Technology Fiber Distributed Data Interface- the first local area network technology that used fiber optic cable as a data transmission medium.
Attempts to use light as a medium carrying information have been made for a long time - 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.
Work on the use of light to transmit information intensified in the 1960s in connection with the invention of a laser that could provide light modulation at very high frequencies, that is, create a broadband channel for transmitting a large number information at high speed. Around the same time, optical fibers appeared that could transmit light in cable systems, similar to how copper wires transmit electrical signals in traditional cables. However, the light loss in these fibers was too great to be used as an alternative to copper strands. Low Cost Optical Fibers for Low Power Loss light signal and wide bandwidth (up to several GHz) appeared only in the 1970s. In the early 1980s, the industrial installation and operation of fiber optic communication channels for territorial telecommunication systems began.
In the 1980s, work also began on the creation of standard technologies and devices for using 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 National 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.
Nowadays, most networking technologies support fiber optic cables as one of the options. physical layer, but FDDI remains the most mature 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 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:
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 ring, therefore this mode is called the mode Thru- "through" or "transit". The secondary ring (Secondary) is not used in this mode.
In the event of some type 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 (figure 2.1), forming a single ring again. This network mode is called Wrap, i.e. "folding" or "folding" rings. The folding operation is performed by the 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.
The FDDI standards place a lot of emphasis on various procedures to determine if a network has failed and then reconfigure as necessary. The FDDI network can fully restore its operability in the event of single failures of its elements. With multiple failures, the network breaks up into several connected networks.
Rice. 2.1. Reconfiguration of FDDI Rings on Failure
Rings in FDDI networks are considered as a common shared data transmission medium, so a special access method is defined for it. This method is very close to the access method of Token Ring networks and is also called the token (or token) ring method - token ring (Figure 2.2, a).
The station can start transmitting its own data frames only if it has received a special frame from the previous station - an access token (Figure 2.2, b). After that, she can transmit her frames, if she has them, for a time called the token hold 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.
Rice. 2.2. Frame processing by FDDI ring stations
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. This case is shown in the figure (Figure 2.2, c). 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 the protocol of the level above FDDI (for example, IP), and then transmits the original frame over the network of the subsequent station (Figure 2.2, d). 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 (Figure 2.2, e). 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.
Figure 2.3 shows the structure of the FDDI technology protocols 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.
Rice. 2.3. Structure of FDDI technology protocols
The physical layer is divided into two sublayers: the media-independent sublayer PHY (Physical), and environment dependent sublayer PMD (Physical Media Dependent). The operation of all levels is controlled by the station control protocol SMT (Station Management).
PMD level provides the necessary means to transfer data from one station to another over fiber optics. Its specification defines:
The TP-PMD specification defines the possibility of transmitting data between stations over twisted pair in accordance with the MLT-3 method. The PMD and TP-PMD layer specifications have already been discussed in the sections on Fast technology ethernet.
PHY level performs encoding and decoding of data circulating between the MAC layer and the PMD layer, and also provides timing of information signals. Its specification defines:
MAC level is responsible for network access control, as well as for receiving and processing data frames. It defines the following parameters:
SMT level 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:
The fault tolerance of FDDI networks is ensured 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.
The following table compares FDDI technology with Ethernet and Token Ring technologies.
| Characteristic | FDDI | Ethernet Token Ring |
| bit rate | 100 Mb/s | 10 Mb/s16 Mb/s |
| Topology | double ring trees |
Tire/starStar/ring |
| Access method | Share of time token turnover |
CSMA/CDPriority Reservation System |
| Transmission medium data |
Multimode optical fiber, unshielded twisted pair |
thick coax, thin coax, twisted pair, optical fiber Shielded and Unshielded Twisted Pair, optical fiber |
| Maximum network length (without bridges) | 200 km (100 km per ring) |
2500 m1000 m |
| Maximum distance between nodes | 2 km (-11 dB loss between nodes) |
2500 m 100 m |
| Maximum number of nodes |
500 (1000 connections) | 1024260 for shielded twisted pair, 72 for unshielded twisted couples |
| Clocking and failure recovery |
Distributed implementation of clocking and failover |
UndefinedActive Monitor |
All stations in the FDDI network are divided into several types according to the following criteria:
If the station is connected only to the primary ring, then this option is called a single connection - Single Attachment SA(Figure 2.4, a). If the station is attached to both the primary and secondary rings, then this option is called double attachment - Dual Attachment, D.A.(Figure 2.4, b).
