Networking
Introduction
A
collection of two or more computers interconnected by the telephone lines,
co-axial cable, satellite links, radio and microwave transmission and some
other communication techniques. A
computer network is a group of computers that are connected together and that
communicate with one another for a common purpose.
Although
the computer industry is young compared to anther industries (e.g., automobiles
air transportation), computer have made spectacular progress in a short
time. During the first two decades of
their extrinsic, computer system highly centralized, usually a single large
room. A medium size company or
university might have had one or two computers, while large instructions had at
most a few dozen. The idea that with in 20 years equally powerful computers
smaller than postage stamps would be mass produced by the millions was pure
science fiction.
The
merging of computers & communications has had a profound influence on the
way computer systems are organized. The concept of the computer center as a
room with a large computer to which users bring their work for processing is
now totally obsolete. The old model of single computer serving all of the
organizations computational needs has been replaced by one in which a large
number of separate but interconnected computers do the job. These systems are
called computer networks.
Two
computers are said to be interconnected if they are able to exchange information’s.
The connection need not be via a copper wire; fiber optics, microwaves, and
communication satellites can also be used. By requiring the computer to be
autonomous, we which to execute from our definitions systems in which there is
a clear master/slave relation. If one computer can forcibly starts , stop, or
control another one, the computers are not autonomous. A system with one
control unit and many slaves is not a network; nor is a large computer with
remote printers and terminals.
There
is considerable confusion in the literature between a computer network and a distributed
system. The key distinction is that in a distributed system, the extence of
multiple autonomous computers is transparent to the user. He can type a command
to run a program. And it runs. It is up to the operating system to select the
best processor, find and transport all the input files to the processor, and
put the result in the appropriate place. In other words, the user of a
distributed system is not aware that there are multiple processors; it looks
like a virtual uni-processer. A location of jobs to processors and files to
disks, moment of files between where they are stored and where they are needed,
and all other system functions must be automatic.
With
a network, user must explicitly log on to one machine, explicitly submit jobs
remotely, explicitly move files around and generally handle all the network
management personally. With the distributed system, nothing has to be done
explicitly it is all automatically done by the system without the users
knowledge.
In
effect, a distributed system is a software system built on top of a network.
The software gives it a high degree of cohesiveness and transparency. Those
distinction between a network and a distributed system lies with the software
rather than with the hardware. Nevertheless, there is considerable our lap
between the two subjects. E.g., both distributed system and computers networks
need to move file around. The difference lies in whom in vokes the movement,
the system or the user.
Local Area Networks (LAN):
Local
area network, generally called LANs, is privately-owned networks with in a
single building or campus of up to a few KM in size. They are widely used to
connect personal computers and workstation in company offices and factories to
share resources (e.g., printers) and exchange information. LANs are
distinguished from other kinds of networks by three characteristics:
1.
Their size,
2.
Their
transmission technology,
3.
Their topology.
LANs are restricted in size, which means that the
worst-case transmission time is bounded and known in advance. Knowing this
bound makes it possible to use certain kinds of designs that would not
otherwise be possible. It also simplifies network management.
Metropolitan
Area Networks (MAN):
A metropolitan
area network, or MAN (plural:
MANs, not MEN) is basically a bigger version of a LAN and normally uses similar
technology. It might cover a group of nearby corporate offices or a city and
might be either private or public. A MAN can support both data and voice, and
might even be related to the local cable television network. A MAN just has one
or two cables and does not contain switching elements, which shunt packets over
one of several potential output lines.
Wide Area
Networks (WAN):
A wide area
network, or WAN, spans a large
geographical area, often a country or continent. It contains of machines
intended for running user (i.e., application) programs. We will follow
traditional usage and call these machines hosts.
The term end system is sometimes
also used in the literature. The hosts are connected by a communication subnet, or just subnet
for short. The job of the subnet is to carry messages from host to host,
just as the telephone system carries words from speaker to listener. By
separating the pure communication aspects of the network (the subnet) from the
application aspects (the hosts), the complete network design is greatly
simplified.
