Copper Access Technologies

8/10/2019 Copper Access Technologies

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Copper Access Technologies

We are quite used to voice-grade data modems, and their limitations. Voice grade modemspresently transmit up to 28.8 kbps over a common telephone line, but the practical limit only twentyyears ago was 1.2 kbps. o one believes we can go much !aster than "".# kbps in the !uture,however. Voice grade bandwidth does not e$ceed "." k%&. 'odems like V."( achieve 1) bits per%ert& o! bandwidth, a startling !igure that approaches theoretical limits. ot only that, V."( modems

transmit and receive simultaneously, in the same band. *nd you can buy one !or under +2)). Wehave these modems because o! almost sublime advances in algorithms, digital signal processing,and semiconductor technology.

Voice grade modems operate at the subscriber premises end o! voice grade lines and transmitsignals through the core switching network without alteration the network treats them e$actly likevoice signals. his has been their singular power, that, despite rather slow speeds compared toterminals today, they can be connected immediately anywhere a telephone line e$ists, and there arenearly #)) million such locations.

andwidth limitations o! voice band lines do not come !rom the subscriber line, however. hey come!rom the core network. /ilters at the edge o! the core network limit voice grade bandwidth to "."k%&. Without !ilters, copper access lines can pass !requencies into '%& regions, albeit withsubstantial attenuation. 0ndeed, attenuation, which increases with line length and !requency,dominates the constraints on data rate over twisted pair wire. ractical limits on data rate in one

direction compared to line length o! 2( gauge twisted pair3 are4

561 13 1.7(( 'bps 18,))) !eet

1 2.)(8 'bps 1#,))) !eet

562 #."12 'bps 12,))) !eet

2 8.((8 'bps 9,))) !eet

1:( 66-1 12.9#) 'bps (,7)) !eet

1:2 66-1 27.92) 'bps ",))) !eet

66-1 71.8() 'bps 1,))) !eet

6ubscriber loop plant con!igurations vary tremendously around the world. 0n some countries 18,)))!eet covers virtually every subscriber in others, such as the ;nited 6tates, 18,))) !eet covers lessthan 8)< o! subscribers. %owever, the 2)< or so remaining have lines with loading coils whichcannot be used !or any 56= service including 0653 without removing the coils. 'ost telephonecompanies have had programs to shrink average loop length underway !or a number o! years,largely to stretch the capacity o! e$isting central o!!ices. he typical technique involves installationo! access nodes remote !rom central o!!ices, creating so-called 5istribution *reas with ma$imumsubscriber loops o! #))) !eet !rom the access node. >emote access nodes are !ed by 1:1 lines



now using %56=3 or !iber. 0n suburban communities a 5istribution *rea connects an average o!17)) premises in urban areas, the !igure is double, about "))) premises. ?! course the number o!premises served dwindles as service data rates increase. * /iber to the @urb system /@3 o!!ering66-1 rates may only be within reach o! twenty homes in some suburban areas.

Aou now have enough in!ormation to be a network planner, presuming the marketing department

has handed you a stable list o! applications. 0! that list does not include digital live television or%5V but does include video on demand and 0nternet access3, then a data rate o! 1.7 'bps persubscriber terminal downstream may su!!ice, and you can o!!er it to virtually everyone within 18,)))!eet, the nominal range o! 065. /or subscribers with shorter lines, either to a central o!!ice orremote access node, you can o!!er more than one channel to more than one premises terminal. 0!digital live television is on the list, then you have to o!!er at least # 'bps, and you may be limited to(7)) !oot distances to supply more than one channel at a time. his !act is the heart o! telephonecompany interest in wireless broadcast digital V, and the consequent alkani&ation o! its !uturenetwork.3 @learly %5V, demanding as much as 2) 'bps, only goes over the shortest loop length.

?! course, this o!!ering o! digital services over e$isting twisted-pair lines requires transceivers,special modems capable o! da&&ling data rates when one considers the age and original intentions o! twisted-pair wiring technology. 0t turns out that this e!!ort to use twisted pair !or high speedin!ormation began many years ago.

