Information Technology Reference
In-Depth Information
Physical Connections
IBM Token Ring network stations are directly connected to MAUs, which can be wired together to form
one large ring (as shown in Figure 6-2). Patch cables connect MAUs to adjacent MAUs. Lobe cables
connect MAUs to stations. MAUs include bypass relays for removing stations from the ring.
Figure6-2
IBM Token Ring Network Physical Connections
MAU
MAU
Ring
in
Ring
out
Ring
in
Ring
out
1
2
3
4
5
6
7
8
1
2
3
4
5
6
7
8
Stations
Stations
Patch
cables
MAU
MAU
Ring
in
Ring
out
Ring
in
Ring
out
1
2
3
4
5
6
7
8
1
2
3
4
5
6
7
8
Lobe
cables
Stations
Stations
The Priority System
Token Ring networks use a sophisticated priority system that permits certain user-designated,
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 is 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 pass around the network.
When the next token is generated, it includes the higher priority of the reserving station. Stations that
raise a token's priority level must reinstate the previous priority after their transmission is complete.
Fault Management Mechanisms
Token Ring networks employ several mechanisms for detecting and compensating for network faults.
For example, one station in the Token Ring network is selected to be the active monitor. This station,
which can potentially be any station on the network, acts as a centralized source of timing information
for other ring stations and performs a variety of ring maintenance functions. One of these functions is
the removal of continuously circulating frames from the ring. When a sending device fails, its frame may
