high-speed copper access: a tutorial overview

Hicrh-sDeed comer access: a tutorial overview

by 1. K. Czajkowski

Telecommunication companies (telcos) are beginning to exploit further their existing access networks by using new technologies capable of delivering data a t up to

30 Mbit/s over unshielded twisted copper wire pairs. This tutorial paper provides a high-level overview of the key issues related to these digital subscriber line (DSL) technologies, including the environment in which they must operate, line codes,

standards, and the current state of the market.

1 Introduction

In recent years we have seen a phenomenal growth in the Internet. Not only has the number of users grown, but ever increasing multimedia content (such as pictures, video- clips and audio) on the Internet means that these users are accessing ever more data. This is therefore driving a demand for higher access speeds to the Internet. A wealth of new competitive service and access providers compete to secure access to customers. As new channels for data- access, such as h e d wireless, broadband satellite and coax cable-modem access, emerge, so there has been an ever increasing demand to deliver higher speeds through existing telephone networks (i.e. those belonging to the telcos) to existing customers. Telcos are now looking for technologies capable of delivering high-speed data over networks often designed a hundred years ago to carry only voice with a bandwidth of less than 4 kHz. This has led to the development of a family of digital subscriber line (DSL) technologies, collectively referred to as xDSL, capable of delivering up to tens of megabits per second over existing unshielded twisted pair (UTP) telco networks.

The scope of this tutorial paper is limited to the xDSL high-speed copper access technologies suitable for access over twisted pair networks, and does not include the cable- modems operating over coax networks, which face very different technical issues.

2 xDSL overview

Drivers for high-speed access The 1990s have witnessed a move towards interactive

applications accessible from the home. Emerging applications have illustrated more and more a convergence of entertainment and information. For example, TV cards are now available for PCs, whilst WebTV gives Internet access through aTV.

In the early to mid 1990s there was significant expectation placed on video on demand (VoD) to the home

as a ‘killer’ application for broadband. It seemed very reasonable that with newly available enabling digital technologies, we would all soon want to have access to a network that gave us TV and video entertainment with a ‘what we wanted when we wanted it’ philosophy. This interactive TV market was an initial ‘raison d’2tre’ for ADSL (asymmetric digital subscriber line), a technology capable of delivering data over existing telephone networks at speeds of several megabits per second. Unfortunately, this market never really materialised. A large number of VoD trials failed to justlry the business cases on which the broadband networks were predicated. One view is that in those ‘early’ broadband days, technologies such as ADSL were pushing the market, rather than the market requirements pulling the technologies. What is certain, however, is that the false start for broadband around 1995 1eftADSLlooking for new market drivers.

The past three years have seen an explosion in both the size and the popularity of the ‘Internet’. What only a few years ago was a difficult to navigate and sporadic pool of diverse information has blossomed into a more navigable and complete system that actually enhances both work and leisure activities. Key behind the success has been the massive growth in the use of the Internet as a commercial marketplace. Almost every major company is now represented on the Internet and, most importantly, it is becoming a major trading channel. An often quoted example’ is that Dell Computers sell some US$5 million of PC equipment daily through the Internet.

It is fair to say that the Internet, together with all that it promises to deliver, is today almost single-handedly driving the need for faster and faster access to residential customers. Additionally, for business customers, both intranet (private network) and Internet access demands high-speed access to the business premises. Already in excess of 40% of homes in the USA (source: IDC) have a PC, and 98% have a TV. More than 20% of US homes have Internet access, and it is estimated that around 15% use the Internet ‘seriously’. Most of these users are currently

ELECTRONICS & COMMUNICATION ENGINEERING JOURNAL JUNE 1999 125

2B1Q 4B3T ADSL ADSL-Lite AM ANSI ATM ATM-25 AWG BB CAP CDSL CLEC CM CO CPE CT1

DAVIC DFE DLC DM DMT DSL DWMT E l EMC ETSI

FCC FDD FEC FEXT FFT FIR FSAN FlTB FlTC FTTCab FlTEx ETTH FTTK Gx HDSL HF HFC HSCA IDC IEC IFFT

ILEC

~~

Abbreviations

= a line code = a line code = asymmetric digital subscriber line = limited-feature version of ADSL = amplitude modulation =American National Standards Institute = asynchronous transfer mode = ATM Forum 25 Mbit/s interface = American wire gauge = broadband = carrierless amplitude and phase = customer digital subscriber line = competitive local exchange camer = common mode = central office = customer premises equipment = Cordless Telephone standard 1

= Digital Audio-visual Council = decision feedback equaliser = digital loop carrier = differential mode = discrete multitone = digital subscriber loop = discrete wavelet multitone = 2 Mbit/s European transport unit = electromagnetic compatibility = European Telecommunications

= Federal Communications Commission = frequency division duplex = forward error correction = far-end crosstalk = fast Fourier transform = finite impulse response = full services access network = fibre to the building (or basement) = fibre to the curb (same as F'ITK) = fibre to the cabinet = fibre to the exchange = fibre to the home = fibre to the kerb = group of partnering telcos = high-speed digital subscriber line = high frequency = hybrid fibre coax = high-speed copper access = International Data Corporation = inter exchange carrier = inverse fast Fourier transform

(frequency to time domains) = incumbent local exchange carrier

(analogue)

Standards Institute

IP ISDN ISP ITU-T

MC LAN LEC LTE LW MDF MPEG1

Mw NB NEXT NT NTE ONU PAM PC PON POTS PSD PTT

QAM QoS RADSL RBOC RF RFI RMS RS SDSL

SNR SSB STB SU-32 sw T1

TDD Tv UAWG UN1 UTP VDSL VoD xDSL

= Internet Protocol = Integrated Services Digital Network = internet service provider = International Telecommunications

= inter exchange carrier = local-access network = local exchange carrier = line terminating equipment = long wave = main distribution frame = Motion Picture Experts Group standard

for video compression = medium wave = narrowband = near-end crosstalk = network termination = network termination equipment = optical network unit = pulse amplitude modulation = personal computer = passive optical network = plain old telephone service = power spectral density = post, telephone and telegraph

(European incumbent telcos) = quadrature amplitude modulation = quality of service = rate-adaptive ADSL = Regional Bell Operating Company = radio frequency = radio frequency interference = root mean square = Reed-Solomon = symmetric digital subscriber line

(HDSL for 2-pairs) = signal-to-noise ratio = single sideband = set-top box = a line code = short wave = 1.5 Mbit/s North American

transmission unit =time division duplex = television = Universal ADSL Working Group = user-network interface = unshielded twisted pair = very-high-speed digital subscriber line =video on demand = collective group of digital subscriber

loop technologies

Union

126 ELECTRONICS & COMMUNICATION ENGINEERING JOURNAL JUNE 1999

connected through analogue modems running at speeds between 28-8 kbit/s and 56.6 kbit/s. The ever increasing multimedia-rich content of the Internet makes access at higher speeds not only desirable but ultimately a necessity. Most users with an analogue dial-up modem are desperate for (i.e. are willing to pay for!) higher bandwidths. The growth in the Internet coupled with an expected PC penetration of greater than 60% by the year 2002 is the primary driver behind the recent growth of and extraordinary interest in ADSL, and to an extent VDSL (very high-speed digital subscriber line).

How fast is fast enough? Having established that there is a driver for high-

speed to the Internet, the natural question is then ‘how fast is fast enough?’. There are a number of digital modem technologies emerging, capable of delivering data to the home at speeds up to 50 Mbit/s. To illustrate the impact that a fast modem will have compared with the analogue modems widely in use today, compare the times taken to download a file of 10 Mbytes (which might represent an MPEG-1 video clip lasting around one and a half minutes, or a graphics-rich presentation slide-pack) . Downloading this file would take around 46 minutes with a 28.8 kbit/s analogue modem and around 23 minutes with a 56.6 kbit/s modem, ignoring protocol overheads. It is clear that with the modem technologies widely deployed today ‘we’re sucking information through a straw when what we really need is a fire hose’2. Even with ISDN (Integrated Services Digital Network) operating at 128 kbit/s the file would take around 10 minutes to download. The recently proposed ‘ADSL-Lite’ technology running at around 1.5 Mbit/s would give you the file in under 1 minute, and ADSL at 6 Mbit/s would transfer it in around 13 seconds. VDSL (very high speed digital subscriber line) operating at, for example, 26 Mbit/s would require only 3 seconds to download the file. For those of us who have experienced using analogue modems to transfer large files, the prospect of near instantaneous file transfer represents Internet heaven. However, it can be argued that all the xDSL technologies represent a significant improvement in the quality of access. Compared with 46 minutes, 1 minute seems virtually instantaneous. Added to this, their ability to carry out the transfer without disabling access to the telephone for normal (narrowband) calls is a significant advantage.

The above example is just for one application. The possible service requirements placed on xDSL depend very much on the country, and reflect the local competitive environment. For example, in one

simultaneous video channels to compete with cable companies. In another, VDSL may be ideally suited to providing a high-bandwidth pipe to be shared by multiple users at a small office.

Factors driving the reuse of telco copper The telco copper access networks are but one of a

number of competing access channels for higher speed data access that are or will be available to the customer. It is this competition, enabled by deregulation (e.g. the 1996 Telecoms Reform Act in the USA), that is the main driver moving telcos towards broadband. Also, there is a new form of competition in unbundling of the local loop, where telcos are faced with providing access to competitive access providers such as CLECs (competitive local exchange carriers) on equal terms with their own businesses.

