network chapter6 - introduction to data network

8/9/2019 Network Chapter6 - Introduction to Data Network

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Introduction to Data Networks

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Objective

At the end of this chapter, you will be able to identify

the differences between switched and leased lines

and choose between the two given a deployment

scenario and describe VoIP technology, its benefits

and the tradeoffs involved in ensuring speech

quality.


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Scope

Switched and Leased Lines

Circuit Switching

Packet Switching

Voice over Internet Protocol (VoIP) Integrated Services Digital Network (ISDN)

Asynchronous Transfer Mode (ATM) and ISDN


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An important parameter about the lines is whetherthey are public switched lines or not. Voice gradeand digital lines can either be switched through

public exchanges (central offices) or permanentlyconnected.

Facilities for switching broadband channels are inoperation in some countries, although most

broadband channels today are permanentconnections (point-to-point).


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When you dial a friend and talk to him on the

telephone, you speak over a line connected by means

of the public exchanges.

This line, referred to as a "public" or "switched"line, could be used for the transmission of data.

Alternatively, a "private" or "leased" line could be

connected permanently or semi-permanentlybetween the transmitting machines.



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Pros and Cons of Leased Lines

Leased voice lines have certain advantages for datatransmission over switched telephone connections.

The advantages are summarized as follows : If it is to be used for more than a given number of

hours per day, the leased line is less expensive thanthe switched line. If it is used for only an hour or soper day then it is more expensive .

The break-even point depends on the actual chargeswhich in turn depend on the mileage of the circuit,but it is likely to be of the order of several hours perday.



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Because the leased line is permanently connected

there need be no delay associated with switching

times. Leased lines are therefore better than

telephone switching systems for applicationsrequiring fast access to a distant computer .

Private lines can be specially treated or

"conditioned" to compensate for the distortion that is

encountered on them. The common carriers chargeextra for conditioning. In this way the number of

data errors can be reduced, or, alternatively, a higher

transmission rate can be made possible.



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Switched voice lines usually carry signaling within

the bandwidth that would be used for data. Data

transmission machines must be designed so that the

form in which the data are sent cannot interfere withthe common carrier's signaling .

The leased line may be less perturbed by noise and

distortion than the switched line. The switching

circuits and mechanisms can cause impulse noise that

results in errors in data.



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With a leased line the user is often able to attach his

own performance monitoring and network

management devices to the circuit .



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Functions of a Switching Office (Telephone Exchange)

The basic functions performed by a switching office

are the same whether it is manual,

electromechanical, or electronic. The basic stagesthat a call must go through are as follows:

When the subscriber picks up her telephone, the

office must detect that service is needed. In anautomatic office, the dialing tone is switched to that

line, and the mechanism waits for the subscriber to

dial.



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The dialed telephone number must now be used to

set up an interconnection path. The number is

received as a train of pulses from a rotary dial or

train of frequency pairs from a push-buttontelephone. These signals cause the equipment to set

up a path through the exchange to the appropriate

outgoing line.



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When we place a telephone call, the switchingequipment within the system seeks out a physical"copper" (include fiber and radio) path all the way

from our telephone to the receiver telephone. Thistechnique is called circuit switching .,

An important property of circuit switching is theneed to set up an end-to-end path before any

information (voice or data) can be sent. The lapsedtime as most of us know between end of dialing andstart of ringing can be more than 5 seconds .

Circuit Switching


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During this time the telephone system is hunting fora copper path. Once the call is set up (the calledparty answers), a dedicated path between both endsexist and will continue to exist until the call is

terminated circuit switching was primarily designedand used for voice switching.

However, as we have seen is session 1 there arefundamental differences between voice traffic and

data traffic. In order to facilitate better and moreefficient data transmission, another type of switchingcalled packet switching came into being calledPacketSwitching.

Circuit Switching


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Packet: A group of Binary digits including data

and control signaling which is switched as a

composite whole. The data call control signals and

possibly error control information are arranged ina specified format. Packets may be thought of as

envelopes into which data are placed .

Packed switching: The transmission of data by

means of addressed packets, whereby a

transmission channels is occupied for the duration

of transmission of the packet only.

Packet Switching


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Operation of a Packet switched network

A packet-switched network is made up of three main

elements: Network Computers, the Host computers,

and Terminals. A network computer has two main

functions :

Firstly, it acts as a link between the network and the

data-processing equipment, which uses the network.Secondly, it carries out the switching operation,

determining the route by which the data will be sent

and transmitting them.

Packet Switching


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The customer's computers that the network serves

are called host computers. When one host computer

sends data to another, it passes the data with a

destination address to its local network computer . The network computer formats the data into one or

more blocks, called packets.

Each packet contains the control information neededto transmit the data correctly. The packets are

transmitted from one network computer to another

until they reach their destination .

Packet Switching


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The final network computer strips the transmission-

control information from the packets, assembles the

data, and passes them to the requisite host computer.

A network computer receiving a packet places it in aqueue to await attention. When it reaches the head of

the queue, the computer examines its destination

address, selects the next network computer on the

route, and places the packet in an output queue forthat destination .

Packet Switching


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Comparison between Circuit Switching and Packet

Switching:

Packet Switching

d k


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The public telephone network and the equipment

that makes it possible are taken for granted in most

parts of the world. Availability of a telephone and

access to a low-cost, high-quality worldwidenetwork is considered to be essential in modern

society (telephones are even expected to work when

the power is off).

Anything that would jeopardize this is usually

treated with suspicion.

Voice over Internet Protocol

I d i D N k


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VoIP can be defined as the ability to make telephone

calls (i.e., to do everything we can do today with the

PSTN) and to send data over IP-based data networks

with a suitable quality of service (QoS) and a muchsuperior cost/benefit.


I d i D N k


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Benefits of VoIP :

Cost Reduction .

Simplification .

Consolidation .


I d i D N k


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Acceptable Speech Quality and Costs Vs Quality

Tradeoffs :

The goal for developers is to add telephone calling

capabilities (both voice transfer and signaling) to IP-based networks and interconnect these to the public

telephone network and to private voice networks in

such a way as to maintain current voice quality

standards and preserve the features everyone expectsfrom the telephone .


I t d ti t D t N t k


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For the greater part of the last century, the primary

international telecommunication infrastructure was

the Public Switched Telephone System. Anticipating

considerable user demand for an end-to-end digitalservice,

The worlds telephone companies got together along

with the Consultative Committee on International

Telegraphy and Telephony (CCITT) to build a newfully digital, circuit-switched telephone system .

Integrated Services Digital Network



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This new system, called Integrated Services DigitalNetwork (ISDN), was created with the main aim ofintegrating voice and non-voice traffic.

The key ideal behind ISDN is that of the digital bitpipe, a conceptual pipe between the customer andthe carrier through which bits flow. Whether the bitsoriginated from a digital telephone, a digital

terminal; a digital facsimile machine or some otherdevice is irrelevant. All that matters is that bits canflow through the pipe in both directions.

Integrated Services Digital Network



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Summary

A switched line offers connectivity using the

resources of the PSTN on an as and when required

basis while a leased line reserves the resources

permanently irrespective of whether you use it ornot. Hence a switched line is cheaper and less

reliable than a leased line .

Circuit switching statically reserves the required

bandwidth in advance, whereas packet switching

acquires and releases it, as it is needed .



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VoIP can be defined as the ability to make telephone

calls (i.e., to do everything we can do today with the

PSTN) and to send facsimiles over IP-based data

networks with a suitable quality of service (QoS)and a much superior cost/benefit.

Summary

network chapter6 - introduction to data network

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