1.6 REFERENCE MODELS
One of the most difficult software to be developed is the networking software.
Reference models were designed to standardize the layer of functions and activities.
The following section describes one of the most important reference models used in
Computer Networks.
OSI (Open System Interconnection) Model
The OSI model is an abstract description for layered communications and computer
network protocol design open system means that it can communicate with any other
system that follows the specified standards, formats and semantics. Protocols give the
rules that specify how the different parties may communicate.
In its most basic form, it divides network architecture into seven layers which from
top to bottom are the Application, Presentation, Session, Transport, Network, Data-Link, and Physical Layers. It is therefore often referred to as the OSI Seven Layer
Model (Figure 1.13).
A layer is a collection of conceptually similar functions that provide services to the
layer above it and receives service from the layer below it. On each layer an instance
provides services to the instances at the layer above and requests service from the layer below. For example, a layer that provides error-free communications across a
network provides the path needed by applications above it, while it calls the next
lower layer to send and receive packets that make up the contents of the path. Any two
instances at one layer are connected by a horizontal protocol connection on that layer.
layer below. For example, a layer that provides error-free communications across a
network provides the path needed by applications above it, while it calls the next
lower layer to send and receive packets that make up the contents of the path. Any two
instances at one layer are connected by a horizontal protocol connection on that layer.
The following are the layers of OSI model:
In transmission side data flows from layer 7 to layer 1, then to cabling or suitable
medium. When data reaches the reception side, it flows from layer 1 to layer 7.
Application Layer: This layer is the layer for user interaction. We must have application software for dealing with the data.
Presentation Layer: It converts the data into suitable format. It does tasks like compression, decompression, encryption and decryption.
Session Layer: This layer manages connections between different application layers. Transport Layer: This layer converts data into segments and re-assembles the data stream. TCP and UDP are theprotocols used in this layer. In this layer, data is converted into so called segments.
Network Layer: This layer translates logical address into physical address. This layer also fixes the route for data path. Router works in this layer. In this layer data is called a packet.
Data-Link Layer: This layer provides physical identification of a device using Media Access Control Address. It adds source and destination address to packets and convert them into frames. This is the layer that provides error free transmission.
Physical Layer: This layer provides the functional requirements for activating a physical link. In this layer, data is carried from one device to another.
Now, we can better understand the OSI layer with an example. Consider that you have
to send a word document to a different network or through internet. The following are
the process that will take place:
1. In the APPLICATION LAYER, the user can edit the file by using application
software like a word processor.
2. In the PRESENTATION LAYER, user can compress the word file by using
WINRAR or WINZIP and convert the data into different format for example.zip
or .rar. You can also convert the word document into different formats.
3. In the SESSION LAYER, the particular file has to be integrated with browser
for attaching it to email or likewise clients.
4. In the TRANSPORT LAYER, data is converted to segments. Source IP and
destination IP are added to each packet. Frame checks and parity bits are also
added in this layer.
5. In the NETWORK LAYER, the data is handed over to a router. The router
calculates the best path for data transmission
6. In the DATA-LINK LAYER, transmission errors are handled and also flow of
data is regulated so that receivers are not swamped by fast senders.
7. In the PHYSICAL LAYER, frames are transmitted as bits through media such
as Optical fibre.