DSL -- Digital Subscriber Line

he basic acronyms !or all 56= arrangements came !rom ellcore, so we must blame them !or thebasic con!usion between a line and its modems. 0n general we say that 56= signi!ies a modem, or amodem pair, and not a line at all. Aes, a modem pair applied to a line creates a digital subscriberline, but when a telephone company buys 56=, or *56=, or %56=, it buys modems, quite apart !romthe lines, which they already own. 6o, 56= is a modem, not a line. his con!usion becomes quiteimportant to avoid when we talk about prices. * B56=B is one modem a line requires two.

56= itsel!, apart !rom its later siblings, is the modem used !or asic >ate 065. * 56= transmitsduple$ data, i.e., data in both directions simultaneously, at 1#) kbps over copper lines up to 18,)))!eet o! 2( ga wire. he multiple$ing and demultiple$ing o! this data stream into two channels #(kbps each3, a 5 channel 1# kbps3, and some overhead takes place in attached terminal equipment.

y modern standards 56= does not press any transmission thresholds, but its standardimplementation *60 1.#)1 or 0; 0.("13 employs echo cancellation to separate the transmitsignal !rom the received signal at both ends, a novelty at the time 56= !irst !ound its way into thenetwork.

56= modems use twisted-pair bandwidth !rom ) to about 8) k%&. 6ome uropean systems use 12)k%& o! bandwidth.3 hey there!ore preclude the simultaneous provisioning o! analog ?6. %owever,56= modems are being used today !or so-called pair gain applications, in which 56= modemsconvert a single ?6 line to two ?6 lines, obviating the physical installation o! the second linewiring. he telephone company Cust installs the analog:digital voice !unctions at the customerpremises !or both lines, and presto, two !rom one.

T1 or E1

0n the early si$ties engineers at ell =abs created a voice multiple$ing system that !irst digiti&ed a



voice signal into a #( kbps data stream representing 8))) voltage samples a second with eachsample e$pressed in 8 bits3 and then organi&ed twenty !our o! them into a !ramed data stream, withsome conventions !or !iguring out which 8 bit slot went where at the receiving end. he resulting!rame was 19" bits long, and created an equivalent data rate o! 1.7(( 'bps. he structured signalwas called 561, but it has acquired an almost colloquial synonym -- 1 -- which also describes theraw data rate, regardless o! !raming or intended use. *D deployed 561 in the intero!!ice plant

starting in the late si$ties almost all o! which has since been replaced by !iber3, and by the mid-seventies was using 561 in the !eeder segment o! the outside loop plant.

0n urope, and at @@0 now 0;3, the collection o! world s other than * modi!ied ell =absoriginal approach, as they were wont to do, and de!ined 1, a multiple$ing system !or ") voicechannels running at 2.)(8 'bps. 0n urope 1 is the only designation, and stands !or both the!ormatted version and the raw data rate.

;ntil recently, 1 and 1 circuits were implemented over copper wire by using crude transceiverswith a sel!-clocking *lternate 'ark 0nversion *'03 protocol. *'0 requires repeaters "))) !eet !romthe central o!!ice and every #))) !eet therea!ter, and takes 1.7 '%& o! bandwidth, with a signalpeak at E7) k%& ;.6. systems3. o a transmission purist, this is pro!ligate and ugly, but it hasworked !or many years and hundreds o! thousands o! lines 1 and 13 e$ist in the world today.

elephone companies originally used 1:1 circuits !or transmission between o!!ices in the coreswitching network. ?ver time they tari!!ed 1:1 services and o!!ered them !or private networks,connecting Fs and 1 multiple$ors together over the Wide *rea etwork W*3. oday 1:1circuits can be used !or many other applications, such as connecting 0nternet routers together,bringing tra!!ic !rom a cellular antenna site to a central o!!ice, or connecting multimedia servers intoa central o!!ice. *n increasingly important application is in the so-called !eeder plant, the section o!a telephone network radiating !rom a central o!!ice to remote access nodes that in turn servicepremises over individual copper lines. 1:1 circuits !eed 5igital =oop @arrier 5=@3 systems thatconcentrate 2( or ") voice lines over two twisted pair lines !rom a central o!!ice, thereby savingcopper lines and reducing the distance between an access point and the !inal subscriber.

