- The polling MAC Protocols requires one of the nodes to be designated as a master node.
- The master node polls each of the nodes in a round-robin fashion.
- In particular, the master node first sends a message to node 1, saying that it (node 1) can transmit up to some maximum number of frames.
- After node 1 transmits some frames, the master node tells node 2 it (node 2) can transmit up to the maximum number of frames.
- The master node can determine when a node has finished sending its frames by observing the lack of a signal on the channel.
- The procedure continues in this manner, with the master node polling each of the nodes in a cyclic manner.
- The polling protocol eliminates the collisions and empty slots that plague random access protocols.
- This allows polling to achieve a much higher efficiency.
- The first drawback is that the MAC Protocols introduces a polling delay—the amount of time required to notify a node that it can transmit.
- The second drawback, which is potentially more serious, is that if the master node fails, the entire channel becomes inoperative.
- In the taking-turns protocol, there is no master node.
- A small, special-purpose frame known as a token exchanged among the nodes in some fixed order.
- For example, node 1 might always send the token to node 2, node 2 might always send the token to node 3, and node N might always send the token to node 1.
- When a node receives a token, it holds onto the token only if it has some frames to transmit; otherwise. It immediately forwards the token to the next node.
- If a node does have frames to transmit when it receives the token, it sends up to a maximum number of frames and then forwards the token to the next node.
- Token passing decentralized and highly efficient. But it has its problems as well. For example, the failure of one node can crash the entire channel. Or if a node accidentally neglects to release the token. Then some recovery procedure must be invoked to get the token back in circulation.