For the incumbent telcos, such as the incumbent local exchange carriers (ILECs) in North America and PTTs in Europe, the route to broadband is (certainly for the medium-term) to maximise the reuse of the copper metallic access network that it already has in the ground. There are some 790 million copper pair loops installed in the world, and this copper infrastructure represents the telcos’ key asset. It is already there, and therefore it would seem sensible to reuse it as much as possible. Although many broadband architectures require optical fibre to be brought at least some of the way towards the home from the exchange, bringing the fibre increasingly closer to the home becomes prohibitively expensive. The cost of fibre deployment is dominated not by the material costs but by the cost of civil works (i.e. the digging of the road/ pavement and the laying of the fibre) - a cost which can range between E25 and in excess of &75 per metre, depending on the circumstances. An added incentive for re-using the installed copper is that it derisks the route to broadband by employing a modular and evolvable deployment strategy, without placing all the risk on day one.

overhead distr

overhead drop

/50 rn

may be suited to a telco wishing to deliver multiple

Fig. 1 from a typical customer.

Access architecture for a typical European telco. The distances are those


exchanoe or cabinet

e a 4 existing hqme wiring

narrowband network

NI3 (POTS) line card L

a \frequency

NB+xDSL network n splitter

termination e q u i p m e Y Q A

line termlnatlng e g active NT, with new home wiring carrying Ethernet or ATM-25

Fig. 2 Schematic illustration of the principle of xDSL as an overlay technology, where broadband services are delivered over the same copper access pairs as the existing narrowband services

The route for reuse of the existing copper to deliver high-speed data services using xDSL technologies has led to the now familiar phrases ‘copper may be buried, but its not dead’, and ‘making the copper sweat’. The latter refers to the increasing closeness to the maximum achievable bandwidth that xDSL technologies transmit over the copper.

Overview of the telco access architecture In order to understand the proposed architectures for

xDSL in the access network, a cursory understanding of the typical telco access network is required.

Fig. 1 shows a schematic diagram of a typical European telco network. It is important to understand that the specifics of the network will vary from telco to telco.

Customers (residential or business) are connected via unshielded twisted pair copper wires to a central exchange (central office (CO) in North America) where they are connected to a switch via a main distribution frame (MDF) . Fig. 3 Schematic depiction of different xDSL deployment architectures: FlTH, F l T K , FlTCab, FTTB and FTTEx

The total length of the copper loop from the exchange to the customer will depend on the telco, the demographics of the environment and the location of the customer. However, it is typically less than 4 or 5 km, a limitation derived from the lengths of copper cable over which narrowband telephony could be delivered. The copper loop is typically made up of a number of separately joined sections, running in bundles with other copper pairs beside them. A typical bundle may contain 50 pairs. The copper pairs are typically around 0.3 to 0-5 mm in diameter, with the thinner gauges near the exchange.

There are a number of key ‘flexibility points’ in the network, where cross-connection is made between pairs in bundled lengths of cable. Most customers will be on a loop that passes through a main cross-connect, usually sited in a metallic cabinet by the road. Some will also pass through a second cross-connect cabinet. The cabinets are typically less than 1.5 km from the customers. The network between the exchange and the cabinet is typically called the feeder network, and the last dendritic part of the access network between the cabinet flexibility point and the customer is called the distribution network. Additionally, there is often also a flexibility point nearer the customer (e.g. a junction box, often underground), usually within 300 m of the customer.

An important feature of the access network is that the copper pairs can be laid either underground or strung overhead between poles. The latter is often the case for the final-drop part of the loop, typically covering the last 50 m or less. However, it is also not uncommon to have overhead distribution all the way from the cabinet to the customer.

4 c6 km + L

c300m -

FTTK BB

BB = broadband NB = narrowband


Table 1: Overview of the key features and attributes of HDSL, ADSL and VDSL, including ISDN for comparison

Standard

Spectral range

Symmetry

Upstream

Downstream

Line code Deployment

Notes

ISDN HDSL, SDSL ADSL VDSL

1987

- DC - - 160 kHz

symmetric

144 kbit/s

144 kbit/s

4B3T, ZBlQ, SU32

f u I I-reach

single-pair

1993

- DC - 784 kHz

symmetric

I 2 M bit/s

I 2 M bit/s

2B1 Q, CAP

full-reach (FTTEx) pair-gain

multipair (< 3) (si ng le- pa i r H DS L-2)

r

1995 ANSI T1.413 1999/2000? 1999 ITU G.992.1 (G.dmt) 1999 ITU G.992.2 (G.Lite)

- 25 kHZ - 1.1 M H z - 300 kHz - > 10 MHz (max. 30 MHz)

asymmetric symmetric or asymmetric

I 1 Mbit/s I 2 Mbit/s asymmetric 526 Mbit/s symmetric

58 Mbit/s I 5 0 Mbit/s asymmetric 526 Mbit/s symmetric

DMT, (CAPIQAM) DMT, QAM full-reach (FTTEx) FTTCab, FlTEx

single-pair single-pair data above POTS/ISDN

The flexibility points are important for xDSL, because they represent key points at which the xDSL line-card modems could be deployed, either in existing cabinets where space allows, or in new, adjacent cabinets. At these points, access to the copper loop is convenient, and this therefore places on xDSL technologies requirements to deliver high-speed data over reaches of 4-5 km, around 1.5 km and less than 300 m, depending on the architecture selected.

xDSL access architecture Except for HDSL (high-speed digital subscriber line),

xDSL is predominantly seen as an overlay technology, operating on the same pairs as narrowband services such as POTS (plain old telephony service) or ISDN. This is achieved by utilising frequency spectrum above that used for POTS on the copper pair, as schematically illustrated in Fig. 2. This shows the basic principle, where the same customer copper pair is used for broadband connection using an xDSL modem to a broadband (BB) network and a narrowband (NB) connection using a POTS line card. The signals from the two are combined using a POTS splitter. The narrowband signal uses the low part of the spectrum, whilst the broadband signal uses the higher frequencies. At the customer premises, a POTS splitter allows these separate signals to be passed to an xDSL modem and to the telephones. The figure shows an active NT (network termination) example, where the broadband signal passes over a separate new network inside the house from the NT to the broadband device (e.g. PC).

Where spare pairs exist, there is also a market for xDSL on additional pairs, removing the requirement to carry narrowband and broadband services on the same pairs. This scenario would therefore obviate the need for the POTS splitters.

Depending on the type of xDSL and the bandwidths required to the customer, different deployment architectures are possible for xDSL. These are illustrated

in Fig. 3. All the possible architectures rely on reusing some portion of the access copper network, usually from a flexibility point such as the exchange or a cabinet. The connection from the broadband network to the flexibility point is typically made using optical fibre, either through a ring or PON (passive optical network) topology. An optical network unit (ONU) at the termination of the fibre houses the xDSL line cards, and the xDSL signal can be coupled into the existing flexibility point through a POTS splitter.

FTTH (fibre to the home) places the ONU at the customer premises, with no access-network copper reuse. FTTK (fibre to the kerb, or curb) uses a flexibility point within some 300 m of the home, and would typically serve 10 to 50 customers. FTTCab (fibre to the cabinet) uses the main cross-connect cabinet flexibility point, and is a possible deployment architecture for VDSL. FITB (fibre to the building) places the ONU in the basement of a tower block, running fibre to the building and reusing the internal vertical wiring with xDSL. FTTEx (fibre to the exchange) relies on reusing the copper all the way from the exchange to the customer premises, and is the normal architecture considered for ADSL and ADSLLite.

There is a fundamental trade-off between the length of copper and the achievable bit-rate. The closer the fibre reaches to the customer, so the shorter the length of copper reused, and so the higher the achievable bit-rate. However, the closer the fibre reaches to the customer, so the fewer customers per ONU, and so the more expensive the deployment per customer, especially at low penetrations.

xDSL overview The xDSL alphabet (or acronym) soup is regularly

getting larger. However, most of the newer xDSL technologies are related to HDSL, ADSL or VDSL. Table 1 shows some of the key features of these three xDSL technologies, and also includes basic-rate ISDN.

The table shows that as we move from HDSL through


ADSL to VDSL, higher and higher bit-rates are achievable, but that these require the use of progressively more bandwidth in the copper pair. For ADSL, the frequency spectrum up to 1.1 MHz is utilised, whilst for VDSL the spectrum up to as high as 30 MHz can be used. ISDN and HDSL offer symmetric data rates (upstream the same as downstream), whilst ADSL is designed to offer asymmetric data, with a larger rate downstream. VDSL is likely to be able to offer either symmetric or asymmetric modes of operation, with as much as 52 Mbit/s downstream and 2 Mbit/s upstream asymmetric, or 26 Mbit/s symmetric operation.

It can be seen that both single-carrier line codes (CAP and QAM) and multicarrier line codes (DMV are utilised. These are discussed in more detail in Section 4. HDSL and ADSL are designed for full-reach (FITEX) deployment, whilst VDSL could reach significant numbers of customers from FTTEx as well as F"ITCab deployment.