Rice. 2.4. Single (SA) and double (DA) connection of stations
Obviously, a station can only use the fail-safe features provided by having two FDDI rings if it is dual-connected.
Rice. 2.5. Reconfiguration of dual connected stations in the event of a cable break
As can be seen from Figure 2.5, the reaction of the stations to a cable break is to change the internal ways of transmitting information between individual components stations.
In order to be able to transmit its own data to the ring (and not just relay the data of neighboring stations), the station must have at least one MAC node that has its own unique MAC address. Stations may not have a single MAC node, and, therefore, participate only in the relay of foreign frames. But usually all stations on an FDDI network, even hubs, have at least one MAC. The hubs use the MAC node to capture and generate service frames, such as ring initialization frames, ring fault finding frames, and the like.
Stations that have one MAC node are called SM (Single MAC) stations, and stations that have two MAC nodes are called DM (Dual MAC) stations.
The following combinations of attachment types and number of MAC nodes are possible:
| SM/SA | The station has one MAC node and joins only the primary ring. The station cannot take part in the formation of a common ring of two. |
| SM/DA | The station has one MAC-node and joins immediately to the primary and secondary rings. In normal mode, it can only receive data on the primary ring, using the second for failover. |
| DM/DA | The station has two MAC nodes and is attached to two rings. It can (potentially) receive data simultaneously on two rings (full duplex mode), and in case of failures, participate in the reconfiguration of the rings. |
| DM/SA | The station has two MAC nodes, but is only attached to the primary ring. Illegal combination for the end station, a special case of the hub. |
Depending on whether the station is a hub or an end station, the following designations are accepted depending on the type of their connection:
SAS (Single Attachment Station)- end station with single connection,
DAS (Dual Attachment Station)- end station with dual connection,
SAC (Single Attachment Concentrator)- hub with single connection,
DAC (Dual Attachment Concentrator)- hub with double connection.
The FDDI standard describes four types of ports that differ in their purpose and ability to connect with each other to form the correct network configurations.
| Port type | Connection | Purpose |
| A | PI/SO - (Primary In/Secondary Out) Primary Ring Inlet / Secondary Ring Outlet |
rings |
| B | PO/SI - (Primary Out/Secondary In) Primary Ring Outlet/Secondary Ring Inlet |
Connects devices with dual connection with trunk rings |
| M | Master-PI/PO |
Hub port that connects it to devices with single connection; uses only the primary ring |
| S | Slave-PI/PO Primary Ring Inlet/Primary Ring Outlet |
Connects a device to a single connecting to a hub; uses only the primary ring |
Figure 2.6 shows a typical use of different types of ports to connect SAS and DAS stations to a DAC.
Rice. 2.6. Using different types of ports
An S-S port connection is valid because it creates an isolated primary ring that connects only two stations, but is not normally used.
The connection of ports M - M is prohibited, and connections A-A, B-B, A-S, S-A, B-S, S-B - undesirable, as they create inefficient combinations of rings.
Connections type A-M and B-M correspond to the case of the so-called Dual homing connections when a dual-capable device, that is, with ports A and B, uses them for two connections to the primary ring through the M ports of another device.
Such a connection is shown in figure 2.7.
It has two hubs, DAC4 and DAC5, connected to the hubs DAC1, DAC2 and DAC3 according to the Dual Homing scheme.
