The link layer of fddi technology. Features of the fddi access method. FDDI networks - principle of operation, equipment used, use cases Mode of operation of the fddi network in the case of

networks FDDI . Protocols, history, status

In Russia, the process of intensive introduction of new and modernization of existing local area networks (LAN) continues. Increasing sizes of networks, applied software systems, requiring ever-higher information exchange speeds, increasing requirements for reliability and fault tolerance are forcing us to look for an alternative to traditional Ethernet and Arcnet networks. One type of high-speed network is FDDI (Fiber Distributed Data Interface).

Network computer complexes become an integral means of production of any organization or enterprise. Quick access to information, its reliability increases the likelihood of making the right decisions by staff and, ultimately, the likelihood of winning in the competition. In their managers and information systems firms see the means of strategic superiority over competitors and consider investment in them as a capital investment.

Due to the fact that the processing and transfer of information using computers are becoming faster and more efficient, there is a real information explosion. LANs are beginning to merge into geographically distributed networks, the number of servers, workstations and peripheral equipment connected to the LAN is increasing.

Today in Russia, the computer networks of many large enterprises and organizations are one or more LANs built on the basis of Arcnet or Ethernet standards. The network operating environment is typically NetWare v3.12 or Windows NT with one or more file servers. These LANs either have no connection to each other at all, or are connected by a cable operating in one of these standards through internal or external NetWare software routers.

Modern operating systems and application software require the transfer of large amounts of information for their work. At the same time, it is required to ensure the transmission of information at high speeds and over long distances. Therefore, sooner or later, the performance of Ethernet networks and software bridges and routers ceases to meet the growing needs of users, and they begin to consider the possibility of using faster standards in their networks. One of them is FDDI.

General information.

FDDI (Fiber Distributed Data Interface- Fiber-optic data interface) - a standard for data transmission in a local network stretched over a distance of up to 200 kilometers. In this area, the FDDI network is capable of supporting several thousand users.

FDDI technology is largely based on Token Ring technology, developing and improving its main ideas. Token ring - Local area network (LAN) ring technology with "token access" - a local area network protocol that resides at the data link layer (DLL) of the OSI model. A station can start transmitting its own data frames only if it has received a special frame from the previous station - an access token. After that, she can transfer her frames, if she has them, for a time called the token holding time - Token Holding Time (THT). After the expiration of the THT time, the station must complete the transmission of its next frame and pass the access token to the next station. If, at the time of accepting the token, the station does not have frames to transmit over the network, then it immediately broadcasts the token of the next station. In an FDDI network, each station has an upstream neighbor and a downstream neighbor determined by its physical links and direction of information transfer.

Each station in the network constantly receives the frames transmitted to it by the previous neighbor and analyzes their destination address. If the destination address does not match its own, then it broadcasts the frame to its subsequent neighbor. It should be noted that if the station has captured the token and transmits its own frames, then during this period of time it does not broadcast incoming frames, but removes them from the network.

If the frame address matches the address of the station, then it copies the frame to its internal buffer, checks its correctness (mainly by checksum), passes its data field for further processing to a protocol lying above the FDDI level (for example, IP), and then transmits the original frame over the network of the subsequent station. In a frame transmitted to the network, the destination station notes three signs: address recognition, frame copying, and the absence or presence of errors in it.

After that, the frame continues to travel through the network, being broadcast by each node. The station, which is the source of the frame for the network, is responsible for removing the frame from the network after it, having made a full turn, reaches it again. In this case, the source station checks the signs of the frame, whether it reached the destination station and whether it was damaged. The process of restoring information frames is not the responsibility of the FDDI protocol, this should be handled by higher layer protocols.

The FDDI network is built on the basis of two fiber optic rings, which form the main and backup data transmission paths between network nodes. The use of two rings is the main way to increase fault tolerance in an FDDI network, and nodes that want to use it must be connected to both rings. In the normal mode of network operation, data passes through all nodes and all sections of the cable of the primary (Primary) ring, so this mode is called end-to-end or "transit". The secondary ring (Secondary) is not used in this mode.

In the event of some form of failure where part of the primary ring is unable to transmit data (for example, a cable break or node failure), the primary ring is combined with the secondary, forming a single ring again. This network mode is called Wrap, i.e. "folding" or "folding" the rings. The folding operation is performed by the forces of concentrators and/or network adapters FDDI. To simplify this procedure, data on the primary ring is always transmitted counterclockwise, and on the secondary - clockwise. Therefore, when a common ring is formed from two rings, the transmitters of the stations still remain connected to the receivers of neighboring stations, which makes it possible to correctly transmit and receive information by neighboring stations.

Since the FDDI network uses fiber optic cable as a transmission medium, the moment of technology development was largely delayed due to the long introduction of fiber optic cables and the elimination of errors associated with new fiber optic technology.

Back in 1880, Alexander Bell patented a device that transmitted speech over a distance of up to 200 meters using a mirror that vibrated synchronously with sound waves and modulated the reflected light. And only in the 1980s, work began on the creation of conventional technologies and devices for the use of fiber optic channels in local networks. Work on the generalization of experience and the development of the first fiber optic standard for local networks were concentrated at the American State Standards Institute - ANSI, within the framework of the X3T9.5 committee created for this purpose.

The initial versions of the various components of the FDDI standard were developed by the X3T9.5 committee in 1986-1988, and at the same time the first equipment appeared - network adapters, hubs, bridges and routers that support this standard.

Currently, most network technologies support fiber optic cables as one of the physical layer options, but FDDI remains the most established high-speed technology, the standards for which have stood the test of time and are well-established, so that equipment from different manufacturers shows a good degree of compatibility.

FDDI protocols

The figure shows the protocol structure of FDDI technology in comparison with the seven-layer OSI model. FDDI defines the physical layer protocol and the media access sublayer (MAC) protocol of the link layer. Like many other LAN technologies, FDDI uses the 802.2 Data Link Control (LLC) protocol defined in the IEEE 802.2 and ISO 8802.2 standards. FDDI uses the first type of LLC procedures, in which nodes operate in datagram mode - connectionless and without recovering lost or corrupted frames.

The physical layer is divided into two sublayers: the media-independent PHY (Physical) sublayer, and the media-dependent PMD (Physical Media Dependent) sublayer. The operation of all levels is controlled by the SMT (Station Management) station management protocol.

The PMD layer provides the necessary means to transfer data from one station to another over fiber. Its specification defines:

Optical power requirements and 62.5/125 µm multimode optical fiber.

