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Start Delimiter

The 1-octet start delimiter consists of signaling patterns that are always distinguishable from data. The format of this byte is NN0NN000, where N is a nondata media access control symbol and 0 is a zero.

Control Frame

This 1-octet field indicates which of four classes of frame is being sent. Its four data frame classes are media access control data, logical link control data, station management data, and special-purpose data (which is reserved for future use). The media access control frame format is 00CCCCCC, where C is the type of media access control field and 0 is a zero. The bits CCCCCC (positions 3 through 8) indicate the type of media access control frame. The meanings of the bit patterns are shown in Exhibit 1-1-21.


Exhibit 1-1-21.  Media Access Control Frame Format

Destination Address and Source Address Field

The destination or source address may be 16 or 48 bits in length. On a given LAN, the destination and the source address must be the same length.

Data Frame

The data frame carries the actual information. The first byte indicates the priority and the action to be taken by the station. The format for the first byte is shown in Exhibit 1-1-22.


Exhibit 1-1-22.  Data Frame (First Byte)

Frame Check Sequence

The 32-bit frame check sequence is based on the standard generator 32-degree polynomial:

G(x) = X32 + X26 + X23 + X22 + X16 + X12 + X11 +

X10 + X8 + X7 + X5 + X4 + X2 + X + 1

End Delimiter

The 1-byte end delimiter ends the frame and specifies the starting position of the frame check sequence. The signaling pattern of this field is always different from data. The format is NN1NN11E, where N is the nondata media access control symbol, 1 is a one, I is the intermediate bit (1 means more to transmit, 0 means end of transmission), and E is the error bit (i means error, 0 means no error).

Media Access Control Services

The services provided by the media access control sublayer of the token bus are more complex than the CSMA/CD protocol. Most of the complexities are pertinent to the management and maintenance of the token bus, and they include:

  Fault management:
  Multiple token.
  Lost token.
  Token pass failure
  Deaf station.
  Duplicate station addresses.
  Classes of service and a priority algorithm.
  Addition of a station into the logical ring.
  Deletion of a station.
  Ring initialization.

A station hands over the token to its successor using the media access control frame. The sending station then listens for evidence that the successor has received the frame. If there is no valid response frame, the sender repeats the token passing operation and performs the same monitoring as during the first attempt.

If the intended recipient, or successor, fails to respond, the sender assumes that its successor has malfunctioned and designates a new successor. The sender then broadcasts a who-follows frame using its address in the set-successor frame. This removes the failed station from the logical ring.

If the sending station fails to get a response to a who-follows frame, it retries. If the second trial is unsuccessful, the station initiates a new strategy by sending a solicit-successor-2 frame with its own address in the DA and SA fields and inviting any station in the network to respond.

An active station that hears the request and wants to join the logical ring may respond during the next response window. The sending station then transmits the remaining data frames and resumes the token passing process. At the end of its activity, the station switches to an idle state and waits for another station’s transmission. If all attempts to solicit a successor fail, however, the station assumes the occurrence of one of the following events:

  All stations have failed.
  All stations have left the logical ring.
  The medium has broken.
  The station’s own receiver has failed and cannot receive any frame.

The station then ceases its attempt and switches to an idle state.


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