Access Control
There are two methods of controlling the access to a network.
1. CSMA/CD (Carrier Sense Multiple Access/Collision Detection) This is a simple but effective protocol where the node that wants to transmit listens for any other transmission that may be occurring on the bus. If this node does not hear any other activity, it transmits its message. If during the transmission of its message, it detects a collision (or another node transmitting at the same time), it stops its transmission for a random length of time before retrying to transmit.
Operation of CSMA/CD
2. Control Token Access
A special bit pattern called a 'control token' is passed from node to node around a logical ring until it is received by a node wishing to transmit a frame. The transmitting node then sends the frame using the physical ring and on conclusion of the transmission, it passes the control token onto the next node in the sequence.
Control Token Access
The control token is used with both the ring and bus physical topologies. Token Ring LANs use a physical ring to connect the nodes.
On Token Bus LANs the nodes are connected to a physical bus but the control token is still passed from node to node in a logical ring.