Section 5.4. Classification of MAC Protocols

5.4. Classification of MAC Protocols

MAC protocols can be broadly classified as centralized or distributed based on the type of network architecture. In the centralized scheme, a central controller controls the operation of the users, and users communicate only with the controller. Users adhere to the transmission schedule specified by the controller. In a distributed network, users talk to each other and dynamically determine the transmission schedule. Centralized schemes are used in applications that require a simple access scheme, QoS, and guaranteed capacity. However, centralized schemes are vulnerable to a single point of failure. Distributed schemes have no single point of overall failure. Yet, coordinating access among devices is a complex process for distributed schemes, and the network cannot provide a QoS guarantee.

MAC protocols can also be characterized as synchronous or asynchronous . In the synchronous scheme, such as time-division multiplexing and frequency-division multiplexing, capacity is preassigned to each connection. The synchronous scheme is rarely used in LANs, as users' transmission requirements vary dynamically. Rather, it is optimal to assign capacity to users asynchronously, based on demand.

In terms of access to the shared medium, MAC protocols can be broadly classified into two types:

• Contention access , by which each user needing to transmit data must contend to access the medium

• Round- robin access , by which each user is given a chance to transmit data in a round-robin fashion

The details of these two protocols are presented in Sections 5.5 and 5.6.

A third category of access method is reservation-access MAC. This application-specific method is used mainly for streaming traffic. With this method, as in synchronous time-division multiplexing, time slots are used to access the medium. A user can reserve time slots for transmission in advance. The control mechanism for the reservations can be either centralized or decentralized.