OSI Reference Model
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
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
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
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).