Data Link Layer
- The Data Link layer assigns a meaning to the bits and bytes that are being transmitted over the network and ensures that they are structured.
- In fact the Physical layer only deals with bits of data (1s and 0s) but the Data Link layer organises data into frames.
- The main task of the Data Link layer is to take a raw transmission facility and transform it into a line that appears free of transmission errors in the Network layer.
- It adds control information in the form of fields such as a source and destination hardware address and error checking values.
- Other information includes the frame length Network layer protocol identifier and a preamble at the start of the frame.
Other functions of the Data Link layer include:
- Addressing frames to specific computers on the network (using their hardware address)
- Recognizing when the destination address in a frame matches the hardware address of the installed network card and ignoring frames that do not match.
- Error control allows the detection and correction of errors between adjacent nodes.
- Typical errors include frames lost on route and those recognizable as damaged frames from using cyclic redundancy checks (CRCs)
- Controlling the data flow prevents fast transmitters from slow receivers with data.
IEEE divides the function of the Data Link layer into two sub layers.
- These two sub layers are known as Media Access Control (MAC) and Logical Link Control (LLC).
- The Media Access Control sub layer defines the way in which multiple NICs share a single transmission medium.
Media Access Control covers the following:
- Logical topology
- bus or ring
- Media access method
- contention token passing or demand priority.
- Addressing
- the hardware address of the NIC (also known as the ‘MAC address’)
The logical link control sub layer is responsible for establishing and maintaining a link between the communicating devices for the transmission of frames.
This includes the following:
- Service level
- connection-oriented or connectionless
- Flow control and error control
Connectivity devices working at the Data Link layer include:
Switches
Bridges