Requirements for optical bypass switches and optical transceivers.

Parameters of optical connectors MIC (Media Interface Connector), their marking.

The wavelength of 1300 nanometers at which the transceivers operate.

Representation of signals in optical fibers according to the NRZI method.

The PHY layer performs encoding and decoding of data circulating between the MAC layer and the PMD layer, and also provides timing for information signals. Its specification defines:

encoding information in accordance with the scheme 4B/5B;

signal timing rules;

requirements for the stability of the clock frequency of 125 MHz;

rules for converting information from parallel to serial form.

The MAC layer is responsible for network access control and for receiving and processing data frames. It defines the following parameters:

Token transfer protocol.

Rules for capturing and relaying a token.

Frame formation.

Rules for generating and recognizing addresses.

Rules for calculating and verifying a 32-bit checksum.

The SMT layer performs all the functions of managing and monitoring all other layers of the FDDI protocol stack. Each node of the FDDI network takes part in ring management. Therefore, all hosts exchange special SMT frames to manage the network. The SMT specification defines the following:

Algorithms for detecting errors and recovering from failures.

Rules for monitoring the operation of the ring and stations.

Ring management.

Ring initialization procedures.

Fault tolerance of FDDI networks is provided by managing the SMT layer by other layers: using the PHY layer, network failures are eliminated for physical reasons, for example, due to a cable break, and using the MAC layer, logical network failures, for example, the loss of the desired internal token transfer path and data frames between hub ports.

State.

Technology developers tried to implement the following:

· Increase the bit rate of data transfer up to 100 Mb/s;

· To increase the network fault tolerance due to standard procedures for restoring it after failures of various kinds - cable damage, incorrect operation of a node, hub, high noise level on the line, etc.;

· Make the most of potential network bandwidth for both asynchronous and synchronous traffic.

Based on this, the advantage of FDDI technology is the combination of several properties that are very important for local networks:

1. high degree of fault tolerance;

2. The ability to cover large areas, up to the territories of large cities;

3. High speed data exchange;

4. Deterministic access, allowing the transfer of delay-sensitive applications;

5. Flexible mechanism for distributing the bandwidth of the ring between stations;

6. Possibility of work at the load factor of the ring close to one;

7. The ability to easily translate FDDI traffic into graphics of such popular protocols as Ethernet and Token Ring due to the compatibility of station address formats and the use of a common LLC sublayer.

So far, FDDI is the only technology that has managed to combine all of these properties. In other technologies, these properties also occur, but not in combination. For example, Fast Ethernet technology also has a data transfer rate of 100 Mbps, but it does not allow the network to be restored after a single cable break and does not make it possible to work with a high network load factor (if you do not take Fast Ethernet switching into account).

The PMD layer provides the necessary means to transfer data from one station to another over fiber. Its specification defines:

Optical power requirements and 62.5/125 µm multimode optical fiber.

Requirements for optical bypass switches and optical transceivers.

Parameters of optical connectors MIC (Media Interface Connector), their marking.

The wavelength of 1300 nanometers at which the transceivers operate.

Representation of signals in optical fibers according to the NRZI method.

The PHY layer performs encoding and decoding of data circulating between the MAC layer and the PMD layer, and also provides timing for information signals. Its specification defines:

encoding information in accordance with the scheme 4B/5B;

signal timing rules;

requirements for the stability of the clock frequency of 125 MHz;

rules for converting information from parallel to serial form.

The MAC layer is responsible for network access control and for receiving and processing data frames. It defines the following parameters:

Token transfer protocol.

Rules for capturing and relaying a token.

Frame formation.

Rules for generating and recognizing addresses.

Rules for calculating and verifying a 32-bit checksum.

Algorithms for detecting errors and recovering from failures.

Rules for monitoring the operation of the ring and stations.

Ring management.

Ring initialization procedures.

Fault tolerance of FDDI networks is ensured by controlling the SMT layer by other layers: using the PHY layer, network failures are eliminated for physical reasons, for example, due to a cable break, and using the MAC layer, logical network failures, for example, the loss of the desired internal token transfer path and data frames between hub ports.

State.

Technology developers tried to implement the following:

· Increase the bit rate of data transfer up to 100 Mb/s;

· Increase network fault tolerance due to standard procedures for restoring it after failures of various kinds - cable damage, incorrect operation of a node, hub, a high level of interference on the line, etc.;

· Make the most of potential network bandwidth for both asynchronous and synchronous traffic.

Based on this, the advantage of FDDI technology is the combination of several properties that are very important for local networks:

1. high degree of fault tolerance;

2. The ability to cover large areas, up to the territories of large cities;

3. High speed data exchange;

4. Deterministic access, allowing the transfer of delay-sensitive applications;

5. Flexible mechanism for distributing the bandwidth of the ring between stations;

6. Possibility of work at the load factor of the ring close to one;

The disadvantages include one - the high cost of equipment. There is a price to pay for this unique combination of features - FDDI technology remains the most expensive 100-Mbit technology. Therefore, its main areas of application are the backbones of campuses and buildings, as well as the connection of corporate servers. In these cases, the costs are justified - the backbone of the network must be fault-tolerant and fast, the same applies to a server built on the basis of an expensive multiprocessor platform and serving hundreds of users. Due to the high cost of hardware, FDDI-based solutions are inferior to Fast Ethernet solutions in the construction of small LANs, when the Fast Ethernet standard provides the optimal solution.

In Russia, the process of intensive introduction of new and modernization of existing local area networks (LAN) continues. The growing size of networks, application software systems that require ever-higher speeds of information exchange, increasing requirements for reliability and fault tolerance force us to look for an alternative to traditional Ethernet and Arcnet networks. One type of high-speed network is FDDI (Fiber Distributed Data Interface). The article discusses the possibilities of using FDDI in the construction of corporate computer systems.

According to Peripheral Strategies forecasts worldwide by 1997 to local computer networks more than 90% of all personal computers(currently - 30-40%). Network computer systems are becoming an integral means of production of any organization or enterprise. Quick access to information and its reliability increase the likelihood of making the right decisions by staff and, ultimately, the likelihood of winning in the competition. Firms see their control and information systems as a means of strategic superiority over competitors and view investment in them as a capital investment.

Due to the fact that the processing and transmission of information using computers are becoming faster and more efficient, there is a real information explosion. LANs are beginning to merge into geographically distributed networks, the number of servers, workstations and peripheral equipment connected to the LAN is increasing.

