This article is from the Token Ring
FAQ, by James Messer James@NetworkUptime.com with numerous
contributions by others.
5.1] What is a token? (Token Ring Data Link Layer)
A token frame is a three byte frame that takes this format:
| SDEL | AC | EDEL |
| 1 byte | 1 byte | 1 byte |
The Starting Delimiter (SDEL) byte is coded as JK0JK000, where the J
and K bits are intentional Manchester encoding violations. These
intentional violations delineate the token from normal traffic data.
J is the encoding violation of a 1, and K is the encoding violation
of a 0.
The Access Control (AC) byte is coded as PPPTMRRR. The priority bits
(PPP) provide eight levels of priority (000 through 111). The token
indicator bit (T) of 0 determines that the following information is
a token, a 1 designates the following information is a frame. The
Monitor bit (M) is used to prevent frames from constantly circling
the ring. The Priority Reservations bits (RRR) provide token
reservation to ring stations.
The Ending Delimiter (EDEL) byte is coded as JK1JK1IE, where the J
and K bits are encoding violations and the I and E bits are the
intermediate frame and error detection bits, respectively. The
intermediate bit is set to 1 if there are more frames to transmit in
this set. The error detection bit is set to 1 by a station that
recognizes a CRC error in the frame so other stations downstream do
not report the same error.