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09: Data-link Layer - LLC and MAC sublayers part 2


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LLC (Logical Link Control) and MAC (Media Access Control) sublayers.

In the previous lesson we laid the ground for a deeper coming explanation to the Data Link Layer. And it is important to point out that this layer acts as a connecting layer between the software processes of the above layers and the hardware of the Physical Layer.


This layer takes the PDU (which is packet) of the Network Layer and encapsulates it with a header and trailer to be called a frame (which is the PDU of the Data Link Layer).  Data Link layer is divided into two sublayers, LLC ( Logical Link Control) and MAC (Media Access Control).

LLC (Logical Link Control) sublayer

As for LLC, this sublayer acts as a link between the upper layer (Network Layer)  and the bottom layer (Physical Layer). It is responsible for identifying Network layer protocols. Put another way, when a frame coming from a remote host (source) reaches a computer in a local network (destination), as you know, the frame is first generated at the source machine starting  from the Application Layer down to the Physical Layer, it is slipped off of the header and trailer of the Data-link Layer ( which you will see), then it is passed on to the Network layer. Now, the question is how the frame will find the intended protocol (the same one that was used at the Network layer of the source machine)  at the Network layer ? the LLC answers this question by being responsible for that.  It is the one that sends the frame to its correct protocol  at the Network Layer - for example, IP protocol. To remove ambiguity, take a look at the following figure :
By and large, LLC provides the frame with information whose purpose is to identify which Network Layer protocol is being used for the frame. The role of this information is allowing the protocols that reside at the Network Layer, such as IPv4, IPv6, IPX, to be used appropriately, and to be given the data that were attached to the concerned protocol at the source machine. Also, to use the same network interface and media

LLC (Logical Link Control) sublayer has also another function which is providing flow control and error detection. I know now that you are wondering how come flow control to take place at the Data-link Layer since it is already done at the Transport layer. But do not worry ! everything will be clear because every single concept is adequately explained in the folowing lessons. Besides, flow control and error detection are devoted a full lesson on the grounds of their importance and huge content.

MAC (Media Access Control) sublayer

In addition to LLC  (Logical Link Control), MAC sublayer has, equally, significant functions. It provides Data-link Layer addressing and puts Frames on the media to takes their journey across the network towards the destination.

Addressing in the Data-link Layer is providing the frame, generated at the sending machine, with a source address and destination address called MAC address so that the frame is destined to its correct receiving machine.  More precisely, as you saw in the previous lesson, MAC  address or hardware address  is a unique identifier which consists of 48 bits in Hexadecimal. It looks like this 00:0a:9f:9d:6a:15. This address is burned on any network device, which means that it is unchangeable. NIC (Network Interface Card) card or Wireless card, for example, has a unique MAC address which identifies it. Routers, switches, hubs all have MAC addresses.

The primary function of this address is to enable devices to be located inside a local network (LAN). For example, when a frame coming from a remote network gets at the router of the receiving host, it needs to continue its path along the concerned host. To do that, the routers, in this case, depends on the MAC address to reach the concerned host (in this case the correct term is node rather than host). The actions of the router is analogous to what follows : the router will ask, whose MAC  address is  this ?( the router means by this  which computer does have the same MAC address), of course, the question is addressed to all devices in a given network. The computer  that has the same MAC address will respond, it is me ! my MAC address matches the one you are asking for. Lastly, the frame will be directed to the computer. All that takes place using a method called ARP (it will be explained in the forthcoming lessons). MAC address structure and much more is explained in the next lessons for the purpose of giving an in-depth detail.

As for the second function of the MAC sublayer, which is putting frames on the media, after data passes by all the above layers and being encapsulated by all of them, it is also encapsulated at the Data-link Layer with a header and trailer to become a frame. This latter needs to be put on wires or wirelessly to travel to the destination. This process is the job of MAC sublayer. It passes the frame on to the Physical Layer to be placed on the appropriate medium. But that does not happen randomly, it is takes places according to some rules which use a set of methods for the purpose. For example, media through which frames are to be traveling differ ; we have a plethora of technologies for that, such as Ethernet cables, wireless, satellite, WAN cables like fiber optics etc.  Accordingly, it is of paramount importance to mention that each single medium has its distinct frame, which meas that frames are not all the same for all media. Consider the following example :
As you see is the above figure, frames may travel through many different media. In the right, there is a laptop connected to the internet wirelessly, then the wireless router is connected to a dish which is connected through radio waves to the satellite. This latter is itself connected to another dish, which is, in turn, connected to a router that is finally connected to a computer.

In other words, the laptop is connected wirelessly to the router, which means that the frame is special to wireless connection. Then, the wireless router is connected to the dish through fiber optics as an example, which means that the frame generated at the laptop and arriving at the wireless router is decapsulated ( removing the header and trailer), and encapsulated again (adding header and trailer) with a header and trailer that is suitable to the fiber optics technology. When the frame leaves the fiber optics, it is decapsulated another time to be encapsulated with another header and trailer that is appropriate to satellite connection. After that, once that frame arrives at the dish on the left, it is decapsulated and encapsulated in a way suitable to fiber optics technology. Next, when the frame arrives at the router on the left, it is decapsulated and encapsulated with a frame that is fitting local wired local network.

Obviously, saying that frames change from a medium to another, it means that there are different types of frames, and they differ in their structures. That is why there is a lesson to each frame. For example, we have Ethernet frame, HDLC frame, ATM frame, PPP frame, Frame relay. All of which will be covered thoroughly in the coming lessons.

Finally, you are now questioning how the Data-link layer can know the characteristics  of each medium, then formatting the appropriate frame. For all of the aforementioned frames to be placed on media, there are equally different methods of accessing the media or MAC techniques, which gives each medium its suitable frame. So, see the following lesson which covers all that in detail.

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09: Data-link Layer - LLC and MAC sublayers part 2 Reviewed by BOUFTIRA on 10:01:00 AM Rating: 5

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