ote, however, that 1:1 is not a very suitable service !or connecting to individual residences. /irsto! all, *'0 is so demanding o! bandwidth, and corrupts cable spectrum so much, that telephonecompanies cannot put more than one circuit in a single 7) pair cable, and must put none in any

adCacent cables. ?!!ering such a system to residences would be equivalent to pulling new wire tomost o! them. 6econdly, until recently no application going to the home demanded such a data rate.hirdly, even now, as data rate requirements accelerate with the hope o! movies and high speeddata !or everyone, the demands are highly asymmetric -- bundles downstream to the subscriber,and very little upstream in return -- and many situations will require rates above 1 or 1. 0ngeneral, high speed data rate services to the home will be carried by *56= or V56= or similar typeso! modems over @*V lines3.

HDSL -- High data rate Digital Subscriber Line

%56= is simply a better way o! transmitting 1 or 1 over twisted pair copper lines. 0t uses lessbandwidth and requires no repeaters. ;sing more advanced modulation techniques, %56= transmits1.7(( 'bps or 2.)(8 'bps in bandwidths ranging !rom 8) k%& to 2() k%&, depending upon thespeci!ic technique, rather than the greedy 1.7 '%& absorbed by *'0. %56= provides such rates over

lines up to 12,))) !eet in length 2( ga3, the so-called @arrier 6erving *rea @6*3, but does so byusing two lines !or 1 and three lines !or 1, each operating at hal! or third speed.

'ost %56= will go into the !eeder plant, which connect subscribers a!ter a !ashion, but hardly in thesense o! an individual using a phone service.

ypical applications include F network connections, cellular antenna stations, digital loop carriersystems, intere$change ?s, 0nternet servers, and private data networks. *s %56= is the mostmature o! 56= technologies with rates above a megabit, it will be used !or early-adopter premisesapplications !or 0nternet and remote =* access, but will l ikely give way to *56= and 656= in the



near !uture.

SDSL -- Single line Digital Subscriber Line

?n its !ace 656= is simply a single line version o! %56=, transmitting 1 or 1 signals over a single

twisted pair, and in most cases3 operating over ?6, so a single line can support ?6 and 1:1simultaneously. %owever, 656= has the important advantage compared to %56= that it suits themarket !or individual subscriber premises which are o!ten equipped with only a single telephone line.656= will be desired !or any application needing symmetric access such as servers and powerremote =* users3, and it there!ore complements *56= see below3. 0t should be noted, however,that 656= will not reach much beyond 1),))) !eet, a distance overwhich *56= achieves rates above# 'bps.

ADSL -- Asymmetric Digital Subscriber Line

*56= !ollowed on the heels o! %56=, but is really intended !or the last leg into a customerGspremises. *s its name implies, *56= transmits an asymmetric data stream, with much more goingdownstream to the subscriber and much less coming back. he reason !or this has less to do withtransmission technology than with the cable plant itsel!. wisted pair telephone wires are bundled

together in large cables. /i!ty pair to a cable is a typical con!iguration towards the subscriber, butcables coming out o! a central o!!ice may have hundreds or even thousands o! pairs bundledtogether. *n individual line !rom a @? to a subscriber is spliced together !rom many cable sectionsas they !an out !rom the central o!!ice ellcore claims that the average ;.6. subscriber line hastwenty-two splices3. *le$ander ell invented twisted pair wiring to minimi&e the inter!erence o!signals !rom one cable to another caused by radiation or capacitive coupling, but the process is notper!ect. 6ignals do couple, and couple more so as !requencies and the length o! line increase. 0tturns out that i! you try to send symmetric signals in many pairs within a cable, you signi!icantlylimit the data rate and length o! line you can attain.