Today in Russia, the computer networks of many large enterprises and organizations are one or more LANs built on the basis of Arcnet or Ethernet standards. The network operating environment is typically NetWare v3.11 or v3.12 with one or more file servers. These LANs either have no connection to each other at all, or are connected by a cable operating in one of these standards through internal or external NetWare software routers.

Modern operating systems and application software require the transfer of large amounts of information for their work. At the same time, it is required to ensure the transmission of information at ever higher speeds and over ever greater distances. Therefore, sooner or later, the performance of Ethernet networks and software bridges and routers ceases to meet the growing needs of users, and they begin to consider the possibility of using faster standards in their networks. One of them is FDDI.

How an FDDI Network Works

The FDDI network is a fiber optic token ring with a data transfer rate of 100 Mbps.

The FDDI standard was developed by the X3T9.5 committee of the American National Standards Institute (ANSI). FDDI networking supported by all leading manufacturers network equipment. The ANSI X3T9.5 committee has now been renamed X3T12.

The use of fiber optics as a propagation medium can significantly expand the cable bandwidth and increase the distance between network devices.

Let's compare the throughput of FDDI and Ethernet networks with multi-user access. The allowable level of utilization of the Ethernet network is within 35% (3.5 Mbps) of the maximum throughput (10 Mbps), otherwise the probability of collisions becomes not too high and the cable throughput will drop sharply. For FDDI networks, allowable utilization can reach 90-95% (90-95 Mbps). Thus, the throughput of FDDI is approximately 25 times higher.

The deterministic nature of the FDDI protocol (the ability to predict the maximum delay when transmitting a packet over a network and the ability to provide a guaranteed bandwidth for each of the stations) makes it ideal for use in real-time network control systems and in time-critical applications (for example, for video transmission). and sound information).

FDDI inherited many of its key properties from Token Ring networks (IEEE 802.5 standard). First of all, it is a ring topology and a marker method of accessing the medium. Marker - a special signal rotating around the ring. The station that received the token can transmit its data.

However, FDDI also has a number of fundamental differences from Token Ring, which makes it a faster protocol. For example, the data modulation algorithm at the physical layer has been changed. Token Ring uses a Manchester coding scheme that requires doubling the bandwidth of the transmitted signal relative to the transmitted data. FDDI implements a "five out of four" - 4V / 5V coding algorithm that provides the transmission of four information bits by five transmitted bits. When transmitting 100 Mbps of information per second, 125 Mbps are physically transmitted to the network, instead of 200 Mbps, which would be required when using Manchester coding.

Optimized and medium access control (VAC). In Token Ring it is based on a bit basis, while in FDDI it is based on the parallel processing of a group of four or eight transmitted bits. This reduces the hardware performance requirements.

Physically, the FDDI ring is formed by a fiber optic cable with two light-conducting windows. One of them forms the primary ring (primary ring), is the main one and is used for the circulation of data tokens. The second fiber forms the secondary ring, is redundant and is not used in normal mode.

Stations connected to the FDDI network fall into two categories.

Class A stations have physical connections to the primary and secondary rings (Dual Attached Station - doubly connected station);

2. Class AND stations are connected only to the primary ring (Single Attached Station - once connected station) and are connected only through special devices called hubs.

On fig. 1 shows an example of connecting a concentrator and stations of classes A and B in a closed loop, through which the marker circulates. On fig. Figure 2 shows a more complex network topology with a branched structure (Ring-of-Trees - a ring of trees) formed by class B stations.

Ports of network devices connected to the FDDI network are classified into 4 categories: A ports, B ports, M ports and S ports. Port A is the port that receives data from the primary ring and sends it to the secondary ring. Port B is the port that receives data from the secondary ring and sends it to the primary ring. M (Master) and S (Slave) ports transmit and receive data from the same ring. The M port is used on the hub to connect the Single Attached Station via the S port.

The X3T9.5 standard has a number of limitations. The total length of a double fiber optic ring is up to 100 km. Up to 500 class A stations can be connected to the ring. The distance between nodes when using a multimode fiber-optic cable is up to 2 km, and when using a single-mode cable, it is mainly determined by the parameters of the fiber and the transceiver equipment (it can reach 60 or more km).

Fault tolerance of FDDI networks

The ANSI X3T9.5 standard regulates 4 basic fault-tolerant properties of FDDI networks:

1. The ring cable system with class A stations is fault-tolerant to a single cable break anywhere in the ring. On fig. Figure 3 shows an example of both primary and secondary fiber breaks in a ring cable. Stations on either side of the cliff reconfigure the token and data path by connecting a secondary fiber optic ring.

2. A power outage, failure of one of the class B stations, or a broken cable from the hub to that station will be detected by the hub and the station will be disconnected from the ring.

3. Two class B stations are connected to two hubs at once. This special type of connection is called Dual Homing and can be used for fault-tolerant (to failures in the hub or in the cable system) connection of class B stations by duplicating the connection to the main ring. In normal mode, data exchange occurs only through one hub. If for any reason the connection is lost, then the exchange will be carried out through the second hub.

4. A power outage or failure of one of the class A stations will not cause the rest of the stations connected to the ring to fail, as the light signal will be passively transmitted to the next station via the optical switch (Optical Bypass Switch). The standard allows up to three sequentially located switched off stations.

Optical switches are manufactured by Molex and AMP.

Synchronous and asynchronous transmission

Connecting to the FDDI network, stations can transmit their data to the ring in two modes - synchronous and asynchronous.

Synchronous mode is arranged as follows. During the initialization of the network, the expected token round trip time is determined - TTRT (Target Token Rotation Time). Each station that captures the token is given a guaranteed time to transmit its data to the ring. After this time, the station must complete the transmission and send the token into the ring.

Each station at the time of sending a new token turns on a timer that measures the time interval until the token returns to it - TRT (Token Rotation Timer). If the token returns to the station before the expected TTRT bypass time, then the station may extend the time it takes to send its data to the ring after the end of the synchronous transmission. This is what asynchronous transmission is based on. The additional time interval for transmission by the station will be equal to the difference between the expected and real time bypassing the ring with a marker.

From the algorithm described above, it can be seen that if one or more stations do not have enough data to fully use the time slot for synchronous transmission, then the bandwidth not used by them immediately becomes available for asynchronous transmission by other stations.

cable system

The FDDI PMD (Physical medium-dependent layer) substandard defines a multimode fiber optic cable with a diameter of 62.5/125 µm as the basic cable system. It is allowed to use cables with a different fiber diameter, for example: 50/125 microns. Wavelength - 1300 nm.

