The Open Systems Interconnection (OSI) model
Introduction
The OSI model was based on a proposal
by the International Standards Organization (ISO) as the first step
toward international standardization of the protocols used in the
various layers and was given that name because it deals with
connecting open systems (i.e. systems which are open for
communication with other systems).
OSI divided network communication into seven components called
layers. Every layer consists of
protocols for communicating with the preceding and succeeding layer.
There are seven layers in the OSI model , each representing a
step in the network communications process. The seven layers are
shown in the following table :
Each layer performs a specific task and then
passes the data up or down to the next layer, depending on
whether the layer is operating in the sending or the receiving
computer. As the data passes through the layers, each layer adds
its own information in the form of
headers, which are added in front of the original data. As
each layer adds header information the data is
encaptulated or enclosed within
a larger unit. When the data reaches the receiving computer the
process occurs in reverse and as it does so the encaptulated
information is stripped off, one layer at a time, in the reverse
order in which it was added.
Here is an analysis of the tasks performed by each layer
starting from the top where the process begins on the sending
computer :
The application layer
This layer provides for interaction between the application program
and the network. Protocols that function in the applica-tion layer
perform functions like file transfers , printing access and
messaging services. Such protocols(Shinder 2002,p86) are :
- FTP(File transfer protocol) which is used to transfer files
between computers that do not necessarily run on the sameoperating
system or platform.
- Telnet is used for terminal emulation and to enable access and
applications and files on another computer.
- SMTP(Simple Mail Transfer Protocol) is a non-vector specific
protocol used for sending e-mail on the internet.
- SNMP(Simple Network Management Protocol) gathers information about
the network.
The above are only a few application-layer protocols. Others
include HTTP(Hypertext transfer protocol) and NNTP (network news
transfer protocol).
The presentation layer
It handles issues that have to do with the packaging or the
presentation of the data. It hides differences between different
kinds of computers who want to talk to each other by converting
data into or from a machine's native internal format.
It also performs (Shinder 2002,p87) :
a) Data compression : This is the reduction in the size of data
in order to achieve faster transmission over the network.
b) Data encryption : It is the conversion of data into an encoded
form that cannot be read by an unothorized person.
c) Protocol translation : This is the conversion of data from one
protocol to another that it can be transfered between dissimilar
platforms or operating systems.
The session layer
The protocols at this layer have the responsibility of
establishing a one-to-one session between the sending and receiving
computers. It uses its computer's identification to call another
computer. The call is called a session-an exchange of dialog or
messages between two workstations. The functions in this layer
enable applications running at two workstations to coordinate their
their communications into a single session. Another function is to
control whether a transmission is sent as half-dublex or full-dublex
communication. Finally the layer is responsible for data expedition ,
class of service and the repor-
ting of problems in itself and the layers above it in the
networking model. (Nance 1997,p326)
The transport layer
Its primary responsibility is to provide for reliable end-to-end
error control and flow control , making sure that the information
send by a computer on the network is received properly by
another computer. The layer knows the maximum size of each data
packet and breaks up a work request message into smaller packets
as necessary. It also keeps track of such things as validity of
data packets , sequencing and the handling of duplicate packets.
There are two protocol types used in this layer-connection oriented
( TCP protocol ) and connectionless ( UDP protocol ). Another duty of
the transport layer is the resolution of computer names to logical
network addresses through the Domain Name System (DNS) that ope-
rates in this layer. (Shinder 2002,p89)
The network layer
It is responsible for getting the data packets to their
destination and also handles the routing of the packets. Therefore
most routing protocols operate on this layer. It keeps track of
the different routes that a packet may need to travel by
inserting routing information into each packet to help the
intermediate computers and devices (i.e. routers) to forward it to
its destination. (Nance 1997,p328)
The data link layer
It is the most complex layer. It encompasses the sending of the
characters that make up a data packet on a character-by-character
basis. Because of its complexity it is broken down into a Media
Access Control (MAC) layer and a Logical Link Control (LLC) layer.
The MAC layer manages network access and network control by using
the MAC address or hardware address of computer. The LLC layer
operating at a higher layer than the MAC layer , sends and
receives the user
data messages and packets. It is here that the logical topology
of the network is defined.(Nance 1997,p329)
The physical layer
This is where the data is translated into transmittable signals
and put on the network media (i.e. wire) to travel across the
network. It is therefore dealing with signaling issues including :
analog versus digital signaling , baseband versus broadband technology
, asynchronous versus synchronous transmission and multiplexing.
Another physical layer issue is network topology.This refers to
the physical layout of the network , as opposed to the logical
topology addressed at the data link layer. (Shinder 2002,p95)
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