEE committee (Institute of Electrical and Electronic Engineers) and ECMA (European Computer Manufacturers Association).

The standard was named IEEE 802.3 (in English it reads as eight oh two dot three). It defines multiple access to a mono-channel of the bus type with collision detection and transmission control, that is, with the already mentioned CSMA / CD access method. Some other networks also met this standard, since the level of detail is low. As a result, networks of the IEEE 802.3 standard were often incompatible with each other, both in terms of design and electrical characteristics... Recently, however, the IEEE 802.3 standard has been considered the standard for the Ethernet network.

Key features of the original IEEE 802.3 standard:

topology - bus;
transmission medium - coaxial cable;
transmission speed - 10 Mbit / s;
maximum network length - 5 km;
the maximum number of subscribers is up to 1024;
network segment length - up to 500 m;
number of subscribers on one segment - up to 100;
access method - CSMA / CD;
transmission is narrowband, that is, without modulation (mono channel).

Strictly speaking, there are minor differences between the IEEE 802.3 and Ethernet standards, but they usually prefer not to be remembered.

In addition to the standard bus topology, passive star and passive tree topologies are increasingly being used. This assumes the use of repeaters and repeater hubs connecting different parts (segments) of the network. As a result, a tree-like structure can be formed on segments of different types (Fig. 7.1).

Rice. 7.1. Classic Ethernet topology

The maximum cable length of the network as a whole (maximum signal path) can theoretically reach 6.5 kilometers, but practically does not exceed 3.5 kilometers.

The standard Manchester code is used to transmit information on an Ethernet network.

Access to the Ethernet network is carried out using a random CSMA / CD method, which ensures the equality of subscribers. The network uses packets of variable length with the structure shown in Fig. 7.2. (numbers show the number of bytes)

Rice. 7.2. Ethernet packet structure

The Ethernet frame length (that is, the packet without the preamble) must be at least 512 bit intervals or 51.2 µs (this is the limit for the double transit time in the network). Provides individual, multicast and broadcast addressing.

The Ethernet packet includes the following fields:

The preamble consists of 8 bytes, the first seven are the code 10101010, and the last byte is the code 10101011. In the IEEE 802.3 standard, the eighth byte is called the Start of Frame Delimiter (SFD) and forms a separate field of the packet.
The recipient (receiver) and sender (transmitter) addresses include 6 bytes each and are built according to the standard described in the Packet Addressing section of Lecture 4. These address fields are processed by the subscribers' equipment.
The control field (L / T - Length / Type) contains information about the length of the data field. It can also determine the type of protocol used. It is generally accepted that if the value of this field is not more than 1500, then it indicates the length of the data field. If its value is more than 1500, then it determines the frame type. The control field is processed programmatically.
The data field must contain between 46 and 1500 bytes of data. If the packet is to contain less than 46 bytes of data, then the data field is padded with padding bytes. According to the IEEE 802.3 standard, a special padding field (pad data) is allocated in the packet structure, which can have a length of zero when there is enough data (more than 46 bytes).
The Frame Check Sequence (FCS) field contains a 32-bit cyclic packet checksum (CRC) and is used to check the correctness of the packet transmission.

Thus, the minimum frame length (packet without preamble) is 64 bytes (512 bits). It is this value that determines the maximum allowable double delay of signal propagation over the network in 512 bit intervals (51.2 μs for Ethernet or 5.12 μs for Fast Ethernet). The standard assumes that the preamble may shrink as the packet passes through various network devices, so it is ignored. The maximum frame length is 1518 bytes (12144 bits, i.e. 1214.4 μs for Ethernet, 121.44 μs for Fast Ethernet). This is important for choosing the size of the buffer memory of network equipment and for assessing the overall network load.

