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Introduction to Copper Access Technologies and ADSL

Distributed Multimedia Course Notes

Michael Rorke, Computer Science Honours, Rhodes University, 1997

IntroductionWith the advent of the Internet, more and more people want to connect their computers up to othercomputers. Also, the volumes of data that they want to send between these computers are growing,beyond the capabilities of the traditional analogue modem. To address this problem, the majortelephone companies have proposed new methods for sending data quickly (and is large volumes)over copper cable. These new methods are loosely titled xDSL, where the x is replaced with adifferent letter to denote a different version of the product. For example, the first product was calledHDSL (High data rate Digital Subscriber Line), followed closely by the main focus of thisdocument, ADSL (Asymmetric Digital Subscriber Line), and then by an emerging technologyVDSL (Very high data rate Digital Subscriber Line). The original research into what are nowknown as xDSL modems actually began long ago and these types of modems have been in use formany years as part of the core switching networks of many of the major world telcos. But, whereasbefore they were used to carry mainly voice type traffic, they are now being adapted to the needs ofdata traffic carriers. Table 1 lists the main forms of xDSL and the capabilities of each:

Name Meaning Data Rate Mode Applications

V.22, V.32,V. 34

Voice band modems 1200 BPS to28,800 BPS

Duplex Data Communications

DSL Digital Subscriber Line 160 KBPS Duplex ISDN, voice and data services

HDSL High data rate DigitalSubscriber Line

1.544 MBPS2.048 MBPS

Duplex T1/E1 service, feeder plant,WAN/LAN access and serveraccess

SDSL Single line DigitalSubscriber Line

1.544 MBPS2.048 MBPS

Duplex Same as HDSL, plus premisesaccess for symmetric services

ADSL Asymmetric DigitalSubscriber Line

1.5 – 9 MBPS16 – 640 KBPS

Down Up Internet access, Video OnDemand, simplex video, remoteLAN access, interactivemultimedia

VDSL Very high data rate DigitalSubscriber line

13 – 52 MBPS1.5 – 2.3 MBPS

Down Up Same as ADSL plus HDTV

Table 1 - Generic List of Copper Access Technologies

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Introduction to Copper Wire NetworksThe reason that xDSL technologies are able to provide a much better service than voice gradeanalogue modes is due mainly to the layout of the copper cable system. Voice grade modemoperate over the normal telephone switching system, with signals travelling from the site of thesending node, into the core network of the telephone company, and from there, out again to thedestination node, with the signals treated the same as normal voice signals. This is an advantage inthat almost everywhere in the world is linked via the telephone system. However, the core networkimposes certain bandwidth limitations on signals coming into it. For instance, there are filters at theedge of the core network that limit the incoming bandwidth to 3.3kHz, and without these filters,copper cables would be able to pass signals in the MHz regions, albeit with substantial attenuation.This attenuation, which increases with line length, is what limits the practical data rates over copperto those shown in table 2:

Access Type Speed /MBPS Usable Distance /km

DS1 (T1) 1.544 5.5

E1 2.048 5

DS2 6.312 3.5

E2 8.448 3

¼ STS-1 12.960 1.3

½ STS-1 25.920 0.9

STS-1 51.840 0.3

Table 2 - Practical Limits on Data Rate and Line Length

The average length of the subscriber loop varies tremendously depending on where in the worldyou are. In many countries, most (if not all) of the local loops are smaller than 5.5 km. In others,like the USA, 5.5 km only covers 80% of the people, with the other 20% or so having lines, whichcontain loading coils, making them useless for any form of xDSL technology, unless the loadingcoils are removed. There are moves underway in many countries to try and lower the length of thesubscriber loop with projects such as the USA’s ‘fibre to the curb’, where the idea is to lay fibreoptic lines into a central point in every neighbourhood, from which the individual subscribers canbe fed off with copper cable. This trend to shorten the length of the subscriber loop is going a longway to assist the introduction of copper access technologies, which typically only operate (at highdata rates) for distances less than 6 km.