In most wide area networks, the subnet consists of two
distinct components: transmission lines & switching elements. Transmission lines
(also called circuits, channels, or trunks) move bits between machines.
The switching elements are specialized computers used
to connect two or more transmission lines. When data arrive on an incoming
line, the switching element must choose an outgoing line to forward them on.
In most WANs, the network contains numerous cables or
telephone lines, each one connecting a pair of routers. If two routers that do
not share a cable nevertheless wish to communicate, they must do this
indirectly, via other routers. When a
packet is send from one router to another via one or more intermediate routers,
the packet is received at each intermediate router in its entirety, stored there
until the required output line is free, and the forwarded. A subnet using this principle is called a Point to Point, store and forward or packet
switched subnet. Nearly all wide
area networks (except those using satellite) have store and forward
subnet. When the packet are small and
all the same size, they often called cells.
Wireless
network:
Mobile computers, such as notebook computers and
personal digital assistants (PDA) are the fastest growing segment of the
computer industries. Many of the owners
of these computers have desktop machines on LAN’s and WAN’s back at the office
and want to be connected to their home base even when away from home or en
route. Since having a wired connection
is impossible in cars and airplanes, there is a lot of interest in wireless
network.
Wireless networks come in many forms. Some
universities are already installing antennas all over campus to allow students
to sit under the trees and consult the libraries card catalogs. Here the computers communicate directly with
the wireless LAN in a digital form.
Another possibility is using a cellular (i.e. portable) telephone with a
traditional analog modem. Direct digital
cellular service, called CDPD (Cellular
Digital Packet Data) is becoming available in many cities.
Network Hardware
It
is now time to turn our attention from the application and social aspects of
networking to the technical issues involved in network design. There is no
generally excepted taxonomy into which all computer network fit, but two
dimensions stand out as important: transmission technologies and scale. We will
now examine each of these in turns.
Broadly
speaking, there are two types of transmission technologies:
- Broadcast networks.
- Point-to-point
networks.
Broadcast
networks: It have a single
communication channel that is shared by all the machines on the network. Short
messages, called packets in certain
contexts, sent by any machine are received by all the others. An address field
within the packet specifies for whom it’s intended. Upon receiving a packet, a
machine checks the address field. If the packet is intended for itself, it
process the packet, if the packet is intended for the other machine, it is just
ignored.
Broadcast systems generally also allow the possibility
of addressing a packet to all destinations by using a special code in the
address field. When a packet with this code is transmitted, it is received and
processed by every machine on the network. This mode of operation is called broadcasting. Some broadcast system
also support transmission to a subset of a machines, something now has multicasting.
Point-to-point
networks: It consists of many
connections between individual pairs of machines. To go from the source to the
destination, a packet on this type of network may have to first visit one or
more intermediate machines. Often multiple routes, of different lengths are
possible, so routing algorithm play an important role in point-to-point
networks. As a general rule (although there are many exceptions), smaller,
geographically localized networks tend to use broadcasting, where larger
usually are point-to-point.
Whenever we want two devices – transmitting and
receiving device to communicate with each other, we need hardware’s to achieve
that. We would be discussing about the
various hardware such as:
1.
Sender and
Receiver hardware
2.
Communication
devices
3.
Communication
channels
Sender and
Receiver Hardware:
Following hardware are used for handling communication
messages, data transfer etc.
Nodes and
workstations:
Data communication is done using various communication
devices and softwares interconnected for information exchange. The devices used to communicate a data in
communication network are called Workstations.
These workstations may be computer, terminal, printer, telephones and other
communication devices. Each workstation
is connected to something called a Data Communication Network Mode.
Multiplexer:
As the name suggests, multiplexing is a form of data
transferring which are communication channels and is used for several
transmission. For e.g. the telephone
lines that we used for our daily conversation can carry 100’s and even 1000’s
of conversations using multiplexing. In
other words multiplexing is a type of network which allows a number of simple,
low cost terminals to share each communication lines introduces almost no delay
and requires no special computer softwares.