The choice of the preamble format is not accidental. The fact is that the sequence of alternating ones and zeros (101010 ... 10) in the Manchester code is characterized by the fact that it has transitions only in the middle of the bit intervals (see Section 2.6.3), that is, only information transitions. Of course, it is easy for the receiver to tune (synchronize) with such a sequence, even if for some reason it is shortened by a few bits. The last two unit bits of the preamble (11) differ significantly from the sequence 101010 ... 10 (transitions also appear at the border of the bit intervals). Therefore, an already tuned receiver can easily select them and thereby detect the beginning useful information(start of frame).

For an Ethernet network operating at a speed of 10 Mbit / s, the standard defines four main types of network segments, focused on different media:

10BASE5 (thick coaxial cable);
10BASE2 (thin coaxial cable);
10BASE-T (twisted pair);
10BASE-FL (fiber optic cable).

The segment name includes three elements: the number 10 means the transmission rate of 10 Mbit / s, the word BASE means transmission in the main frequency band (that is, without modulation of the high-frequency signal), and the last element means the permissible segment length: 5 - 500 meters, 2 - 200 meters (more precisely, 185 meters) or the type of communication line: T - twisted pair (from English twisted-pair), F - fiber optic cable (from English fiber optic).

Likewise, for an Ethernet network operating at a speed of 100 Mbps (Fast Ethernet), the standard defines three types of segments, differing in the types of transmission media:

100BASE-T4 (twisted pair);
100BASE-TX (twisted pair);
100BASE-FX (fiber optic cable).

Here, the number 100 stands for a transmission rate of 100 Mbps, the letter T for a twisted pair, and the letter F for a fiber optic cable. The types 100BASE-TX and 100BASE-FX are sometimes combined under the name 100BASE-X, and 100BASE-T4 and 100BASE-TX under the name 100BASE-T.

The features of Ethernet equipment, as well as the CSMA / CD exchange control algorithm and the cyclic checksum (CRC) calculation algorithm will be discussed in more detail later in special sections of the course. It should be noted here only that the Ethernet network does not differ in either record characteristics or optimal algorithms; it is inferior in a number of parameters to other standard networks. But thanks to strong support, the highest level of standardization, huge production volumes technical means, Ethernet stands out favorably among other standard networks, and therefore any other network technology is usually compared with Ethernet.

The evolution of Ethernet technology is moving away from the original standard. The use of new transmission media and switches can significantly increase the size of the network. Abandoning the Manchester code (on Fast Ethernet and Gigabit Ethernet) results in higher data rates and reduced cable requirements. Rejection of the CSMA / CD control method (with full-duplex exchange mode) makes it possible to dramatically increase the efficiency of work and remove restrictions on the length of the network. However, all of the newer types of networking are also referred to as Ethernet.

Token-Ring network

IBM has done everything to make its network as widespread as possible: detailed documentation has been released up to schematic diagrams adapters. As a result, many companies, for example, 3COM, Novell, Western Digital, Proteon and others, started to manufacture adapters. By the way, the NetBIOS concept was developed specifically for this network, as well as for another IBM PC Network. Whereas in the previously created PC Network, NetBIOS programs were stored in the read-only memory built into the adapter, in the Token-Ring network, a NetBIOS emulation program was already used. This made it possible to more flexibly respond to the peculiarities of the hardware and maintain compatibility with higher-level programs.

Rice. 7.3. Star-ring token-ring network topology

At the same time, the hub (MAU) allows you to centralize the configuration task, disconnect faulty subscribers, monitor network operation, etc. (fig. 7.4). It does not perform any information processing.

Rice. 7.4. Ringing Token-Ring Subscribers Using a Hub (MAU)

For each subscriber, a special Trunk Coupling Unit (TCU) is used as part of the hub, which provides automatic inclusion of the subscriber in the ring if it is connected to the hub and is working properly. If a subscriber disconnects from the hub or fails, the TCU automatically restores the integrity of the ring without the participation of this subscriber. The TCU is triggered by a DC signal (the so-called phantom current), which comes from a subscriber who wants to join the ring. The subscriber can also disconnect from the ring and carry out a self-test procedure (the far right subscriber in Fig. 7.4). The phantom current does not affect the information signal in any way, since the signal in the ring does not have a constant component.

