Access Control (1 byte) (Token-Ring)

Protocol: Token-Ring MAC Token-Ring PCI

Field: access control

Length: 1 byte

Contents: media access control.

a

Token-Ring's second byte controls the station's access to the token-ring media.

1.

Each station has a priority (P) in a range between 000 (low) and 111 (high). Access is related to this priority value.

000 (dec: 0): ring stations / configurable / assigned by RPS.

001 (dec: 1): " "

010 (dec: 2): " "

011 (dec: 3): " "

100 (dec: 4): bridges

101 (dec: 5): (reserved)

110 (dec: 6): (reserved)

111 (dec: 7): station management (SMT MAC frames)

2.

In order to reserve the next token for itself, a ring station may write its internal priority value into the three 'reservation' bits (R). Any other station which has to release the next free token will shift reservation bits' value into the 'priority' field. Ring stations are only allowed to send data if their internal priority value is equal or higher than the 'priority' bit's value.

3.

The token bit (T) tells if the frame is a free token (T=0) or if the frame contains data (T=1) which could be user data (LLC) or MAC data (vectors).

4.

The monitor bit (M) is set to zero (M=0) each time when a frame is sent by a ring station. If the 'Active Monitor' detects a M=0 bit, it changes the monitor bit to M=1. If the 'Active Monitor' detects a M=1 bit, it discards the frame, purges the ring and releases a new, free token.

Token-Ring: Token Passing Ring