HDSL: High-speed digital subscriber line is the oldest of the digital subscriber line technologies, and is designed to deliver up to 2 Mbit/s symmetrically. It was originally a 3-pair technology, specified in 1993. A 2-pair HDSL was specified in 1996, and most recently ANSI (American National Standards Institute) through the TlE1.4 committee has a drafted standard for HDSL-2 using only one single copper pair. Whilst this will deliver single-pair 1.5 Mbit/s e l ) operation in North America, it is unclear whether a standard for 2 Mbit/s (El) operation over a single pair will be pursued in Europe. Unlike traditional HDSL, HDSL-2 includes forward error correction (FEC).

HDSL is primarily used for T1 and E l deployment, and for pair-gain (a technique for multiplexing several narrowband POTS calls onto a single pair in the feeder network). As many as 50% of the T1 lines deployed today in North America are deployed using HDSL. HDSL can give T1 over 4 km spans with bridge taps present without the need for repeaters. Native T1 requires repeaters every 1 km and does not tolerate many bridge taps. Additionally, HDSL is tolerant to poor quality cables (e.g. it was used by the US army for rapid deployment in the field in Bosnia). HDSL is largely a mature technology, with numerous manufacturers offering commercial systems.

Some telcos (e.g. US West in North America) are using HDSL to deliver up to 784 kbit/s to residential customers.

ADSL: Asymmetrical digital subscriber line was standardised by ANSI (T1.413) in 1995 with DMT line code3. It is capable of delivering up to 6 to 8 Mbit/s downstream, and 640 kbit/s to 1 Mbit/s upstream. It was originally driven by the VoD requirements, but is ideally suited in many ways to the fast delivery of Internet service.

Modems are available that utilise both the standard- based DMT and non-standards based CAP/QAM modulation schemes. Most telcos are now actively pursuing either trials or deployment of ADSL. Already 1999 has seen significant commercial deployments launched in North America and Europe. To date, these have been in selective markets (i.e. only in targeted locations or exchanges).

A number of CLECs and other second operators are offering ADSL over unbundled loops, which they rent from the incumbent telcos. However, a number of issues remain, such as co-location of equipment and qualification of loops.

Recently, ANSI completed issue 2 of the T1.413 standard3.

VDSL: Very high speed digital subscriber line is the latest and most challenging technology, capable of offering immense bit-rates over conventional UTPs. Bit-rates in excess of 20 Mbit/s are possible in each direction in symmetric operation, and asymmetric rates up to 52 Mbit/s are achievable for short reaches. VDSL is likely to offer the possibility of both short-reach symmetric and long-reach asymmetric operation.

In the past year, the standards bodies defining VDSL have moved from functional requirement specification to the drafting of a full standard, though this is not yet complete.

VDSL faces a number of challenges unique when compared with the other xDSL technologies. For example, the large bandwidth utilised requires careful consideration of the interference environment (see Section 3), and deployment in a FTTCab architecture will require active electronics outside the exchange, something with which most telcos have little experience.

VDSL trial systems are available using both DMT and QAM technology. To achieve the high bit-rates over conventional copper, VDSL maximises the use of the available spectrum, stretching the capabilities of copper transmission close to the limits. This has been enabled by recent advances in state-of-the-art digital signal processing.

ADSL-Lite: As has been alluded to earlier, ADSL was initially developed to address video on demand (VoD). As such, it has been specifically targeted at high-spec service delivery, such as full-screen broadcast-quality MPEG, which requires a high quality of service (QoS). For this reason, standard (ANSI T1.4133) ADSL includes a number of features which might be regarded as a luxury for many applications today, such as dual latency and data-rates of up to 8 Mbit/s.

Indeed, recently there has been a move to adapt ADSL to address more directly the requirements of the emerging Internet-access market. With Internet delivery, there is scope to revisit some of the ADSL specification, with possible corresponding gains in terms of cost and ease of deployment. Inevitably, such gains require a careful balance of various trade-offs, such as ease of deployment and reach at the expense of delivered bit-rate. However, going from high QoS MPEG delivery for VoD to Internet delivery presents a possible paradigm-shift in QoS philosophy. The very nature of most Internet applications allows for some degree of 'best-effort' delivery. It can be argued, for example, that robust systems relying on layer 2 or layer 3 (e.g. IP packet) retransmission on packet loss can give completely adequate service quality for Internet applications, with all the corresponding savings in moving


away from a highly specified guaranteed error-rate.

This new wave of ADSL development proposes sufficient changes to the ADSL spec to warrant it becoming a new ‘flavour’ of xDSL, which has become known as ADSL-Lite.

A major initiative in standardising ADSLLite was announced with the formation of the Universal ADSL Working Group OJAWG) in January 1998, which heralded the standardisation activity within the ITU (the G.Lite activity). The first version of the

network is used in the home to deliver the telephony service. The broadband xDSL signal is passed through the high-pass filter in the POTS splitter, and is then passed over new wiring to the broadband equipment such as a PC or set-top box (STB). The xDSL modem can either be located at the NT (active NT) with some standard protocol (e.g. Ethernet or ATM-25) used for in-house distribution of the broadband data over the new wiring, or the xDSL modem can be located in the equipment,

xDSL POTS I

no splitter at home entry point

xDSL+narrowband signal on all home

wiring

Ethernet or ATM-25

+narrowband

narrowband (POTS or ISDN)

Fig. 4 Typical customer-premises architecture for traditional ADSL requiring a POTS splitter

G.Lite standard (G.992.2) is expected at the end of June 1999. The emphasis has been on reaching a standard for customer-installable ADSL modems capable of around 1 to 1-5 Mbit/s downstream over the vast majority of telco access lines. This targets the possibility of a customer going out to a store and purchasing the ADSLLite modem, plugging it in, ringing his telco, and having the service switched on without the need for a telco employee to visit the customer’s premises. The first ADSL-Lite concepts were proposed by Rockwell (CDSL-Customer Digital Subscriber Line) and Nortel, who’s 1-Meg-Modem was the first (initially prestandard) ADSL-Lite product available.

One of the key features of ADSL-Lite is the possibility of dispensing with POTSsplitters at the customer’s premises. This is a prospect that warrants a little more discussion.

ADSL operation with POTS-splitter: Fig. 4 shows schematically a typical customer-premises architecture for xDSL requiring a POTS splitter and a separate in-premises overlay network. This is the scenario for traditional full-spec ADSL. The narrowband signal is passed through the low- pass filter of the POTS splitter, and the existing narrowband

This traditional architecture for ADSL requires a POTS splitter at the point of entry to the premises. This is considered unattractive by some telcos, as it requires a telco engineer to visit the premises to install the service. Anecdotal evidence suggests that in North America an installation team can cover between two and three lines per day for CPE (customer premises equipment) installation. Additionally, requiring a POTS splitter at each end adds component cost, and the architecture requires new wiring to be installed in the home for the broadband distribution.

Splitterless operation for ADSL-Lite: Fig. 5 shows a schematic diagram of ADSL-Lite in a splitterless deployment. There is no splitter at the entry point to the premises, and the existing in-home wiring is used to deliver both the narrowband-telephony and the broadband signals. The xDSL modem can be connected to any of the existing telephone sockets.

The key advantages of this architecture are that it avoids a splitter, and it reuses the in-house wiring. No visit is therefore required by the telco service engineer. However, there is a potential issue with the modem affecting the telephony performance when both are used


-75

N

h: E m -0

.g -100 c -0

z 2 Q

-125

-1 50

10

9 - 8 -

s 7 - 5 6 -

.g 5

0 4 - 3 - 2 - 1 -

I I l l 1 1 -

-

-

1 2 3 4 5 6 7 8 9 10

frequency, MHz

Fig. 6 Example of the received signal and noise for a VDSL system operating over a typical 1 km copper loop comprising sections of 0-4 mm and 0.5 mm UTP. The shaded region indicates the available spectrum for data- modulation.

simultaneously. This can happen where nonlinearities in the handsets cause frequencies in the ADSL signal to be down-converted into the voice frequencies, hence becoming audible. With ADSL-Lite, therefore, some lines may require low-pass filters to be placed at each telephone socketwhere a telephone is connected. An alternative is for the modem to detect the off-hook state, and reduce its power correspondingly.

Additionally, there is the potential of a reciprocal process where impulse noise can affect the modem performance when the telephone is used, especially with older loopdisconnect handsets. This can also be

limit of modelled capacity '\. &------

1 I I I I I

1000 2000 3000 4000 5000 total loop length, m

Fig. 7 Capacity of ADSL against reach, assuming ANSI standard loop comprising 0.32 mm, 0.4 mm and 0.5 mm sections of UTP, and including Model A noise and multiple self-FEXT noise and AM broadcast RFI (simulated results)

addressed by placing low-pass filters directly in front of each telephone socket where a telephone is connected, as above. However, where the service is not significantly affected by the associated loss of packets from the impulse noise, the retransmission of packets will provide adequate robustness.

Because the ADSL-Lite signal is carried over the in-house wiring when splitterless deployment is used, there is a possibility of egress RFI from that wiring, particularly where the cablebalance is poor. The ownership of the in-house wiring varies from country to country and is often of unknown quality. This consideration, coupled with the decreasing (worsening) cable balance with frequency places a requirement on ADSL-Lite to use only the lower portion of the spectrum, e.g. below 500 kHz. This is in part the reason why ADSL-Lite is limited to around 1.5 Mbit/s operation.

