09: Data-link Layer - Framing and addressing part 1

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Data link layer is the second layer of OSI reference model. It is one of the most complicated layers to explain. But no worries ! everything will be clearly and thoroughly expounded so that you have a firm grasp. This layer comes between the first layer ( Physical layer) and the third layer ( Network layer) as shown in the following figure ; obviously, it plays the role of a medium between those layers. It takes data (packets) coming from the third layer and gives them to the first layer, and vice versa. In other words, It is the one responsible for preparing data to be transmitted on media.

We said  in the previous lessons that Data has different names at different layers. It is called Segment at Transport layer, Packet at the Network layer, and it is referred to as Frame in The Data link layer. When a packet reaches the Data link layer, it is encapsulated with a header and a trailer (which you will see later)  to be called a frame.

The role of this layer is to transfer data between adjacent network nodes ( it is a name to refer  to end devices) in Wide Area Networks (WANs), or between nodes in the same Local Area Networks (LANs). By and large, its job is to deliver Data to hosts within Local Area Networks. After Data segment is encapsulated with the Network layer’s header (to become packet), it is added a destination IP address to direct it to the correct remote network, but once it arrives at the network, this latter consists of a group of nodes (e.g. computers, laptops, printers…), so how the intended node within the network (LAN) will be identified among other nodes ? here where the Data link layer comes in handy. This layer can easily reach the correct node by means of an address generated at the same layer at the source machine for the purpose of delivering Frames to their final receiver (node). This address is called MAC (Media Access Control), it will be explain in detail. Take a look at the following figure.

 The above figure shows that a source machine whose IP address 10.0.0.2 wants to send a message to a laptop (destination), whose ip address 13.0.0.4, located in a remote network whose ip address 13.0.0.0 (this is the ip address of the whole network, which includes the three hosts). When the message travels through the network, it stops at the router to which the three machines belong. The message has to be transmitted to its correct node. Thus, the router will see which machine has a MAC address that matches the one contained in the frame (through a method called ARP request, which will be in-depth explained). Once the machine is found, the router will send the message to the switch (if any) shown in the figure, then it will be directed to the intended machine with the meant MAC address.

Generally, the function of this layer is to change packets into frames, by adding an additional header and trailer,  and convert them into bit streams to be ready to travel across the network after, of course, being subjected to physical layer role, which is transforming bits into signals.
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