Hubs DAC1, DAC2 and DAC3 are connected in the usual way to both rings, forming the root trunk of the FDDI network. Usually such concentrators are called in the English-language literature rooted concentrators .
Hubs DAC4 and DAC5 are connected in a tree-like manner. It could also be formed using SAC4 and SAC5 hubs, which in this case would be connected to the M-port of the root hubs using the S.
Connecting DAC hubs in a tree-like manner, but using Dual Homing, allows you to increase the fault tolerance of the network, and retain the advantages of a tree-like multi-level structure.
Rice. 2.7. Dual homing connection
DAC4 is connected via classical pattern dual homing. This scheme is designed for such a hub to have only one MAC node. When ports A and B of the DAC4 are connected to the M ports of the DAC1, a physical connection is established between these ports, which is constantly monitored by the physical PHY layer. However, only port B is brought up with respect to frame flow over the network, and port A remains in a logical standby state. The default preference for port B is defined in the FDDI standard.
If the physical connection on port B does not work correctly, the DAC4 hub transfers it to a standby state, and port A becomes active. After that, port B constantly checks the physical state of its communication line, and if it is restored, it becomes active again.
The DAC5 hub is also included in the Dual Homing scheme, but with more complete functionality to control the connection of the backup port A. The DAC5 has two MAC nodes, so not only port B is active in the primary ring, transmitting frames to the primary MAC node from the DAC3 port M, but also port A is also in the active state, receiving frames from the same primary ring, but from DAC2 port M. This allows the secondary MAC node to constantly monitor the logical state of the backup link.
It should be noted that devices that support Dual Homing mode can be implemented in several different ways, so there may be incompatibility of these modes among different manufacturers.
When a new station joins the FDDI network, the network temporarily suspends its work, passing through the ring initialization process, during which the main parameters of the ring are agreed between all stations, the most important of which is the nominal token rotation time around the ring. This procedure can be avoided in some cases. An example of such a case is the connection of a new SAS station to the port M of the hub with the so-called "roaming" node MAC (Roving MAC), which is also called the local MAC node.
An example of such a connection is shown in Figure 2.8.
Rice. 2.8. Attaching a station to a roaming MAC node
The DM/DAC1 hub has two MAC nodes: one is involved in the normal operation of the primary ring, and the second, local, is attached to the path connecting port M to the SAS3 station. This path forms an isolated ring and is used for local verification of the health and parameters of the SAS3 station. If it is operational and its parameters do not require reinitialization of the main network, then the SAS3 station is included in the operation of the primary ring "smoothly" (smooth-insertion).
The fact that a station with a single connection is powered off will be immediately noticed by the physical layer facilities serving the corresponding M-port of the hub, and this port, at the command of the SMT layer of the hub, will be bypassed along the internal data path through the hub. This fact will not have any effect on the further fault tolerance of the network (Figure 2.9).
Rice. 2.9. Optical Bypass Switch
If you turn off the power at the DAS station or DAC hub, then the network, although it will continue to work, switching to the Wrap state, but the fault tolerance margin will be lost, which is undesirable. Therefore, for dual-connected devices, it is recommended to use optical bypass switches - Optical Bypass Switch, which allow you to short-circuit the input and output optical fibers and bypass the station if it is turned off. The optical bypass switch is powered by the station and consists, in the simplest case, of reflective mirrors or a moving optical fiber. When the power is off, such a switch bypasses the station, and when it is powered on, connects the inputs of ports A and B to the internal PHY circuits of the station.
Consider the physical sublayer PMD (Physical Media Dependent layer), defined in the FDDI standard for optical fiber - Fiber PMD.
This specification defines the hardware components for creating physical connections between stations: optical transmitters, optical receivers, cable parameters, optical connectors. For each of these elements, design and optical parameters are indicated that allow stations to interact stably at certain distances.
The physical connection is the basic building block of an FDDI network. A typical physical connection structure is shown in Figure 2.10.
Rice. 2.10. Physical connection of the FDDI network
Each physical connection consists of two physical links - primary and secondary. These communications are one-way - data is transmitted from the transmitter of one PHY device to the receiver of another PHY device.
