Specifications

2-17
TOKEN RING OVERVIEW
Therefore, the station with the highest address eventually receives its own
Claim Token MAC frame back, allowing it to reissue a sufficient number
(3) of successive Claim Token MAC frames to ensure the integrity of the
ring. It then becomes the new AM.
To ensure that all stations receive information about the AM status, an
AMP MAC frame is transmitted by the AM on a periodic basis. When this
timer expires in 7 seconds, an AMP frame is released by the AM. If stations
do not receive an AMP frame for 18 seconds then Monitor Contention is
entered. The AMP frame also allows stations to receive the address of their
nearest active upstream neighbor.
Frame Transmission
After the station has access to the network, it can now proceed with frame
transmission, as long as those frames are of sufficient Access priority.
Access Priority
The basic Token Ring protocol allows for the use of different access
priorities. The priorities allow for a station that requires immediate
transmission to receive the next available token. Priority is determined by
looking at the next frame, or to be more precise, the Protocol Data Unit
(PDU), queued by a station for transmission.
Priority is from 000 lowest to 111 highest. The left bit is the most significant
so 110 has a higher priority than 011. The following sequence of events
depicts the priority system:
• Station A is transmitting to Station C, and the present priority is set to
0, lowest priority.
• Station B wants to transmit when the token becomes free, so as the
frame is regenerated by Station B, it changes the Priority reservation
field in the frame to 1. No other data is changed within the field.
• Station C receives the frame and copies the data. Station C sets the
frame copied bit as normal and regenerates the frame back onto the
ring.
• Station D also wants to transmit and has authority to send a higher
priority than Station B. It therefore sets the priority reservation field to
3, overriding the priority set by Station B. Station D then regenerates
the frame back onto the ring.
• When Station A receives the frame it regenerates a new token with a
priority of 3 and sets the reservation field back to 0.