Information Technology Reference
In-Depth Information
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.
Figure6-3
IEEE 802.5/Token Ring Frame Formats
Data/command frame
Field length,
in bytes
1
1
1
6
6
0
4
1
Start
delimiter
Access
control
Frame
control
Destination
address
Source
address
Data
FCS
End
delimiter
Token
Start
delimiter
Access
control
End
delimiter
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).
