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What is Media Access Control? What is the reason for MAC?

About Media Access Control or MAC

– Media access control or MAC is a data communication protocol that is used in the 7 layer OSI computer networking model.
– This is implemented in the data link layer’s (which is layer 2 itself) sub – layer.
– The purpose of this MAC sub layer is to provide channel access control and addressing mechanisms.
– These mechanisms are important for various network nodes and terminals that use them for establishing communication in a shared medium that lies within a multiple access network such as Ethernet.
– The hardware used for the implementation of the MAC is called the medium access controller.
– The MAC sub – layer resides at the interface intermediate to that of the physical layer of the network and the LLC i.e., the logical link control.
– In a multi – point network this layer is responsible for the emulation of the full – duplex logical communication channel.

Functions of MAC Sub-Layer

The following functions are carried out in the MAC sub – layer:
– Receiving and transmitting the normal frames.
– Back off and half – duplex transmitting functions.
– Checking and appending frame check sequence or FCS.
– Inter – frame gap enforcement
– Discarding the frames that are not formed properly.
– Removing and appending SFD, padding and preamble.
– Appending and removing the MAC address i.e., the half – duplex compatibility

Addressing Mechanism in the MAC Sub–Layer

– Mac address is the local network address that is particularly used in IP – Ethernet.
– MAC address is nothing but another name for this addressing mechanism.
– It is actually a unique serial number.
– This number is assigned to every network interface at the time of manufacturing so that it becomes uniquely identifiable among all the devices in the world.
– All the devices in a network thus have unique identities.
– This is required for making possible the delivery of the data packets to the exact destination host in the network.
– When the data packet reaches the destination, its IP address is resolved by either of the two in to the destination’s MAC address:
1. By address resolution protocol (IPv4)
2. Neighbor discovery protocol (IPv6)

– Ethernet provides the best example for a physical network that is extendable by WLAN access points and network adapters.
– This is so because both have the same hierarchy of 48 – bit MAC address.
– However, in a full–duplex point–to–point communication there is no use for MAC layer but it does require some address fields for maintaining compatibility in point – to – point protocols.
– Channel access control mechanism, also referred to as the multiple access protocol can connect several stations that are connected with the same medium can share it.
– Some examples of such shared media are:
1. Ring networks
2. Bus networks
3. Wireless networks
4. Hub networks
5. Half – duplex point – to – point links

– This whole mechanism is dependent up on a multiplex scheme for physical layer.
– The most widely used protocol in this category is CSMA/ CD based up on contention.
– However, this mechanism is supposed to operate only in a network collision domain and is not of any use in the full-duplex networks.
– But still it is supplied along with the equipment for the reasons of compatibility.
– Some other such protocols are:
1. Token passing
2. Token bus
3. Token ring
4. Dynamic TDMA
5. CDMA
6. OFDMA

Thus, the two major functions of the MAC can be summarized as shown:
1. Data encapsulation i.e., transmitting and receiving. It further includes:
– Framing operations such as frame synchronization and boundary delimitation.
– Addressing operations like handling various addresses.
– Error detection including physical medium transmission errors.

2. Media access management: It consists of two sub – functions:
– Medium allocation i.e., collision avoidance
– Contention resolution or the collision handling
There is one optional MAC control sub layer that might be placed between the MAC and the LLC.

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