The History of the xDSL Modem

DSL or Digital Subscriber LineBefore going on, there is an important distinction that needs to be made between the name digitalsubscriber line, and what we are actually talking about when we say DSL. In general, DSL refers to

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the hardware on either end of the copper line, and not, as its name would suggest, to the actual lineitself, which is plain old Unshielded Twisted Pair (UTP).

The original DSL was the modem used to implement Basic Rate ISDN. A generic DSL transmitsdata in both directions simultaneously i.e. full duplex, at 160 KBPS over copper lines up to 5.5 kmof 24 (0.5-mm) gauge wire. The modems at either end break the bandwidth up into two B channels(64 KBPS each) and a D channel (16 KBPS). The original DSL modems used the bandwidth from0 to about 80 kHz, and thus were not able to provide the simultaneous plain old telephone servicethat is a feature of xDSL modems i.e. the ability to use the copper line for both voice and datasimultaneously and independently of each other.

While the specifications of the DSL modem may seem trivial by today’s standards, at the time oftheir release they were fairly revolutionary and are, in fact, still in use today, mainly in what aretermed pair-gain applications.

T1 and E1The engineers at Bell labs created a multiplexing system, which send digitised voice through 24framed 64 KBPS streams. The resulting frame was 193 bits long and created an equivalent data rateof 1.544 Mbps. This signalling method was named DS1, which was later expanded to T1 andincludes different framing methods, etc. This was in wide use in the mid-1970’s. In Europe, amodified T1 method was developed using 30 voice channels and giving an equivalent data rate of2.048 Mbps. T1 and E1 circuits require a repeater 900 m from the node, and further repeaters forevery 1800 m thereafter, making them expensive and maintenance intensive. Despite this, theywere, and still are, widely implemented, although they are being phased out, and replaced withHDSL links.

HDSL (High data rate Digital Subscriber Line)Simply put, HDSL is just a better way of transmitting T1 or E1 over twisted pair copper cables.Developed by Bellcore in the late 1980’s, it uses less bandwidth than the traditional methods andoperates without the need for costly repeaters. The idea behind HDSL was simply to develop amethod of delivering a high-performance, cost-effective, 2 Mbps data stream over copper cables.

Initially, HDSL was used by Bellcore to provide T1/E1 links to remote areas, and later, HDSL wasused for all new T1/E1 links. Today, HDSL is used mainly to provide advanced digital services tolocal loop customers and corporate end users. HDSL works by creating a mathematical model ofthe noise characteristics over the copper wire, allowing the transmitting device to preciselycompensate for copper-based distortion. This adjustment occurs dynamically all the time, allowingthe equipment to adjust to changes in the copper environment.

HDSL operates with a bandwidth of 1.544 Mbps up to 3.6 km on standard 24 gauge copper wire,and with certain enhancements or heavier gauge copper, up to 7 km. This was the first technologyto provide fibre optic level network technologies over plain copper wires.

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ADSL (Asymmetric Digital Subscriber Line)

An Introduction to ADSLADSL followed soon after the deployment of HDSL. ADSL is specifically tailored as a last leg linkinto customer premises. As its name suggests, the link provided by an ADSL modem is asymmetrici.e. it offers a single 6 Mbps link from the network to the subscriber, and a pair of 640 KBPS links,one from the subscriber to the network and the other from the network to the subscriber. Thereasoning behind this asymmetric system is that the amount of information coming into the endusers node is greater than the amount of information going out of the end users node, to the rest ofthe network. Studies on end user nodes running TCP/IP networking applications showed that theactual ratio of incoming to outgoing data, was often as high as 10:1!

Specifications of ADSLAn ADSL circuit is a point to point link connecting two nodes over a single twisted pair coppercable. This cable must be a dedicated line, and a normal telephone cable i.e. going through anexchange, will not suffice. ADSL provides a dedicated line type of connection. The ADSL modemcreates three channels, a high speed downstream (from the end user node to the rest of the network)channel, a medium speed duplex channel and an ordinary telephone channel (see figure 1).