Some of the issues related to ADSL-Lite still need to be resolved, but the forthcoming G.992.2 standard is an important step towards an ADSL technology suitable for user plug-and-play installation.

xDSL data capacity The data capacity that can be delivered by an xDSL

system running over a real network line will depend on many factors. Predominantly, the capacity will be dictated by the attenuation of the copper pair, the crosstalk environment resulting from interference from signals in other copper pairs in the bundle, other noise sources within the network, and the interference environment attributable to external sources (such as broadcast radio).

Fig. 6 shows as an example the available spectrum for a VDSL system operating over a typical 1 km loop comprising sections of 0.4 and 0.5 mm cable. The graph shows the usable bandwidth and the signal-to-noise ratio (SNR) .

The available spectrum is bounded by the attenuation of the pairs and the noise. For VDSL, the self-FEXT (far-end crosstalk from other VDSL systems in the bundle - see Section 3) is the limiting noise at low frequencies. At higher frequencies, the limiting noise is the white noise. The best part of the spectrum is at the lower frequencies, and this is the valuable spectrum for data modulation. At higher frequencies, the received signal strength falls off rapidly due to line attenuation. The available spectrum per unit frequency range is very much lower at higher frequencies, but this nonetheless represents valuable spectral real estate that cannot be ignored when delivering maximum bit-rates.

The practical lower frequency of operation is limited by the requirement to operate over narrowband services (POTS, ISDN, etc.), and the need to make the POTS splitter simple, small and cheap. The practical upper limit will depend on the length of the line. For longer loops, the attenuation increases so rapidly that the frequency above which there is no longer any useful spectral SNR can be lower than that which VDSL could use on shorter loops.

AM broadcast and amateur radio bands also fall within the spectral range of VDSL, and add additional noise- concerns. These issues are discussed in more detail in Section 3.


radio amateur RFI

operate is a complicated one, and includes alarge number of contributing factors, some intrinsic (related to the network and the services operating on that network) and some extrinsic (related to sources outside of the network, but nonetheless impinging on it). An understanding of the environment is important both for specification and design of xDSL systems and for planning deployment.

Fig. 8 shows schematically the key

Extrinsic environment AM/SW-broadcast RFI

insic environment

/(d,t)rxyd+ ~ y% V(d, t) G 6 d V + S d g

V other interferers

The above example is for VDSL, but similar arguments apply for the other xDSL technologies, bounded by the spectrum allocated to them (e.g. up to 1.1 MHz for ADSL). The data capacity (bit/$ will depend on the coding scheme used to maximise the use of the available spectrum. For example, standard (T1.413) ADSL uses DMT (see Section 4) with up to 15 bits per carrier. Fig. 7 shows a modelled simulation of capacity against loop length for the downstream capacity assuming a typical loop comprising sections of 0.32 mm, 0.4 mm and 0.5 mm copper UTP, and assumes ANSI standard Model A noise and multiple ADSL self-FEXT and AM broadcast radio frequency interference (RFI)3. For 3 km loops, a capacity of greater than 6 Mbit/s is possible in this scenario, and the effect of increasing length on capacity can be seen. Also shown is the effect of crosstalk.

The actual achievable capacity of an xDSL system will depend on a large number of factors and is ultimately dictated by the environment within which the xDSL system must operate.

Fig.8 Thekey environmental factors with which xDSL systems must co-exist, including those attributable to the 'intrinsic environment' (the environment directly due to the network and services on that network), and the 'extrinsic environment' (the environment from outside the network)

Intrinsic environment The intrinsic environment can be thought of as that

directly attributable to the network and the services that share the network with the xDSL signals. It includes the physical plant characteristics, such as loss and cable balance, as well as noise sources such as crosstalk, white noise and some impulsive noise effects. It must be borne in mind that no two telco networks are the same, and the complete environment for xDSL will be different for each network. However, it is important to understand the key issues and trends, and the bounds on the factors which can be used in the successful design and deployment of xDSL systems.

Cable 1oss:Almost universally, the cables used in the access networks are unshielded twisted pairs (UTP) of various gauges. Most common are pairs of diameter 0.32 mm (near the exchanges), 0.4 mm (close to AWG 26 gauge in North America) and 0.5 mm (close to AWG 24 gauge in North America). Other gauges are also deployed, but these are less common. Thinner pairs are normally found nearer the exchanges, and thicker pairs nearer the customers (both because of handling issues where large numbers of pairs aggregate near exchanges, and because of the better

3 The xDSL environment

The environment within which an xDSL system must


-0

-10

-20

5 -30 B ._ s -40 c

3

-50

-60

-70

1 2 3 4 5 6 7 8 9 10

frequency, MHz

Fig. 10 Typical loss as a function of frequency (to 10 MHz) for 1 krn sections of 0.5 mm and 0.4 mm UTP, assuming source- and terminating-impedances of 100 Q

transmission properties of the thicker gauges). Most pairs are twisted to improve their longitudinal balance, and to reduce crosstalk, emissions and pickup.

The cable characteristics can be modelled using an equivalent circuit for a twisted pair. Fig. 9 shows the equivalent circuit. The loss or leakage conductance (G) is nearly negligible for most UTP cable. The resistance (R) increases with frequency and is the dominant component, resulting in increased loss at higher frequencies. Typical values for R are around 280 R/km @C) and 570 R/km at 1 MHz for 0.5 mm UTP. The inductance (L) decreases with frequency (e.g. approximately 600 pH/km (DC) and 500 pH/km at 1 MHz for 0.5 mm UTP). The capacitance (C) is constant with frequency in the range of interest, typically 50 nF/km for 0.5 mm UTP.

Fig. 10 shows the typical loss for frequencies up to 10 MHz for 1 km lengths of 0-5 mm and 0.4 mm UTP, assuming source and termination impedances of 100 R.

The effect of the loss on available capacity for an xDSL system has already been highlighted in Section 2.

For some systems (e.g. ADSL-Lite), the in-premises wiring must also be considered. This raises many issues, especially as the quality of the wiring varies from customer to customer. Ownership of this wiring also varies, but it can be customer-owned. Additionally, in the UK, in-house wiring is 3-wire, which is unbalanced.

Cable balance: Consideration of the balance of access cables is crucial, as it directly relates to the degree of interference present from RFI sources such as AM broadcast radio (e.g. for ADSL and VDSL) and single sideband (SSB) amateur radio broadcasts (e.g. VDSL).

10 2o 1 01 I I I I I

0 2 4 6 a 10

frequency, MHz

Fig. 11 a 0.5 mm aerial drop UTP, showing balance decreasing with increasing frequency, including spectrally localised features

Measured balance as a function of frequency for

The cable balance relates the degree of coupling between the common mode (cable to ground) and differential mode (between the two wires of the pair) signals.

Numerically, balance is given by:

balance (dB) = 2010g !'!EEEE 0 vdifferentiul

where Vcommon and Vd@erential are the common-mode and differential-mode voltages, respectively.

The balance of a UTP decreases with frequency. Generally below 1 MHz the balance is greater than 50 dB, but it can be less than 30 dB at higher frequencies. It is important to realise that the balance varies from pair to pair and will depend on many factors, such as the cable type, the quality of the pair, the age of the pair, the installation procedure used, and even environmental factors such as rain-water ingress. Additionally, although the trend is for worse balance at higher frequencies, it is not a smoothly varying function. Fig. 11 shows the measured balance of a selected access pair. The balance can be seen to display sharp features, often localised in frequency. For example, the pair shown has a localised peak in balance of 55 dB at around 5 MHz, and a localised dip below 30 dB at around 7 MHz. It should be noted that this particular example has been selected to show a balance below 30 dB, and this is not typical of the majority of access pairs.

For VDSL, the requirements state (e.g. Reference 4) that the balance should be better than 30 dB. The example shown illustrates that this is true over the majority of the frequency range, and indeed that the balance is better than 40 dB for the most part. When considering the effect of

134 ELECTRONICS & COMMUNICATION ENGINEERING JOURNAL- JUNE 1999

balance on both ingress and egress radio frequency interference 0, it is important to appreciate that the scale of any effect will depend on the likelihood of many cumulated effects, for example the existence of a region of poor cable balance for a pair, the characteristics of any possible interference source in that frequency range, the probability of there being auser of that spectrum within the

data-block. Interleaving involves redistributing the data such that the bytes within the period of a noise burst are interleaved from sufficient separately protected data blocks as to enable full data recovery. For example (Fig. 12), if protection is required for a complete loss of symbol data during a burst of 300 ps, then using a Reed-Solomon overhead of 10% gives protection of 5% of each symbol. The

specific range of importance (only tens of metres for radio amateur broadcasts), the combined orienta- tions and polarisations of the cable and the source antenna, and the simultaneous operation of the xDSL system and the other Fig. 12 Example of interleaving, where data from individually

Reed-Solomon-protected data-blocks are interleaved across 6 ms to give protection against 300 ps bursts of noise (assumes 10%

RS bytes added to data, to give 5% RS error protection)

user ofthe spectrum.

ImPulse noise: All xDSL systems must operate in the presence of impulsive noise. Impulsive noise is essentially the term used to describe random noise events resulting from a number of possible intrinsic or extrinsic sources, such as tip/ring, loop disconnect, lightning strikes, etc. Integrated small impulses give a Gaussian noise profile. The impulsive noise events are random, and therefore not predictable. Additionally, the lengths of the impulses vary widely. Pulse lengths of 2 1 ms can not be discounted.

