IEEE 802'6 MAN

1
IEEE 802.6 (MAN)

• Random Access Scheme
• packet collision gt waste resources
• retry backoff gt bus idle
• Controlled Access Scheme
• slow access at light load
• overhead of token traveling.

• Distributed Control/Queuing (of messages)
• It forms a single and distributed queue which
  controls the access of packets to the subnet in
  essentially the same order as they were generated
• Under light load, it behaves as CSMA/CD
• Under heavy load, it behaves as token passing
• Different levels of priority

2
A DQDB MAN
3
Network Configuration

• Two buses with unidirectional structure.
• All communications on the subnet are synchronous
  and access is in fixed length packets that repeat
  continually.
• It is the function of the controllers at the bus
  ends to generate all timing signals to ensure
  synchronization.

4
State transmission diagram for packet access
CD 0
5
The Distributed Queue Dual Bus Protocol (DQDB)

• The operation of the access protocol is identical
  and independent for each bus.
• Consider Si accessing bus A
• state transition diagram
• IDLE state no packet to transmit
• keep track of the request bit received on Bus B

• Request Counter (RC) is incremented for each
  request bit received (REQ bit has been set),
  indicating the number of packet queued up for
  transmission on Bus A downstream from Si.
• RC is decremented for each empty packet that
  passes Si on bus A.

6

• COUNTDOWN state when Si has packet for
  transmission and (AQ ¹ 0 and RC ¹ 0)
• AQ of packets queues for transmission
• RC is transferred CD counter and RC is cleared.
• CD counter is decremented by one for each empty
  packet that passes on bus A.
• RC is incremented for each received request bit
  that has been set.
• Initiate the transmission of the REQ bit to
  reserve a free packet (on Bus B).

7

• WAIT and ACCESS state
• when CD counter reaches zero
• Wait for next free packet on bus A in order to
  transmit queued packet.
• Set the busy bit, if available, and transmit the
  packet.
• AQ--
• if (AQ 0) then goto IDLE state
• else if (RC 0) then goto STANDBY state
• else goto COUNTDOWN state

8

• STANDBY state
• when a new packet moves into the first
• queue position and RC 0
• Attempt access in the next packet.
• If it is free, then transmit in the packet
  without sending REQ bit else initiate the sending
  of a REQ bit and go into WAIT state.

9
Priority Controlled Distributed Queueing

• Achieved by operating separate distributed queues
  for each level of priority.
• For each N-level priority system, N separate REQ
  bits, RC, CD counters.
• When REQi is received, each of the RC of priority
  i or lower will be incremented.
• Each of those RC is decremented for each empty
  packet, that passes on bus A.

• The I-level COUNTDOWN state will not be entered
  until all higher priority buffered packets have
  been transmitted.
• Once in the I-level COUNTDOWN state, the function
  is as before.