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The Media Access Control Sublayer Function

The IEEE 802.5 standard defines a set of services to be provided by the MAC sublayer of a Token Ring network. It includes frame transmission, token transmission, stripping, frame reception, priority operation, beaconing, and neighbor notification.

Frame Transmission. This is controlled by the access mechanism for the media. When a station receives a transmit request from a higher layer, the medium access layer prefixes the higher-layer protocol data unit with the appropriate header and puts it into the transmission queues.

The station then waits for the appearance of a free token (i.e., the access control field token it set to 0) in the ring. The node reserves the token by altering the bit to 1. It copies the token’s starting delimiter and access control, places them at the front of the data frame, and initiates transmission. The station continues to transmit until it has nothing to transmit or the token-holding timer expires. After the station receives the access control field of the last transmitted frame, it places a free token (T set to 0) into the ring and appends the appropriate bits to the end delimiter byte of the token.

Token Transmission. At the end of transmission token-holding time, the station checks to verify that its address has been returned in the source address field, as indicated by the MA__FLAG. In the event of a failure, the station transmits fill frames until the MA__FLAG is set. At the end the station places a token into the ring.

Stripping. After transmission of the frames, the station remains active until it has removed all transmitted frames from the ring. This simplifies the recovery mechanism that would be required if frames were allowed to circulate continuously on the ring.

Frame Reception. Stations, while repeating the incoming signal stream, check it for frames they should copy or act on. If the frame-type bits indicate a media access control frame, the control bits are interpreted by all stations on the ring. In addition, if the frame’s DA field matches the station’s individual address, relevant group address, or broadcast address, the FC, DA, SA, information, and FS fields are copied into a receive buffer and subsequently forwarded to the appropriate sublayer.

Priority Operation. The 802.5 specification supports a three-component priority scheme. The priority bits (PPP) and the reservation bits (RRR) contained in the access control field are utilized by the priority algorithm. The algorithm supports three types of priorities:

  Pm: Priority of the message to be transmitted by a station.
  Pr: Received priority.
  Rr: Received reservation.

The priority algorithm is a four-step process;

1.  A ready-to-transmit station waits for a free token with received priority, Pr, less than or equal to the message priority, Pm.
2.  During the waiting state, a station may receive an occupied token at its own priority level, Pm. However, if the station detects a free token, it sets the reservation field to its message priority (Rr equal to Pm) only if Rr is less than Pm and Pm is less than Pr. This is equivalent to preempting any lower-priority reservation. If the station detects a passing data frame, it sets the frames reservation field to priority Rr equal to Pm. It does that only if the priority of the reservation field is found to be less than its priority (Rr less than Pm).
3.  When a station captures a token, it sets bits T to 1 and M to 0 and leaves the priority bits unchanged.
4.  At the end of a successful transmission, a station issues a new token with the priority set to the maximum of Pr, Rr, and Pm and a reservation set to the maximum of Rr and Pm.

One possible drawback of the algorithm was the potential for a station to set the priority level at its highest value and maintain at that level. To avoid this situation, the 802.5 specification requires that each station maintain two stacks, one for reservation and the other for priority. Each station must ensure that no token circulates indefinitely because of its high priority. A station is able to detect this situation by remembering the priority of an earlier transmission. A station now can detect this condition and downgrade the priority to a lower priority or reservation.

Beaconing and Neighbor Notification. For proper Token Ring operation, hard failure must be detected and isolated. Failure can occur at the station reporting the failure (the beaconing station), at the station upstream of the beaconing station, or on the intervening ring medium.


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