LAYER NUMBER 2 – THE DATA LINK LAYER
ARE YOU TAKING A CAR, TRAIN OR AEROPLANE?
“Defines how data is transported over a physical media”
Simply put, the Data Link layer creates the containers into which data is placed before being sent across the physical medium. These containers are called frames.
The kind of frame created will depend on what physical means are being used. This will in turn depend on how far the data is going, and what technology will be used to get it there. For example, a local area network (LAN) will usually only span a single building. The technology used here would be a type suited to LANs such as Ethernet or Token Ring (in which case the kind of container created would be an Ethernet frame or Token Ring frame).
For LANs, the Data Link layer has been divided further into two sub-layers because the container is created in two steps, labelling and addressing:
Step 1. Logical Link Control (LLC) sub-layer – Labelling
The Logical Link Control sub-layer identifies and labels data from the network layer. This is a little like what the transport layer did using port numbers. Instead of port numbers though, different kinds of labelling systems exist that use different kinds of codes to identify what protocol was used at the network layer
Again like the transport layer, the Logical Link sub-layer can also do flow control to upper layers and dictate whether data transfer will be reliable or unreliable.
Step 2. Media Access Layer (MAC) sub-layer – Addressing
This is the layer that defines exactly how the data is transported over the physical link. It sits right in front of the physical layer.
This layer stamps a physical address onto the container. Physical addressing is different from the logical addressing that we found at the network layer. The difference between the two is that a logical address is used to find exactly where a particular computer is, rather like a postal address while the physical address uniquely defines the computer, rather like a name.
This MAC address is found on the network interface card (NIC) of every computer. It is also called a hardware address or burnt-in-address (bia) because during manufacture, it is "burnt in" into the circuitry of a network interface card.
Other functions of the MAC layer include defining network topology (what a diagram of your network will look like), line discipline, error notification, orderly delivery of frames and even flow control.
These two sub-layers do not exist for WANs.
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