Sign in Subscribe

Layer 2: Media Access Control – MAC

Last updated on 

Layer 2 of the OSI (Open Systems Interconnection) model is often referred to as the Data Link Layer. Within this layer, one of the most crucial components is the Media Access Control (MAC) sublayer. Let's dive deep into the technical aspects of the MAC layer.

Media Access Control (MAC) Layer:

1. Functionality:

The primary function of the MAC layer is to control the access of devices to the physical network medium. In other words, it determines which device gets to transmit data over the shared medium (like a cable or wireless spectrum) and when.

2. MAC Address:

  • Every device (such as a network card or interface) has a unique identifier called a MAC address (Media Access Control address).
  • The MAC address is a 48-bit (or 6-byte) hexadecimal number, often represented as six groups of two hexadecimal digits separated by colons (e.g., 00:1A:2B:3C:4D:5E).
  • This address is hard-coded into the device's hardware by the manufacturer and is used for addressing and delivering data packets within a local network segment.

3. Frame Structure:

  • Data at the Data Link Layer is encapsulated into frames. A frame typically consists of:
    • Preamble: A sequence of bits used to signal the beginning of the frame, helping devices synchronize their clocks.
    • Destination MAC Address: Specifies the MAC address of the intended recipient.
    • Source MAC Address: Indicates the MAC address of the sender.
    • Type/Length: Indicates either the type of the payload or its length.
    • Payload: Actual data being transmitted.
    • Frame Check Sequence (FCS): A section that contains error-checking information.

4. MAC Protocols:

Different MAC protocols determine how devices access and transmit data over a shared medium:

  • Carrier Sense Multiple Access (CSMA): Devices listen to the medium before transmitting. If they detect another transmission, they wait for a random period before retrying. Variants include CSMA/CD (Collision Detection) used in Ethernet.
  • Token Passing: A token is passed from one device to the next. Only the device with the token can transmit data.
  • Polling: A central device (e.g., a hub or switch) polls devices in sequence, asking if they have data to transmit.

5. Ethernet and MAC:

  • In Ethernet networks, the MAC layer protocol is used to address devices in the same local network segment.
  • Ethernet frames use the MAC address to ensure that data packets reach the correct destination within the same LAN.

6. Switching and MAC:

  • Ethernet switches operate primarily at the MAC layer.
  • When a switch receives a frame, it looks at the destination MAC address to determine which port to forward the frame to, ensuring efficient data delivery within a LAN.

7. Address Learning and Aging:

  • Ethernet switches maintain a MAC address table that maps MAC addresses to switch ports.
  • When a frame enters a switch port, the switch learns the MAC address from the source and updates its MAC address table.
  • Entries in the MAC address table have a certain aging time. If a switch doesn't see traffic from a MAC address for a while, it removes that entry to ensure the table's accuracy.

Conclusion:

The Media Access Control (MAC) layer is pivotal in the OSI model, responsible for the identification of devices on a network through MAC addresses and controlling access to shared network resources. Through protocols like CSMA, token passing, and polling, the MAC layer ensures orderly and efficient data transmission within local network segments.