Lesson 2: OSI Reference Model

The Layered Model | Physical & Data Link Layers | Network Layer | Transport, Session, Presentation, and Application Layers

Peer-to-Peer Communications

Let’s see how these layers work in a Peer to Peer Communications Network. In this exercise we will package information and move it from Host A, across network lines to Host B.
Each layer uses its own layer protocol to communicate with its peer layer in the other system. Each layer’s protocol exchanges information, called protocol data units (PDUs), between peer layers.
This peer-layer protocol communication is achieved by using the services of the layers below it. The layer below any current or active layer provides its services to the current layer.
The transport layer will insure that data is kept segmented or separated from one other data. At the network layer we get packets that begin to be assembled. At the data link layer those packets become frames and then at the physical layer those frames go out on the wires from one host to the other host as bits

Data Encapsulation

This whole process of moving data from host A to host B is known as data encapsulation – the data is being wrapped in the appropriate protocol header so it can be properly received.
Let’s say we compose an email that we wish to send from system A to system B. The application we are using is Eudora. We write the letter and then hit send. Now, the computer translates the numbers into ASCII and then into binary (1s and 0s). If the email is a long one, then it is broken up and mailed in pieces. This all happens by the time the data reaches the Transport layer.

At the network layer, a network header is added to the data. This header contains information required to complete the transfer, such as source and destination logical addresses.

The packet from the network layer is then passed to the data link layer where a frame header and a frame trailer are added thus creating a data link frame.

Finally, the physical layer provides a service to the data link layer. This service includes encoding the data link frame into a pattern of 1s and 0s for transmission on the medium (usually a wire).

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