The average power of the optical signal at the station input must be at least -31 dBm. With such an input power, the probability of an error per bit when retransmitting data by the station should not exceed 2.5 * 10 -10 . With an increase in the input signal power by 2 dBm, this probability should decrease to 10 -12 .

The standard defines the maximum allowable signal loss level in the cable as 11 dBm.

The FDDI substandard SMF-PMD (Single-mode fiber Physical medium-dependent layer) defines the requirements for the physical layer when using a single-mode fiber optic cable. In this case, a laser LED is usually used as a transmitting element, and the distance between stations can reach 60 or even 100 km.

FDDI modules for single-mode cable are produced, for example, by Cisco Systems for their Cisco 7000 and AGS+ routers. Singlemode and multimode cable segments can be interleaved in an FDDI ring. For these Cisco routers, you can select modules with all four port combinations: multimode-multimode, multimode-singlemode, singlemode-multimode, singlemode-singlemode.

Cabletron Systems Inc. releases Dual Attached repeaters - FDR-4000, which allow you to connect a single-mode cable to a class A station with ports designed to work on a multimode cable. These repeaters make it possible to increase the distance between the nodes of the FDDI ring up to 40 km.

The CDDI (Copper Distributed Data Interface) physical layer substandard defines the requirements for the physical layer when using shielded (IBM Type 1) and unshielded (Category 5) twisted pair. This greatly simplifies the installation process of the cabling system and reduces the cost of it, network adapters and hub equipment. Distances between stations when using twisted pairs should not exceed 100 km.

Lannet Data Communications Inc. releases FDDI modules for its hubs, which allow you to work either in standard mode, when the secondary ring is used only for fault tolerance in case of a cable break, or in advanced mode, when the secondary ring is also used for data transmission. In the second case, the bandwidth of the cable system is expanded to 200 Mbps.

Connecting equipment to the FDDI network

There are two main ways to connect computers to the FDDI network: directly, and also through bridges or routers to networks of other protocols.

Direct connection

This connection method is used, as a rule, to connect files, archiving and other servers, medium and large computers to the FDDI network, that is, key network components that are the main computing centers that provide services to many users and require high I / O speeds over the network .

Workstations can be connected in the same way. However, since network adapters for FDDI are very expensive, this method is used only in cases where high network speed is a prerequisite for normal operation of the application. Examples of such applications: multimedia systems, video and audio transmission.

To connect personal computers to the FDDI network, specialized network adapters are used, which are usually inserted into one of the free slots on the computer. Such adapters are produced by the following companies: 3Com, IBM, Microdyne, Network Peripherials, SysKonnect, etc. There are cards on the market for all common buses - ISA, EISA and Micro Channel; there are adapters for connecting class A or B stations for all types of cable system - fiber optic, shielded and unshielded twisted pairs.

All leading manufacturers of UNIX machines (DEC, Hewlett-Packard, IBM, Sun Microsystems, and others) provide interfaces for direct connection to FDDI networks.

Connecting through bridges and routers

Bridges (bridges) and routers (routers) allow you to connect to FDDI networks of other protocols, such as Token Ring and Ethernet. This makes it possible to cost-effectively connect a large number of workstations and other network equipment to FDDI in both new and existing LANs.

Structurally, bridges and routers are manufactured in two versions - in a finished form, which does not allow further hardware growth or reconfiguration (the so-called standalone devices), and in the form of modular hubs.

Examples of standalone devices are Hewlett-Packard's Router BR and Network Peripherals' EIFO Client/Server Switching Hub.

Modular hubs are used in complex large networks as central network devices. The hub is a housing with a power supply and a communication board. Network communication modules are inserted into the slots of the hub. The modular design of the hubs makes it easy to assemble any LAN configuration, combine cable systems of various types and protocols. The remaining free slots can be used for further expansion of the LAN.

Hubs are manufactured by many companies: 3Com, Cabletron, Chipcom, Cisco, Gandalf, Lannet, Proteon, SMC, SynOptics, Wellfleet and others.

The hub is the central node of the LAN. Its failure can bring the entire network, or at least a significant part of it, to a halt. Therefore, most hub manufacturers take special measures to improve their fault tolerance. Such measures are the redundancy of power supplies in load sharing or hot standby mode, as well as the ability to change or reinstall modules without turning off the power (hot swap).

In order to reduce the cost of the hub, all its modules are powered from a common power source. The power elements of the power supply are the most likely cause of its failure. Therefore, the redundancy of the power supply significantly extends the uptime. During installation, each of the power supplies of the hub can be connected to a separate source uninterruptible power supply(UPS) in case of malfunctions in the power supply system. Each of the UPS is desirable to connect to the hotel power electrical networks from different substations.

The ability to change or reinstall modules (often including power supplies) without turning off the hub allows you to repair or expand the network without interrupting service for those users whose network segments are connected to other hub modules.

FDDI-to-Ethernet bridges

Bridges operate on the first two layers of the open systems interconnection model - physical and channel - and are designed to link multiple LANs of single or different physical layer protocols, such as Ethernet, Token Ring and FDDI.

According to their principle of operation, bridges are divided into two types (Sourece Routing - source routing) require that the node-sender of the packet place information about the path of its routing in it. In other words, each station must have built-in packet routing capabilities. The second type of bridges (Transparent Bridges - transparent bridges) provide transparent communication between stations located in different LANs, and all routing functions are performed only by the bridges themselves. Below, we will discuss only such bridges.

All bridges can add to the table of addresses (Learn addresses), route and filter packets. Smart bridges can also filter packets based on criteria set through the network management system to improve security or performance.

When a data packet arrives on one of the bridge ports, the bridge must either forward it to the port to which the packet's destination host is connected, or simply filter it out if the destination host is on the same port that the packet came from. Filtering avoids unnecessary traffic on other LAN segments.

Each bridge builds an internal table of physical addresses of nodes connected to the network. The filling process is as follows. Each packet has in its header the physical addresses of the origin and destination hosts. Having received a data packet on one of its ports, the bridge works according to the following algorithm. In the first step, the bridge checks to see if its internal table contains the host address of the packet's sender. If not, then the bridge enters it into a table and associates with it the port number on which the packet arrived. The second step checks to see if the address of the destination node is entered in the internal table. If not, the bridge forwards the received packet to all networks connected to all other ports. If the destination host address is found in the internal table, the bridge checks whether the destination host's LAN is connected to the same port that the packet came from or not. If not, then the bridge filters the packet, and if so, then it transmits it only to the port to which the network segment with the destination host is connected.