%appily, the preponderance o! target applications !or digital subscriber services are asymmetric.Video on demand, home shopping, 0nternet access, remote =* access, multimedia access,speciali&ed @ services all !eature high data rate demands downstream, to the subscriber, butrelatively low data rates demands upstream. 'H movies with simulated V@> controls, !ore$ample, require 1.7 or ".) 'bps downstream, but can work Cust !ine with no more than #( kbps

or 1# kbps3 upstream. he 0 protocols !or 0nternet or =* access push upstream rates higher, buta ten to one ratio o! down to upstream does not compromise per!ormance in most cases.

6o *56= has a range o! downstream speeds depending on distance4

;p to 18,))) !eet 1.7(( 'bps 13

1#,))) !eet 2.)(8 'bps 13

12,))) !eet #."12 'bps 5623

9,))) !eet 8.((8 'bps

;pstream speeds range !rom 1# kbps to #() kbps. 0ndividual products today incorporate a varietyo! speed arrangements, !rom a minimum set o! 1.7((:2.)(8 'bps down and 1# kbps up to ama$imum set o! 9 'bps down and #() kbps up. *ll o! these arrangement operate in a !requencyband above ?6, leaving ?6 service independent and undisturbed, even i! a premises *56=modem !ails.

*s *56= transmits digitally compressed video, among other things, it includes error correction



capabilities intended to reduce the e!!ect o! impulse noise on video signals. rror correctionintroduces about 2) msec o! delay, which is much too much !or =* and 0-based datacommunications applications. here!ore *56= must know what kind o! signals it is passing, to knowwhether to apply error control or not this problem obtains !or any wire-line transmissiontechnology, over twisted pair or coa$ial cable3. /urthermore, *56= will be used !or circuit switchedwhat we have today3, packet switched such as an 0 router3 and, eventually, *' switched data.

*56= must connect to personal computers and television set top bo$es at the same time. akentogether, these application conditions create a complicated protocol and installation environment !or*56= modems, moving these modems well-beyond the !unctions o! simple data transmission andreception.

VDSL -- Very high data rate Digital Subscriber Line

V56= began li!e being called V*56=, because at least in its !irst mani!estations, V56= will beasymmetric transceivers at data rates higher than *56= but over shorter lines. While no generalstandards e$ist yet !or V56=, discussion has centered around the !ollowing downstream speeds4

12.9# 'bps 1:( 66-13 (,7)) !eet o! wire

27.82 'bps 1:2 66-13 ",))) !eet o! wire

71.8( 'bps 66-13 1,))) !eet o! wire

;pstream rates !all within a suggested range !rom 1.# 'bps to 2." 'bps. he principal reason11.( decided against BV*56=B was the implication that V56= would never be symmetric, whensome providers and suppliers hope !or !ully symmetric V56= someday, recogni&ing that line lengthwill be compromised.

0n many ways V56= is simpler than *56=. 6horter lines impose !ar !ewer transmission constraints,so the basic transceiver technology is much less comple$, even though it is ten times !aster. V56=

only targets *' network architectures, obviating channeli&ation and packet handling requirementsimposed on *56=. *nd V56= admits passive network terminations, enabling more than one V56=modem to be connected to the same line at a customer premises, in much the same way ase$tension phones connect to home wiring !or ?6.

%owever, the picture clouds under closer inspection. V56= must still provide error correction, themost demanding o! the non-transceiver !unctions asked o! *56=. *s public switched network *'has not begun deployment yet, and will take decades to become ubiquitous, V56= will likely beasked to transmit conventional circuit and packet switched tra!!ic. 0ndeed, a recent telephonecompany >/I describes a V56=-type transceiver with three circuit-switched video channels and asingle *' channel.3 *nd passive network terminations have a host o! problems, some technical,some regulatory, that will surely lead to a version o! V56= that looks identical to *56= withinherent active termination3 e$cept its capability !or higher data rates.

V56= will operate over ?6 and 065, with both separated !rom V56= signals by passive !iltering.