Multiplexing is used in two major ways:
FDM (Frequency Division Multiplexing)
TDM (Time Division Multiplexing)
FDM: By dividing a communication channel into various
smaller segments of different frequencies.
TDM: By taking groups of bytes from each sender and send
or transmit them over the channel one after another. Each group of data bytes are tagged at the
beginning and end with start and stop bytes.
These by control bytes are then separated at the receiving end and send
to their respective places. This type of
multiplexing occurs so fast that the transmission seems continuous.
Communication
devices:
There
are several types of communication devices or interface used in data
communication. These interfaces are
connection between receiver and sender hardware involved in data communication
and the communication network. Some of
these interface or communication devices are discussed below:
1.
Modems:
Modulator and Demodulators are devices which converts digital signals in to
analog for transmission over the analog transmission facilities such as
telephones. At the receiving end, a
modem performs the reverse function and converts analog signals into digital
form.
2.
Codec performs
the opposite function of a modem. A
communication port / adapter is a connector on the computer, which is used as
an entry to departure point for data involved in data communication. A common type of communication port is
RS-232c. This adapter is used for
transfer of data between the computer and the telephone lines.
3. Line Drivers are simple devices used to transmit digital signals
over short distances. These devices do
not modulate or demodulate signals, but use filters to reduce the high
frequency components and the modified signal is transmitted directly over the
media. These devices are used for the
distances up to 1 KM and can achieve data transfer rates of up to 19200 BPS. It is mainly used for connecting VDU
terminals with a computer at a distance of more than 100 feet or so.
Communication
channels:
The most basic hardware required for communication is
the media through which data is transferred.
There are several types of media, and the choice of the right media
depends on many factors such as cost of transmission media, efficiency of data
transmission and the transfer rate.
Some of the following transmission Medias is as
follows:
1.
Two wire open line: This is the simplest of all the
transmission media. It consists of a
simple pair of metallic wires made of copper or some times aluminums of between
0.4 and 1mm diameter, and each wire is insulated from the other. There are variations to this simplest form
with several pairs of wire enclosed in a single protected cable called a multi
core cable or molded in the form of a flat ribbon.
This type of media is used for communication within a
short distance, up to about 50 M, and can achieve a transfer rate of up to
19200 bits per second.
2.
Twisted Pair cable: A twisted pair consists of a pair of insulated conductors that are
twisted together. The advantages of a
Twisted Pair cable over the Two Wire Open Lines are; it provides better
immunity from spurious noise signals. As
the Two Wires are closed to each other, both pick equal interferences caused by
extraneous signal sources and this reduces the differential signal added by the
noise.
Twisted Pair cable is used for communication up a
distance of 1 KM and can achieve a transfer rate of up to 1-2 MBPS. But as the speed increased the maximum
transmission distances reduced, and may require repeaters.
Twisted pair cables are widely used in telephone
network and are increasingly being used for data transmission.
3.
Co-axial Cable: A co-axial cable consists of a solid conductor running co-axial inside
a solid or braided our annular conductor.
The space between the two conductors is filled with a dielectric
insulating material. Larger the cable
diameter, lower is the transmission loss, and higher transfer speeds can be
achieved. A co-axial cable can be used
over a distance of about 1 KM and can achieve a transfer rate of up to 100
MBPS.
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A co-axial cable is of two types- a 75 Ohm cable which
is used by the cable TV operators and 50 Ohm cable which is used in high speed
broad band networks and is low loss cables.
4.
Fiber Optic Cables:
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A fiber optic cable carries signals in the form of
fluctuating light in a glass or plastic cable. An optical fiber cable consists
of a glass or plastic core surrounded by a cladding of a similar material but
with a lower refractive index. The core transmits the light while the change in
refractive index. The core transmit the light while the change in refractive
index between the core and the cladding
causes total internal reflection, thus minimizing the loss of light from fiber.
As light waves gave a much wider wand width then the electrical
then the electrical signal and are immune from electromagnetic interferences,
this leads to high data transfer rate of about 1000 mega bites per second &
can be used for long & medium distance transmission links.