Three main parameters of the bridge:
- size of the internal address table;
- filtration speed;
- packet routing rate.

The size of the address table characterizes the maximum number of network devices whose traffic can be routed by the bridge. Typical address table sizes range from 500 to 8000. What happens if the number of connected nodes exceeds the address table size? Since most bridges store in it the network addresses of the hosts that last transmitted their packets, the bridge will gradually "forget" the addresses of the hosts as other transmit packets. This may lead to a decrease in the efficiency of the filtering process, but will not cause fundamental problems in the network.

Packet filtering and routing rates characterize the performance of a bridge. If they are below the maximum possible packet rate on the LAN, then the bridge can cause latency and performance degradation. If it is higher, then the cost of the bridge is higher than the minimum required. Let's calculate what the performance of the bridge should be for connecting several Ethernet protocol LANs to FDDI.

Let us calculate the maximum possible intensity of packets in the Ethernet network. The structure of Ethernet packets is shown in Table 1. The minimum packet length is 72 bytes or 576 bits. The time required to transmit one bit over an Ethernet LAN at 10 Mbps is 0.1 µs. Then the transmission time of the minimum packet length will be 57.6*10 -6 sec. The Ethernet standard requires a pause between packets of 9.6 µs. Then the number of packets transmitted in 1 second will be 1/((57.6+9.6)*10 -6 )=14880 packets per second.

If the bridge connects N Ethernet protocol networks to the FDDI network, then, respectively, its filtering and routing rates should be equal to N * 14880 packets per second.

Table 1.
Packet structure in Ethernet networks.

On the FDDI port side, the packet filtering rate should be much higher. In order for the bridge not to degrade network performance, it should be about 500,000 packets per second.

According to the principle of packet transmission, bridges are divided into Encapsulating Bridges and Translational Bridges. Physical layer packets of one LAN are completely transferred to physical layer packets of another LAN. After passing through the second LAN, another similar bridge removes the shell from the intermediate protocol, and the packet continues its movement in its original form.

Such bridges allow two Ethernet LANs to be connected by an FDDI backbone. However, in this case, FDDI will only be used as a transmission medium, and stations connected to Ethernet networks will not "see" stations directly connected to the FDDI network.

Bridges of the second type convert from one physical layer protocol to another. They remove the header and trailing overhead of one protocol and transfer data to another protocol. Such a conversion has a significant advantage: FDDI can be used not only as a transmission medium, but also for direct connection of network equipment, transparently visible to stations connected to Ethernet networks.

Thus, such bridges provide transparency of all networks over network and higher layer protocols (TCP / IP, Novell IPX, ISO CLNS, DECnet Phase IV and Phase V, AppleTalk Phase 1 and Phase 2, Banyan VINES, XNS, etc.).

One more important characteristic bridge - the presence or absence of support for the Spannig Tree Algorithm (STA) IEEE 802.1D. It is also sometimes referred to as the Transparent Bridging Standard (TBS).

On fig. Figure 1 shows a situation where there are two possible paths between LAN1 and LAN2 - via bridge 1 or via bridge 2. Situations similar to these are called active loops. Active loops can cause serious network problems: duplicate packets break the logic network protocols and lead to a decrease in the throughput of the cable system. STA enforces blocking of all possible ways, except one. However, in case of problems with the main communication line, one of the backup paths will immediately be set as active.

Intelligent bridges

So far, we have discussed the properties of arbitrary bridges. Intelligent bridges have a number of additional features.

For large computer networks, one of the key problems that determine their effectiveness is to reduce the cost of operation, early diagnosis possible problems, reducing troubleshooting time.

For this, centralized network management systems are used. As a rule, they work using the SNMP protocol (Simple Network Management Protocol) and allow the network administrator from his workplace:
- configure hub ports;
- produce a set of statistics and traffic analysis. For example, for each station connected to the network, you can get information about when it last sent packets to the network, the number of packets and bytes received by each station with a LAN different from the one to which it is connected, the number of broadcasts sent (broadcast) packages, etc.;

Install additional filters on hub ports by LAN numbers or by physical addresses of network devices in order to enhance protection against unauthorized access to network resources or to improve the efficiency of individual LAN segments;
- promptly receive messages about all emerging problems in the network and easily localize them;
- carry out diagnostics of concentrator modules;
- view in graphical form image of the front panels of modules installed in remote hubs, including the current status of indicators (this is possible due to the fact that the software automatically recognizes which module is installed in each particular slot of the hub, and receives information about the current status of all module ports);
- view the system log, which automatically records information about all problems with the network, the time of turning on and off workstations and servers, and all other important events for the administrator.

These features are common to all intelligent bridges and routers. Some of them (for example, Gandalf's Prism System) also have the following important advanced features:

1. Protocol priorities. According to separate protocols network layer some hubs act as routers. In this case, the setting of priorities of some protocols over others can be supported. For example, you can set TCP/IP to take precedence over all other protocols. This means that TCP/IP packets will be transmitted first (this is useful in case of insufficient cable system bandwidth).

2. Protection against "broadcast storms"(broadcast storm). One of characteristic faults network equipment and errors in software- spontaneous generation with a high intensity of broadcast packets, i.e. packets addressed to all other devices connected to the network. The network address of the destination host of such a packet consists of only ones. Having received such a packet on one of its ports, the bridge must address it to all other ports, including the FDDI port. In normal mode, such packets are used by operating systems for service purposes, for example, to send messages about the appearance of a new server on the network. However, with a high intensity of their generation, they will immediately occupy the entire bandwidth. The bridge provides network congestion protection by including a filter on the port from which such packets are received. The filter does not pass broadcast packets and other LANs, thereby protecting the rest of the network from overload and maintaining its performance.

3. Collection of statistics in the "What if?" This option allows you to virtually install filters on bridge ports. In this mode, filtering is not physically performed, but statistics are collected about packets that would be filtered if the filters were actually enabled. This allows the administrator to pre-evaluate the consequences of turning on the filter, thereby reducing the chance of errors when filtering conditions are set incorrectly and without leading to malfunctions of connected equipment.

FDDI Usage Examples

Here are two of the most typical examples of the possible use of FDDI networks.

