CS485 Sylabus

IEEE 802.4 - Token Bus architecture

The IEEE 802.4 Token bus architecture addresses the manual node addition/deletion problem in the IEEE 802.5 token ring

Architecture of the IEEE 802.4 Token bus:

Notes:

Stations (computers) are connected using a bus (like Ethernet)
Stations (computers) form a logical ring
The order of the nodes in the ring is always descending according to the ID of the nodes that are in the ring

Each station maintains:

the ID of its logical predessor
(The ID of the predecessor is needed for station (node) removal)
the ID of its logical successor
(The ID of the successor is needed for general operation and station insertion)

IEEE 802.4 - Token Bus protocol

Protocol

When a node receives the token message, it can transmit upto 1 messages
Unlike in the Token ring system, the token holder does not wait for the message to return
The token holder sends the token to its successor

So it's very simple if the token ring is set up...
The problem is maintaining logical ring automatically
We will study these problems in token bus:

Node insertion

Node insertion procedure:

From time to time, a node that is not the last member of the ring, sends out a Solicit_successor1(myID, mySucc) message
Example:
A node with ID between 1 and 4 that wants to join will then reply.
Example:
The node is inserted by the inviting node by sending an Set_successor() message.
Example:
The inserted station must make itself known to it's downstream neightbor
Example:

Node insertion procedure - last node:

When the last node (it can tell by: my ID < succ ID), it sends a different Solicitation message - called Solicit_successor2()
Example:
The Solicit_successor2(0,4) message will invite nodes:
Beside this difference, the insert procedure is identical to the one described abovbe...

Collision resolution

When multiple nodes are invited, there will be collision
Example:
Responses from nodes 2 and 3 will collide:

Collision Resolution Protocol:

Executed by nodes that are involved in a collision:

Select the first 2 bits of your own ID
Let this value be x
Transmit for (x+1) × &tau time
If the medium is still busy after (x+1) × &tau time, remove yourself from the contention
(I.e., you have lost the competition)
If just before (x+1) × &ta, you only sensed your own transmission, declare yourself to be the winner
Otherwise (you sensed another transmission just before you ended):
- Select the next 2 bits from your own ID
- Repeat the above precedure...

Node deletion
- When a node wants to leave the token bus, it sends a Set_successor(my Successor) message to its predecessor.
  Example:
- The notified node will then make itself known to its new successor:
  Example:
Ring initialization
- When the token ring is first started, there are no stations in the ring...
- When the first node on the token bus comes up, it sends a Claim_token packet to initialize the ring.
- If more than one station send this packet at the same time, there is a collision.
- The Collision Resolution Protocol (tournament) is run...
  And the station with the largest ID will be the winner.
- This station will form a ring consisting of one node...
- Then the station invites the other stations into the ring

Node failure....

When a node fails, it will not pass the token on and the whole token ring is dead...

Responsibility of the token holder:

When the token holder passes the token to its successor, it sets a timeout
When the timeout expires without any network activity, the token holder will retransmit the token again.
When the token holder times out for the second time, it will assume that its successor has failed
.... and initializes a recovering protocol

Recovery Protocol:

The Token holder send a Who_follows(x) message, with x equals to the ID of the down node
The successor of the down node x (say node C), will now respond with:
The token holder will set its successor node to C, and the logical ring is restored.