Specifications

TOKEN RING OVERVIEW
2-28
Ending Delimiter (ED)
The ED has the following format:
J K 1 J K 1 | I | E
The first 6 bits, J K 1 J K 1, are always the same. Note that the J and K bits
are non-data symbols and violate the Differential Manchester Encoding
scheme.
I is the Intermediate Frame bit which notes if the frame is part of a
multiple frame transmission. If this bit is set to 0 then the frame is the last
or only frame in the transmission sequence. If set to ‘1’ then it is the first of
several or an intermediate frame.
E is the Error Detected bit which is set to ‘0’ by the sending station. If any
station, while regenerating the frame, detects an error, it changes this bit to
a ‘1’. Note that this field is after the FCS calculation, hence the station has
time to perform the FCS task and modify this bit if necessary. The station
that sets the error bit will log the error by type.
Data Frame Format
The minimum length of a Token Ring frame is 21 octets. The maximum
frame size is 18,000 bytes for 16 Mbps and 4500 bytes for 4 Mbps
networks. Also the time required to transmit a frame must be no greater
than the token holding period that has been established for the station,
default is 10 ms. A frame is made up of the following fields:
• Starting Delimiter (SD) 8 bits
• Access Control (AC) 8 bits
• Frame Control (FC) 8 bits
• Destination Address (DA) 48 bits
• Source Address (SA) 48 bits
• Optional Routing Info (RIF) 0 or more bits
• Information (Info) 0 or more bits
• Frame Check Sequence (FCS) 32 bits
• Ending Delimiter (ED) 8 bits
• Frame Status (FS) 8 bits
This format shown in Figure 2-6 is used to transmit both MAC and LLC
messages to the destination station(s). MAC frames are covered in a later
section.