The Fiber PMD standard does not explicitly define distance limits between a pair of cooperating devices over a single physical link.
Instead, the standard defines a maximum level of optical power loss between two stations communicating over the same physical link. This level is -11 dB, where
dB = 10 log P 2 /P 1 ,
and P1 is the signal strength at the transmitter station, and P2- signal strength at the input of the receiving station. Since the power decreases as the signal is transmitted over the cable, the attenuation is negative.
According to the Fiber PMD cable attenuation parameters and commercially available connectors, it is considered that to provide attenuation of -11 dB, the length of the optical cable between neighboring nodes should not exceed 2 km.
The correctness of the physical connection between nodes can be more accurately calculated by taking into account the exact attenuation characteristics introduced by the cable, connectors, cable spikes, as well as transmitter power and receiver sensitivity.
The Fiber PMD standard defines the following physical connection element parameter limits (called the FDDI Power Budget):
The absolute values of the power of optical signals (for the output of the transmitter and for the input of the receiver) are measured in decibels in relation to the standard power of 1 milliwatt (mW) and are denoted as dBm:
dBm = 10 log P/1,
where power R also measured in milliwatts.
It can be seen from the values in the table that the maximum loss between stations of -11 dB corresponds to the worst combination of transmitter (-20 dBm) and receiver (-31 dBm) power limits.
The main type of cable for the Fiber PMD standard is a multimode cable with a core diameter of 62.5 µm and a reflective sheath diameter of 125 µm. The Fiber PMD specification does not specify requirements for cable attenuation in dB per km, but only requires an overall attenuation requirement of -11 dB between stations connected by cable and connectors. The bandwidth of the cable must be no worse than 500 MHz per km.
In addition to the basic cable type, the Fiber PMD specification allows the use of multimode cables with a core diameter of 50 µm, 85 µm and 100 µm.
The Fiber PMD standard defines optical connectors as connectors. MIC (Media Interface Connector). The MIC connector connects the 2 fibers of the cable connected to the MIC plug to the 2 fibers of the station port connected to the MIC socket. Only MIC receptacle design parameters are standardized, and any MIC plugs that fit standard MIC receptacles are considered usable.
The Fiber PMD specification does not define the level of loss in the MIC connector. This level is the manufacturer's business, the main thing is to keep allowable level-11 dB loss across the entire physical connection.
MIC connectors should be keyed to indicate the type of port to prevent misconnection of connectors. Four identified various types key:
Key types for these types of connectors are shown in Figure 2.11.
Rice. 2.11. MIC socket keys
In addition to MIC connectors, commercially available ST and SC connectors can be used.
Light-emitting diodes (LED) or laser diodes with a wavelength of 1.3 µm can be used as a light source.
In addition to multi-mode cable, a higher quality single-mode cable can be used. (Single Mode Fiber, SMF) and SMF-MIC connectors for this cable. In this case, the range of the physical connection between neighboring nodes can increase up to 40 km - 60 km, depending on the quality of the cable, connectors and connections. The requirements defined in the SMF-PMD specification for transmitter output and receiver input power are the same as for single mode cable.
The Fiber PMD specification requires this layer to execute the Signal_Detect function to determine if there are optical signals at the input of the station's physical connection. This signal is transmitted to the PHY layer, where it is used by the Line State Detect function (Figure 2.12).
The PMD layer generates for the PHY an indication of the presence of an optical Signal_Detect signal if the power input signal exceeds -43.5 dBm, and removes it when this power decreases to -45 dBm and below. Thus, there is a hysteresis of 1.5 dBm to prevent frequent line status changes when the input signal power fluctuates around -45 dBm.