5.
Radio, Microwaves & Satellite Channels
Radio, Microwaves & Satellite Channels use
electromagnetic propagation in open space. The advantage of these channels lie
in their capability to cove large geographical areas & being inexpensive than
the wired installation.
The demarcation between radio, Microwave &
satellite channels lie in the frequencies in which they operate. Frequency
bellow 1000 MHZ are radio frequencies & higher the Microwave frequencies.
The radio frequency transmission may be bellowing 30
MHZ above 30 MHZ & thus the techniques of transmission are different. Owing
to the characteristics of the ionosphere, frequencies bellow 30MHZ are
reflected back towards the surface of the earth. Above 30Mhz propagation is on
line of sight paths. Antennas are placed in between the line-of- sight paths to
increase the distance. Radio frequencies are prone to attenuation and, thus,
they require repeats along the path to enhance the signal. Radio frequencies
can achieve data transfer rate of 100 Kbps to 400 Kbps.
Microwave links use line- of- sight transmission with
repeaters placed every 100-200 Kms. Microwave links can achieve data transfer
rates of about 1000 Mbps.
Satellite links use microwave frequencies is the order
of 4-12 GHz with the satellite as a repeater. They can achieve data transfer
rates of about 1000 Mbps.
Network
Concept and Classifications:
Communication
using computer has brought a revolution in the world of computer technology,
particular in the field of computers. We
have always heard of networking or the term network, a network is a way or
means of transmitting or receiving information one or more sources. As an e.g. car salesman after years in the
business, have developed a network of associates. When the car salesman needs a
car to make a sale the car salesman calls out to his network to retrieve
information on the location of the car. Employment agents also develop a
network. Their customers become their network. Employment agents will
frequently keep in touch with their clientele for possible openings or to locate
a candidate for an opening. Without the capability of networking, these two
people would have a difficult time. It is the same in computing. Networks
provide the means for locating transporting information.
In
computing networks, the origin of the information request utilized the service
of a network to locate & return the information. This is done with
addresses. In the two previous examples of the car sales man & the
employment agent, a telephone number can be considered the address of their associate
or client. Addresses in computer networking are used in the same manner. These
addresses identify the network resource. There are two popular architectures
for networking – hierarchical & peer.
Hierarchical
addressing is defined in a master slave relationship. In hierarchical network,
the master controls the network & therefore assigns addresses to the
network resource. This architecture has the maniframe as the master & all
network resources as slave. The bases of this is that if the master does not
know before hand of a network resource existence through a pre- defined address
then that resource can not participate in the network.
Peer
networking does not need pre- defined networking addressing. Instead, each
resource on the network is seen as a peer. Each network resource is a peer to
the other network resource. When a new network resource joins the network it
introduced itself & notifies its peer to any other network resources that
it knows about peer networks are share network information.
The
entire computer network can be classified into two board categories. They are (a)
LAN (Local Area Network) (b) WAN (Wide Area Network)
LAN (Local Area Network)
As
numbers of system grow within an organization, a need is felt for sharing
expensive resource and exchanging data and information between systems. This
need of information exchange and resource and sharing with in a organization
has given birth to a local area network or LAN.
A
LAN is a data communication network, which connects many computers or
workstation ( Computer’s Terminals, Printer etc.) and permits exchange of data
& information among themselves, with in a localized area, typically
confined to a building, or a cluster of buildings. The distance between two
communications prints connected on the same LAN channels is usually up to 02 to 05 kms.
LANs
are not rigidly defined but tend to share most of all of the following
characteristics.
§ The transmission media is shared by
all the connected devices in the network.
§ Each device is connected in the
network can either operate stand alone or in the network.
§ Area covered is small.
§ Data transfer rates are high, usually
1 Mbps- 100 Mbps (Million of bits per second).
§ Each device connected in the network
can communicate with any other device in network.
§ Cost of setting up the network is
usually low.
LAN Topology
A
network topology refers to the physical lay out of the network in which all the
devices are connected. This includes all the hardware that makes up the network.
