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`Introduction to Copper Access Technologies and ADSL
`
`Distributed Multimedia Course Notes
`
`Michael Rorke, Computer Science Honours, Rhodes University, 1997
`
`Introduction
`With the advent of the Internet, more and more people want to connect their computers up to other
`computers. 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 major
`telephone 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 a
`different letter to denote a different version of the product. For example, the first product was called
`HDSL (High data rate Digital Subscriber Line), followed closely by the main focus of this
`document, ADSL (Asymmetric Digital Subscriber Line), and then by an emerging technology
`VDSL (Very high data rate Digital Subscriber Line). The original research into what are now
`known as xDSL modems actually began long ago and these types of modems have been in use for
`many years as part of the core switching networks of many of the major world telcos. But, whereas
`before they were used to carry mainly voice type traffic, they are now being adapted to the needs of
`data traffic carriers. Table 1 lists the main forms of xDSL and the capabilities of each:
`
`Name
`
`V.22, V.32,
`V. 34
`
`Meaning
`
`Data Rate
`
`Mode
`
`Applications
`
`Voice band modems
`
`1200 BPS to
`28,800 BPS
`
`Duplex
`
`Data Communications
`
`DSL
`
`HDSL
`
`SDSL
`
`ADSL
`
`VDSL
`
`Digital Subscriber Line
`
`160 KBPS
`
`High data rate Digital
`Subscriber Line
`
`1.544 MBPS
`2.048 MBPS
`
`Duplex
`
`Duplex
`
`Single line Digital
`Subscriber Line
`
`Asymmetric Digital
`Subscriber Line
`
`1.544 MBPS
`2.048 MBPS
`
`1.5 – 9 MBPS
`16 – 640 KBPS
`
`Duplex
`
`Down Up
`
`ISDN, voice and data services
`
`T1/E1 service, feeder plant,
`WAN/LAN access and server
`access
`
`Same as HDSL, plus premises
`access for symmetric services
`
`Internet access, Video On
`Demand, simplex video, remote
`LAN access, interactive
`multimedia
`
`Very high data rate Digital
`Subscriber line
`
`13 – 52 MBPS
`1.5 – 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 Networks
`The reason that xDSL technologies are able to provide a much better service than voice grade
`analogue modes is due mainly to the layout of the copper cable system. Voice grade modem
`operate over the normal telephone switching system, with signals travelling from the site of the
`sending node, into the core network of the telephone company, and from there, out again to the
`destination node, with the signals treated the same as normal voice signals. This is an advantage in
`that almost everywhere in the world is linked via the telephone system. However, the core network
`imposes certain bandwidth limitations on signals coming into it. For instance, there are filters at the
`edge 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 copper
`to those shown in table 2:
`
`Access Type
`
`Speed /MBPS
`
`Usable Distance /km
`
`DS1 (T1)
`
`E1
`
`DS2
`
`E2
`
`¼ STS-1
`
`½ STS-1
`
`STS-1
`
`1.544
`
`2.048
`
`6.312
`
`8.448
`
`12.960
`
`25.920
`
`51.840
`
`5.5
`
`5
`
`3.5
`
`3
`
`1.3
`
`0.9
`
`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 world
`you 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, which
`contain loading coils, making them useless for any form of xDSL technology, unless the loading
`coils are removed. There are moves underway in many countries to try and lower the length of the
`subscriber loop with projects such as the USA’s ‘fibre to the curb’, where the idea is to lay fibre
`optic lines into a central point in every neighbourhood, from which the individual subscribers can
`be fed off with copper cable. This trend to shorten the length of the subscriber loop is going a long
`way to assist the introduction of copper access technologies, which typically only operate (at high
`data rates) for distances less than 6 km.
`
`The History of the xDSL Modem
`
`DSL or Digital Subscriber Line
`Before going on, there is an important distinction that needs to be made between the name digital
`subscriber 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 line
`itself, which is plain old Unshielded Twisted Pair (UTP).