Client-server applications. FDDI is used to connect equipment that requires a wide bandwidth from a LAN. Typically, these are NetWare file servers, UNIX machines, and large mainframes. In addition, as noted above, some workstations that require high data exchange rates can also be connected directly to the FDDI network.

User workstations are connected via multiport FDDI-Ethernet bridges. The bridge performs filtering and transmission of packets not only between FDDI and Ethernet, but also between different Ethernet networks. The data packet will only be transmitted to the port where the destination node is located, saving the bandwidth of other LANs. From the side of Ethernet networks, their interaction is equivalent to communication through the backbone (backbone), only in this case it does not physically exist in the form of a distributed cable system, but is entirely concentrated in a multiport bridge (Collapsed Backbone or Backbone-in-a-box).

FDDI (Fiber Distributed Data Interface) technology- fiber optic distributed data interface is the first LAN technology in which the data transmission medium is a fiber optic cable.

Work on the creation of technologies and devices for the use of fiber-optic channels in local networks began in the 80s, shortly after the start of industrial operation of such channels in territorial networks. The XZT9.5 problem group of the ANSI Institute developed in the period from 1986 to 1988. the initial versions of the FDDI standard, which provides the transmission of frames at a rate of 100 Mbit / s over a double fiber-optic ring up to 100 km long.

FDDI technology is largely based on Token Ring technology, developing and improving its main ideas. The developers of FDDI technology set themselves the following goals as the highest priority:

Increase the bit rate of data transfer to 100 Mbps;

Increase the fault tolerance of the network due to standard procedures for restoring it after failures of various kinds - cable damage, incorrect operation of the node, hub, the occurrence of a high level of interference on the line, etc.;

Make the most of potential bandwidth

network capability for both asynchronous and synchronous (delay sensitive) traffic.

The FDDI network is built on the basis of two fiber optic rings, which form the main and backup data transmission paths between network nodes. Having two rings is the primary way to increase resiliency in an FDDI network, and nodes that want to take advantage of this increased reliability potential should be connected to both rings.

In the normal mode of the network, data passes through all nodes and all sections of the cable only the primary (Primary) ring, this mode is called the Thru mode - “through” or “transit”. The secondary ring (Secondary) is not used in this mode.

In the event of some kind of failure where part of the primary ring is unable to transmit data (for example, a cable break or node failure), the primary ring is combined with the secondary (see figure), again forming a single ring. This mode of network operation is called Wrap, that is, "folding" or "folding" rings. The folding operation is performed by means of hubs and/or FDDI network adapters. To simplify this procedure, data on the primary ring is always transmitted in one direction (in the diagrams, this direction is shown counterclockwise), and on the secondary - in the opposite direction (shown clockwise). Therefore, when a common ring is formed from two rings, the transmitters of the stations still remain connected to the receivers of neighboring stations, which makes it possible to correctly transmit and receive information by neighboring stations.

Features of the access method.

For the transmission of synchronous frames, the station always has the right to acquire the token when it arrives. The holding time of the marker has a predetermined fixed value. If the station of the FDDI ring needs to transmit an asynchronous frame (the frame type is determined by the protocols of the upper layers), then to find out the possibility of capturing the token at its next occurrence, the station must measure the time interval that has passed since the previous arrival of the token. This interval is called the Token Rotation Time (TRT). The TRT interval is compared with another value - the maximum allowable token rotation time around the T_Opr ring. While in Token Ring technology the maximum allowable token rotation time is a fixed value (2.6 s based on 260 stations in the ring), in FDDI technology the stations agree on the T_Opr value during ring initialization. Each station can offer its own value of T_Opr, as a result, the minimum of the times offered by the stations is set for the ring.

Fault tolerance technology.

To ensure fault tolerance, the FDDI standard provides for the creation of two fiber optic rings - primary and secondary.

The FDDI standard allows two types of connection of stations to the network:

Simultaneous connection to the primary and secondary rings is called dual connection - Dual Attachment, DA.

Attaching only to the primary ring is called a single connection - Single Attachment, SA.

The FDDI standard provides for the presence of end nodes in the network - stations (Station), as well as hubs (Concentrator). For stations and hubs, any type of network connection is allowed - both single and double. Accordingly, such devices have the corresponding names: SAS (Single Attachment Station), DAS (Dual Attachment Station), SAC (Single Attachment Concentrator) and DAC (Dual Attachment Concentrator).

Typically, hubs are dual-wired and stations are single-wired, as shown in the figure, although this is not required. To make it easier to properly connect devices to the network, their connectors are marked. Connectors of type A and B must be for devices with a dual connection, an M (Master) connector is available for a hub for a single connection of a station, in which the mating connector must be of type S (Slave).

The physical layer is divided into two sublayers: the media-independent PHY (Physical) sublayer and the media-dependent PMD (Physical Media Dependent) sublayer

13. Structured cabling system /SCS/. Hierarchy in the cable system. Choice of cable type for different subsystems.

Structured cabling system (SCS) - the physical basis of the information infrastructure of the enterprise, which allows you to bring together single system many information services for various purposes: local computing and telephone networks, security systems, video surveillance, etc.

SCS is a hierarchical cable system of a building or a group of buildings, divided into structural subsystems. It consists of a set of copper and optical cables, cross panels, connecting cords, cable connectors, modular jacks, information outlets and auxiliary equipment. All of these elements are integrated into a single system and operated according to certain rules.

A cable system is a system whose elements are cables and components that are connected to the cable. Cable components include all passive switching equipment used to connect or physically terminate (terminate) a cable - telecommunication sockets at workplaces, crossover and patch panels (jargon: “patch panels”) in telecommunication premises, couplings and splices;

Structured. A structure is any set or combination of related and dependent constituent parts. The term “structured” means, on the one hand, the ability of the system to support various telecommunication applications (voice, data and video), on the other hand, the ability to use various components and products from different manufacturers, and on the third, the ability to implement the so-called multimedia environment, in which uses several types of transmission media - coaxial cable, UTP, STP and optical fiber. The structure of the cabling system is determined by the infrastructure information technologies, IT (Information Technology), it is she who dictates the content of a particular cabling project in accordance with the requirements of the end user, regardless of the active equipment that can be used subsequently.

14. Network adapters /CA/. Functions and characteristics of SA. SA classification. Principle of operation.

Network adapters act as a physical interface between the computer and the network cable. They are usually inserted into the expansion slots of workstations and servers. To provide a physical connection between the computer and the network, a network cable is connected to the appropriate port on the adapter after it is installed.

Functions and characteristics of network adapters.

