Network Access Control Techniques

Access control methods are required in a network to ensure that only one user of the network is able to transmit at any one time.

In ring-based local area networks (LANs), two commonly employed techniques are:

• Token passing
• Slot passing

Token Passing

In this technique, a token i.e. a special bit pattern, is circulated. When a station wishes to transmit, it waits until it receives the token, and then transmits the data with the token attached to its end. Another station that wishes to transmit removes the token from the package of data and transmits its own data with the token attached to its end.

Slot Passing                  

This technique involves empty slots being circulated. When a station wishes to transmit data it deposits it in the first empty slot that comes along.

In bus or tree networks a technique that is frequently employed is:

Carrier Sense Multiple Access with Collision Detection (CSMA/CD)

With CSMA/CD technique, stations have to listen for other transmissions before transmitting, with any station being able to gain control of the network and transmit, hence the term multiple access. If no activity is detected then transmission can occur. If there is an activity, then the system has to wait until it can detect no further activity. Even with this listening before transmitting, it is still possible for two or more systems to start to transmit at the same time. If such a condition is detected, both stations cease submitting and wait a random time before attempting to transmit again. This method is commonly associated with Ethernet LAN bus.

Related: Topological Network Structures used in Fieldbus Systems