Rice. 2.12. PMD input signal detection function
FDDI network
The FDDI (Fiber Distributed Data Interface) 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 FDDI standard was originally focused on high transmission speed (100 Mbps) and on the use of the most promising fiber optic cable. The choice of optical 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 allows many tasks that are not possible with slower networks, such as real-time image transmission. 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, while having all the advantages of local networks (in particular, 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 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 allowable network sizes. In addition, the FDDI marker access method, unlike CSMA / CD, provides guaranteed access time and the absence of conflicts at any load level.
Basic technical characteristics of the FDDI network.
The maximum number of network subscribers is 1000.
The maximum length of the network ring is 20 (100) kilometers.
The maximum distance between network subscribers is 2 kilometers.
The transmission medium is a multimode fiber optic cable (it is possible to use an electric twisted pair cable).
The access method is marker.
Information transfer rate - 100 Mbps (200 Mbps for duplex transmission mode).
It is also possible to use a single-mode cable, in which case the distance between subscribers can reach 45 kilometers, and the total length of the ring is 200 kilometers.
Frame formats
Rice. Information frame format (Frame) and marker format (Token)
Purpose of fields:
The Preamble is used for synchronization. Initially, it contains 64 bits, but the subscribers through which the packet passes can change its size.
The start delimiter (SD-Start Delimiter) performs the function of a sign of the beginning of the frame.
The control byte (FC - Frame Control) contains information about the packet (address field size, synchronous / asynchronous transmission, packet type - service or information, command code).
Receiver and source addresses (SA - Source Address and DA - Destination Address) can be 6-byte (similar to Ethernet and Token-Ring) or 2-byte.
The data field (Data) has a variable length (from 0 to 4478 bytes). In service (command) packets, the data field has zero length.
The Frame Check Sequence (FCS) field contains the 32-bit cyclic checksum of the packet (CRC).
The end delimiter (ED - End Delimiter) defines the end of the frame.
The frame status (FS) byte includes an error detection bit, an address recognition bit, and a copy bit (similar to Token-Ring).
Format of the FDDI network control byte (Fig. 3):
The packet class bit determines whether the packet is synchronous or asynchronous.
The address length bit determines which address (6-byte or 2-byte) is used in this packet.
The Packet Type field (two bits) determines whether the packet is a control packet or an information packet.
The command code field (four bits) indicates which command the receiver should execute (if it is a control packet).
Rice. 3. Control byte format
Building a network
The FDDI standard was based on the token access method provided for by the international standard IEEE 802.5 (Token-Ring). The topology of the FDDI network is a double ring, where the network uses two multi-directional fiber optic cables. 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 Thru mode - "through" or "transit". The secondary ring (Secondary) is not used in this mode. These rings provide transmission redundancy to each other, that is, if some problems occur on one ring, the other will be included in the transmission. FDDI itself recognizes and eliminates the problems that have arisen. This mode of network operation is called "folding" or "folding" rings. The folding operation is performed by hubs and/or FDDI network adapters. 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. 
This solution also allows the use of full-duplex transmission of information (simultaneously in two directions) with twice the effective speed of 200 Mbps (each of the two channels operates at a speed of 100 Mbps). A star-ring topology is also used with hubs included in the ring (as in Token-Ring).
The FDDI standard, in order to achieve high network flexibility, provides for the inclusion of two types of subscribers in the ring:
Class A subscribers (stations) (dual connection subscribers, DAS) are connected to both (internal and external) network rings. In this case, the possibility of exchanging at speeds up to 200 Mbps or redundant network cable is realized (if the main cable is damaged, a backup cable is used). The equipment of this class is used in the most critical parts of the network in terms of speed.
Subscribers (stations) of class B (single connection subscribers, SAS -) are connected to only one (outer) ring of the network. They are simpler and cheaper than class A adapters, but do not have their capabilities. They can only be connected to the network through a hub or a bypass switch that turns them off in case of an accident.
In addition to the subscribers themselves (computers, terminals, etc.), the network uses communication hubs, the inclusion of which allows you to collect all connection points in one place in order to monitor network operation, 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 to optical signals and vice versa. Hubs also come in dual connection (DAC) and single connection (SAC).