The network adapter and its driver in a computer network perform the function of the physical layer and the MAC layer. The network adapter and the driver receive and transmit the frame. This operation takes place in several stages. Most often, the interaction of protocols with each other inside a computer occurs through buffers located inside the RAM.

It is known that network adapters implement protocols, and depending on which protocol they work with, adapters are divided into: Ethernet - adapters, FDDI - adapters, Token Ring - adapters, and many others. Most modern Ethernet adapters support two speeds, and therefore they also contain the prefix 10/100 in their name.

Before installing a network adapter on a computer, you need to configure it. In the event that the computer operating system and the network adapter itself support the Plug-and-Play standard, the adapter and its driver are automatically configured. If this standard is not supported, then you must first configure the network adapter, and then apply exactly the same parameters in configuring the driver. AT this process a lot depends on the manufacturer of the network adapter, as well as on the parameters and capabilities of the bus for which the adapter is intended.

Classification of network adapters.

Four generations have been noted in the development of Ethernet network adapters. For the manufacture of the first generation of adapters, discrete, logic microcircuits were used, so they were not very reliable. Their buffer memory was designed for only one frame, which already indicates that their performance was very low. In addition, this type of network adapter was configured using jumpers, which means manually.

So, we have already noted that the technology FDDI took much as a basis from technology token ring developing and improving her ideas. Technology Developers FDDI set the following goals as top priorities:

first, - increase the bit rate of data transmission to 100 Mbps;

secondly, to increase the fault tolerance of the network due to standard procedures for restoring it after failures of various kinds - cable damage, incorrect operation of the node, hub, the occurrence of a high level of interference on the line, etc.;

And also, to maximize the potential network bandwidth for both asynchronous and synchronous (delay-sensitive) traffic.

Network FDDI built on the basis two fiber optic rings that form basic and spare data transfer paths between network nodes.

It is the presence of two rings that has become the main way to increase fault tolerance in the network FDDI. Nodes that wish to take advantage of this increased reliability potential must be connected to both rings. Now we will consider this feature of building a network.

In normal network operation, data passes through all nodes and all sections of the cable only on the primary (primary) rings.

This mode is called the mode Thru - "through" or "transit". secondary ring (Secondary) not used in this mode.

In the event of some form of failure where part of the primary ring is unable to transmit data (for example, a cable break or node failure), the primary ring is merged with the secondary to form a single ring again.

This network mode is called Wrap, i.e. "folding" or "folding" rings.

Operation coagulation produced by means of hubs and/or network adapters technology FDDI.

To simplify this procedure, data on the primary ring is always transmitted in one direction (in the diagrams, this direction is shown counterclockwise), and on the secondary - in the opposite direction (shown clockwise). Therefore, when a common ring is formed from two rings, the transmitters of the stations still remain connected to the receivers of neighboring stations, which makes it possible to correctly transmit and receive information by neighboring stations.

So, let's take a look at the operation of stations in the network in general terms. FDDI:

Rings in networks FDDI, as in networks 802.5 are considered as a common shared data transmission medium, an access method is defined for it, very close to the access method of networks token ring and also called token ring method.

A station can start transmitting its own data frames only if it has received a special frame from the previous station - an access token (also commonly called a token). After that, she can transmit her frames, if she has them, during a time called token holding time - Token Holding Time (THT).

After the expiration of time THT the station must complete the transmission of its next frame and pass the access token to the next station. If, at the moment of accepting the token, the station does not have frames to transmit over the network, then it immediately broadcasts the token of the next station. Online FDDI each station has an upstream neighbor and a downstream neighbor determined by its physical links and the direction of information transfer.

If the frame address matches the address of the station, then it copies the frame to its internal buffer, checks its correctness (mainly by checksum), passes its data field for further processing to the protocol above FDDI level (for example, IP), and then transmits the original frame over the network of the subsequent station. In the frame transmitted to the network (as well as in the frame token ring) the destination station notes three signs: address recognition, frame copying, and the absence or presence of errors in it.

After that, the frame continues to travel through the network, being broadcast by each node. The station, which is the source of the frame for the network, is responsible for removing the frame from the network after it, having made a full turn, reaches it again. In this case, the source station checks the signs of the frame, whether it reached the destination station and whether it was damaged. The process of restoring information frames is not the responsibility of the protocol. FDDI, this should be handled by higher layer protocols.

Structure of technology protocols FDDI in the projection on the seven-level model OSI defines the physical layer protocol and the media access sublayer (MAC) protocol of the link layer. Like many other LAN technologies, the technology FDDI uses protocol 802.2 data link control (LLC) sublayer defined in standards IEEE 802.2 and ISO 8802.2. FDDI uses the first type of procedures LLC, in which nodes operate in datagram mode - without establishing connections and without recovering lost or corrupted frames.

In standards FDDI much attention is paid to the various procedures that allow you to determine the presence of a failure in the network, and then make the necessary reconfiguration.

Network FDDI can fully restore its performance in the event of single failures of its elements.

With multiple failures, the network breaks up into several unrelated networks.

Technology FDDI complements technology failure detection mechanisms token ring mechanisms for reconfiguring the data transmission path in the network, based on the presence of backup links provided by the second ring.

Access Method Differences FDDI are that marker holding time online FDDI is not a constant value, as in the network token ring.

Here, this time depends on the load of the ring - with a small load, it increases, and with large overloads, it can decrease to zero.

Changes to the access method only affect asynchronous traffic, which is not sensitive to small frame delays. For synchronous traffic, the token hold time is still a fixed value.

The frame priority mechanism that was present in the technology token ring, in technology FDDI absent. The developers of the technology decided that the division of traffic into 8 priority levels redundant and it is enough to simply divide the traffic into two classes - asynchronous and synchronous. Synchronous traffic is always serviced, even if the ring is congested.

Otherwise, the forwarding of frames between the stations of the ring at the level MAC, as we have already considered, is fully consistent with the technology token ring.

Stations FDDI use an early token release algorithm like networks token ring with speed 16 Mbps.

Addresses MAC level have a standard technologies IEEE 802 format.

Frame format FDDI also close to frame size token ring, the main differences are the absence of priority fields. Signs of address recognition, frame copying and errors allow you to save existing networks token ring procedures for processing frames by the sending station, intermediate stations and the receiving station.

Frame format

PA - Preamble: 16 or more blank characters.

SD - Starting Delimiter: sequence of "J" and "K".