The ordinary telephone channel (or Plain Old Telephone Service, POTS) is split off from the rest ofthe digital modem channels by means of filters in order to guarantee that, even if the modem shouldfail, the ordinary telephone service will be able to continue uninterrupted.

The actual bandwidth of the high-speed channel can range from 1.5 to 6.1 Mbps, while the duplexchannel bandwidth ranges from 16 to 640 KBPS. Each of these channels can be submultiplexed toform several smaller channels if required.

The actual downstream data rate depends on a number of factors such as the length of the copperline, its wire gauge, the presence of bridged taps and cross-coupled interference. Ignoring theeffects of bridged taps, an ADSL modem will perform as detailed in table 3.

ADSL employs forward error correction, which enables the receiving end to not only detect, but tocorrect errors in the transmitted data, thus dramatically reducing the effects of burst noise. Theforward error correction was included to facilitate such real time applications as digital video. Error

1.5-6.1 MBPS

16-640 KBPSPOTSSplitter

POTSSplitter

Service Provider End User

Figure 1 - Simple ADSL Schematic

Telephone Service

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Data Rate /MBPS Wire Gauge /AWG Wire Size /mm Distance /km

1.5 or 2 24 0.5 5.5

1.5 or 2 26 0.4 4.6

6.1 24 0.5 3.7

6.1 26 0.4 2.7

Table 3 - ADSL Performance / Distance Table

correction on a symbol by symbol basis also reduces errors caused by continuous noise coupledinto the line.

How ADSL WorksTo the user, an ADSL modem looks deceptively simple - a ‘black box’ giving synchronous datapipes at various data rates, over ordinary telephone type copper lines. The actual inner working ofthe modem relies on sophisticated digital signal processing and a number of rather creativealgorithms! Since copper cable lines tend to attenuate signals at 1 MHz (the outer edge of theADSL band) by as much as 90dB, the analogue sections of the modem especially, have to workhard to maintain the large dynamic ranges, separate channels, and low noise figures, required totransmit such high bandwidths.

To provide the separate channels, the modem divides up the available bandwidth in one of twopossible ways:

� Frequency Division Multiplexing (FDM): FDM works by assigning one band of frequencyto the upstream data, and another separate band to the downstream data. The downstreampath may then further subdivided, using Time Division Multiplexing (TDM) into one ormore high-speed channels and a corresponding number of low speed channels, giving theillusion of multiple channels over a single connection. Likewise, the upstream path wouldalso multiplexed into corresponding low speed channels. Figure 2 illustrates the FDMbandwidth.

� Echo Cancellation: Echo cancellation assigns the upstream band to overlap the downstreamband, and separates the two using a method known as local echo cancellation, an establishedtechnique used in V.32 and V.34 analogue modems. Figure 3 illustrates echo cancellationbandwidth division.

Figure 2 - FDM Bandwidth Division

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Using either technique the overall net effect is N high-speed downstream channels, and N lowspeed duplex channels, and a further 4 kHz region on the DC end of the band, set aside for theordinary telephone service.

Since ADSL is an asymmetric connection, the modems at each end are of different types. The userend modem is called an ATU-R (ADSL Transceiver Unit), while the service provider’s end iscalled an ATU-C. A simple schematic of a generic ATU-C is shown in Figure 4 1

ADSL Line ModulationThere are two methods for encoding the data for carrying down the copper cable2. These are DMT(Discrete Multi-Tone) and CAP (Carrierless AM/PM). CAP was the first of the two methods to beimplemented, but this was before the technology was standardised. The ANSI standards body 3

1For a more detailed schematic of the individual units, consult the article ADSL by Kimmo K. Saarela of TampereUniversity of Technology’s Telecommunications Laboratory2 Known as modulating the signal3 ANSI/T1E1.4/94-007, Asymmetric Digital Subscriber line (ADSL) Metallic Interface

Figure 3 - Echo Cancellation Bandwidth Division

D/A & A/DLine

Coupler &Channel

Separator(FD orecho)

POTSSplitter

Receiver

Multiplexor& ErrorControl

Demultiplexor& Error Control

Transmitter

Figure 4 - ADSL Transceiver Schematic

send

receive

Copper line

Phone line

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decided, rather, to go with the DMT technology, so that it what I will be discussing in thisdocument.

