Chapter v Final

8/17/2019 Chapter v Final

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CHAPTER-V

DATA COMMUNICATION &DIGITAL SUBSCRIBER LINE

TECHNOLOGY

Principles o D!"! Co##$nic!"ions:

The first data communications system was the telegraph. It was invented more than

100 years ago. The letters to be transmitted were converted into a code called Morse code. The

codes were transmitted as pulses along a wire or as radio-frequency bursts in the case of wireless

telegraph. Then the development of data communications slowed but during the last few decades

data communications have e!panded rapidly as computers have become tools for everyone in

both business and residential environments.

Co#p$"er Co##$nic!"ions%

Modern computers manipulate bits binary symbols of electrical energy. "hen a

computer communicates with another computer it sends these bits along a cable between them.

This is relatively easy if the computers are within the same room or a building. If the distance is

longer a telecommunications networ# is required that provides an end-to-end communications

channel. $ata communications can be accomplished by means of many various alternatives

some of which we discuss in the following sections

Seri!l !n' P!r!llel D!"! Co##$nic!"ions%

In a transmission networ# only one channel is usually allocated for one end to-end

connection in each direction. %et us use as an e!ample source of data a simple &merican

'tandard (ode for Information Interchange )&'(II* terminal. "e press #eys on the #eyboard

and each #eystro#e generates a +-bit binary word ), bits with parity* corresponding to the

letter or number of the #ey pressed. or e!ample character a corresponds to the binarysequence 1000011 )the first bit on the left* .

"hen serial transmission is used between a computer and its peripheral device a

parallel cloc# signal may be used for timing. In serial transmission over longer distances we want

to manage with one channel and we have to use a line code to insert timing information into the

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data stream. This synchroniation information enables the receiver to determine when it has to

detect each individual received bit. /ow we implement this depends on whether we use an

asynchronous or synchronous transmission mode.

arallel data transmission is much quic#er than serial but we can typically use it only over short

distances. The ma!imum is usually of the order of 10m.

(ommunicating terminal devices in data communications are called data

terminal equipment )$T* and the equipment that terminates the transmission channel that

goes through the networ# is called data circuit terminating equipment )$(*. & modem

that we use for data transmission over a telephone networ# is a typical e!ample of $(. Many

different interface specifications e!ist for $T and $( and the most common standards are

defined by the IT2-T and the lectronic Industries &ssociation )I&*.



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As(nc)rono$s !n' S(nc)rono$s D!"! Tr!ns#ission%

3ver longer distances we use serial transmission either in an asynchronous or

synchronous transmission mode. 'erial transmission over long distance requires that the timing

information for the receiver be transmitted together with the data so that a separate cloc# signal

is not required.

In asynchronous transmission only a small number of bits are transmitted at a time

usually , bits that correspond to one &'(II character. In the beginning of each bloc# of , bits of

data a start bit is sent to indicate to the receiver that it should prepare to receive , bits of data .

or synchroniation the receiver has to #now the data rate which has to be set in advance so that

when it detects the start bit it is able to receive the few following bits. &fter these bits a stop bit is

sent that terminates the ,-bit data bloc#. The ne!t bloc# of data is synchronied independently

with the help of a new start bit preceding the data bits.

In asynchronous transmission a simple error-detecting scheme called parity can be

used. "e may use even or odd parity error chec#ing. If even parity is used the total number of

415 bits in the bloc# including data bits and the parity bit is set to be even with the help of the



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parity bit. In the case of odd parity the parity bit is set to 415 or 405 so that the total number of

415bits in the bloc# is odd. To detect possible transmission errors the receiver determines

whether the received number of 415 bits is even or odd depending on the parity agreed.

