Specifications

2-27
TOKEN RING OVERVIEW
Starting Delimiter (SD)
The SD is always the same pattern of 8 bits - J K 0 J K 0 0 0.
Both J and K violate the Differential Manchester Encoding scheme since
they have no mid-bit polarity transition. J has the same polarity as the
previous bit and K has the opposite polarity of the preceding bit.
Access Control (AC)
The AC field shown in Figure 2-5 has the following format:
PPP | T | M | RRR
Where PPP are the priority bits. These indicate the priority of the token
and therefore which stations are allowed to use it. The bits are set by the
issuing station and cannot be modified by other stations. There are eight
levels of priority, 000 to 111.
Where T is the token bit indicating whether the field is part of a free token
frame or a busy token frame. This bit is set by the issuing station to a ‘0’ in
a free token and a ‘1’ in a busy token with data contained in the frame.
Where, RRR are the priority reservation bits. These bits are used to allow
stations with high priority data awaiting transmission to request that the
next token be issued at the required priority. These bits can be modified as
they pass through the station’s repeater buffer. There are eight (8) levels of
binary priority reservation from 000 to 111.
Figure 2-5. Access Control Field Format in Token Frame Structure
SD
P P P T M R R R
AC ED
SD - Starting Delimiter (one octet)
AC - Access Control (one octet)
ED - Ending Delimiter (one octet)
P - Priority Bits - 000 through 111
T - Token Bit - 0 defines token as free (not busy)
M - Monitor Bit - 1 if circulated past the AM
R - Request Priority Bit - 000 through 111