`The original DSL was the modem used to implement Basic Rate ISDN. A generic DSL transmits
`data in both directions simultaneously i.e. full duplex, at 160 KBPS over copper lines up to 5.5 km
`of 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 from
`0 to about 80 kHz, and thus were not able to provide the simultaneous plain old telephone service
`that is a feature of xDSL modems i.e. the ability to use the copper line for both voice and data
`simultaneously and independently of each other.
`While the specifications of the DSL modem may seem trivial by today’s standards, at the time of
`their release they were fairly revolutionary and are, in fact, still in use today, mainly in what are
`termed pair-gain applications.
`
`T1 and E1
`The engineers at Bell labs created a multiplexing system, which send digitised voice through 24
`framed 64 KBPS streams. The resulting frame was 193 bits long and created an equivalent data rate
`of 1.544 Mbps. This signalling method was named DS1, which was later expanded to T1 and
`includes different framing methods, etc. This was in wide use in the mid-1970’s. In Europe, a
`modified T1 method was developed using 30 voice channels and giving an equivalent data rate of
`2.048 Mbps. T1 and E1 circuits require a repeater 900 m from the node, and further repeaters for
`every 1800 m thereafter, making them expensive and maintenance intensive. Despite this, they
`were, and still are, widely implemented, although they are being phased out, and replaced with
`HDSL 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 and
`operates without the need for costly repeaters. The idea behind HDSL was simply to develop a
`method 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 was
`used for all new T1/E1 links. Today, HDSL is used mainly to provide advanced digital services to
`local loop customers and corporate end users. HDSL works by creating a mathematical model of
`the noise characteristics over the copper wire, allowing the transmitting device to precisely
`compensate for copper-based distortion. This adjustment occurs dynamically all the time, allowing
`the 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 technology
`to provide fibre optic level network technologies over plain copper wires.
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`ADSL (Asymmetric Digital Subscriber Line)
`
`An Introduction to ADSL
`ADSL followed soon after the deployment of HDSL. ADSL is specifically tailored as a last leg link
`into customer premises. As its name suggests, the link provided by an ADSL modem is asymmetric
`i.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. The
`reasoning behind this asymmetric system is that the amount of information coming into the end
`users node is greater than the amount of information going out of the end users node, to the rest of
`the network. Studies on end user nodes running TCP/IP networking applications showed that the
`actual ratio of incoming to outgoing data, was often as high as 10:1!
`
`Specifications of ADSL
`An ADSL circuit is a point to point link connecting two nodes over a single twisted pair copper
`cable. This cable must be a dedicated line, and a normal telephone cable i.e. going through an
`exchange, will not suffice. ADSL provides a dedicated line type of connection. The ADSL modem
`creates 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).
`
`Service Provider
`
`Telephone Service
`
`1.5-6.1 MBPS
`
`End User
`
`POTS
`Splitter
`
`16-640 KBPS
`
`POTS
`Splitter
`
`Figure 1 - Simple ADSL Schematic
`
`The ordinary telephone channel (or Plain Old Telephone Service, POTS) is split off from the rest of
`the digital modem channels by means of filters in order to guarantee that, even if the modem should
`fail, 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 duplex
`channel bandwidth ranges from 16 to 640 KBPS. Each of these channels can be submultiplexed to
`form several smaller channels if required.
`The actual downstream data rate depends on a number of factors such as the length of the copper
`line, its wire gauge, the presence of bridged taps and cross-coupled interference. Ignoring the
`effects 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 to
`correct errors in the transmitted data, thus dramatically reducing the effects of burst noise. The
`forward error correction was included to facilitate such real time applications as digital video. Error
`
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`Data Rate /MBPS Wire Gauge /AWG
`
`Wire Size /mm
`
`Distance /km
`
`1.5 or 2
`
`1.5 or 2
`
`6.1
`
`6.1
`
`24
`
`26
`
`24
`
`26
`
`0.5
`
`0.4
`
`0.5
`
`0.4
`
`5.5
`
`4.6
`
`3.7
`
`2.7
`
`Table 3 - ADSL Performance / Distance Table
`correction on a symbol by symbol basis also reduces errors caused by continuous noise coupled
`into the line.