'ynchronous transmission is a more modern principle for transmitting a large

amount of information in a frame )see igure*. ach frame starts with a special start-of-frame bit

sequence and the frame may contain more than 1000 bytes of information. ach frame also

contains error control words and an end-of-frame sequence. The receiver uses the error control

section of the frame to detect if errors have occurred in transmission. The most common

detection method for error detection is a cyclic redundancy chec# )(6(*. It is much more

reliable than the parity chec# method discussed previously. In the case of errors the transmitter

retransmits the frame in error. In the most common protocols the receiver sends an

ac#nowledgment to the transmitter in the other transmission direction for received error-free

frame or frames. If errors have occurred the frame is not ac#nowledged in a predefined period of

time and the transmitter sends it again.

In asynchronous transmission the start bit provided the required timing information

for each byte of data. 2nique start-of-frame and end of frame sequences or flags are used to

provide frame synchroniation. These flags should be unique and actual data must

not include similar data sequences. 3ne common method used to avoid frame misalignment

is to use bit stuffing or ero insertion as shown in igure. (onsider a flag )01111110* used

in the popular high-level data lin# control )/$%(* protocol. &fter the start-of-frame flag the

sequence of si! subsequent 17s is not allowed in the data section of the frame. To avoid that a

0 is inserted in the end of each sequence of five subsequent 17s. In the receiver each 0 following

five subsequent 17s is discarded. If binary 1 follows five subsequent 17s the frame is declared to

be finished )end-of-frame flag*.



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:

CIRCUIT S*ITCHING NET*OR+S%

(ommunication via circuit switching involves three phases

Circ$i" es"!,lis)#en"

D!"! "r!nser

Circ$i" 'isconnec"



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The best-#nown e!ample of a circuit-switching networ# is the public telephone networ#.

S$,scri,ers: The devices that attach to the networ#. It is still the case that most subscriber

devices to public telecommunications networ#s are telephones but the percentage of data

traffic increases year by year.

Loc!l loop: The lin# between the subscriber and the networ# also referred to as the subscriber

loop. &lmost all local loop connections used twisted-pair wire. The length of a local loop is

typically in a range from a few #ilometers to a few tens of #ilometers.

Ec)!n.es: The switching centers in the networ#. & switching center that directly supports

subscribers is #nown as an end office. Typically an end office will support many thousands of

subscribers in a localied area. There are over 18000end offices in the 2nited 'tates so it is

clearly impractical for each end office to have a direct lin# to each of the other end offices.

Tr$n/s: The branches between e!changes. Trun#s carry multiple voice frequency circuits

using either $M or synchronous T$M. arlier these were referred to as carrier systems.

'ubscribers connect directly to an end office which switches traffic between

subscribers and between a subscriber and other e!changes. The other e!changes are responsible

for routing and switching traffic between end offices9 this distinction is shown in igure. To

connect two subscribers attached to the same end office a circuit is set up between them in the

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same fashion as described before. If two subscribers connect to different end offices a circuit

between them consists of a chain of circuits through one or more intermediate offices. In the

figure a connection is established between lines a and b by simply setting up the connection

through the end office. The connection between c and d is more comple!. In cs end office a

connection is established between line c and one channel on a T$M trun# to the intermediate

switch. In the intermediate switch that channel is connected to a channel on a T$M trun# to ds

end office. In that end office the channel is connected to line d.

Circ$i" es"!,lis)#en"

(ircuit-switching technology has been driven by those applications that handle voice traffic.

3ne of the #ey requirements for voice traffic is that there must be virtually no transmission delay

and certainly no variation in delay. & constant signal transmission rate must be maintained as

transmission and reception occur at the same signal rate. These requirements are necessary to

allow normal human conversation. urther the quality of the received signal must be sufficiently

high to provide at a minimum intelligibility.

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PAC+ET S*ITCHING%

ac#et-switched networ#s are specially designed for data communication. The source data are

split into pac#ets containing route or destination identifications. The pac#ets are routed toward

the destination by pac#et-switching nodes on the path through the networ#. There are two basictypes of pac#et-switched networ#s as illustrated in igure

;irtual circuits

$atagram transmission.