The points of the connection to the network by the station are calls Nodes or
Link stations. There are several types of topographical design & strategies
are used to implement LAN. The majority of these are based on three types of
topologies.
a) Star b) Bus c) Ring
Each
topology has its advantages & disadvantages.
Star Topology
Star topology is shown bellow. In this topology. A
number of stations are connected directly to a central station or controller.
Communication on the connecting links between the stations & the central
station of the star topology can be bi- directional and are point to point. A
station on this type of network passes an information frame to the central
controller, which then forwards the information to the destination station. The
central controller manage and controls all communication between stations on
the network.
Star Network
Failure of a station on a star network is easy to
detect and can be remove from the network. However, failure of the central
controller will disable communication through out the whole network.
Bus Topology
A bus topology
is shown bellow all stations are connected to a single communication line. This
single communication line is referred to a bus. Information frames originating
at a station are propagated away from the station in both directions on the
bus. Each station on the bus interrogates the information frame destination
address failed for its on addresses. If the destination failed does not mach
the station address, the station discards the information frame back on to the
bus. If the destination address matches the station addresses, it accepts the
information frame & processes the frame.
An extension to the bus topology is tree topology is
in the following figure. Tree topology extends the branches of the bus topology
allowing more stations to access the bus.
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Bus Topology
On a bus or tree network there is no central point for
management & control. These function
ions are distributed to each station on the bus. A brake in the bus can be
difficult to locate but limits the outage to communications between stations
that traverse the broken point.
Ring Topology
A ring topology is shown bellow. Local area network
that have each station attached to an adjacent station using point to point links
from a physical ring. Each station attached and active to the ring regenerate
the information frame, then retransmits the information frame on the ring. The
ring itself is logically circle and the information travels in one direction.
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Failure of a station in a ring topology disrupts the
ring because the information frame is not regenerated. Additions or deletions
of stations of the ring can be disruptive, if the changes are not managed
properly.
LAN Hardware
and Software
As
we have seen so far, to realize a LAN process, several functions are to be
performed. These are so specialized in nature the require hardware specially
built for such purpose. Here we will discuss briefly the basic hardware
components of LAN, these are:
(A)
Transfermission
channel
(B)
Network Interface
Unit (NIU)
(C)
Servers
(D)
Work Station
(A) Transfermission Channels
Generally
following four types of channels are used for data communication in a LAN. They
are:
(i)
Twisted Pair Cable
(ii)
Coaxial Cable
(iii)
Fiber Optic Cable
(iv)
Radio waves
(B)
Network Interface Units (NIU)
Network
interface units connect each device in the LAN network to shared transmission
device. It contains the rules or logic to access the LAN. NIU is also used for
to implement LAN protocols and for device attachments. Its functions depend on
the type of topology used in LAN.
( C) Servers
One
of the major benefits of implementation of LAN is sharing expensive resources
such as storage device, printer etc. this is achieved through providing servers
on the LAN. It is a dedicated computer that controls on or more resources. This
contains both hardware & software for LAN. Three major categories of
servers used in LANs are
(i)
File Server
(ii)
Printer Server
(iii)
Modem Server
In
a networking file server is used to share storage space for files. Beside
providing storage space for files in a LAN environment, it is used for talking
periodical backup, and also to provide gateway to other servers with in &
between LANs.
Similarly
printer server is use to handle printing works of all workstations connected in
the network.
In
the LAN environment also modem is required to get connected to other network or
simply to use a telephone. A modem server is used to share these expensive
resources by all connected workstation in a networking ring.
LAN Software/ Operating
System
As
the name suggest LAN operating system is required to operate on the LAN system,
manage the tremendous work load with a number of various types of server
attached to it. It has basically two aspects (i) Server Software (ii) Work
station Software. As case of other multi user operating system, LAN operating
system also facilitate the sharing of expensive resources such as printer,
storage space etc. among all LAN users, provides security of data permits connections
to other network.
