What are different types of multiple access protocols?

Multiple access protocols are used to coordinate access to shared communication channels, where multiple nodes regulate their transmission onto broadcast channels. These protocols are essential in both wired and wireless LANs, as well as satellite networks.

When multiple nodes transmit frames simultaneously, collisions occur at the receiver. During a collision, transmitted frames are lost and no acknowledgement is sent, wasting the broadcast channel during the collision interval. Multiple access protocols solve this fundamental problem by providing structured methods for channel sharing.

Common shared channels include Ethernet cables, wireless frequencies, and satellite links. Multiple access protocols also apply to computer systems, storage facilities, and servers where resources are shared among independent users.

Random Access MAC Protocols

In random access protocols, transmitting nodes always transmit at full channel strength. When collisions occur, involved nodes retransmit their frames after waiting random delays.

The collision recovery process works as follows:

• Collision detection − Nodes detect when their transmissions collide

• Random backoff − Each colliding node waits a randomly chosen delay before retransmitting

• Independent delays − Nodes choose delays independently, increasing the probability that one will transmit successfully

• No transmission guarantee − Success depends on avoiding future collisions

Key random access protocols:

• ALOHA
• Slotted ALOHA
• CSMA (Carrier Sense Multiple Access)
• CSMA/CD (CSMA with Collision Detection)
• CSMA/CA (CSMA with Collision Avoidance)

Channel Partitioning Protocols

Channel partitioning protocols divide the shared medium among users to eliminate collisions entirely. Each participant receives a dedicated portion of the channel based on time, frequency, or code.

Three main partitioning methods:

• Time Division Multiplexing (TDM) − Transmission time is divided into slots, with each node assigned specific time periods

• Frequency Division Multiplexing (FDM) − Frequency bandwidth is divided into slots, with each node transmitting on assigned frequencies

• Code Division Multiple Access (CDMA) − Each node uses a unique code, allowing simultaneous transmissions without collisions

This approach ensures predictable access but may waste capacity when some nodes are inactive.

Taking Turns Protocols

Taking turns protocols combine benefits of random access and channel partitioning by coordinating transmission opportunities among nodes through polling or token passing mechanisms.

Functions of MAC Protocols

Conclusion

Multiple access protocols solve the fundamental problem of coordinating shared channel access among multiple nodes. The three main categories − random access, channel partitioning, and taking turns − each offer different trade-offs between simplicity, efficiency, and collision avoidance for various network scenarios.