Lecture 28: Data Communication/Computer Networks
Objectives of this lecture
q Introduce the concept of data communication/Networks
q Learn the different types of networks
q Learn Network Topologies and Media
q Learn the OSI reference Model
What is Data Communication/Network?
q Data communication is the process of transmitting and
receiving information from one computing device to another.
q This could be within the same computer (e.g. keyboard
to CPU)
q More importantly, it is can be from one computer to
another through a computer network.
q This idea of connecting devices together for the
purpose of sharing data is one of the most important inventions ever made by
man.
q It has broken the barrier of distance, that the world
has been reduced to a village.
q It has touched on almost all aspect of our lives that
it is tagged a revolution.
q In this and next lecture, we shall take an overview of
the underlying concepts behind this very important area of computer science.
Types of Networks
q Communication networks may be categorized based on the
architecture used to transfer data:
q Switched networks: In this
approach, data is transferred from source to destination through a series of
intermediate nodes. These type of
networks can be further divided into circuit-switched networks and packet switched networks.
Ø
Circuit switch
networks require that a dedicated
path be established between two nodes.
Data is then transmitted from source to destination along this path as
rapidly as possible. Example is the telephone
network.
Ø
Packet-switched
network: In this case, it is not necessary to dedicate a path between the
two nodes. Rather, data is sent in a
sequence of small chunks, called packets. Each packet contains in addition to the data
bits, the destination and source addresses. A packet is passed from node to node along
some path leading from source to destination.
At each node, the packet is received, stored briefly and then
re-transmitted to the next. Example of
a packet switch network is the Internet.
q Broadcast networks: In this approach, there are no intermediate
switching nodes. At each station, there
is a transmitter/receiver that communicates over a shared medium. A transmission from any station is received
by all other stations. Only the station
whose address is contained in the packet being transmitted will respond and the
others simply ignore the packet. Since
the medium is shared, only one station can transmit at a time. Example of broadcast networks are: Packet radio networks, Satellite networks and Local area
networks.
Ø
In packet radio and
satellite networks, stations
transmits and receive via antenna, and all stations share the same channel or
radio frequency. In packet radio,
stations are within transmission range of each other and broadcast directly to
each other. In satellite network, data
is not transferred directly from transmitter to receiver but relayed via
satellite.
Ø
Another common broadcast
network is local area network which we discuss next.
Local Area Networks (LANS):
q These are networks within a single building or a
cluster of buildings such as campus of up to four kilometers in size.
q They are widely used to connect personal computers in
an organization for the purpose of sharing information. LAN can be formed using any of the following
topologies.
q 
Bus Topology
: In this topology, all stations are attached to a single cable
q At any point, only one station (called the host) is
allowed to transmit at a time and the rest are expected to listen. If there is a collection (two machine)
trying to transmit at a time), it has to be resolved using same conflict
resolution mechanism. When a station
generates a message, it moves on both directions of the cable and is removed
from the network when it reaches the terminal ends.
q A problem with this approach is that, if there is any
breakage in the cable, the whole network breaks.
q Ring Topology:
q This consists of a closed loop, with each station
attached to a repeating element. A
transmission from any station circulates around the ring past all stations and
is eventually removed by the originating machine. It suffers from the same disadvantage as the Bus topology.
q Star Topology:
This is set-up where stations are connected to one another through a hub.
q The advantage of this topology is that, if there is
any breakage in any cable segment, it only affects the computer connected to
that particular segment. The
disadvantage is that it requires more cable.
q Communication Medium
q LANS involve connecting the machines using any of the
following communication cables medium
Ø
Twisted Pair: These are
two copper similar to telephone line wires that are twisted along their length.
Ø
Coaxial Cable: This is a
solid copper wire enclosed in a braided outer conductor covered by an insulated
coating. It is best known for its
protection against noise interference.
Ø
Fibre-optic Cable: This
is the newest type of cable. It uses
light transmitted along thin silica fibres.
A light-emitting diode (LED), is used as the transmitting source
converting electrical signals to light pulses.
Fibre-optic cable is very efficient, flexible and capable of handling
thousands of communication lines simultaneously
Wide Area Network:
q WAN: A network of computers that communicate with each
other and that are geographically dispersed over a country, continent or the
entire globe is called Wide-area-network
q WANS have become very popular because of the features
they offer, such as e-mail, on-line search, www, file transfer etc. Internet is an example of WAN
q For transmission over long distances (over 50miles) a
variety of media are used. Some of
these are;
q Telephone lines: This involves the use of normal public telephone lines.
q The problem with telephone lines is that it is usually
an analog medium To solve this problem, a modem (modulator-demodulator) is
used. This converts from digital
transmission to analog on one end and converted from analog to digital on the
other end.
q Other medium used for long distance are microwave and
satellites.
Protocols
q It is clear that there must be a high degree of
cooperation among the various computer systems for computer networks to
work. This is achieved by adhering to a
set of rules called protocols.
q The rules are set by international standard bodies
such as the International Standard Organization (ISO).
q ISO consists of National Standard organizations,
example, ANSI. These national organizations
ensure that networking components are produced according to the agreed
standards.
q One communication standard created by ISO is the Open
Systems Interconnection (OSI) model.
q OSI model has seven layers, each with its own set of
protocols.
These are as
follows:
|
7. Application Layer |
|
6. Presentation |
|
5. Session |
|
4. Transport |
|
3. Network |
|
2. Data Link |
|
8. Physical |
q Layer 1
(Physical Layer): This is concerned with transmission of unstructured
bit stream over a physical medium; deals with electrical, mechanical,
functional and procedural characteristics to access the physical medium. Example, how many volts would be used to
represent 1, and how may for zero. How
many pins a network connector should have, etc.
q Layer 2 (Data Link Layer): This provides
for reliable transfer of information across the physical link; sends blocks of
data (called frames) with necessary synchronization, error control and flow
control. Example it has to take care of damaged or duplicated frames, etc.
q Layer 3 (Network Layer): This is responsible for establishing, maintaining and
terminating connections. It provides
the upper layers with independence from the data transmission and switching
technology used to connect the two systems.
q Layer 4 (Transport Layer): It specifies the rules for information exchange and
manage end-to-end delivery of information within and between networks,
including error recovery and information flow.
Example, multiple data streams on a single channel.
q Layer 5 (Session Layer): Provides the control structure for communication
between applications; establishes and manages and terminates connections
(sessions) between operating applications.
q Layer 6 (Presentation Layer): Provides independence to application processes from
differences in data representation. Example, ASCII to ABCDIC character
conversion, data encryption and compression, etc.
q Layer 7 (Application Layer): Supplies functions for particular applications such
as file transfer, WWW, remote access, etc.
q When sending or receiving information according to the
OSI model, the sender and the reciver must use the seven layer-protocols and
interfaces. The following diagram shows
basic communication between two machines is achieved
q Suppose a host A is sending information to Host B both
of which are using the OSI model. At
layer 7, host A begins to send information down to layer 1. From the site of A, the physical transmission
takes place along a medium to site B. At site B, the information goes from
layer 1 to layer 7, where the information is received.
q There are also other proprietary systems such as the
IMB’s System Network Architecture (SNA) and the Transmission Control
Protocol/Internet Protocol (TCP/IP) which is used on the Internet.