An example of an FDDI network configuration is shown in fig. 4 
Rice. 4. FDDI network configuration example
The principle of information transfer
FDDI uses what is known as multiple token passing.
A station can only start sending its own data frames if it has received a token (access token) from a previous station. After that, it can transmit its frames, if it has them, for a time called the token hold 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.
A subscriber wishing to transmit waits for the token that follows each packet.
When the token arrives, the subscriber removes it from the network and transmits his packet.
Immediately after sending his packet, the subscriber sends a new token.
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.
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), and then transmits the original frame over the network of the next 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.
Having received his packet back around the ring, the sending subscriber destroys it. In the packet status field, it has information about whether there were errors and whether the receiver received the packet.
In conclusion, it should be noted that despite the obvious advantages of FDDI this network not widely used, which is mainly due to the high cost of its equipment. The main scope of FDDI now is the basic, backbone (Backbone) networks that combine several networks. FDDI is also used to connect powerful workstations or servers that require high-speed exchange.
FDDI (Fiber Distributed Data Interface) is a standard, or rather a set network standards, focused primarily on data transmission over fiber optic cable at a speed of 100 Mbps. The vast majority of the FDDI standard specifications were developed by the X3T9.5 (ANSI) problem group in the second half of the 1980s. FDDI has become a LAN using optical fiber as the transmission medium.
Currently, most network technologies support fiber optic interface 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, and equipment from different manufacturers shows a good degree of compatibility.
When developing FDDI technology, the following goals were set as the highest priority:
- Increasing the bit rate of data transfer up to 100 Mbps;
— Increasing network fault tolerance due to standard recovery procedures after failures of various kinds — cable damage, incorrect operation of the network node, the occurrence of a high level of interference on the line, etc.;
-Maximize the potential throughput with both asynchronous and synchronous schedules.
FDDI technology is largely based on Token Ring technology, developing and improving its main ideas. The FDDI protocol also has significant differences from Token Ring. These differences are related to the requirements that are necessary to support the high speed of information transfer, long distances and the ability to conduct synchronous transmission along with asynchronous data transfer. The two main differences in token control protocols between FDDI and IEEE 802.5 Token Ring are:
- in Token Ring, the station transmitting frames holds the token until it receives all sent packets. In FDDI, the station releases the token directly at the end of the transmission of the frame (frames);
- FDDI does not use the priority and reservation fields that Token Ring uses to allocate system resources.
In table. 6.1. the main characteristics of the FDDI network are indicated.
Table 6.1. Main characteristics of the FDDI network
|
Transfer rate |
|
|
Environment access type |
marker |
|
Maximum data frame size |
|
| Maximum number of stations | |
| Maximum distance between stations | 2 km (multimode fiber) 20 km* (single mode fiber) 100 m (UTP Cat.5 UTP) 100 m (IBM Shielded Twisted Pair Type 1) |
| Maximum marker traversal path length | 200 km |
| Maximum network length for ring topology (perimeter) | 100 km** (double FDDI loop) |
|
Optical fiber (multi-mode, single-mode), twisted pair (UTP Cat.5, IBM Type 1) |
* Some manufacturers produce equipment for a transmission distance of up to 50 km.
** With the specified length, the network will continue to work correctly and maintain integrity when a single ring break occurs or when one of the ring stations is turned off (WRAP mode) - while the length of the marker bypass path will not exceed 200 km.
The classic version of the FDDI network is built on the basis of two fiber-optic rings (double ring), the light signal through which propagates in opposite directions, Fig. 6.1 a. Each node is connected to receive and transmit to both rings. It is this ring physical topology that implements the main way to increase network fault tolerance. In normal operation, data travels from station to station on only one of the rings, which is called the primary ring. For definiteness, the direction of data movement in the primary ring is set counterclockwise. The data path reflects the logical topology of the FDDI network, which is always a ring. All stations, except for the transmitting and receiving stations, retransmit data and are end-to-end. The secondary ring (secondary) is redundant and is not used for data transmission in normal operation of the network, although it is used to continuously monitor the integrity of the ring.