There
are various types of LAN operating systems for example Novel Netware, LAN
server, omni met, PC Net, IBM PC LAN, Etherlik plus etc.
WIDE Area Network
As
the name suggest, WAN spread across countries and continents satellites being
one of the transmission media.
A
wide area network WAN is a network that links separate geographical location. A
WAN can be a public system such as the Public Switched Telephone Network (PSTN)
or one of the various packet switched services provided by the public
telecommunication authorities. WANs can also use most other types of other
types of circuit including satellite networks, ISDN, Value Added Networks
(VANs/VADs).
The
network can be a private system made up from the local telephone company or set
up using public systems as virtual private network. A virtual private network
is one which operates in the same way as a private network but which uses
public switched services for the transmission of information.
The
main distinguishing feature between a LAN and WAN is that, the LAN is under the
complete control of the owner, whereas the WAN needs the involvement of another
authority like the telephone company. LANs are also able to handle very high
data transfer rates at low cost because of the limited area covered. LANs have
a lower error rate then WANs.
Communication Switching Techniques
In
a WAN, two computing devices are not connected directly. A network of switching
nodes provides a transfer path between the two devices. The process of transferring
data blocks from one node to another is called data switching.
There
are three switching techniques commonly employed and these are:
1.
Circuit Switching
In
circuit switching there is a dedicated communication path between the sending
and receiving devices. The dedicated path is a connected sequence of links
switching nodes. A conventional telephone network, where a dedicated path is
set between the called party for the duration of a telephone call is an example
of circuit switching.
Communication
viz. circuit switching involves three steps-circuit establishment; data
transfer; and circuit termination. Circuit switching is mainly used for voice
telephone network, but is not that effective for data communication network, as
channel capacities are not fully utilized, as data communication equipments do
not generate data continuously.
2.
Massage Switching
Massage
switching is an alternative switching techni8que, where it is not necessary to
establish a dedicated path between the sending and receiving devices. In
massage switching, the sending device appends the destination address to the
massage and passes to the network; the massage is then passed through the
network from one node to another till it reaches the intended destination. Each
switching are electronic mails, computer files, telegrams and transaction
queries and responses. A complete exchange may consist of several messages.
The
basic disadvantage of massage switching is the variable delay at intermediate
switching nodes.
3.
Packet Switching
Packet
switching combines the advantages of message & circuiting switching. Packet
switching is functionally similar to message switching, in which data is
transmitted in blocks, stored by the first switching node it meets in the
network and it forwarded to the next and subsequent downstream nodes until it
reaches the destination. The length of data block is limited in packet
switching network. Typical maximum length of packets is between 128 bytes to
4096 bytes. There are two approaches to packet switching:
§ Datagram
§ Virtual circuit
In
datagram approach, each packet is treated independently and may follow a
different path through the network. Packets may be reordered, dropped or
delivered in wrong sequence. The communication protocols provide the error recovery
sequencing of packets at the receiving device.
In
virtual circuit approach, a fixed logical path through the network from the
sender to the receiver is established before any packets are sent. This path
remains unchanged for the duration of the session. This is quite like circuit
switching, but no resources are reserved along the path. Packets are buffered
at intermediate nodes awaiting transmission.
WAN Devices / Hardware
The
switching techniques utilized the routine technology of data transfer. Routing
is responsible for searching a path between two computing devices that wish to
communicate and for following the data packets on this path. Devices such as
bridges, router and gateway provide this routing function.
1. Bridges
Bridges are used to connect two LANs that use
identical LAN protocols over a wide area. The bridge acts as an address filter
which picks up packets from one LAN that are identical for a destination on the
another LAN and passes these packets on the network. Bridges operate at the data
link layer (layer 2) of the OSI model. As all devices use the same protocols,
the amount of processing required at the bridge is minimal. If the distance
between the two LANs is large, the user would require two additional bridges at
either end of the communication link.
Besides a point-to-point link, the intervening
communication facility can be a network such as a wide area packet switching
network in such case the bridges need to add X. 25 link layer header and
trailer.