Structurally, the hub is a self-contained unit with ten connectors on the front panel (Fig. 7.5).

Rice. 7.5. Token-Ring Hub (8228 MAU)

Eight central connectors (1 ... 8) are intended for connecting subscribers (computers) using adapter cables or radial cables. The two extreme connectors: input RI (Ring In) and output RO (Ring Out) are used to connect to other hubs using special trunk cables (Path cables). Wall-mount and desktop-mount options are available.

There are both passive and active MAUs. An active hub recovers the signal coming from the subscriber (that is, it acts as an Ethernet hub). The passive hub does not perform signal recovery, it only re-switches the communication lines.

The hub in the network can be the only one (as in Figure 7.4), in this case only the subscribers connected to it are closed in the ring. Outwardly, this topology looks like a star. If more than eight subscribers need to be connected to the network, then several hubs are connected by trunk cables and form a star-ring topology.

As noted, ring topology is very sensitive to ring cable breaks. To increase the survivability of the network, Token-Ring provides a so-called ring folding mode, which allows you to bypass the break point.

In normal mode, the hubs are connected in a ring by two parallel cables, but information is transmitted only through one of them (Fig. 7.6).

Rice. 7.6. Combining MAUs in Normal Mode

In the event of a single damage (breakage) of the cable, the network transmits through both cables, thereby bypassing the damaged section. At the same time, the order of bypassing subscribers connected to concentrators is even preserved (Fig. 7.7). True, the total length of the ring increases.

In the event of multiple damage to the cable, the network splits into several parts (segments) that are not connected to each other, but remain fully operational (Fig. 7.8). The maximum part of the network remains connected, as before. Of course, this no longer rescues the network as a whole, but it allows, with the correct distribution of subscribers to concentrators, to preserve a significant part of the functions of the damaged network.

Several hubs can be structurally combined into a group, a cluster, within which subscribers are also connected in a ring. The use of clusters allows you to increase the number of subscribers connected to one center, for example, up to 16 (if the cluster includes two hubs).

Rice. 7.7. Collapsing the ring when the cable is damaged

Rice. 7.8. Ring disintegration with multiple cable damage

The main technical characteristics of the classic version of the Token-Ring network:

the maximum number of IBM 8228 MAU type hubs is 12;
the maximum number of subscribers in the network is 96;
maximum cable length between the subscriber and the hub - 45 meters;
maximum cable length between hubs - 45 meters;
the maximum length of the cable connecting all the hubs is 120 meters;
data transfer rate - 4 Mbit / s and 16 Mbit / s.

Compared to Ethernet hardware, Token-Ring hardware is noticeably more expensive, since it uses a more complex method of exchange control, so the Token-Ring network is not so widespread.

The Token-Ring network uses the classic token access method, that is, a token constantly circulates around the ring, to which subscribers can attach their data packets (see Fig. 7.8). This implies such an important advantage of this network as the absence of conflicts, but there are also disadvantages, in particular, the need to control the integrity of the token and the dependence of the functioning of the network on each subscriber (in the event of a malfunction, the subscriber must be excluded from the ring).

Each subscriber of the network (its network adapter) must perform the following functions:

identification of transmission errors;
network configuration control (network restoration in case of failure of the subscriber that precedes him in the ring);
control of numerous time relationships adopted in the network.

The large number of functions, of course, complicates and increases the cost of the network adapter hardware.

To control the integrity of the token in the network, one of the subscribers is used (the so-called active monitor). At the same time, his equipment is no different from the rest, but his software monitor the temporal relationships in the network and form, if necessary, a new marker.

The active monitor performs the following functions:

launches a marker into the ring at the beginning of work and when it disappears;
regularly (every 7 seconds) informs about his presence with a special control package (AMP - Active Monitor Present);
removes from the ring a packet that was not removed by the subscriber who sent it;
monitors the allowed packet transmission time.

