System information

Frame Formats
Book Title
6-82
device fails, its frame may continue to circle the ring. This can prevent other stations from
transmitting their own frames and essentially lock up the network. The active monitor can detect
such frames, remove them from the ring, and generate a new token.
The IBM Token Ring network’s star topology also contributes to overall network reliability. Because
all information in a Token Ring network is seen by active MAUs, these devices can be programmed
to check for problems and selectively remove stations from the ring if necessary.
A Token Ring algorithm called beaconing detects and tries to repair certain network faults.
Whenever a station detects a serious problem with the network (such as a cable break), it sends a
beacon frame. The beacon frame defines a failure domain, which includes the station 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 in an attempt to reconfigure the network around the failed areas. Physically, the
MAU can accomplish this through electrical reconfiguration.
Frame Formats
Token Ring networks define two frame types: tokens and data/command frames. Both formats are
shown in Figure 6-3.
Figure 6-3 IEEE 802.5/Token Ring Frame Formats
Tokens
Each token is 3 bytes in length and consists of a start delimiter, an access control byte, and an end
delimiter.
The start delimiter serves to alert each station to the arrival of a token (or data/command frame). This
field 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 fields, as well as a token bit (used to
differentiate a token from a data/command frame) and a monitor bit (used by the active monitor to
determine whether a frame is circling the ring endlessly).
Finally, the end delimiter signals the end of the token or data/command frame. It also contains bits
to indicate a damaged frame and a frame that is the last in a logical sequence.
Token
Data/command frame
Field length,
in bytes
Start
delimiter
1
End
delimiter
1
Access
control
1
Start
delimiter
End
delimiter
Access
control
Data
≥0
FCS
4
Frame
control
1
Destination
address
6
Source
address
6