To operate in the presence of impulsive noise, xDSL systems require error correction and interleaving. A forward error correction (FEC) code such as the Reed-Solomon (RS) code is commonly applied to protect against errors resulting from transmission. Such a code can recover up to a specific number of errored, corrupted or lost bytes of data, and will protect against impulsive noise events up to a certain length, depending on the implementation of the Reed-Solomon code.

The Reed-Solomon code is a ‘block-code’ requiring the addition of redundant parity symbols to the transmitted data symbols, enabling subsequent error correction. It operates on a multibit symbol rather than on individual bits. Adding A!?? extra parity check symbols to a data block enables the correction of up to x/2 %errors. For example, the addition of 20 Reed-Solomon bytes to 200 bytes of data gives protection of up to 11 errored bytes.

Whilst use of forward error correction protects against spu-

interleaving depth (7) required for full protection is given by (300 ps)/T < 5%. This gives T> 6 ms. In other words, interleaving to distribute the data from each protected symbol across 6 ms is required.

Although the above techniques of FEC and interleaving are used regularly with

xDSL, they do have the disadvantage that they addlatency to the end-to-end link. For this reason, ADSL caters for delay-sensitive services by providing a channel without interleaving.

To date, very little characterisation data of impulses on real telco networks has been published, and the design of existing xDSL systems is based on a broad understanding of impulsive noise events. Better measurement data is required to help define error correction and interleaving requirements for future xDSL systems, but this will be too late for early standards.

Crosstalk: Crosstalk is the coupling of signals between different copper pairs in the same cable. Coupling can take place between signals propagating in opposite directions (NEXT, or near-end crosstalk), or between signals propagating in the same direction (FEXT, or far-end crosstalk). These are illustrated in Fig. 13.

NEXT coupling increases according to f i e q u e n ~ y ) ~ ’ ~ , while FEXT coupling increases according to fiequency)2.

The NEXT and FEXT environment depends on the characteristics of the services in the different pairs

copper pairs /I

FEXT ----

NEXT ~ - .

Fig. 13 Schematic representation of NEXT (near-end crosstalk) and FEXT (far-end crosstalk) between two copper pairs in close

proximity

(duplexing scheme, spectral plan, etc.), and the cable construction, service mix and fill among the pairs in the bundle. ANSI gives expressions for NEXT and FEXT for different services within the standards documents. The noise models stip ulated within standards assume a particular

rious errors and very short impulses, interleaving is required to protect against longer impulses. This gives added protection against burst noise, which may occupy periods longer than those protected within an individual

mix of services, and usually a worstcase cable fill (all the adjacent pairs providing crosstalk).

The crosstalk noise in the ADSL frequency range with an example service mix is shown in Fig. 14. FEXT is loop-


r fl -100

s; -110

% -120

. E m U I .- U)

- 2

B -130

?I

c

-140

-1 50

XT

0 500 frequency, kHz

1000

Fig. 14 Crosstalk noise from a typical service mix in the ADSL frequency range (under 1 MHz)

length dependent and is worse for shorter loops. For ADSL, NEXT is usually dominant, because the valuable spectrum utilised by ADSL is shared with HDSL, ISDN, etc. For VDSL, NEXT from other services is not so important, because the VDSL spectrum starts above 300 kHz (an example where NEXT is possible from ADSL into VDSLat the exchange end is discussed in Section 5). VDSL self-NEXT (NEXT into VDSL from other pairs carrying VDSL) is avoided by either frequency division duplex (FDD) or time division duplex (TDD) operation, and the dominant crosstalk is therefore FEXT.

Bridge taps: Bridge taps are unterminated copper pair tails attached to copper pairs in service and are a particular concern in North America where they are commonplace. Estimates vary, but for some operators, more than 50% of lines are shared with a bridge tap.

Bridge taps are usually the direct result of copper deployment practice in North America. Fig. 15 illustrates how they arise. Typically, a sheathed bundle is deployed past the properties of the customers. This bundle is left unterminated at the end of the cable-run. When providing service to a customer, the sheath is breached, and a pair selected. This pair is tapped and a separate drop wire coupled to it for the final drop to the customer’s premises. The remainder of the copper pair used in the bundle is left attached, and runs to the end of the cable-run. The length

Fig. 15 Schematic illustration of deployment practice of copper pairs in North America leading to unterminated bridge taps

of the bridge tap depends on the distance of the customer from the end of the cable-run and might be a few tens of metres for the final customers in the run. This practice causes no ill-effects for narrowband services, but creates problems for broadband services using higher frequency spectrum in the copper.

To understand the importance of bridge taps for xDSL services, it is helpful to look at the spectral characteristics of the taps. Fig. 16 shows the modelled effect on the insertion loss of bridge taps of different lengths. The example is for a 900 m loop, with a bridge tap of varying length placed 100 m from the customer (this is an ANSI standard test-loop) . As bridge taps

get shorter, so the notches they produce in the insertion loss get deeper and move to higher frequencies. For ADSL, which uses the spectrum only below 1.1 MHz, only the longer bridge taps are important, but these have relatively shallow notches in insertion loss and a corresponding impact on available spectrum. For VDSL, however, the shorter bridge taps, which are wider and deeper, can have a very detrimental effect on the available spectrum. Additionally, the higher harmonics from the longer bridge taps also fall within the spectrum used by VDSL.

Possible solutions for bridge taps include removing the bridge taps altogether, but this is universally considered to be expensive and impractical. Termination of the bridge taps has also been proposed, which could be done at the end of the cable-run. However, this does not overcome the loss associated with the bridge tap, but only the resonant effects. Fig. 16 also shows the effect of termination on the insertion loss (a 3dB increase in insertion loss across the band), and this suggests that the net improvement on capacity compared with tolerating the bridge tap might be marginal. Another possibility is to require xDSL to operate in the presence of bridge taps. This would be particularly applicable with a line code capable of tailoring the spectral distribution of data to the localised features in the SNR.

For VDSL, ANSI specifically requires VDSL to operate in the presence of (standard) bridge taps. ETSI does not mention bridge taps.

shielded cable containing - -. . - - - I , \ twisted copper pairs \ I ,

cable shield breached to

central office

. . I ,

left unterminated

136 ELECTRONICS & COMMUNICATION ENGINEERING JOURNAL, JUNE 1999

Extrinsic environment The intrinsic environmental factors

discussed above are related to the telco’s network, and are to a large degree under the control of the telco. The extrinsic environment within which the xDSL system must operate is, by definition, governed by external factors, and again these willvary widely according to the local conditions.

RFI issues: The most important extrinsic-environment factors are those related to radio frequency interference (RFl). Fig. 17 shows the RF spectrum used by ADSL and VDSL, together with the most widely used radio bands.

Immediately evident is the overlap of VDSL with the LW (in Europe), Mw and SWAM broadcast radio bands, and the SSB amateur broadcast bands. In addition to these commonly considered bands, in most countries, there are no unassigned frequency bands between 150 kHz and 30 MHz, and these assignments are often for undisclosed applications.

For a telecommunication system (such as an xDSL system), which must

bridge tap (length varies)

800 m

0

-20

!2 U5

f -40

; 0 ._ I

.- 4 0

-80

10 m (3 dB loss) I I I I I I

A S L 2 4 6 6 10 .12 VDSL

frequency, MHz

Fig. 16 Simulated insertion loss (as a function of frequency) for an ANSI test loop incorporating a bridge tap, showing result for 10 m, 20 m, 30 m and 40 m bridge taps. Also shown is the insertion loss assuming termination of the bridge taps. A schematic diagram of the test-loop is shown at the top of the figure. The spectral allocation to ADSL and VDSL is indicated at the bottom of the figure.

be considered an unintentional transmitter of RF, the legal requirements are clear. They must conform to EMC requirements (e.g. FCC Title 47, part 15 in the USA, and EN 55022 and EN 58802-1 in Europe). However, in addition to conforming to the legal EMC requirements, they must also cause no interference to other licensed users of the RF spectrum.

It is important to consider both ingress and egress when considering the interference environment. In other words, not only must the system be robust in the presence of interference from other users of the spectrum, but additionally it must be flexible in mitigating against interference to others. These issues are particularly

employ both underground and overhead cables and, as already discussed, these could be in the distribution network as well as just the final drops.

AM broadcast: AM broadcast interference is an ingress rather than an egress issue. The transmitted powers from AM stations are large, and the field strengths from the transmitters at the receivers of the listeners who are likely to be listening to those stations are very much (orders of magnitude) higher than any likely interference level from an xDSL system. However, it is important that an xDSL system be capable of the flexibility not to broadcast in specific spectral regions of concern, either as a local policy or to overcome

important because xDSL must operate on networks which interference in a particular and unrepresentative instance.