The active monitor is selected when the network is initialized; it can be any computer on the network, but, as a rule, it becomes the first subscriber connected to the network. The subscriber, who has become an active monitor, includes its buffer (shift register) into the network, which guarantees that the marker will fit into the ring even with the minimum ring length. The size of this buffer is 24 bits for 4 Mbps and 32 bits for 16 Mbps.

Each subscriber constantly monitors how the active monitor performs its duties. If the active monitor fails for some reason, a special mechanism is activated through which all other subscribers (spare, backup monitors) decide on the appointment of a new active monitor. To do this, the subscriber who detects the failure of the active monitor transmits a control packet (token request packet) over the ring with its MAC address. Each subsequent subscriber compares the MAC address from the packet with its own. If its own address is less, it passes the packet on unchanged. If more, then it sets its own MAC address in the packet. The active monitor will be the subscriber whose MAC-address value is higher than that of the others (he must receive back a packet with his MAC-address three times). A sign of failure of the active monitor is its failure to perform one of the listed functions.

The Token-Ring network token is a control packet containing only three bytes (Figure 7.9): the Start Delimiter byte (SD), Access Control byte (AC), and End Delimiter byte (ED). All these three bytes are also included in the information package, although their functions in the marker and in the package are somewhat different.

The leading and trailing separators are not just a sequence of zeros and ones, but contain signals of a special kind. This was done so that the delimiters could not be confused with any other packet bytes.

Rice. 7.9. Token-Ring Token Format

The initial SD delimiter contains four non-standard bit intervals (Figure 7.10). Two of them, denoted by J, represent a low signal level during the entire bit interval. The other two bits, labeled K, represent a high signal level for the entire bit interval. It is clear that such timing failures are easily detected by the receiver. Bits J and K can never occur among the bits of useful information.

Rice. 7.10. Leading (SD) and Ending (ED) Delimiter Formats

The final delimiter ED also contains four special bits (two J bits and two K bits), as well as two one bits. But, in addition, it also includes two information bits, which are meaningful only as part of an information package:

Bit I (Intermediate) is a sign of an intermediate packet (1 corresponds to the first in a chain or an intermediate packet, 0 - to the last in a chain or a single packet).
The E (Error) bit is a sign of a detected error (0 corresponds to the absence of errors, 1 to their presence).

The Access Control (AC) byte is divided into four fields (Figure 7.11): a priority field (three bits), a marker bit, a monitor bit, and a reservation field (three bits).

Rice. 7.11. Access Control Byte Format

The priority bits (field) allow the subscriber to assign priority to his packets or token (priority can be from 0 to 7, with 7 being the highest priority and 0 being the lowest). The subscriber can attach his package to the marker only when his own priority (the priority of his packages) is the same or higher than the priority of the token.

The marker bit determines whether a packet is attached to the marker or not (one corresponds to a marker without a packet, zero - to a marker with a packet). The monitor bit, set to one, indicates that this marker was transmitted by the active monitor.

Reservation bits (field) allow the subscriber to reserve his right to further seize the network, that is, to take a queue for service. If the subscriber's priority (the priority of his packets) is higher than the current value of the reservation field, then he can write his priority there instead of the previous one. After looping around the ring, the highest priority of all subscribers will be recorded in the reservation field. The content of the reservation field is similar to the content of the priority field, but indicates the future priority.

As a result of the use of priority and reservation fields, only subscribers with the highest priority packets for transmission are able to access the network. Lower priority packets will be served only when higher priority packets are exhausted.

The format of the information packet (frame) Token-Ring is shown in Fig. 7.12. In addition to the start and end delimiters, and the access control byte, this packet also includes the packet control byte, receiver and transmitter network addresses, data, checksum, and packet status byte.

Rice. 7.12. Packet (frame) format of the Token-Ring network (the length of the fields is given in bytes)

Purpose of the fields of the packet (frame).