In the case of virtual circuits the virtual connection is established at the beginning of each

conversation or it is permanently set up and every pac#et belonging to a certain connection is

transmitted via the same established route. The main difference between circuit-switched

physical circuits and virtual circuits is that many users share the capacity of the transmission

lines and channels between networ# nodes if virtual instead of physical circuits are used. &t a

certain moment active users may use all the available capacity if other users are not transmitting

anything. The complete address information is not needed in the pac#ets when the connection is

established. 3nly a short connection identifier is included in each pac#et to define the virtual

circuit to which the pac#et belongs.

&nother method for pac#et-switched data communications is connectionless datagram

transmission in which routing devices perform routing procedures and each pac#et contains a

full destination address.

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P!c/e" S0i"c)in.

< &dvantages:

< 'ecurity

< =andwidth used to full potential

< $evices of different speeds can communicate

< >ot affected by line failure )rediverts signal*

< &vailability ? do not have to wait for a direct connection to becomeavailable

< $uring a crisis or disaster when the public telephone networ# might stop

wor#ing e-mails and te!ts can still be sent via pac#et switching

$isadvantages

< 2nder heavy use there can be a delay

< $ata pac#ets can get lost or become corrupted

< rotocols are needed for a reliable transfer

< >ot so good for some types data streams e.g real-time video streams canlose frames due to the way pac#ets arrive out of sequence.

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1 Circ$i" S0i"c)in.

< &dvantages:

< (ircuit is dedicated to the call ? no interference no sharing

< @uaranteed the full bandwidth for the duration of the call

< @uaranteed Auality of 'ervice

$isadvantages:

< Inefficient ? the equipment may be unused for a lot of the call if no data is

being sent the dedicated line still remains open

< Ta#es a relatively long time to set up the circuit

< $uring a crisis or disaster the networ# may become unstable orunavailable.

< It was primarily developed for voice traffic rather than data traffic.

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Co#p!rison o co##$nic!"ion s0i"c)in. "ec)ni2$es D!"!.r!# p!c/e" Vir"$!l-circ$i"

Circ$i" s0i"c)in. s0i"c)in. p!c/e" s0i"c)in.

$edicated transmission path >o dedicated path >o dedicated path(ontinuous transmission of Transmission of pac#ets Transmission of pac#ets

data

ast enough for interactive ast enough for interactive ast enough for interactive

Messages are not stored ac#ets may be stored until ac#ets stored until

delivered delivered

The path is established for 6oute established for each 6oute established for entire

entire conversation pac#et conversation

(all setup delay: negligible ac#et transmission delay (all setup delay9 pac#et

transmission delay transmission delay=usy signal if called party 'ender may be notified if 'ender notified of

busy pac#et not delivered connection denial

3verload may bloc# call 3verload increases pac#et 3verload may bloc# call

setup9 no delay for delay setup9 increases pac#et

established calls delay

lectromechanical or 'mall switching nodes 'mall switching nodes

(omputeried switching nodes

2ser responsible for message >etwor# may be responsible >etwor# may be responsible

loss protection for individual pac#ets for pac#et sequences2sually no speed or code 'peed and code 'peed and code

conversion conversion conversion

i!ed bandwidth $ynamic use of bandwidth $ynamic use of bandwidth

transmission

>o overhead bits after call 3verhead bits in each 3verhead bit in each

setup pac#et pac#et

ACCESS METHODS

Di.i"!l s$,scri,er line "ec)nolo.(%



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& family of technologies #nown as $'% or digital subscriber line has been

developed to increase the data transmission rate over ordinary local loops to the order of a few

megabits per second and it is simultaneously available for ordinary telephone service. This is far

beyond the capacity of I'$> subscriber lines. The I'$> data channels are e!pensive dial-up

circuits that are switched by I'$> e!changes and each connection minute increases the

subscriber7s telephone bill. In the case of $'% data and speech are separated at the local

e!change site. Then the data portion is connected to the data networ# for Internet access.

(ustomers pay a fi!ed monthly fee for a high-data-rate connection that is always on.