0 public radio broadcasts international SW broadcasts

0 amateur bands

0 air and maritime fixed and

0 TV broadcast

mobile bands, and other

frequency 3 kHz 300 kHz 3 MHz 30 MHz 300 MHz

Fig. 17 Illustration of the principal allocations of spectrum below 30 MHz, together with an indication of the spectral overlap with ADSL and VDSL

ELECTRONICS & COMMUNICATION ENGINEERING JOUWAL JUNE 1999 137

;:i 60

0 z c

E 40 PI

Cable balance

0 50dB n 0 40dB

0 30dB

worst case -30 expectation -40

r

d -40 -50 -50 -60

interference level, dBm

Fig. 18 Results of a simple demographic study of the proportion of homes at risk of RFI from AM broadcast radio interference (Raleigh, North Carolina, USA). The bottom plot shows the locations of homes (dark circles) and transmitters (light towers). The top plot shows the proportion of homes at risk assuming different values of balance in the AM band.

A better understanding of the ingress issues related to AM radio can be gained from a consideration of the demographics of the siting and operation of AM transmitters. Firstly, AM broadcast transmitters (MW, LW and SW bands) are fixed at known sites, and typically broadcast 24 hours a day. They are therefore a predictable and constant (long-term) problem. Additionally, there is a carrier present all the time, which can be identified by an xDSL system during training. Some transmitters do have switchable power, direction and frequency, and vary these during the day. High powers are common (with additional antenna gain from directionality), with transmitted powers up to several hundred kilowatts in the UK, and up to 50 kW in the USA

The demographics of transmitter distribution varies between countries. For example, in the UK there are less than 200 transmitter sites, and the transmitters are generally sited outside towns, and away from highly populated areas. In North America, the situation is very different. There are in excess of 2000 transmitters in the USA alone, and the trend is for more local stations, with lower powers than in the UK. Additionally, the transmitters are often sited within the populated areas, often in among the residential housing. This therefore increases the likelihood of powerful radio transmissions close to the final drops (or aerial distribution cables) carrying xDSL services.

An additional consideration with AM radio is that multiple transmitters are often sited at one transmitter site. This therefore means that where an xDSL system suffers ingress at one frequency, it is likely to suffer ingress at several frequencies.

Standards bodies typically include multifrequency models with 10 AM interferers. It is important that the system is flexible in operating in the possibly complicated interference environment in the proximity of multiple AM radio stations.

To illustrate the possible scale of the AM radio interference environment, we have performed some demographic modelling of a typical deployment area, Raleigh, North Carolina. Fig. 18 shows the location of homes (dark circles) and AM transmitters (lighter towers) in Raleigh. The modelling has included all the transmitters up to one hundred kilometres from Raleigh. There are seven AM transmitters within 5 km of the town, and 141 000 homes have been included in the analysis. The detailed discussion of the analysis and the assumptions made would be out of place here, except to say that all the final drops are assumed to be aerial, and field strength is assumed to fall off as the inverse square of the distance from the transmitter.

The results of the model are shown at the top of the figure, which gives the proportion of homes at risk of differing levels of worst case and expectation levels of interference. We have found from measurements that the expectation level is typically 10 dB lower than the worst case, which assumes maximum coupling and orientation alignment of the dropwire with the transmitter antenna. The dependence on the balance of the dropwire in the AM radio frequency-range is evident.

The results show that xDSL systems whose frequency usage overlaps with AM bands must be able to operate error-free in the presence of numerous simultaneous interferers. This requirement is reflected in both the ANSI ADSL standard and the various VDSL draft standards (e.g.

Although the potential risk of interference could be significant, it is through an appreciation of the AM environment that practical xDSL systems have been designed to operate in the presence of these potential interferers. Standards-based ADSL is based on the DMT coding-scheme (see Section 4), which is both resilient and flexible in the presence of AM RFI, allowing data not to be coded onto regions of the frequency spectrum where AM

m13.


interference is experienced. Similarly, early prototype VDSL modems have been shown to operate error-free in the presence of strong AM RFI.

Radio amateur broadcast: The consideration of radio amateur transmissions requires very different treatment from the AM broadcast situation. The radio amateur community is a large one, whose members are very experienced users of the spectrum allocated to them, comprising a number of separate bands of the spectrum, many of which fall in the HF spectrum under 30 MHz. Radio amateur transmissions are not a relevant consideration for ADSL, but must be considered for VDSL.

Again, it is instructive to consider the demographic attributes of radio amateur broadcasters. Firstly, unlike AM broadcast transmitters, they are distributed among the population (they are after all people living in homes among nonbroadcasters). There are some 30 000 amateur broadcasters licensed to use the HF bands in the UK, a figure typical of many countries in Europe. In the USA, this figure is around 830000. The interference sources (the amateur antennas, usually dipoles) are close to telephone network dropwires (possibly within tens of metres). Bkcause radio amateurs often listen to very weak signals, the issue of egress must be taken as seriously as ingress. Radio amateurs move house along with everyone else (in the UK people move house on average every ten years or so). The interference environment related to radio amateurs is therefore neither a predictable or constant problem in any particular area.

Radio amateur transmissions use the single sideband (SSB) modulation, and therefore there is no carrier to lock on to, and the presence of an amateur can not be detected except during broadcast. Additionally, amateurs tend to have unusual and somewhat unique usage patterns. They are very frequency agile, often jumping between bands, and to complicate matters are often listening rather than transmitting. Given the possible proximity of the drop wires, powers are high. In the UK, transmission can be up to 400 W, in the USA up to 1.5 kW for some classes of operator, and in Canada up to 2 kW.

ETSI is proposing a noise model for VDSL including a radio amateur interferer at 0 dBm differential mode signal. Within ANSI, signals as high as -5 dBm have been discussed.

It is the careful consideration of the possible risks of egress interference in the amateur bands that has led to an inclusion of notching of the transmitted power spectral density in the draft standards requirements. VDSL systems will be required not to transmit powers greater than -80 dBm/Hz in the amateur bands, whilst transmitting at up to -60 dBm/Hz in other parts of the spectrum.

Fig. 19 show the results of modelling the risk of ingress interference from radio amateurs in the region of Raleigh, North Carolina. The analysis shows a breakdown of results for two specific, but very different, localities: a section of inner-city housing (few amateurs) and a wealthy suburban area (many amateurs) which might represent the typical desirable deployment area for xDSL services. Again, all

drops are assumed to be aerial. Only amateurs licensed to broadcast in the VDSL bands are included.

The results at the top of the figure show the proportion of homes at risk of -5 dBm worst case interference assuming a 30 dB balance in the amateur bands (or -25 dBm interference assuming 50 dB cable balance). Whilst the proportion of homes at risk in the inner city is small (less than l%), in the suburban area the risk is much more significant (nearly 10%). This figure will be reduced when accounting for underground drops, and additionally does not account for amateurs holding a licence but not active. However, this illustrates that the possibility of radio amateur broadcasts resulting in a sizeable interference level on an access line is a real one, and that VDSL systems will need to be completely robust in the presence of amateur radio signals. Such robustness has already been

risk of -5 dBm worst case interference assuming 30 dB

interference assuming 50 dBm cable balance (-25 dBm

inner-city suburban Raleigh Raleigh Raleigh (entire) + +

Fig. 19 Results of a simple demographic study of the proportion of homes at risk of RFI from radio-amateur interference (Raleigh, North Carolina, USA). The bottom plot shows the locations of homes (dark circles) and amateurs (darker towers). The top plot shows the proportion of homes at risk of -5 dBm worst-case interference (assuming 30 dB cable balance in the amateur band) for different regions of Raleigh.


Fig. 20 Schematic illustration of the concept behind QAM

Fig. 21 Schematic illustration of the concept behind CAP

constellation of modulated and modulated symbols filtered signal

modulated, filtered and frequency-shifted signal

constellation of modulated symbols modulated signal

modulated and filtered signal

filter response

demonstrated in early VDSL modems, such as Nortel’s prototype DMT VDSL modem that uses a proprietary enhancement of the DMT line code (see Section 4).

The above discussion should also serve to show that radio amateurs exist in sufficient numbers to require a notched power spectral density (PSD) to ensure a happy coexistence. Additionally, it is desirable to ‘turn off transmissions in the amateur bands all together. Such measures mitigate any possible egress interference from VDSL in the amateur bands. It cannot be overstated that this position has been arrived at by careful assessment of the environment, and by consultation with the relevant amateur radio interest groups when drafting standards requirements.

Other RFI: As discussed earlier, most of the radio frequency spectrum below 30 MHz has been licensed to various users. The vast majority of this allocation is not of broad-sweeping relevance, and includes maritime communications, etc. However, xDSL systems should be designed to be flexible in mitigating against undesirable ingress or egress interference related to unforeseen RFI sources. In other words, it is one thing to design a system to operate in the presence of the foreseen, but it is quite another to cater for the unforeseen. The ANSI VDSL noise model includes two RFI signals at -60 dBm from unspecified sources. ETSI draws attention to the maritime distress bands (e.g. 500 kHz and 2.182 MHz).

An additional factor is that xDSLmust operate over some in-house wiring before terminating at the NT and will be close to domestic sources of interference. The requirement for VDSL systems to be flexible in coping with unforeseen RFI has been demonstrated by experiences at Nortel. By chance, the interference threat from/to CT1 cordless telephones was identified as an issue for VDSL. CT1 base stations in the UK transmit to the handset in the

1.6 to 1-8 MHz band, and the base stations and their antennas are typically sited very near the entry point of telephone cables into the home. We have measured interference levels in the laboratory from CT1 greater than those from AM broadcast stations. This example highlights the requirement for the VDSL line code to be flexible in dynamically notching out regions of spectrum with unforeseen RFI.