The leading delimiter (SD) is the start of the packet, the format is the same as in the marker.
The Access Control (AC) byte has the same format as the token.
The Packet Control Byte (FC - Frame Control) defines the type of packet (frame).
The six-byte source and destination MAC addresses of a packet follow the standard format described in Chapter 4.
The data field (Data) includes the transmitted data (in an information packet) or information for control of the exchange (in a control packet).
The Frame Check Sequence (FCS) field is a 32-bit cyclic packet checksum (CRC).
The trailing separator (ED), as in the marker, indicates the end of the packet. In addition, it determines whether the given packet is intermediate or final in the sequence of transmitted packets, and also contains a sign of packet error (see Fig. 7.10).
The packet status byte (FS - Frame Status) tells what happened to the given packet: whether it was seen by the receiver (that is, whether there is a receiver with the specified address) and copied into the receiver's memory. From it, the sender of the packet knows whether the packet arrived at its destination and without errors, or if it needs to be transmitted again.

It should be noted that the larger allowable size of the transmitted data in one packet compared to an Ethernet network can be a decisive factor in increasing network performance. Theoretically, for transfer rates of 16 Mbit / s and 100 Mbit / s, the length of the data field can even reach 18 Kbytes, which is essential when transferring large amounts of data. But even at 4 Mbps, Token-Ring often delivers faster actual transfer rates than 10 Mbps Ethernet, thanks to token-based access. The advantage of Token-Ring is especially noticeable at high loads (over 30-40%), since in this case the CSMA / CD method requires a lot of time to resolve repeated conflicts.

A subscriber wishing to transmit a packet waits for a free token to arrive and captures it. The captured marker is transformed into the frame of the information packet. Then the subscriber transmits the information packet to the ring and waits for its return. It then releases the token and sends it back to the network.

In addition to the token and the usual packet, a special control packet can be transmitted in the Token-Ring network, which serves to interrupt the transmission (Abort). It can be sent anytime and anywhere in the data stream. This package consists of two one-byte fields - the initial (SD) and final (ED) delimiters of the described format.

Interestingly, the faster version of Token-Ring (16 Mbps and higher) uses the so-called Early Token Release (ETR) method. It avoids network overhead while the data packet is looped back to its sender.

The ETR method boils down to the fact that immediately after transmitting its packet attached to the token, any subscriber issues a new free token to the network. Other subscribers can start transmitting their packets immediately after the end of the packet of the previous subscriber, without waiting for him to complete the traversal of the entire network ring. As a result, there can be several packets on the network at the same time, but there will always be no more than one free token. This pipeline is especially effective on long-haul networks that have significant propagation delay.

When a subscriber is connected to the hub, it performs the procedure of autonomous self-test and cable testing (it does not turn on in the ring yet, since there is no phantom current signal). The subscriber sends himself a number of packets and checks the correctness of their passage (his input is directly connected to his output by the TCU, as shown in Fig. 7.4). After that, the subscriber includes himself in the ring, sending a phantom current. At the moment of switching on, the packet transmitted over the ring can be corrupted. Next, the subscriber sets up synchronization and checks for an active monitor on the network. If there is no active monitor, the subscriber starts the competition for the right to become one. Then the subscriber checks the uniqueness of his own address in the ring and collects information about other subscribers. After which he becomes a full participant in the exchange over the network.

In the course of the exchange, each subscriber monitors the health of the previous subscriber (around the ring). If he suspects a failure of the previous subscriber, he starts the automatic ring recovery procedure. A special control package (buoy) tells the previous subscriber to conduct a self-test and, possibly, disconnect from the ring.

The Token-Ring network also provides for the use of bridges and switches. They are used to divide a large ring into several ring segments that can exchange packets with each other. This allows you to reduce the load on each segment and increase the proportion of time provided to each subscriber.

As a result, it is possible to form a distributed ring, that is, the union of several ring segments into one large backbone ring (Figure 7.13) or a star-ring structure with a central switch to which the ring segments are connected (Figure 7.14).

