Token Ring/FDDI Networks

An illustration of a token ring's token passing

General

A token passing ring LAN is a group of computers connected in a loop. The group uses a token passing access mechanism. A computer wishing to send data should first receive permission. When it gets control of the network it may transmit a frame. Each frame transmitted on the ring is transmitted from one computer to the next, until it ultimately returns to the initiator of the transmission.
The Token Passing Ring Network was originally developed by IBM and only Ethernet LANs are more popular. The IEEE 802.5 specification which was modeled after IBM's Token Ring is almost identical and the term Token Ring is used to refer both specifications.
FDDI (Fiber Distributed Data Interconnect) is an improved token ring specification based on fiber as the physical medium. As opposed to Token Ring's single ring, FDDI, uses two to achieve better results. CDDI, yet another standard, resembles FDDI, but uses a copper wire for its ring.

Characteristics

• Comparison of basic characteristics

• Topology

Token Ring LANs use a ring topology, i.e. each station is connected to two other stations which are all together arranged in a loop. Each station can send a signal along the loop after receiving permission to do so (Only one station may have control on the network at
a specific point of time). The signal will travel from one station to the other until it reaches its initiator.

• Priority System

Token Ring networks has a priority system that allows stations with high priority to use the network more frequently. The priority is defined by the frame's priority and reservation fields. In order to seize a token a station must have priority which equals or is higher than the priority field of the token. Only than the station can reserve the token for the next pass around the network. This way when the next token is generated, it includes the higher reserving station. Stations must change the priority back to its previous value after their transmission has completed.

• Fault Management Mechanisms

In order to detect and correct network faults Token Ring networks may dedicate a station for monitoring frames which are circling around without being dealt with. This monitor removes such frames and allow the network to function in a normal manner all over again.

• FDDI - Self healing

As described above FDDI networks implements a recovery mechanism which enable the network to function properly even under a broken ring. FDDI uses two rings to achieve recovery capabilities. As shown a token is passed simultaneously on the network's inner and outer rings which backup each other.
As shown in the following figure in case of broken connection or station malfunction, the closest station closes the network loop by sending the token it received from the outer/inner ring back using the inner/outer ring. This feature is called Self healing.

Frame Format

Token Ring Frame Formats (tokens and data/command)

Tokens consist of:

1. Start delimiter - which alerts the stations of a token arrival (or data/command frame).
2. Access control byte - which contains the priority and reservation fields, a token bit to differentiate token from data/command frame and a monitor bit checking whether a frame is circling the ring endlessly.
3. End delimiter - which signals the end of a frame, end of a logical sequence and damaged frames.

Data/Command Frames carry information for upper-layer protocols.
After the Access control byte a frame control byte arrives and indicates whether it is a data or control information (and which) frame. Then arrives two address fields (source & destination) each 6 bytes long. Data follows these fields (its length depends on the time each station can hold a token) and then a FCS (frame check sequence) field. At the end as in tokens, an end delimiter completes the frame.

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