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09: Data-link Layer - Framing : Role of the header and trailer part 5

Framing : Role of the header and trailer
For the network layer PDU – packet –  to reach its destined receiver, it necessarily needs to be encapsulated with a header and  trailer – to form a frame.  This latter contains control information that allows the data packet to arrive correctly and safely at the receiving station. A frame is composed of a header and a trailer, each of which has distinct functions. Think of the frame as the following:
As shown on the above representation of the frame, the header and trailer are added to the already formed PDU, which is packet that contains data coming from the Application Layer down to the Network layer. Thus, providing an adequate demonstration of the role of the frame’s header and trailer is highly pivotal.

Role of the Header

The header consists of control information whose role is to guide the the whole frame to its correct destination. The header is added according to the media used as well as the logical topology. The header, in turn, encompasses a number of fields, each devoted to a job. Notwithstanding  fields differ from one frame format to another, a typical frame header includes the following :

Start of Frame field:  this field identifies the beginning of the frame. When the frame is traveling through the media, it might be influenced by noise, and hence distorted, for frame’s bits would be merged with bits caused by noise. For that reason, the frame is distinguished by bits added at the beginning. (you will learn more about this field in the Ethernet frame lesson).

Source and Destination address field: this field contains the physical address or MAC address of the transmitting machine as well as the receiving machine ; they are used to identify nodes on the media.

Priority/Quality of Service field: this field indicates a particular type of communication service for processing.

Type field: this field responsible for indicating the upper-layer service contained in the frame.

Logical Connection field: this fiel dis responsible for establishing a logical connection between devices (nodes).

Physical Link Control field: this field is utilized to establish media link.

Flow control field: this field serves the purpose of preventing  one station from overwhelming  another station with data by starting and stopping traffic  over the media.

Congestion Control field: indicates congestion in the media.

These above-mentioned fields are not specific to one frame, rather they might be found in different frame formats, not necessarily in that order, or the same fields.

Role of the Trailer

Data-link Layer adds also a trailer at the end of each frame. The trailer is responsible for ensuring that frames are received intact or undamaged. A typical frame includes two fields as a trailer.

Frame Check Sequence field: this field is used to determine whether errors occurred during the transmission of frames. In other words, when a frame leaves the sender, it is added a specific arrangement of numbers, and when it is arrived at the destination, the frames is checked, then a calculation is done to see if the resulted numbers matches the ones included in the header. If the numbers are the same, it means that the frame is intact, but in case the numbers are different, it signifies that the frame is damaged and, as a result, will be discarded. The process of creating numbers or a  logical summary of the contents  of the frame in numbers at the source and the act of calculating the checksum of the frame at the destination is called Cyclic Redundancy Check or CRC, and the outcome is referred to as CRC value. (you will learn more about how to calculate CRC in the coming lessons).

Stop field: this field provides the frame with numbers that indicate where the end of the frame, as opposed to start field.
09: Data-link Layer - Framing : Role of the header and trailer part 5 Reviewed by BOUFTIRA on 8:01:00 AM Rating: 5

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