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OSI Model
When we think of how to send data from one computer to another, there are
many different things involved. There are network adapters, voltages and signals
on the cable, how the data is packaged, error control in case something goes
wrong, and many other concerns. By dividing these into separate layers, it
makes the task of writing software to perform this much easier.
In the Open Systems Interconnect model, which allows dissimilar computers
to transfer data between themselves, there are SEVEN distinct layers.
The Data-Link Layer also provides error-free delivery of data between
the two computers by using the physical layer. It does this by
packaging the data from the Network Layer into a frame
which includes error detection information. At the receiving computer,
the Data-Link Layer reads the incoming frame, and generates its own
error detection information based on the received frames data. After
receiving all of the frame, it then compares its error detection value
with that of the incoming frames, and if they match, the frame has
been received correctly.
A frame looks like,
The Data-Link Layer actually consists of two separate parts, the
Medium Access Control (MAC) and Logical Link Control
Layer (LLC). Example MAC layers are
Ethernet 802.3 and
Token Ring 802.5
Bridges are an example of devices
which works at the MAC layer.
For Ethernet 802.3, the Physical Layer can be represented as
In 1983, the International Standards Organization (ISO) developed a model which
would allow the sending and receiving of data between two computers. It works
on a layer approach, where each layer is responsible for performing certain
functions.
Provides Applications with acess to network services.
Determines the format used to exchange data among networked
computers.
Allows two applications to establish, use and disconnect a
connection between them called a session. Provides
for name recognition and additional functions like security
which are needed to allow applications to communicate over
the network.
Ensures that data is delivered error free, in sequence and with no
loss, duplications or corruption. This layer also repackages data
by assembling long messages into lots of smaller messages for
sending, and repackaging the smaller messages into the original
larger message at the receiving end.
This is responsible for addressing messages and data so they
are sent to the correct destination, and for translating logical
addresses and names (like a machine name FLAME) into
physical addresses. This layer is also responsible for finding
a path through the network to the destination computer.
This layer takes the data frames or messages from the Network Layer
and provides for their actual transmission. At the receiving computer,
this layer receives the incoming data and sends it to the network
layer for handling.
Controls the transmission of the actual data onto the network cable.
It defines the electrical signals, line states and encoding of the data
and the connector types used. An example is 10BaseT.
Repeaters are an example of
devices that work at the Physical Layer.