FC - Frame Control: 2 characters responsible for the type of information in the INFO field

DA - Destination Address: 12 characters indicating to whom the frame is addressed.

SA - Source Address: 12 characters indicating the source address of the frame.

INFO - Information Field: 0 to 4478 bytes of information.

FCS - Checksum (Frame Check Sequence): 8 characters CRC.

ED - Ending Delimiter

Marker Format

Thus, despite the fact that FDDI technology was developed and standardized by the ANSI institute, and not by the IEEE committee, it fits perfectly into the structure of the 802 standards.

Of course, after all, there are also distinctive features of the standard ANSI - FDDI technologies.

One such feature is that technology FDDI another one highlighted station management level - Station Management (SMT).

Exactly the level SMT performs all the functions of managing and monitoring all other layers of the protocol stack FDDI. Specification SMT the following is defined:

Algorithms for detecting errors and recovering from failures;

Rules for monitoring the operation of the ring and stations;

Ring management;

Ring initialization procedures.

Participates in ring management every node networks FDDI. Therefore, all nodes exchange special SMT frames for network management.

Network resiliency FDDI It is provided by protocols of other levels: with the help of the physical layer, network failures due to physical reasons, for example, due to a cable break, are eliminated, and with the help of MAC level- logical failures of the network, such as the loss of the proper internal path for passing the token and data frames between the ports of the hub.

So, we have considered the most common characteristics of the technology FDDI. Let's take a closer look at the distinctive features.

Features of the FDDI Access Method

For the transmission of synchronous frames, the station always has the right to acquire the token when it arrives. The holding time of the marker has a predetermined fixed value.

If the stations of the ring FDDI If it is necessary to transmit an asynchronous frame (the frame type is determined by the protocols of the upper layers), then in order to find out the possibility of capturing the token at its next arrival, the station must measure the time interval that has passed since the previous arrival of the token.

This interval is called token rotation time (TRT).

Interval TRT compared with another value the maximum allowable turnaround time of the marker along the ring Т_0pr.

If in technology token ring we said that the maximum allowable turnaround time of the marker is a fixed value (2.6 s based on 260 stations in the ring), then in technology FDDI stations agree on a value T_0pr during ring initialization.

Each station can offer its value T_0pr, as a result, for the ring, minimum from the times suggested by the stations.

This feature makes it possible to take into account the needs of those applications that operate at the stations of the ring.

In general, synchronous (real-time) applications need to send data to the network more often in small chunks, while asynchronous applications are better off accessing the network less often, but in large chunks. Preference is given to stations transmitting synchronous traffic.

Thus, at the next arrival of the token for the transmission of an asynchronous frame, the actual token turnover time TRT is compared with the maximum possible T_0pr.

If the ring is not overloaded, then the marker arrives before the T_0pr interval expires, that is, TRT smaller T_0pr.

In case of TRT smaller The T_Opr station is allowed to acquire the token and send its frame (or frames) to the ring.

TNT marker retention time is equal to the difference T_0pr - TRT

During this time, the station sends as many asynchronous frames into the ring as it can.

If the ring is overloaded and the marker is late, then the interval TRT will be greater than T_0pr. In this case, the station does not have the right to acquire the token for the asynchronous frame.

If all stations in the network want to transmit only asynchronous frames, and the token has made a turn around the ring too slowly, then all stations skip the token in the repetition mode, the token quickly makes the next turn, and on the next cycle of operation, the stations already have the right to capture the token and transmit their frames.

Access method FDDI for asynchronous traffic is adaptive and well regulates temporary network congestion.

Fault tolerance of FDDI technology

To ensure fault tolerance in the standard FDDI it is planned to create two fiber optic rings - primary and secondary. In the standard FDDI two types of connection of stations to the network are allowed.

Simultaneous connection to the primary and secondary rings is called double connection - Dual Attachment, D.A.. Connecting only to the primary ring is called a single connection - Single Attachment SA.

In the standard FDDI the presence of end nodes in the network - stations (Station), as well as concentrators (Concentrator) is provided.

For stations and hubs, any type of network connection is allowed - both single and double. Accordingly, such devices have the corresponding names: SAS (Single Attachment Station), DAS (Dual Attachment Station), SAC (Single Attachment Concentrator)andDAC (Dual Attachment Concentrator).

Typically, hubs are dual-wired and stations are single-wired, although this is not required.

Usually connected to the ring through a hub. They have one port that works for receiving and transmitting

To make it easier to properly connect devices to the network, their connectors are marked.

Connectors type AND and AT must be for devices with dual connection, connector M(Master) is available on the hub for a single station connection, for which the mating connector must be of type S(slave).

DAS usually connected to the ring through 2 ports A and B, both have the ability to receive and transmit, allowing you to connect to two rings.

Hubs allow SAS and DAS nodes to connect to the dual FDDI ring. Hubs have M(master) ports to connect SAS and DAS ports, and may also have SAS and DAS ports.

In the event of a single cable break between dual-connected devices, the network FDDI will be able to continue normal operation by automatically reconfiguring the internal frame paths between hub ports. Breaking the cable twice will result in two isolated networks FDDI. When a cable breaks to a station with a single connection, it becomes cut off from the network, and the ring continues to work due to the reconfiguration of the internal path in the hub - port M, to which this station was connected, will be excluded from the common path.

In order to keep the network working during a power outage in stations with dual connection, that is, stations DAS, the latter must be equipped with optical bypass switches (Optical Bypass Switch), which create a bypass for the light fluxes in the event of a power failure, which they receive from the station.

And finally, stations DAS or hubs DAC can be connected to two ports M one or two hubs, creating a tree structure with primary and secondary links. Default port AT supports the main link, and the port AND- reserve. This configuration is called connection. dual homing.

Fault tolerance is maintained through continuous level monitoring SMT hubs and stations behind token and frame circulation time intervals, as well as the presence of a physical connection between neighboring ports in the network.

Online FDDI there is no dedicated active monitor - all stations and hubs are equal, and if deviations from the norm are detected, they begin the process of re-initializing the network, and then reconfiguring it.

Reconfiguration of internal paths in hubs and network adapters is performed by special optical switches that redirect the light beam and have a rather complex design.

Computer tips for novice users

How an FDDI Network Works

Fault tolerance of FDDI networks

Synchronous and asynchronous transmission

cable system

Connecting equipment to the FDDI network

Direct connection

Connecting through bridges and routers

FDDI-to-Ethernet bridges

Intelligent bridges

FDDI Usage Examples

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