Ring Maintenance:

Mechanism:

When the 1st node on the token bus comes up, it sends a Claim_token packet to start the ring. If more than one station send this packet simultaneously, there is a collision. Collision is resolved by a contention method, in which the contending nodes send unsystematic data for 1, 2, 3 and 4 units of time depending on the first two bits of their address. The node sending data for the highest time wins. If two nodes have the identical first two bits in their addresses, then argument is done again based on the next two bits of their address and so on.

After the ring is set up, new nodes which are powered up may wish to connect with the ring. For this a node sends Solicit_successor_1 packets from periodically, inviting bids from new nodes to connect with the ring. This packet contains the address of the current node and its current successor, and asks for nodes in among these two addresses to reply. If more than one node replys, there will be collision. The node then sends a Resolve_contention packet, and the conflict is resolved using a similar mechanism as explained previously. Thus at a time only one node gets to enter the ring. The last node in the ring will send a Solicit_successor_2 packet containing the addresses of it and its heir. This packet asks nodes not having addresses in between these two addresses to respond.

A question arises that how frequently should a node send a Solicit_successor packet? If it is sent too regularly, then overhead will be too high. yet again if it is sent too infrequently, nodes will have to stay for a long time before joining the ring. If the medium is not busy, a node will send a Solicit_successor packet after a fixed number of token rotations. This number can be handled by the network administrator. Yet if there is heavy traffic in the network, then a node would postpone the sending of bids for successors to join in.

There may be problems in the logical ring due to sudden failure of a node. What happens when a node got a problem along with the token. After passing the token, a node can be any node, say node A, hearing to the channel to see if its successor either transmits the token or passes a frame. If nothing happens, it resends a token. Still if nothing happens, A sends a Who_follows packet, having the address of the down node. The heir of the down node, say node C, will now replies with a Set_successor packet, having its own address. This causes A to put its successor node to C, and the logical ring is restored. on the other hand, if two successive nodes go down suddenly, the ring will be lifeless and will have to be built  afresh and starting from a Claim_token packet.

When a node wants to shutdown normally, it have to sends a Set_successor packet to its predecessor, have the name of its own successor. The ring then continues unbroken, and now the node goes out of the ring.

The various control frames used for ring maintenance are shown below:

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