What are the different types of MAC protocols?

The data link layer is only effective if there is a dedicated connection between the sender and the receiver. In case there isn't any, numerous stations can access the channel simultaneously. Therefore, it is necessary to use multiple access control (MAC) protocols to reduce collision and prevent crosstalk.

Analogy

When a teacher presents a question in a classroom full of students and all the students start replying simultaneously, chaos is created. The teacher must manage the students and force them to respond one at a time. This is analogous to when MAC protocols become necessary in data link layers.

Classification

MAC protocols are classified into three categories, as illustrated below:

Random access protocol

In random access protocol, no station has a higher priority than another station. Instead, all are equal. There is no predetermined time for sending data; it depends on the channel's status. The order of the stations transmitting data is not predefined. It is sub-divided into the following:

• ALOHA: In ALOHA, a station can transmit whenever data is available. If another station broadcasts at the same time, collision occurs and the packets are lost. ALOHA protocol has two versions: pure ALOHA and slotted ALOHA.

• CSMA: In Carrier Sense Multiple Access (CSMA) protocol, each node must monitor the carrier for some time before attempting to deliver data packets. Each node on the bus gets an equal chance to send the data across the network.

• CSMA/CD: Carrier Sense Multiple Access with Collision Detection (CSMA/CD) protocol is used for carrier transmission via collision detection. It is used in early ethernet technology for Local Area Networking (LAN).

Note: If you want to read more about CSMA/CD, click here.

• CSMA/CA: Carrier Sense Multiple Access with Collision Avoidance (CSMA/CA) is used for carrier transmission using collision avoidance. It has wide application in wireless technology.

Note: To know about the working of CSMA/CA, click here.

Controlled access protocol

In the controlled access approach, all the stations communicate with each other to determine which station has the authority to send data in order to avoid collision. The following protocols fall under the category of controlled access:

• Reservation: In reservation, a station reserves the transmission channel before transmitting the data packets.

• Polling: In polling, one device is marked as the primary station (controller) and the other as the secondary station. The controller is responsible for all data transfers.

• Token passing: In token passing, stations are logically connected in the form of a ring. To avoid collisions, this media access control technique uses token passing. The only computer allowed to communicate is the one with the token.

Channelization protocol

Channelization is a multiple-access mechanism in which the link's available bandwidth is shared among several stations as per time, frequency, or code.

• FDMA: In Frequency Division Multiple Access (FDMA), the available bandwidth is split into various frequency bands. To prevent crosstalk and noise, guard bandsA narrow frequency between two frequency channels is called guard band. It is typically used to prevent interference from adjacent channels. are also used to ensure that no two bands coincide.

• TDMA: In Time Division Multiple Access (TDMA), bandwidth is divided across several stations. Time is divided into slots and stations broadcast data in their allotted slots to prevent collisions.

• CDMA: In Code Division Multiple Access (CDMA), all signals are broadcast concurrently on one channel. It eliminates the concept of time and bandwidth division. It is a code-based communication technique. CDMA codes are utilized to differentiate between various users.