Basically, DMT tries to split up the available bandwidth into a set of smaller subchannels. DMTthen dynamically allocates data to each subchannel, such that the overall data throughput isconstantly maximised. Thus, if some particular subchannel is particularly ‘noisy’, it can beallocated less data to carry, and more data pumped into one of the less ‘noisy’ subchannels. Figure5 shows a set of diagrams that show how the available bandwidth would be allocated by DMTunder particular noise conditions (represented in the second column of Figure 5).

In order to gauge the conditions on the line, the ATU-C modem initially transmits an equal amountof data per tone (the first column of Figure 5). This signal is then received at the end user by theATU-R modem, which processes the signal, and works out an optimised distribution (the thirdcolumn of Figure 5). This optimal distribution is then sent back to the ATU-C using the samephone line, but at a much lower, more secure speed.

The current ADSL-DMT standard calls for the downstream channel to be divided up into 256 4kHz-wide subchannels, and the upstream channels, into 32 subchannels 4

ADSL Data FramesADSL uses a superframe structure whereby each separate data packet is broken up into 68 ADSLdata frames each encoded and modulated into 4 kHz DMT channels, as dictated by the copper line.

Eight bits of each superframe is reserved for the CRC check. A further 24 ‘indicator’ bits are usedfor other assorted control information. There are also two separate data buffers, the fast data bufferand the interleaved data buffer with each users data stream being assigned to either fast orinterleaved buffers during initialisation.

ADSL also uses forward error correction to ensure optimal performance, especially for time criticaldata like real-time video. The ADSL specification demands that this error correction is enforcedand the method used is based on Reed-Solomon coding. The actual operation of this error checkingalgorithm is transparent to the user and fairly complicated, so I will not go into it here 2

4 For more specifics on the actual frequencies used for the carrier, etc. consult the article by Kimmo K. Saarela.

Figure 5 - DMT Example

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The Future of ADSLSeveral ADSL trails are currently underway in the USA and Canada where people are beingallowed to connect to their local ISP’s through ISP supplied ADSL hardware. These trials have thusfar produced very favourable results, aside from the fact that the rest of the Internet is not reallyready to handle speeds of 6 Mbps to the users desktop.

ADSL is fully capable of handling ATM traffic, and there is already a standard, produced by theADSL forum in association with the ATM forum for doing just this. The document describing thestandard can be found at http://www.adsl.com/adsl_atm.html. There are also other interestingdevelopments that have arisen lately in the ADSL field. Dial up ADSL, for instance, solves theproblem that many service providers have with requiring a dedicated ATU-C for each customersubscriber, which is a waste of resources since the user will, most probably, not be online all thetime. The company NetSpeed has developed what it calls ‘Dial Up ADSL’ to help with this. Dialup ADSL works in a similar way to normal telephone lines where a modem waits for a dial tone,dials the end modem and waits for the other end to ‘pick up’. In this scheme, the ATU-C sends the

Figure 6 - ADSL System Diagram

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dial tone, which the ATU-R needs to hear in order to connect. In this way, several ATU-R’s can beserviced by a single ATU-C.5

Figure 6 shows view that most of the ADSL vendors have of the future of home networking.Basically, they envision fibre optic lines into central ‘neighbourhood’ hubs, from which point, theysplit off into copper line ADSL links to the people’s homes or businesses. With this muchbandwidth into the home, it is also feasible to deliver such services as Video On Demand and cabletelevision, all over a single copper line.