Rice. 7.13. Connecting segments with a trunk ring using bridges

Rice. 7.14. Aggregation of segments with a central switch

Arcnet (or ARCnet from Attached Resource Computer Net) is one of the oldest networks. It was developed by the Datapoint Corporation back in 1977. There are no international standards for this network, although it is she who is considered the ancestor of the token access method. Despite the lack of standards, the Arcnet network until recently (in 1980 - 1990) was popular, even seriously competing with Ethernet. A large number of companies (for example, Datapoint, Standard Microsystems, Xircom, etc.) have produced equipment for this type of network. But now the production of Arcnet equipment is practically discontinued.

Hubs are of two types:

Active concentrators (restore the shape of incoming signals and amplify them). The number of ports is from 4 to 64. Active hubs can be interconnected (cascaded).
Passive hubs (just mix the incoming signals without amplification). The number of ports is 4. Passive hubs cannot be connected to each other. They can only link active hubs and / or network adapters.

Bus segments can only be connected to active hubs.

There are also two types of network adapters:

High impedance (Bus) for use in bus segments:
Low impedance (Star) designed for use in a passive star.

Low impedance adapters differ from high impedance adapters in that they contain 93-ohm matching terminators. When using them, external approval is not required. In bus segments, low impedance adapters can be used as terminating adapters for bus termination. High impedance adapters require external 93 ohm termination. Some network adapters have the ability to switch from a high impedance state to a low impedance state, they can work in the bus and in the star.

Thus, the topology of the Arcnet network looks like this (Figure 7.15).

Rice. 7.15. Arcnet network topology of bus type (B - adapters for working in the bus, S - adapters for working in a star)

The main technical characteristics of the Arcnet network are as follows.

Transmission medium - coaxial cable, twisted pair.
The maximum length of the network is 6 kilometers.
The maximum cable length from the subscriber to the passive hub is 30 meters.
The maximum cable length from the subscriber to the active hub is 600 meters.
The maximum cable length between active and passive hubs is 30 meters.
The maximum cable length between active hubs is 600 meters.
The maximum number of subscribers in the network is 255.
The maximum number of subscribers on a bus segment is 8.
The minimum distance between subscribers in the bus is 1 meter.
The maximum length of a bus segment is 300 meters.
The data transfer rate is 2.5 Mbps.

Arcnet uses token access (pass-through), but is slightly different from Token-Ring. This method is closest to the one provided in the IEEE 802.4 standard. The sequence of actions of subscribers with this method:

1. The subscriber who wants to transmit is waiting for the arrival of the token.

2. Having received the token, he sends a request to transmit information to the receiving subscriber (asks if the receiver is ready to receive his packet).

3. The receiver, having received the request, sends a response (confirms its readiness).

4. Having received confirmation of readiness, the sender subscriber sends his packet.

5. On receiving the packet, the receiver sends an acknowledgment of the packet.

6. The transmitter, having received an acknowledgment of packet reception, ends its communication session. After that, the token is passed to the next subscriber in descending order of network addresses.

Thus, in this case, the packet is transmitted only when there is confidence in the readiness of the receiver to receive it. This significantly increases the reliability of the transmission.

The token is generated by a special subscriber - the network controller. It is the subscriber with the minimum (zero) address.

If the subscriber does not receive a free token within 840 ms, then he sends a long bit sequence to the network (to ensure the destruction of the damaged old token). After that, the procedure for monitoring the network and assigning (if necessary) a new controller is carried out.

The Arcnet package size is 0.5 KB. In addition to the data field, it also includes 8-bit receiver and transmitter addresses and a 16-bit cyclic checksum (CRC). Such a small packet size turns out to be not very convenient with a high traffic intensity over the network.

Arcnet network adapters differ from other network adapters in that they need to set their own network address using switches or jumpers (there can be 255 of them, since the last, 256th address is used in the network for broadcasting mode). The control over the uniqueness of each network address is entirely the responsibility of the network users. Connecting new subscribers becomes quite difficult at the same time, since it is necessary to set the address that has not yet been used. The choice of the 8-bit address format limits the number of network subscribers to 255, which may not be enough for large companies.

As a result, all this led to the almost complete abandonment of the Arcnet network. There were 20 Mbit / s versions of the Arcnet network, but these were not widely adopted.