2. Routers
Routers can be used to connect networks that may not
be similar. Routers provide connectivity between two LANs or two WANs over
large geographical distance. Routers operate at the network layer (layer 3) of
the OSI model. All routers participate in routing protocols to access the
network topology, and based on this information routers computes the best route
from a sender to the receiver,
For large wide area network spanning thousands of
kilometers, the normal practice is to put network routers at suitable locations
to minimize link cost for leased link and provide adequate reliability from
link failures. Networks and other system are then connected to the nearest
router.
3. Gateways
Gateways are used to connect to dissimilar LANs. The
term gateway and routers are used interchangeably, though there is a subtle
difference between the two. A router operates at the network layer (layer 3) of
the OSI model, whereas a gateway operates on the application layer (layer 7) of
the OSI model. A gateway is required to convert data packets from one protocol
format to another before forwarding it, as it connects two dissimilar networks.
While discussing the WAN devices we referred to X.25
and protocols. We will now see what x.25 is.
§ What is X.25
Towards the end of the 1960s, the conman telecommunication
carriers around the world recognized the need for data communication service
that would be distinct from voice circuits. The plan was to put together a
series of recommendation during the 1970s using ITTCC (International telegraph
& telephone consultative committee) as the total standards body. This ITTCC
recommendation series to define the new service was doubled the X series.
During the 1970s sum 60 to 70 recommendations had involved. The most famous of
these is X.25, which define an interface in to a particular regular data
network that of a packet switched network.
ITTCC developed X.25 as the standard interface between
the Data Terminal Equipment (DTE, computer system) and Data Circuit Terminating
Equipment (DCET- the network node to which the DTE is connected) the
recommendation has been widely accepted as the industry standard for public
packet switched networks.
X.25 is a protocol for interfacing to a Public Packet
Switched Network. It is not a protocol for implementing a network. Two systems
that support X.25 cannot necessary be connected back – to – back. They can only
be connected through a DCE in a public packet switched.
Type of WIDE
Area Network
The essential purpose of WIDE Area network, regardless
of the size or technology used is to link separate locations in order remove
data around. A WAN allows these locations to access shared computer resources
and provides the asocial infrastructure for developing wide spread distributed
computing system.
We will now discuss the different types of WAN which
are communally used.
1. Public
Networks
Public networks are those networks which are installed
and run by the telecommunication authorities and are made available to any
organization or individual who subscribe. Examples include Public Switched
Telephone Networks (PSTN), Public Switched Data Network (PSDN), Value Added
Service (VANs/ VADs) and the Integrated Service Digital Networks (ISDN). We
would be discussing the main features of there services:
§ Public
Switched Telephone Network (PSTN)
The features of the PSTN are its low speed, the analog
nature of transmission, restricted bandwidth & its wide spread
availability. As PSTN is designate for telephones, modems are required when it
is used for data communication.
§
Public Switched Data Network (PSDN)
The term PSDN covers a number of technologies; all
through currently it is limited to Public Packet Switch Networks available to
the public. The main features of all PSDNs are their high label reliability and
the high quality of the connection provided. The can support both high &
low speeds at appropriate costs.
§ Value Added
Services (VANs/ VADs)
In value added services, the provider of such services
must process, store and manipulate the data that is carried on the network,
that add value to it. The technique can be used in specific types of business
in which it is advantageous to be able to share information with other
companies in the same line.
§ Integrated
Services Digital Network (ISDN)
The ISDN is the networking
concept providing for the integration of voice, video and data servicing using
data service using digital transmission combining both circuits and packet
switching techniques. The motivating force behind ISDN is that telephone
networks around the world have been making a transition towards utilizing
digital transmission facilities for many years.
2. Private
Networks
The basic technique used in all forms of private WAN
is to use private (or more usually leased) circuits to link the locations to be
served by the network. Between these fixed points the owner of the network has
complete freedom to use the circuits in any way they want. They can use the
circuit to carry large quantities of data or for high speed transmission.
Private WIDE area network can be built using what ever
standard technology is available. The way private networks have generally been
set up has to specify
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