Medium Access Control


Medium Access Control (MAC) address is a hardware address use to uniquely identify each node of a network. It provides addressing and channel access control mechanisms to enable the several terminals or network nodes to communicate in a specified network. Medium Access Control of data communication protocol is also named as Media Access Control. In IEEE 802 OSI Reference model of computer networking, the Data Link Control (DLC) layer is subdivided into two sub-layers:

  • The Logical Link Control (LLC) layer and
  • The Medium Access Control (MAC) layer

The MAC sublayer acts as a direct interface between the logical link control (LLC) Ethernet sublayer and the physical layer of reference model. Consequently, each different type of network medium requires a different MAC layer. On networks that don’t conform they are part of IEEE 802 standards but they do conform that they participate OSI Reference Model then the node address is named the Data Link Control (DLC) address. The MAC sublayer emulates a full-duplex logical communication channel in a multipoint network system. These communication channels may provide unicast, multicast and/or broadcast communication services.

Medium Access Control MAC sublayer
LLC and MAC Sublayer

MAC address is suitable when multiple devices are connected with same physical link then to prevent from collisions system uniquely identify the devices one another at the data link layer, by using the MAC addresses that are assigned to all ports on a switch. The MAC sublayer uses MAC protocols to prevent collisions and MAC protocols uses MAC algorithm that accepts as input a secret key and an arbitrary-length message to be authenticated, and outputs a MAC address.

Functions performed in the MAC sublayer:

The primary functions performed by the MAC layer as per the IEEE Std 802-2001 section 6.2.3 are as follows:

  1. Frame delimiting and recognition: This function is responsible to creates and recognizes frame boundaries.
  2. Addressing: MAC sublayer performs the addressing of destination stations (both as individual stations and as groups of stations) and conveyance of source-station addressing information as well.
  3. Transparent data transfer: It performs the data transparency over data transfer of LLC, PDUs, or of equivalent information in the Ethernet sublayer.
  4. Protection: MAC sublayer function is to protect the data against errors, generally by means of generating and checking frame check sequences.
  5. Access control: Control of access to the physical transmission medium form unauthorized medium access.

One of the most commonly used of MAC sublayer for wired networks i.e. Carrier Sense Multiple Access with Collision Detection (CSMA/CD). Through MAC schema, a sender senses the medium (a wire or coaxial cable) before transmission of data to check whether the medium is free or not. If MAC senses that the medium is busy, the sender waits until it is free. When medium becomes free, the sender starts transmitting of data and continues to listen into the medium. If any kind of collision detected by sender while sending data, it stops at once and sends a jamming signal. But this scheme doest work well with wireless networks. Some of the problems that occur when it uses to transfer data through wireless networks are as follow;

  • Signal strength decreases proportional to the square of the distance
  • The sender would apply Carrier Sense (CS) and Collision Detection (CD), but the collisions happen at the receiver
  • It might be a case that a sender cannot “hear” the collision, i.e., CD does not work
  • Furthermore, CS might not work, if for e.g., the terminals are“hidden”.

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