4 xDSL line codes

There are several possible line codes that have been considered for xDSL. These include the simplest pulse amplitude modulated (PAM) line codes such as 2BlQ, single-carrier line codes such as carrierless amplitude and phase (CAP) and quadrature amplitude modulated (QAM), and multicarrier line codes such as discrete multitone (DMT) and discrete wavelet multitone (DWMT).

HDSL systems initially used 2B1Q coding. DMT has been selected by ANSI (“1.413) as the ADSLstandard line code. However, CAP and &AM modems are also available. ForVDSL, the choice of line code will be made by ANSI and ETSI, but is unlikely to be decided before late 2000.

It is important to appreciate that ADSL and VDSL are solutions to very different problems. They utilise very different spectral ranges and operate in different environments. Consequently, it is reasonable that different arguments apply.

The next two subsections give an introduction to the fundamentals of CAP, QAM and DMT line codes, which are the most frequently encountered for xDSL.

The introduction of the line codes is illustrated by drawing out some key advantages and disadvantages of each. Whilst this is intended to be an objective comparison, the author is aware that the subject of line-code


comparison is a contentious one which has led to several heated debates within the standards forums. The reader is therefore reminded that the comparison presented here is merely for introduction. One important point to make, however, is that a fair comparison requires a comparison of like functionality (e.g. the complete system required to achieve a given performance in the presence of a given environment).

data

&AM and CAP Quadrature amplitude modulation (QAM) and

carrierless amplitude and phase (CAP) line codes are different, but related. Both are essentially single-carrier line codes (even though, strictly speaking, CAP does not employ a carrier).

QAM is an extension of pulse amplitude modulation (PAM). Binary data is coded into a base-band signal where the symbols are complex numbers forming a two dimensional QAM constellation. This is illustrated in Fig. 20. In the example shown, 16-QAM is illustrated, in which 16 states (4 bits of data) are represented on a constellation of 16 different states (complex numbers), each with different amplitude and phase. The signal is placed in the frequency domain by frequency-shifting the filtered base- band signal to the required band. This is also illustrated in the figure.

CAP is similar to QAM and is effectively a special case of QAM. CAP also relies on a phase-amplitude constellation, but the signal is generated directly in place in the frequency domain by filtering the base-band data-samples, rather. than by frequency ski ing. This is illustrated schematically in Fig. 21. High performance is achieved by employing nonlinear time domain equalisation (decision feedback equalisation (DFE) or Tomlinson precoding) .

The process involved within the QAM and CAP processes can be better understood by considering the functional blocks of a modem. Fig. 22 shows schematic diagrams of both a QAM and a CAP modem (transmitter only).

For QAM, the data is split into two halt-rate paths. An encoder maps the bits onto symbols that will map onto the amplitude-phase constellation, and these two separate data

sin encoder %

sym bois) %-M x

(maps bits * to

streams are passed through low-pass filters and then orthogonally modulated by digital cosine and sine mixing. After quadrature modulation, the two signals are combined and passed through a digital-to-analogue converter and low-pass filter.

For CAP, the data is also split into two half-rate paths. However, rather than using sine and cosine mixing, digital finite impulse response (FIR) filters are used. These are designed to have impulse responses with the same amplitude, but differing in phase by d 2 .

In many respects there are more similarities between QAM and CAP than there are differences between some implementations of just one of these. It is therefore not unreasonable to look at the strengths and weaknesses of these two line codes together.

Some key advantages of CAP and QAM are as follows:

data

A low peak-to-average transmit power variation, which makes the design of the analogue stage simpler.

0 Easy and quick training of the modem at start up, or when resynchronisation is required. Additionally it is possible to make a ‘blind’ receiver (if Tomlinson precoding is not used). A simple implementation is possible for very ‘easy’ environments (short loops and low RFI). Exactly what qualifies as an easy environment is the subject of debate.

0 The core modem is a very regular structure (gates and logic).

imaginary encoder (maps bits 47

to symbols)

The principal disadvantages are:

0 Rate adaptation giving a fine rate-adjust is difficult (and requires changing the spectrum or the constellation size used). Operation in the presence of RFI requires increasing complexity with an increasing number of agile interferers.

0 The symbols are short, which gives poor resilience to short noise-impulses (before considering error correction).

0 Flexible power spectral density (PSD) control requires

Fig. 22 Schematic diagram of the key functional blocks for (a) a QAM transmitter and (b) a CAP transmitter


Fig. 23 Illustration of the key concept behind DMT

bit allocation by ,DMT coding b!h,,

.% 2

0 5 10 frequency, MHz

orthogonal carriers individually modulated

programmable notch filters.

DMT Discrete multitone (DMT) is a multicarrier line code

using a large number of separate carriers, each individually quadrature amplitude modulated. Modems are efficiently implemented using fast Fourier transform (FFT) processes. Channel equalisation is performed predominantly directly in the frequency domain and, coupled with the use of a cyclic prefix, eliminates the need for timedomain equalisation.

Fig. 23 shows schematically the division of the spectrum into a number of carriers (typically 256 or 512, though not all are used, such as those in the baseband reserved for narrowband services). The frequency response of each carrier is a sinc wave, and the system is designed such that the response of any one carrier is at a zero coincident with the centre of any other carrier. DMT is therefore a system employing orthogonal carriers.

Because, the number of bits per carrier can be varied according to the local (spectrally) available signal-to-noise ratio, DMTcan tailor the allocation of data to efficiently use

channels with rapidly varying capacity with frequency, such as might be found on a long copper access UTP. The example in the figure illustrates this situation, with progressively less bits per carrier allocated to carriers at increasing frequencies.

As with CAP and QAM, a better understanding of the operation of the line code can be gained from a schematic functional block representation of the transmitter. Fig. 24 shows such a representation for DMT.

The data stream (entering from the left) undergoes forward error correction and interleaving, and is then encoded. This involves splitting the data up into the number of bits to be allocated to each separate carrier and mapping these onto the associated quadrature-amplitude- modulated constellation point (complex number) for each separate carrier. The figure shows as an example the case where the lowest frequency carrier has 4 state (2 bits per carrier) encoding, and the state in the top right quadrant has been selected. This is not a realistic scenario, because normally the lowest frequency carrier would not be utilised, but it serves well to illustrate the principle. The data on each carrier is processed through an inverse FFT

forward error

correction - -

prefix

frequency domain corresponding time domain representation of symbol n

representation of a single real DMT carrier (assuming all other carriers are turned off)

Fig. 24 Schematic illustration of the principal functional blocks in a DMT modem (transmitter)


I 50r 50 r 40 40

3 2 g 30 g 30 .- $ .- g 20

5 Fi 20 8 8

10 10

300 500 1000 1500 300 500 1000 1500

a total line length, m b total line length, m

(IFFT) to place the frequency domain data into the time domain. A cyclic prefix is added, which protects against intersymbol interference and obviates the need for time domain equalisation. The figure shows the corresponding time domain representation of a single real DMT carrier (the lowest frequency carrier already considered) and assumes that all the other carriers are turned off (for illustrative purposes). The time domain signal then passes through an analogue-to-digital converter and low-pass filter before being transmitted. The plot at the right-hand end of the figure shows schematically a symbol with all carriers turned on (the prefix is not shown).

The principal strengths of DMT are:

the ability to apply dynamic bit-allocation, giving an inherent ability to maximise the use of available spectrum and to cope with multiple changing interferers flexible power spectral density (PSD) control (e.g. the ability to switch off carriers) a long symbol length, giving some degree of resilience to short noise-impulses an inherent programmable data-rate capability (with fine-adjust) .

Weaknesses are:

0 a large peak-to-RMS transmit signal ratio 0 a long symbol length, giving an inherent latency and

processing delay 0 a high perceived conceptual complexity (not to be

confused with implementation complexity) I

0 a long start-up (training) procedure (several seconds).

DMT lends itself very favourably to flexibility in an unpredictable RFI environment, but performance depends on several factors including the exact position of an interferer relative to a DMT carrier. The left-hand plot in Fig. 25 shows the effect on DMT of a single-tone interferer in the amateur band at 1.894 MHz. It is possible to enhance the performance of DMTin the presence of RFI. The effect

Fig. 25 Comparison of capacity against line length for (a) a modem with classical DMT and (b) a modem with one possible enhancement to DMT. The plots show the capacity with increasing levels of interference from a single-tone interferer at 1.894 MHz. The arrow indicates plots with increasing level of interference.

of implementing one such enhancement to DMT is shown in the right-hand side of the figure, to illustrate what can be achieved. The enhanced RFI rejection capability can be clearly seen. Such techniques may be essential for adequate operation of VDSL.

5 xDSL technical issues

There are a wide range of technical issues that will impact on how xDSL systems can be practicably deployed in the network. Some of these issues relate to choices that must be made in defining the characteristics of types of service that can be carried and the consequent particulars of the data transmission scheme. Examples of such issues include:

what maximum bit-rate to support (with the consequent reach trade-off) whether to support symmetrical operation or asymmetrical operation (or both)

0 choice of bit-rate flexibility to be supported.

Other issues relate to the particular choices of the specific modulation scheme, such as:

linecode 0 duplexing scheme 0 channel latency.

