• Tokens are 3 bytes in length and consists of a start delimiter, an access control

byte, and an end delimiter.

• The start delimiter alerts each station to the arrival of a token, or data/command

frame. This field also includes signals that distinguish the byte from the rest of the

frame by violating the encoding scheme used elsewhere in the frame.

• The access control byte contains the priority and reservation field, and a token

and monitor bit. The token bit distinguishes a token from a data/command frame,

and a monitor bit determines whether a frame is continuously circling the ring.

The bit pattern for access control is PPP T M RRR

PPP- indicate priority of token

T- token bit, T=0 -indicates token frame and T=1 indicates data frame

M- monitor bit used by monitor to remove orphan frames.

RRR- is used for reserving token priority

l Frame control byte has the pattern FF ZZZZZZ to distinguish between data

frame and control frame

FF= 01 indicates data frame

FF=00 indicates control frame then ZZZZZZ indicates type of control frame.

l SA and DA are as in 802.3

l FCS - frame check sequence having CRC checksum

l Ending delimiter has last two bits to be I and E where

E- error bit, this bit is set if any station detects an error like line coding violation or

frame check sequence error.

I- intermediate frame bit , it is set one to indicate last frame in the sequence of frames

that are transmitted.

l Frame status - has the pattern A C XX A C XX and it allows receiving station to

convey the data transfer status to sending station.

A= 1 indicates destination address was recognized by receiving station.

C=1 indicates that the frame was copied to receivers boffer properly

Token ring passing

• Token-passing networks move a small frame, called a token, around the network.

• Possession of the token grants the right to transmit data.

• If a node that receives a token has no information to send, it passes the token to

the next end station.

• Each station can hold the token for a maximum period of time, depending on the

specific technology that has been implemented.

• When a token is passed to a host that has information to transmit, the host seizes

the token and alters 1 bit of it. The token becomes a start-of-frame sequence.

• Next, the station appends the information to transmit to the token and sends this

data to the next station on the ring. There is no token on the network while the

information frame is circling the ring, unless the ring supports early token

releases. Other stations on the ring cannot transmit at this time. They must wait

for the token to become available.

• Token Ring networks have no collisions. If early token release is supported, a new

token can be released when the frame transmission has been completed.

• The information frame circulates around the ring until it reaches the intended

destination station, which copies the information for processing. The information

frame continues around the ring until it reaches the sending station, where it is

removed. The sending station can verify whether the frame was received and

copied by the destination.

• Unlike CSMA/CD networks, such as Ethernet, token-passing networks are

deterministic. This means that you can calculate the maximum time that will pass

before any end station will be able to transmit.

• This feature, and several reliability features, makes Token Ring networks ideal for

applications where any delay must be predictable, and robust network operation is

important. Factory automation environments are examples of predictable robust

network operations.

• Token Ring networks use a sophisticated priority system that permits certain userdesignated,

high-priority stations to use the network more frequently. Token Ring

frames have two fields that control priority - the priority field and the reservation

field.

• Only stations with a priority equal to, or higher than, the priority value contained

in a token can seize that token.

• Once the token has been seized and changed to an information frame, only

stations with a priority value higher than that of the transmitting station can

reserve the token for the next network pass.

• The next token generated includes the higher priority of the reserving station.

Stations that raise a token's priority level must reinstate the previous priority when

their transmission has been completed.

• Token Ring networks use several mechanisms for detecting and compensating for

network faults.

• One mechanism is to select one station in the Token Ring network to be the active

monitor. This station acts as a centralized source of timing information for other

ring stations and performs a variety of ring maintenance functions. The active

monitor station can potentially be any station.

• One of this station’s functions is to remove continuously circulating frames from

the ring. When a sending device fails, its frame may continue to circle the ring

and prevent other stations from transmitting their frames, which can lock up the

network. The active monitor can detect these frames, remove them from the ring,

and generate a new token.

• The IBM Token Ring network's physical star topology also contributes to overall

network reliability. Active MSAUs (multi-station access units) can see all

information in a Token Ring network enabling them to check for problems and to

selectively remove stations when necessary.

• Beaconing - a Token Ring formula - detects and tries to repair network faults.

When a station detects a serious problem with the network (e.g. a cable break) it

sends a beacon frame. The beacon frame defines a failure domain. A failure

domain includes the station that is reporting the failure, its nearest active

upstream neighbor (NAUN), and everything in between.

• Beaconing initiates a process called autoreconfiguration, where nodes within the

failure domain automatically perform diagnostics. This is an attempt to

reconfigure the network around the failed areas.

• Physically, MSAUs can accomplish this through electrical reconfiguration.

• The 4/16 Mbps Token Ring networks use differential Manchester encoding.

• Token Ring uses the differential Manchester encoding method to encode clock

and data bit information into bit symbols.

Token Ring network stations are directly connected to MSAUs and can be wired together to form one large ring.

Patch cables connect MSAUs to other MSAUs that are adjacent.

Lobe cables connect MSAUs to stations. MSAUs include bypass relays for removing

stations from the ring.