Rice. 6.1. Dual ring FDDI: a) normal operation; b) curled ring mode (WRAP)
In the event of any failure in the network, when part of the primary ring is unable to transmit data (for example, a cable break, failure or disconnection of one of the nodes), the secondary ring is activated for data transmission, which complements the primary, forming again a single logical data ring, fig. 6.1 b. This mode of network operation is called WRAP, that is, “folding” the ring. The folding operation is performed by two network devices located on either side of the source of the problem (damaged cable, or failed station/hub). It is through these devices that the primary and secondary rings are combined. Thus, the FDDI network can fully restore its operability and integrity in the event of single failures of its elements. When the fault is eliminated, the network automatically switches to normal operation with data transmission only on the primary ring.
The FDDI standard pays great attention to various procedures, which, thanks to a distributed control mechanism, make it possible to determine the presence of a network failure 5, and then perform the necessary reconfiguration. With multiple failures, the network breaks up into several unrelated networks - the network is microsegmented.
The operation of the FDDI network is based on deterministic token access to the logical ring. First, the ring is initialized, during which a special shortened service data packet, a token, is emitted into the ring of one of the stations. Once the marker has started circulating around the ring, the stations can exchange information.
As long as there is no data transmission from station to station, only one token circulates, fig. 6.2 a, upon receipt of which the station acquires the ability to transmit information. In an FDDI network, each station has an upstream neighbor and a downstream neighbor determined by its physical links and direction of information transfer. In the classical version, this is determined by the primary ring. The transmission of information is organized in the form of data packets up to 4500 bytes long, called frames. If at the time of receiving the token the station has no data to transmit, then having received the token, it immediately broadcasts it further along the ring. If desired, the station, having received the token, can transmit it and, accordingly, transmit frames for a time called the TNT token holding time (Fig. 6.2 b). After the expiration of the TNT time, the station must complete the transmission of its next frame and transmit (release) the token of the next station, Fig. 6.2 c. At any time, only one station can transmit information, namely the one that captured the marker.
Rice. 6.2. Data transfer
Each network station reads the address fields of received frames. In the case when the station's own address - the MAC address - is different from the recipient's address field, the station simply relays the frame further along the ring, fig. 6.2 d. If the station's own address matches the destination address field in the received frame, the station copies to its internal buffer given frame, checks its correctness (by checksum), passes its data field for further processing to a higher-level protocol (for example, IP), and then transmits the original frame over the network of the subsequent station (Fig. 6.2 e), after putting three signs in the special fields of the frame : Address recognition, frame copy, and no or error in the frame.
Further, the frames, broadcast from node to node, return to the original station, which was their source. The source station for each frame checks the signs of the frame, whether it reached the destination station and whether it was damaged, and if everything is fine, it eliminates this frame (Fig. 6.2 e), freeing network resources, or, otherwise, tries to retransmit. In any case, the function of deleting a frame is assigned to the station that was its source.
Token access is one of the most effective solutions. Due to this, the real performance of the FDDI ring at a high load reaches 95%. For example, the performance of an Ethernet network (within the collision domain) reaches 30% of the bandwidth with increasing load.
The FDDI marker and frame formats, the ring initialization procedure, as well as the issues of network resource allocation in the normal data transfer mode are discussed in clause 6.7.
The constituent levels of the FDDI standard and the main functions performed by these levels are shown in fig. 6.3.
Like many other LAN technologies, FDDI technology 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 - without establishing connections and no recovery of lost or corrupted frames.
Rice. 6.3. Components of the FDDI standard
Initially (by 1988), the following levels were standardized (the names of the relevant ANSI / ISO documents for FDDI are given in Table 6.2):
- PMD (physical medium dependent) - the lower sublevel of the physical layer. Its specifications define the requirements for the transmission medium (multi-mode fiber optic cable) to optical transceivers (power rating and operating wavelength 1300 nm), the maximum allowable distance between stations (2 km), connector types, the operation of optical bypass switches (optical bypass switches) , as well as the representation of signals in optical fibers.
