Related Terms        

 

            Token Ring

 

The Token ring network is a type of Local Area Network (LAN), where workstations and network devices are normally physically connected (e.g. using a cable) in a star-wired ring configuration. It uses a technique whereby access to the network is restricted to a single station at a time (only a very short time which is unnoticeable to a user). The data access method used on token ring networks is called Token passing. This is a deterministic access method which ensures collisions cannot occur due to the fact that only one station can transmit at any given time.

 

 

A
Token
B
C
D
E 
F

The tokens are generated by a monitor station (a Node nominated by the other Nodes on the network when it is first switched on). A token packet is passed round the network which grants permission to each station wishing to transmit data.

 

Using the fig x above, if Node A wishes to transmit data to Node D on the network, it must first wait for the Token packet (which should be empty) to get to it. Node A then attaches the data it wants to transmit to the Token with the destination address and passes it to the next Node, B. Node B examines the token packet to see if the data is intended for it, (the data examination process takes unnoticeable seconds to complete) and the packet is passed to the next Node C. The same check is performed by Node C and since it is also not addressed to it, the Token is passed to Node D which in this case is the designated receiver of the message. It examines the data and because it is addressed to it, the data is striped off and an acknowledgment attached to the token. It is then passed to the next Nodes E and F and finally back to Node A which examines the packet again, reads the acknowledgment from D and subsequently declares the token packet free for any other node wishing to transmit data. If any, then the process is repeated again.

 The Token Ring protocol (TRP) provides services at the Physical and Data Link Layers of the OSI Reference model. Signals travel around the network from one station to the next with the cabling ensuring that a logical ring is maintained. IEEE 802.5 networks originally operated at 4Mbps (and indeed some still do), but the standard now covers transmission rates of up to 16Mbps.Workstations connect to central hubs often called Multi-station Access Units (MSAU's). Multiple MSAU's are connected together to create larger multi-station networks.

An advantage of this configuration of local area network over other types is its stable operation under high network load conditions. The token passing mechanism allows all stations to have a turn at passing data.

 A major disadvantage of the token ring network is when failures occur such as a broken cable the network stops operation and so special test routines only available from the network controller are required to assist in fault location.

 

Topics

Physical Connections
Token Ring Operation
Priority System
Fault-Management Mechanisms
Frame Format

Interactive Token-Ring Network Troubleshooting 
Start from a page of common network symptoms and work your way through tests and results until you arrive at a solution that solves your problem.
Token Ring/IEEE 802.5 
PDF file looks at the history of Token Ring and explains how it works.

Token Ring Access Method (802.5)

IEEE 802.5, 1998 Edition 

IEEE 802.5r and IEEE 802.5j, 1998 Edition (ISO/IEC 8802-5:1998/Amd.1),

IEEE 802.5t-2000,

IEEE Std 802.5v-2001, 

IEEE Std 802.5w-2000 

Book:"Understanding Token Ring Protocols and Standards" by James T. Carlo, et.al. (amazon.com)