Token Ring frame format

Prerequisite – Ethernet Frame Format

• Topology – Ring topology
• Transmission – Unidirectional
• Encoding – Differential Manchester encoding
• Access control – Token passing
• Data rates – 4 Mbps, 16 Mbps

Token Ring Frame format:

• Start frame delimiter (SFD) – Alerts each station for the arrival of token(or data frame) or start of the frame. It is used to synchronize clocks.
• Access control (AC) –
  
  

  Priority bits and reservation bits help in implementing priority. Priority bits = reservation bits = 3. Eg:- server is given priority = 7 and client is given priority = 0.

  Token bit is used to indicate presence of token frame. If token bit = 1 –> token frame and if token bit = 0 –> not a token frame.

  Monitor bit helps in solving orphan packet problem. It is covered by CRC as monitor are powerful machines which can recalculate CRC when modifying monitor bit. If monitor bit = 1 –> stamped by monitor and if monitor bit = 0 –> not yet stamped by monitor.

• Frame control (FC) – First 2 bits indicates whether the frame contains data or control information. In control frames, this byte specifies the type of control information.
  
  
• Destination address (DA) and Source address (SA) – consist of two 6-byte fields which is used to indicate MAC address of source and destination.
• Data – Data length can vary from 0 to maximum token holding time (THT) according to token reservation strategy adopted. Token ring imposes no lower bound on size of data i.e. an advantage over Ethernet.
• Cyclic redundancy check (CRC) – 32 bit CRC which is used to check for errors in the frame, i.e., whether the frame is corrupted or not. If the frame is corrupted, then its discarded.
• End delimiter (ED) – It is used to mark the end of frame. In Ethernet, length field is used for this purpose. It also contains bits to indicate a damaged frame and identify the frame that is the last in a logical sequence.
• Frame status (FS) – It Is a 1-byte field terminating a data frame.
  
  It makes use of 2 copies of AC bits are used as a error detection mechanism (100% redundancy) as CRC does not cover FS byte so that destination does not have to recalculate CRC when modifying AC bits.