Token Ring

Token Ring (IEEE 802.5)

Overview

Originally developed at IBM.

A token is something like a "potty pass", used to prevent collisions.

Typically wired with twisted pair.

Digital signal with differential Manchester encoding.

Transmission rate is 4Mbps or 16Mbps.

Characterized by a ring topology. Ring is unidirectional.

signal comes into NIC, which regenerates and transmits. When NIC powered down, switch closes so signal can pass thru

Physical Configuration may be Star

Sometimes configured like a star with a special hub (MAU: multistation Access Unit), to mitigate problem of cut cable :

imagine pulling the cables between each pair of stations into the center of the ring and running them through a hub.

If a cable is cut, the hub can sense this and close a by-pass switch that isolates the connected station.

The token

Token is special bit pattern (3 bytes long) that circulates on idle ring.

Collisions cannot occur because station must have possession of token to proceed.

Frame transmission proceeds thusly

Sender reads token on ring and removes it.

Sender transmits frame onto ring. For 16Mbps ring, token is released after the frame (early release).

Transmitted frame circulates around the ring.

Receiver copies bits into buffer but regenerates frame on ring. It sets ACK bits in last byte.

All other stations just regenerate frame on ring.

As bits come back to sender, they are "drained" from ring. Error-check here.

For 4Mbps ring, sender releases new token when step 4 completed. (for 16Mbps, see step 2)

Consider performance of this protocol on heavily loaded ring (many stations want to transmit).

Is the protocol fair? The protocol also includes priorities.

Frame Formats

Token ring token

field	bytes	description
start delimiter	1	invalid manchester signal (no transition)
access control	1	token bit plus priority bits plus reservation bits
end delimiter	1	invalid manchester signal

Token ring frame

field	bytes	description
start delimiter	1	same as above
access control	1	same as above
frame control	1	frame type (distinguish data from from ring maintenance frame)
destination	6	all 1’s for broadcast, first bit 1 for group, else individual
source	6	source address
data	0-4500	limited by amount of time station can hold token
checksum	4	CRC-32 checksum
end delimiter	1	same as above
frame status	1	ACKnowlegement that receiver can send as it forwards frame

Priorities

Both token and data frame have 1-byte access control field.

Higher priority station may reserve token by placing its priority into access control byte

Transmission is FCFS among waiting stations with highest priority

Ring Monitor keeps network under control

Designated monitor station plus backup

Regenerates token if none seen before timeout

Removes frame seen for second time (sets frame status bits first time around)

Last reviewed: 16 March 1998

Peter Sanderson ( pete@csc.smsu.edu )