`
`How ADSL Works
`To the user, an ADSL modem looks deceptively simple - a ‘black box’ giving synchronous data
`pipes at various data rates, over ordinary telephone type copper lines. The actual inner working of
`the modem relies on sophisticated digital signal processing and a number of rather creative
`algorithms! Since copper cable lines tend to attenuate signals at 1 MHz (the outer edge of the
`ADSL band) by as much as 90dB, the analogue sections of the modem especially, have to work
`hard to maintain the large dynamic ranges, separate channels, and low noise figures, required to
`transmit such high bandwidths.
`To provide the separate channels, the modem divides up the available bandwidth in one of two
`possible ways:
` Frequency Division Multiplexing (FDM): FDM works by assigning one band of frequency
`to the upstream data, and another separate band to the downstream data. The downstream
`path may then further subdivided, using Time Division Multiplexing (TDM) into one or
`more high-speed channels and a corresponding number of low speed channels, giving the
`illusion of multiple channels over a single connection. Likewise, the upstream path would
`also multiplexed into corresponding low speed channels. Figure 2 illustrates the FDM
`bandwidth.
` Echo Cancellation: Echo cancellation assigns the upstream band to overlap the downstream
`band, and separates the two using a method known as local echo cancellation, an established
`technique used in V.32 and V.34 analogue modems. Figure 3 illustrates echo cancellation
`bandwidth division.
`
`Figure 2 - FDM Bandwidth Division
`
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`Figure 3 - Echo Cancellation Bandwidth Division
`
`Using either technique the overall net effect is N high-speed downstream channels, and N low
`speed duplex channels, and a further 4 kHz region on the DC end of the band, set aside for the
`ordinary telephone service.
`Since ADSL is an asymmetric connection, the modems at each end are of different types. The user
`end modem is called an ATU-R (ADSL Transceiver Unit), while the service provider’s end is
`called an ATU-C. A simple schematic of a generic ATU-C is shown in Figure 4 1
`
`send
`
`Multiplexor
`& Error
`Control
`
`Transmitter
`
`receive
`
`Demultiplexor
`& Error Control
`
`Receiver
`
`D/A & A/D
`Line
`Coupler &
`Channel
`Separator
`(FD or
`echo)
`
`POTS
`Splitter
`
`Copper line
`
`Figure 4 - ADSL Transceiver Schematic
`
`Phone line
`
`ADSL Line Modulation
`There 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 be
`implemented, 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 Tampere
`University of Technology’s Telecommunications Laboratory
`2 Known as modulating the signal
`3 ANSI/T1E1.4/94-007, Asymmetric Digital Subscriber line (ADSL) Metallic Interface
`
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`decided, rather, to go with the DMT technology, so that it what I will be discussing in this
`document.
`Basically, DMT tries to split up the available bandwidth into a set of smaller subchannels. DMT
`then dynamically allocates data to each subchannel, such that the overall data throughput is
`constantly maximised. Thus, if some particular subchannel is particularly ‘noisy’, it can be
`allocated less data to carry, and more data pumped into one of the less ‘noisy’ subchannels. Figure
`5 shows a set of diagrams that show how the available bandwidth would be allocated by DMT
`under particular noise conditions (represented in the second column of Figure 5).
`
`Figure 5 - DMT Example
`
`In order to gauge the conditions on the line, the ATU-C modem initially transmits an equal amount
`of data per tone (the first column of Figure 5). This signal is then received at the end user by the
`ATU-R modem, which processes the signal, and works out an optimised distribution (the third
`column of Figure 5). This optimal distribution is then sent back to the ATU-C using the same
`phone 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 4
`kHz-wide subchannels, and the upstream channels, into 32 subchannels 4
`
`ADSL Data Frames
`ADSL uses a superframe structure whereby each separate data packet is broken up into 68 ADSL
`data 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 used
`for other assorted control information. There are also two separate data buffers, the fast data buffer
`and the interleaved data buffer with each users data stream being assigned to either fast or
`interleaved buffers during initialisation.
