Data Link Layer 2 tutorials

Data received from Network Layer 3 "Packets" is formatted into "Frames" to be transmitted to the Physical Layer 1.

  • It adds the MAC addresses to frames.

  • It performs error detection not correction.

  • The Data Link layer ensures that messages are delivered to the proper device and translates messages from the Network layer into bits for the Physical layer to transmit.

  • It formats the message into data frames and adds a customized header containing the hardware destination and source address.

  • Physical addressing or hardware addressing (rather than logical addressing) ensures that data is delivered to the appropriate node on the LAN.

  • This layer is also responsible for error notification (not correction), network topology, and flow control.

  • Two devices that are used at Data Link Layer 2 are Bridges and Switch.

  • Two domains determine data transport reliability:

1) Broadcast Domain: A group of nodes that can receive each otherís broadcast messages and are segmented by routers.

2) Collision Domain: A group of nodes that share the same media and are segmented by switches. A collision occurs if two nodes attempt a simultaneous transmission. Carrier Sense Multiple Access Collision Detection (CSMA/CD) is an access method that sends a jam signal to notify the devices that there has been a collision. The devices then halt transmission for a random back-off time.

  • This is the only layer of the OSI model that has sublayers. The two sublayers define the IEEE Ethernet 802.3 frame, which in turn provides physical addressing and flow control. Routed protocol information IP, IPX, AppleTalk, and so on is provided to the upper layers.


The IEEE Ethernet 802.3 sublayers are

1) Media Access Control (MAC) and

2) Logical Link Control (LLC).


MAC (Media Access Control)

  • The MAC address is the hard-coded unique address on the network interface controller (NIC) LAN Card.

  • Each MAC address must be unique and it is in following format

  • It consist of 48 bits.

  • It must be displayed by 12 hexadecimal number from (0 to 9 and A to F).

  • The first 6 hexadecimal digits in address are reserved for Vendor code or Manufacture unique identifier and it is assigned by NIC manufacturer.


LLC (Logical Link Control)

  • The LLC sublayer complements the MAC sublayer in the ethernet model; the LLC is responsible for framing, error, and flow control.

  • LLC provides a service access point (SAP) identifier in the frame.

  • The SAP field of the frame consists of one byte that identifies an upper layer protocol (for example, 06 = IP, whereas E0 = IPX). The LLC inserts a destination SAP (DSAP) and a Source SAP (SSAP) in the frame.

  • Following chart shows example of Ethernet frame.