- PHY (physical) - the upper sublayer of the physical layer. It defines the data encoding and decoding scheme between the MAC layer and the PMD layer, the synchronization scheme, and special control characters. Its specification includes: information coding in accordance with the 4V/5V scheme; signal timing rules; requirements for the stability of the clock frequency of 125 MHz; rules for converting information from parallel to serial form.
- MAC (media access control) - the level of access control to the medium. This level defines: token management processes (transmission protocol, token capture and relay rules); formation, reception and processing of data frames (their addressing, error detection and recovery based on the verification of a 32-bit checksum); bandwidth allocation mechanisms between nodes.
— SMT (station management) — station management level. This special overarching layer defines: the communication protocols of this layer
Technology Fiber Distributed Data Interface- the first local area network technology that used fiber optic cable as a data transmission medium.
Attempts to use light as a medium carrying information have been made for a long time - 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.
Work on the use of light to transmit information intensified in the 1960s in connection with the invention of the laser, which could modulate light at very high frequencies, that is, create a broadband channel for transmitting a large amount of information at high speed. Around the same time, optical fibers appeared that could transmit light in cable systems, similar to the way copper wires transmit electrical signals in traditional cables. However, the light loss in these fibers was too great to be used as an alternative to copper strands. Inexpensive optical fibers providing low light signal power loss and wide bandwidth (up to several GHz) appeared only in the 1970s. In the early 1980s, the industrial installation and operation of fiber optic communication channels for territorial telecommunication systems began.
In the 1980s, work also began on the creation of standard technologies and devices for using 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 National 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 networking 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 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:
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 ring, therefore this mode is called the mode Thru- "through" or "transit". The secondary ring (Secondary) is not used in this mode.
In the event of some type 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 (figure 2.1), forming a single ring again. This network mode is called Wrap, i.e. "folding" or "folding" rings. The folding operation is performed by hubs and/or FDDI network adapters. 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.
The FDDI standards place a lot of emphasis on various procedures to determine if a network has failed and then reconfigure as necessary. The FDDI network can fully restore its operability in the event of single failures of its elements. With multiple failures, the network breaks up into several unrelated networks.
Rice. 2.1. Reconfiguration of FDDI Rings on Failure
Rings in FDDI networks are considered as a common shared data transmission medium, so a special access method is defined for it. This method is very close to the access method of Token Ring networks and is also called the token (or token) ring method - token ring (Figure 2.2, a).
The station can start transmitting its own data frames only if it has received a special frame from the previous station - an access token (Figure 2.2, b). After that, she can transmit her frames, if she has them, for a time called the token hold 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.
Rice. 2.2. Frame processing by FDDI ring stations
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. This case is shown in the figure (Figure 2.2, c). 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 the protocol of the level above FDDI (for example, IP), and then transmits the original frame over the network of the subsequent station (Figure 2.2, d). 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 (Figure 2.2, e). 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.
Figure 2.3 shows the structure of the FDDI technology protocols 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.
Rice. 2.3. Structure of FDDI technology protocols
The physical layer is divided into two sublayers: the media-independent sublayer PHY (Physical), and environment dependent sublayer PMD (Physical Media Dependent). The operation of all levels is controlled by the station control protocol SMT (Station Management).
PMD level provides the necessary means to transfer data from one station to another over fiber optics. Its specification defines:
The TP-PMD specification defines the possibility of transmitting data between stations over twisted pair in accordance with the MLT-3 method. The PMD and TP-PMD layer specifications have already been discussed in the Fast Ethernet sections.
PHY level performs encoding and decoding of data circulating between the MAC layer and the PMD layer, and also provides timing of information signals. Its specification defines:
MAC level is responsible for network access control, as well as for receiving and processing data frames. It defines the following parameters.
