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5.2] What are MAC frames? (Token Ring Data Link Layer)




Description

This article is from the Token Ring FAQ, by James Messer James@NetworkUptime.com with numerous contributions by others.

5.2] What are MAC frames? (Token Ring Data Link Layer)

A Media Access Control (MAC) frame is used for management of the
token ring network. MAC frames do not traverse bridges or routers,
since they carry ring management information for a single specific
ring.

The MAC frame has this format:

             +-----+-----+-----+-----+-----+-----+-----+-----+-----+
             | SD  | AC  | FC  | DA  | SA  |Data | FCS | ED  | FS  |
             +-----+-----+-----+-----+-----+-----+-----+-----+-----+
       Size     1     1     1     6     6    >=0    4     1     2
       in bytes

Starting Delimiter (SD) - See section [5.1].

Access Control (AC) - See section [5.1].

Frame Control (FC) - The frame control field consists of eight bits,
coded as TT00AAAA. The Frame Type bits (T) indicate the frame type.
Bits 2 and 3 are reserved, and are always zero. Bits four through
eight are Attention Codes which provide the token ring adapter of
incoming MAC information that can be copied to a special Express
Buffer in the token ring adapter.

Destination Address (DA) - The Destination Address specifies which
station is to receive the frame. The Destination Address can be sent
to a specific station, or a group of stations.

Source Address (SA) - The Source Address is the MAC address of the
sending station.

Data - A MAC frame data field contains token ring management
information, and a non-MAC (LLC) data field contains user data.

Frame Check Sequence (FCS) - A 32 bit Cyclical Redundancy Check
(CRC) is performed on the frame data to provide an integrity check
of the frame data. As each station copies the frame, the CRC is
computed and compared with the value in the FCS frame to verify that
the frame data is correct.

Ending Delimiter (ED) - See section [5.1].

Frame Status (FS) - The Frame Status field provides information for
the sending station regarding the status of the frame as it
circulates the ring. The Frame Status field is coded as AF00AF00.
The bits of the Frame Status field are duplicated, since this field
does not fall under the CRC checking of the Frame Check Sequence
bytes. The Address Recognized Indicator (ARI) is set to 1 by the
destination station if the destination station recognizes the frame.
The Frame Copied Indicator (FCI) is set to 1 if the destination
station was able to copy the frame into the local adapter buffer
memory.

 

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