&pplications of $'%:

The carriers or networ# operators are aiming their $'% services mainly at residential users. or

them it provides high-data-rate access to the Internet and at the same time an ordinary telephone

connection over a local loop. In these applications &$'% which transmits at a higher data rate

downstream than upstream and its variants are preferred. igure illustrates some applications

of $'%: remote access to a data center Internet access and interconnection of %&>s.



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$'% replaces the ordinary local loop and $'% modems are needed at both ends of

the line. If an ordinary telephone connection is to be available simultaneously the low pass filter

splitter at the carrier7s central office splits off the voice channel and routes it to the 'T>. &

$'% access multiple!er )$'%&M* terminates the data channel at the other end of the subscriber

loop and sends traffic onto the carrier7s bac#bone data networ# implemented by I &TM frame

relay technology or fi!ed data circuits where it heads to a remote data center or the Internet.

$'% is mainly designed to improve the utiliation of subscriber cables in the access

networ#. /owever because it requires fewer intermediate repeaters system cost is reduced

and $'% will replace conventional primary rate 1.B- or C-Mbps copper cable transmission

systems inside the core networ# as well.

DSL Tec)ni2$es%$'% technologies are still evolving and many alternative technologies are

available today and new ones are under standardiation. The most important technologies their



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transmission distances and data rates are presented in Table.

Hi.)-Bi"-R!"e DSL 3HDSL4%

The high-bit-rate $'% )/$'%* increases the section length and thusreduces the need for intermediate repeaters. This technology uses C=1A )two bits are

transmitted in each four-level symbol* encoding that has superior spectral and distance

characteristics. /$'% uses two )or sometimes three cable pairs* and thus it is not a consumer



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access technology. It provides the same data rate for 1 or T1 in both directions and is suitable

for small and medium sie enterprises )'Ms* where upstream traffic has equal volume.

/$'% systems use two cable pairs for full-duple! transmission. The data rate is

divided between pairs. In one pair to one direction it is only half of the data rate of conventional

systems that use different cable pairs for each transmission direction. urther improvement is

achieved with the help of an efficient line code. The line code in use is C=1A which means that

each pair of bits is coded into one quaternary symbol with four values to the line. This is the

same line code that is used in I'$> basic rate subscriber lines for 1D0Ebps bidirectional

transmission and each symbol carries two bits of information. That reduces the symbol rate on

the line to half of the binary rate and the lower transmission rate decreases attenuation and

crosstal#. Ta#en together these developments double the transmission distance compared to the

distance of conventional systems.

The /$'% system transmits the same data rate to both directions Fust as

conventional 1.BGC-Mbps copper cable transmission systems. It will replace them in other

applications in the telecommunications networ# such as in I'$> 6I connections because it

requires fewer intermediate repeaters which reduces costs. /$'% is not a consumer access

technology because it is symmetrical uses two pairs and does not allow a voice-band telephone

connection to coe!ist in the same subscriber loop.

R!"e-A'!p"i5e DSL 3RADSL4%

&n often-used term 6&$'% refers to modern $'% technologies such as &$'%.dmt

'$'% and ;$'% that can adapt their operation to ma!imie transmission rates over a cable

pair. To achieve this it adapts loading of each bin to its 'G> as e!plained earlier. /owever the

$'% access data rate is often set to be fi!ed and then 6&$'% technology can ensure that the

defined data rate is achieved in various loop conditions.

Ver(-Hi.)-Bi"-R!"e DSL 3VDSL4 %

;$'% is an evolving technology that aims to provide access to wider band services via ordinary

telephone subscriber pairs. The transmission data rate from the networ# to the subscriber7s

premises is up to BC Mbps and up to D Mbps in the opposite direction over a single pair )see



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igure. Its symmetrical configuration allows an up to H-Mbps data rate in both directions. The

distance over an ordinary cable pair without intermediate repeaters is quite short between 0.1

and C #m depending on the data rate and loop conditions.

'ubscriber loops from e!change site are usually longer than ;$'% can tolerate and the

networ#-side ;$'% equipment has to be installed close to the customer. Then a copper wire $'%

part of the circuit might only include the drop line to a residence or business.


Chapter v Final

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