Download Computer Networks and Distributed Systems

Computer Networks
Some computer networks use intelligent VDU or computer terminals which allows
independent processing power at sites from the central computer.
Other networks distribute even more processing power by linking together
computer systems. Because these networks distribute some processing power to a
number of different sites, they are known as 'distributed' processing system.
Application of computer networks
Networks can be configured to suit almost any application, from the provision of a
world-wide airline booking service to home banking.
Terminals may be only a few hundred feet apart and limited to a single building or
they may be several thousand miles apart.
Btec First Diploma
Introduction to Networking
D Stainton
1
Local Area Networks and Wide Area Networks
A network confined to one building with work stations, in different rooms, are
known as Local Area Networks(LANs).
One particular type of a LAN, known as a 'Ring Network' can extend over a
diameter of about 3 miles. a network distributed nationally or internationally and
which makes use of a telephone or satellite link is referred to as a 'Wide Area
Network' (WAN).
In large organisations with several branches, it is becoming popular to maintain a
LAN at each branch for localized processing and to link each LAN into a WAN
covering the whole organisation. This way, branches of an organisation can have
control over their own processing, yet have access to the organisations main
database at headquarters.
This also makes inter-branch communication possible.
Network Topologies
Computer networks are categorized according to their 'shape' or topology. Each
terminal in a network is known as a 'node'.
If a central computer controls the network it is known as the 'host' computer. The
topology of a network is the arrangement of the nodes and the ways they are
interconnected.
The communication system within a network is known as the 'subnet'. Data can be
transmitted around the subnet either on a point-to-point basis or via a broadcast
channel.
With point-to-point the data passes through each device in the network. Therefore,
if 2 devices wish to communicate they must do so via intervening devices. Each
device must be able to store the entire message and forward it when the output
channel is free.
If a broadcast channel is used, all devices in the network share a common
communication channel. This means that any message sent by a device, is received
by all devices.
The message contains the address of the device intended to receive it, so that the
other devices know to ignore it
Btec First Diploma
Introduction to Networking
D Stainton
2
There are a number of recognized network topologies for example:
Star Network
A star network has a central host computer at the hub, with terminals or nodes
connected directly onto it.
Switched Hub Star Network
Host computer
Switched connection
between nodes
The switched hub star network passes all messages through the host computer,
which interconnects the different users on the network. In this topology the host
computer at the hub has a message switching function. Messages are transmitted
point-to-point. The topology is useful for intercommunications between pairs of
users (via the host).
The network may consist of several computer systems (nodes), connected to a larger
host computer, which switches data and programs between them. The star topology
is less suitable where several nodes require access to another node.
Btec First Diploma
Introduction to Networking
D Stainton
3
Star Computer Network
In the following star topology, the hub performs processing on information fed to it,
via the telephone system. The host computer has a processing , rather than, a
message switching function.
Star network.
Communication
Controller
terminals
The star network is popular for WANs, because most organisations start with a
central computer at the head office, which communicates with other branches, via
the telephone network.
Task 1.
Discuss and list the advantages/disadvantages of the above topology.
Btec First Diploma
Introduction to Networking
D Stainton
4
Ring network
A ring network connects all nodes in a ring
Computer
Printer server
Monitor station
Repeaters
File server
There is no host and none of the nodes need to have overall control of access to the network. A
monitoring station is used for the control of data transmission in the network. The topology is
designed for LANs and the Cambridge Ring is a popular configuration.
The ring has several 'repeaters', which are joined by the physical transmission medium.
Repeaters regenerate messages as they pass round the network. All computers are connected
to repeaters. This means that a message from one node, addressed to another, is passed
continually around the ring until the receiving node flags that it is ready to accept it
Data is transmitted in mini packets, which contain the address of the sending node, the
address of the receiving node and some control bits.
Task 2
Discuss and list advantages/disadvantages of the above network.
Btec First Diploma
Introduction to Networking
D Stainton
5