Encapsulation and Decapsulation Process

Encapsulation and Decapsulation Process:

This is the process of what happens when a host transmits data across a network; i.e. it is wrapped with protocol information at each layer of the OSI Layer. Each Layer communicates only with its peer layer.

Protocol Data Unit (PDU):

Before we discuss the process, it is important to understand PDU (Protocol Data Unit), each layer uses PDU, to communicate and exchange information. These hold the control information attached to the data at each layer of the model. PDU’s are attached to the header or at the trailer.

The Process:

After a data is passed by a User, it passes the higher layers, i.e. the Application Layer, Presentation Layer and the Session Layer, each layer adds some kind of information and then the data stream is passed to the lower layers where the main work of Data Communication starts.

The Data Stream is handed down to the next lower layer, which is the Transport Layer, which sets up a virtual circuit with the receiving device by sending a synch packet. Next the Data Stream is chopped into pieces and a Transport Layer header is attached to each piece. Now each piece of data is called segment. Each segment is sequenced and handed down to the next layer which is the Network Layer.

The segments are received by the Network Layer, and this layer helps is network addressing and routing to the internetwork. Logical Addressing is used here. This Layer adds its PDU to each segments and they are now known as Packets or Datagrams. These packets are then handed to the next layer which is the Data Link Layer.

          

            ·         Transport Layer and Network Layer works together to rebuild a data stream on a receiving host, but its not part of their work to place their PDU’s on a local network segment.

Data Link Layer receives the packets from the Network Layer and places them on a network medium. This layer encapsulates each packets in a Frame. The Frame header contains the MAC Address of the source and the Destination Host. If the Destination Host is on a different network, the frames are sent to the router to be routed through an internetwork. Once it gets to the Detination Network, a new frame is used to get the packet to the Destination Host. The Trailer of the Frame contains a Frame Check Sequence (FCS) field which is the Cyclic Redundancy Check.

To put the Frames on the network it has to be converted to digital signals as frames are logical groups of 0’s and 1’s. The Physical Layer is responsible for encoding these digits into digital signals, which is read by devices on the same local network. The receiving devices will synchronize on the digital and extract (decode) the 1’s and 0’s from the digital signals. At this point the devices build a frames, and runs a CRC, and then checks their answers against the Frame’s FCS field. If it matches the packet is pulled from the frame and whatever left is discarded. This is Decapsulation.

The packet is handed to the Network Layer, it checks the Layer 3 Address which is the IP Address. If the Address matches, the segments are pulled and handed to the Transport Layer, Tranport Layer rebuilds the data stream and acknowledges the sender.

PDU and Layer Addressing

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