System information

Troubleshooting Token Ring 6-81
Physical Connections
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.
Figure 6-2 IBM Token Ring Network Physical Connections
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
Patch
cables
Lobe
cables
Ring
in 1 2 3 4 5 6 7 8
Ring
out
Ring
in 1 2 3 4 5 6 7 8
Ring
out
MAU MAU
Stations Stations
Ring
in 1 2 3 4 5 6 7 8
Ring
out
Ring
in 1 2 3 4 5 6 7 8
Ring
out
MAU MAU
Stations Stations