Some Concrete DataBy way of ending off this section, I have included table 4, which lists some download times forcertain sized files, over ADSL, as a measure of the actual performance that the end user may expectfrom such a system:

File Types File Size/ Mbits

56 KBPS 64 KBPS 384 KBPS 1536 KBPS 2048 KBPS

Digitised Photo 1 17.9 sec 15.6 sec 2.6 sec 0.7 sec 0.5 sec

CAD/CAM File 2 35.7 sec 31.2 sec 5.2 sec 1.3 sec 0.9 sec

CT Scan 5.2 1.5 min 1.4 min 13.5 sec 3.4 sec 2.5 sec

X-Ray 40 11.9 min 10.4 min 1.7 min 26 sec 19.5 sec

Bulk File 500 2.5 hrs 2.2 hrs 21.7 min 5.4 min 4 min

Table 4 - File Transfer Times

VDSL (Very high data rate Digital Subscriber Line)The next step in the line of xDSL products is VDSL. The maximum downstream rate for VDSL isbetween 51 and 55 Mbps, over distances of up to 300m, with the bandwidth dropping to 13 Mbpsfor distances up to 1500m. The ADSL, the upstream rates will, at least initially anyway, be smaller,in the region of 1.6 to 2.3 Mbps. As with ADSL, the data channels will be separated in frequencyfrom the normal telephone channel, but there will also be provision for a frequency separated ISDNchannel, allowing telephone companies to overlay VDSL onto their existing networks, withminimal disruption. The projected capabilities of VDSL are listed in table 5:

Upstream Rates / MBPS Downstream Rates / MBPS Effective Distance / m

1.6 – 2.3 12.96 – 13.8 1500

19.2 25.92 – 27.6 1000

Equal to downstream 51.84 – 55.2 300

Table 5 - VDSL Capabilities

VDSL is very similar to ADSL in the way it works and will include forward error correction. As faras modulating code schemes go, VDSL has four possible candidates; CAP, DMT, DWMT

5The specifics of this scheme can be found on the NetSpeed homepage under http://www.netspeed.com/offhook.html

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(Discrete Wavelet Multitone) and SLC (Simple Line Code). For more specifics on the operation ofVDSL, the ADSL forum offers a VDSL tutorial at http://www.adsl.com/vdsl_tutorial.html. TheVDSL standard is currently under discussion by five of the major standards bodies, namely the U.S.ANSI Standards Group (T1E1.4), the European Telecommunications Standards Institute (ETSI),the Digital Audio-Visual Council (DAVIC), the ATM Forum and the ADSL Forum. As such, noneof these bodies has produced a concrete set of specifications, but there are already companiesworking on producing VDSL products.

ConclusionxDSL is by no means the one and only product in the market for copper access technologies. Cablemodems, which operate over fibre optic lines installed for private television stations, and also wellrepresented, especially in the United States, where, it is estimated, 1 in every 3 houses are alreadyconnected to the cable system. There are many problems with cable modems though, and even inthe U.S. it is thought that ADSL will dominate.

ADSL is certainly the cheapest option for connecting high-speed data links into individual houses.While it is by no means the most desirable option, fibre optic cables to every household andbuilding are still a long way off, and in the mean time, we require the connectivity offered by suchsystems NOW! Certainly, in an African perspective, the prospect of linking all buildings withcopper wire is certainly a more appealing prospect! ADSL offers high-speed connectivity, for areasonable price, over existing cables. For the moment, at least, that is exactly what the worldrequires.

References� ADSL Forum: ADSL Tutorial (Twisted Pair Access to the Information Highway),

http://www.adsl.com/adsl_tutorial.html� ADSL Forum: ADSL Forum TR-002 (ATM over ADSL Recommendation),

http://www.adsl.com/adsl_atm.html� ADSL Forum: General Introduction to Copper Access Technologies,

http://www.adsl.com/general_tutorial.html� ADSL Forum: VDSL Tutorial (Fibre – Copper Access to the Information Highway),

http://www.adsl.com/vdsl_tutorial.html� Westell: ADSL System Diagram, http://www.westell.com/adslDiag.html� Kimmo K. Saarela, Tampere University of Technology (Telecommunications Laboratory),

ADSL.� Imagen Communications: Large File Transfer Times,

http://www.imagen.net/adsl/transfers.html� NetSpeed: Dial Up ADSL, http://www.netspeed.com/offhook.html� PairGain: CopperOptics (Enhancing the Performance of Copper Cable with HDSL),

http://www.pairgain.com/copperop.html