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07: Transport layer - Segmentation, packaging and reassembly part 1

Transport layer is the fourth layer of OSI reference model. This layer is responsible for many important tasks in networking. It comprises several protocols used in data transmission. This lesson is split up into parts for the reason that there are many concepts and protocols to be covered. Transport layer has a myriad of functions and services to provide so that data is guaranteed to be successfully received. This layer takes data received from the upper-layer applications, then put it in a special form to be ready for end-to-end transmission.


One of the transport layer functions is segmentation. The job of this latter is clearly shown in the name ‘segmentation’. Segmentation means to divide something into pieces. When data arrives at the transport layer from the upper layers, it is taken then divided into segments. That is why data at this layer is called segments rather than data. Thus, The PDU (Protocol Data Unit) of this layer is segment(s).

 Undoubtedly you are wondering why data is segmented at this layer. To make it plain for you, take a look at the following picture.  Computer A is sending a video, whose size is 100MB, to computer B. During the transmission, the internet went off and the whole video has not been received. For the video not to be received after a long period of time is really upsetting and disturbing. For this reason, data (videos, pictures, audios, texts..) is divided into segments. Contrariwise, if the video is split into pieces, we will at least guarantee that part of the video is received after the cut of the internet, then the transmission can later be resumed ; consequently, the video will be wholly transmitted thanks to this way.

 Another example of segmentation is when you are downloading somthing from a server. Let us suppose that computer A (in the following figure) is downloading a video from the server B. in this situation, computer A is the receiver on the grounds that data flows from the server towards the computer. After the video download reaches 90%, an error occurs and subsequently the download stops. If the video is not segmented, it will fully be lost ; but if it is segmented, the user can resume downloading because there are some already received segments.
Packaging and reassembly

Packaging and reassembly is another function of transport layer. After data is segmented and sent to the destination machine, this latter reassembles the segments in the correct order even if the segments are in an disorderly way. Segments may take different routes in order to reach the destination. Take a look at the following figure.

The figure shows that data segments may take either Route 1 or Route 2, among others.  Those are routers through which segments travel to the destination machine, which is Pc-2. For instance, a segment may take the first route and another segment may take the second route, thus they will be received at the destination machine at different times and unarranged. the role of the transport layer starts here, through its funcion, packaging &reassembly, it arranges the segments in the right order at the destination station.

07: Transport layer - Segmentation, packaging and reassembly part 1 Reviewed by BOUFTIRA on 8:00:00 AM Rating: 5

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