`ADSL also uses forward error correction to ensure optimal performance, especially for time critical
`data like real-time video. The ADSL specification demands that this error correction is enforced
`and the method used is based on Reed-Solomon coding. The actual operation of this error checking
`algorithm 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.
`
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`The Future of ADSL
`Several ADSL trails are currently underway in the USA and Canada where people are being
`allowed to connect to their local ISP’s through ISP supplied ADSL hardware. These trials have thus
`far produced very favourable results, aside from the fact that the rest of the Internet is not really
`ready 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 the
`ADSL forum in association with the ATM forum for doing just this. The document describing the
`standard can be found at http://www.adsl.com/adsl_atm.html. There are also other interesting
`developments that have arisen lately in the ADSL field. Dial up ADSL, for instance, solves the
`problem that many service providers have with requiring a dedicated ATU-C for each customer
`subscriber, which is a waste of resources since the user will, most probably, not be online all the
`time. The company NetSpeed has developed what it calls ‘Dial Up ADSL’ to help with this. Dial
`up 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 be
`serviced 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, they
`split off into copper line ADSL links to the people’s homes or businesses. With this much
`bandwidth into the home, it is also feasible to deliver such services as Video On Demand and cable
`television, all over a single copper line.
`
`Some Concrete Data
`By way of ending off this section, I have included table 4, which lists some download times for
`certain sized files, over ADSL, as a measure of the actual performance that the end user may expect
`from such a system:
`File Types
`File Size
`/ Mbits
`
`1536 KBPS
`
`2048 KBPS
`
`56 KBPS
`
`64 KBPS
`
`384 KBPS
`
`Digitised Photo
`
`CAD/CAM File
`
`CT Scan
`
`X-Ray
`
`Bulk File
`
`1
`
`2
`
`5.2
`
`40
`
`500
`
`17.9 sec
`
`35.7 sec
`
`1.5 min
`
`11.9 min
`
`2.5 hrs
`
`15.6 sec
`
`31.2 sec
`
`1.4 min
`
`10.4 min
`
`2.2 hrs
`
`2.6 sec
`
`5.2 sec
`
`13.5 sec
`
`1.7 min
`
`21.7 min
`
`0.7 sec
`
`1.3 sec
`
`3.4 sec
`
`26 sec
`
`5.4 min
`
`0.5 sec
`
`0.9 sec
`
`2.5 sec
`
`19.5 sec
`
`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 is
`between 51 and 55 Mbps, over distances of up to 300m, with the bandwidth dropping to 13 Mbps
`for 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 frequency
`from the normal telephone channel, but there will also be provision for a frequency separated ISDN
`channel, allowing telephone companies to overlay VDSL onto their existing networks, with
`minimal disruption. The projected capabilities of VDSL are listed in table 5:
`Upstream Rates / MBPS Downstream Rates / MBPS
`Effective Distance / m
`
`1.6 – 2.3
`
`19.2
`
`Equal to downstream
`
`12.96 – 13.8
`
`25.92 – 27.6
`
`51.84 – 55.2
`
`1500
`
`1000
`
`300
`
`Table 5 - VDSL Capabilities
`VDSL is very similar to ADSL in the way it works and will include forward error correction. As far
`as 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 of
`VDSL, the ADSL forum offers a VDSL tutorial at http://www.adsl.com/vdsl_tutorial.html. The
`VDSL 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, none
`of these bodies has produced a concrete set of specifications, but there are already companies
`working on producing VDSL products.
`
`Conclusion
`xDSL is by no means the one and only product in the market for copper access technologies. Cable
`modems, which operate over fibre optic lines installed for private television stations, and also well
`represented, especially in the United States, where, it is estimated, 1 in every 3 houses are already
`connected to the cable system. There are many problems with cable modems though, and even in
`the 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 and
`building are still a long way off, and in the mean time, we require the connectivity offered by such
`systems NOW! Certainly, in an African perspective, the prospect of linking all buildings with
`copper wire is certainly a more appealing prospect! ADSL offers high-speed connectivity, for a
`reasonable price, over existing cables. For the moment, at least, that is exactly what the world
`requires.
`
`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
`
`
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