Other Terms

V56= had been called BV*65=B or B56=B or even B*56=B prior to June, 1997, when 11.( choseBV56=B as the o!!icial title. he other terms still linger in technical documents created be!ore thattime and media presentations unaware o! the convergence. 60 '", the uropean counterpart to11.(, has also adopted BV56=,B but temporarily appends a lower case BeB to indicate that, untilthe dust settles, the uropean version o! V56= may be slightly di!!erent than the ;.6. version. hisis the case with both %56= and *56=, although there is no convention !or re!lecting the di!!erences



in the name. he di!!erences are su!!iciently small mostly surrounding data rates3 that silicontechnology accommodates both.

K *56= /orum. 2))1

@%? @*@==*0?

I'm doing research on the technologies actual and uture or the local loo!" es!ecially theDSL amily# $or ADSL systems" I %no& that t&o %inds o multi!leing are used ( $D)

$re*uency Di+ision )ulti!leing and the echo cancellation# I &ould li%e to %no& ho& theecho cancellation is im!lemented#

0n simplest terms, echo cancellation means that the upstream and the downstreamsignals are sent on the wire at the 6*' !requencies, i.e. they overlap, whereas /5'method sends the upstream and the downstream separated at di!!erent !reqs. headvantage o! echo cancellation is that the signals are both kept at the lowest possible!requencies since cable loss and crosstalk noise both increase with !requency3 andthere!ore achieves greater cable distance !or a given data rate. %owever echo

cancellation *56= systems are more complicated, and only a !ew vendors haveimplemented them. 0t is an option - @ategory 2- in the *56= 6tandard3. *n *56=receiver will see an incoming signal that is both the incoming signal !rom the !ar endand the outgoing signal !r om the local transmitter. hese are mi$ed together at overthe same !requency range. 0n other words, the received signal composes o! not onlythe signal to be recovered !rom the !ar end but also a local echo due to the localtransmitter. he local echo must be accurately modeled by 56 circuitry and then thisreplica echo is electronically subtracted !rom the composite incoming signal. 0! doneproperly then all that is le!t behind is the incoming data !rom the !ar end *56=system. he process o! modeling the echo is quite complicated since the echo variesdepending on the connected cable type. he 56circuitry automatically adapts toaccount !or this. $act details o! the 56 implementation will vary with vendor.

En los términos más simples, la cancelación del eco significa que las señales por aguas

arriba y enes sentido descendiente están enviadas en el alambre en las MISMASfrecuencias, es decir se traslapan, mientras que el método de !M env"a el por aguas arribay r"o aba#o separados en diversos freqs$ %a venta#a de la cancelación del eco es que lasseñales están guardadas en las frecuencias posibles más ba#as &puesto que la pérdida y lainterferencia del cable divulgan ambo aumento con frecuencia' y por lo tanto alcan(amayor distancia del cable para una tarifa de datos dada$ Sin embargo los sistemas delA!S% de la cancelación del eco son más complicados, y solamente algunos vendedores los)an puesto en e#ecucio*n$ &es una opción + categor"a en el estándar del A!S%'$ -nreceptor del A!S% verá una señal entrante que sea la señal entrante del e.tremo le#ano y laseñal saliente del transmisor local$ /stos se me(clan #untos en el e.cedente la misma gamade frecuencia$ Es decir la señal recibida compone no solamente de la señal de ser

recuperado del e.tremo le#ano pero también de un eco local debido al transmisor local$ Eleco local se debe modelar e.actamente por el tra(ado de circuito de !S0 y entonces esteeco de la reproducción se resta electrónicamente de la señal entrante compuesta$ Si están)ec)os correctamente entonces todo que se de#e detrás son los datos entrantes del le#anoterminan el sistema del A!S%$ El proceso de modelar el eco es absolutamente complicado puesto que el eco var"a dependiendo del tipo conectado del cable$ El !S0circuitry seadapta automáticamente para e.plicar esto$ %os detalles e.actos de la puesta en práctica de!S0 variarán con el vendedor$

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