Finally, there are issues related to practical aspects of deploying a new type of technology in the access network. These issues include:

0 consideration of power provisioning at both the exchange-end and customer-end modems

0 dealing with bridge taps providing liieline services in the event of power failure

0 regulation and control of the frequency spectrum.

Some of these issues are discussed in a little further detail in the following sections.


It is worth noting that there is a difference between issues relating to requirements (which must be clearly defined in a standard) and issues relating to a particular implementation (which can be specific to the network within which the xDSL system is deployed).

Active versus passive NT In most cases, either an active network termination (NT)

or a passive NT is practical, and each has merits. Fig. 26 illustrates the principle of an active NT. The line

code is terminated at the NT, which provides a standardised set of user-network interfaces (UNIs) at the customer’s premises (e.g. Ethernet or ATM-25). In this scheme, a single end-customer xDSL modem serves one or more customer premises equipments (CPEs). This has the advantage that the customer only needs to purchase one modem, and can purchase cheap standard Ethernet (or other standard protocol) cards for his PCs and other broadband equipment. Additionally, an active NT decouples the telco’s network from the in-premises wiring, which is a big bonus for the telco, not wishing to inherit any problems with the premises wiring. The active NT approach is supported by ANSI and ETSI for VDSL.

Fig. 27 illustrates the principle of a passive NT. In this case, the line code is not terminated by the NT but is instead passed directly out to the user terminals (i.e. to the CPEs). In this scheme, each piece of CPE (e.g. TV, STB or PC) requires a separate xDSL modem.

It is also unclear whether the telco would own and have installation obligations for some customers. Telcos

generally do not want to have to open customer’s PCs, with the risk of inheriting future liability. Because the xDSL signal (possibly using frequencies to 10 MHz or more) is running over internal wiring, adequate measures must be taken to control ingress and egress interference from the in-premises wiring. Additionally, the passive NT scheme presents some additional challenges related to upstream access where multiple CPEs share the medium. The passive NT is favoured for VDSL by DAVIC (Digital Audio- Visual Council).

Duplexing: TDD against FDD It is possible to implement xDSL systems using either

frequency domain duplexing (FDD) or time domain duplexing (TDD). Again, the choice is a difficult one, and there are strengths and weaknesses to each approach. The choice of duplexing scheme must be made during standardisation.

TDD relies on a ‘ping-pong’ approach, where alternate time intervals are allocated for upstream or downstream transmission. The advantages of TDD include:

0 software selectable control of relative upstream and downstream periods

0 frequency separation (analogue) filters are not required

0 worse lines degrade upstream and downstream equally, as both directions share the same spectrum.

However, on the negative side for TDD:

active NT

splitter

narrowband

narrowband + xDSL

Fig. 26 Schematic diagram of simplified customer-premises architecture showing an active NT implementation

narrowband + xDSL

xDSL modem

guard periods are required 0 there is need to synch-

ronise across multiple lines (to avoid NEXT).

With FDD, operation is continuous in both directions, but the upstream and downstream transmissions are separated in frequency space, with each allocated different parts of the spectrum. Advantages of this scheme include:

0 the sampling rates for upstream and downstream can be different

0 spectral emissions from upstream transmissions (where powers are great- est on the customer’s final drop) are limited to only part of the spectrum, which can be carefully selected.

Fig. 27 Schematic customer-premises architecture showing a passive NT implementation

The negative aspects of FDD include:

144 ELECTRONICS & COMMUNICATION ENGINEERING JOURNAL- JUNE 1999

0 frequency guard-bands are required, which can eliminate regions of spectrum with high signal-to-noise ratio)

0 frequency separation filters are required.

Standard ADSL operates in FDD (with the possible inclusion of echo-cancellation to allow simultaneous use of regions of spectrum for both upstream and downstream transmission). For VDSL, the decision has not been taken at the time of writing, but will exclude echo-cancellation.

Spectral compatibility With increasingly more diverse tech-

nologies present in the access network,

LTE upstream downstream NTE t ----c

downstream - upstream c

'

i downstream d

VDSL TDD ('ping-pong')

ADSL FDD (continuous)

upstream c- L f

I

Fig. 28 Schematic illustration of possible FEXT and NEXT between a TDD VDSL system and an FDD ADSL system operating in adjacent pairs in a copper access network

each using different transmission schemes and different spectral plans, the issue of spectral compatibility between new and existing systems is a very important one.

XDSL must be designed to co-exist with other services on the same pair, usually narrowband telephony (POTS). However, coexistence of VDSLwith ISDN on the same pair may be important for some telcos. Additionally, xDSL must operate with other services present on adjacent pairs in the same bundle, including VDSL, ADSL, HDSL, ISDN, POTS, and T1. This is a particular concern where xDSL is deployed from the exchange.

With the increasingly deregulated environment for telecommunications, particularly with the unbundling of the local loop, telcos are starting to lose control over the services carried over the copper pairs in their networks. This issue alone is viewed by many observers as the most critical one facing mass implementation of broadband onto telco networks.

As an example of how careful spectral planning can mitigate against spectral compatibility issues, consider the situation where aTDD (ping-pong) VDSL system operates on an adjacent pair to an FDD (continuous) ADSL system, bo# from the exchange. Fig. 28 shows a schematic diagram of the spectral plan for both.

The VDSL spectrum is shown at the top, with the downstream spectrum on line one, and the upstream spectrum on line two. For normal TDD operation, both upstream and downstream use the same regions of spectrum. The third line shows the ADSL downstream spectrum, which overlaps with some of the VDSL spectrum. The bottom line shows the upstream ADSL spectrum, which has no overlap with any of the other spectral allocations.

There is possible NEXT between one downstream and another upstream signal, and FEXT between two upstream or two downstream signals. These are shown clearly in the figure, with regions of shared spectrum indicated by the hatched regions. The NEXT from ADSL into VDSL at the exchange could be significant, because of the large powers transmitted by ADSL (-40 dBm/Hz) . However, the issue can be avoided by operating the VDSL upstream spectrum to start only above the ADSL spectrum (i.e. above

1.1 MHz). This illustrates how careful consideration of the interaction between different services in the network can result in a better coexistence of the services.

Power spectral density As already discussed, xDSL systems must comply with

local EMC requirements, and must additionally not cause interference to any other licensed users of the radio spectrum. In practical terms, this places limits on both the power spectral density (PSD) and the total in-band power.

For HDSL, the total power transmitted is limited to +14 dBm (below 1.168 MHz for 2-pair HDSL), and the PSD is limited to -40 dBm/Hz. For ADSL, the total transmitted power is limited to +20 dBm (below 1-104 MHz), and the PSD is limited to -40 dBm/Hz. For VDSL, draft requirements limit VDSL to +10 dBm total power. There are variations between the ANSI and ETSI proposals for the VDSL PSD, but in broad terms the PSD is limited to -60 dBm/Hz for most of the band (300 kHz to 30 MHz), with the exception that the radio amateur bands are notched, with a limit of -80 dBm/Hz.

Powering For many telcos, deployment of active electronics into

the outside plant represents new territory. Although Cable companies have been doing this for some time, with HFC (hybrid fibre coax) networks, most telcos only have passive cabinets in the outside plant. This is almost universally the case in Europe, though in North America there are active digital loop carrier (DLC) cabinets.

For xDSL deployment using F'ITCab or F'ITK architectures, the line terminating equipment (LTE) modemswill need to be placed at the flexibility points in the outside plant, possibly in new cabinets. These modems will require powering, and it is possible that several hundreds of watts of power will be required at these new active cabinets when full.

Possible power sources for the LTE modems are:

0 network powering with unused copper pairs a dedicated new network power feed

ELECTRONICS 8.1 COMMUNICATION ENGINEERING JOURNAL JUNE 1999 145

0 local powering from the power utility network.

There are arguments for and against each of these, and it is not yet clear which one will become the dominant method.

An additional consideration is that power consumption in outside plant presents difficulties with heat dissipation, particularly in hot climates. Air conditioning, or at least the installation of heat-exchangers, may be necessary. There is anecdotal evidence from the early days of cable television, where the method adopted for cooling of the active cabinets on hot days was to leave the doors wide open!

It is generally assumed that powering at the network termination equipment (NTE) will be local, directly from the customer’s supply.

Bit-rate flexibility Because every network is unique, and every access line

different (in length, attenuation, crosstalk environment, etc.), different telco access lines will be able to support different data-rates. It is desirable for ADSL and VDSL modems to be capable of setting one of a number of possible data-rates. For example, DMT-based ADSL and VDSL can in principle vary the rate with a fine granularity, and CAP-based RADSL (rate-adaptive ADSL) gives some rate setting flexibility.

However, it is likely that telcos will want to limit xDSL service to a small set of rates, sufficient to deliver service in a maximum number of circumstances. Having afinite set of rates suitable for a broad range of circumstances is favoured, where the management of the service delivered to the customer can be made simpler than would be the case for a sliding scale of rates. Telcos will not want the modem to select rates under customer control, but only under network control.

In this model, the selection of the set of rates an xDSL network should deliver will take careful planning. There is always the danger that two neighbours will receive services with very different data-rates, and such circumstances must be managed. The alternative model, whereby a ‘best-effort’ (like for analogue modem access) approach using rate-adaptive ADSL is employed, would favour the new entrant operators and Internet service providers (ISPs).

Symmetrical Venus asymmetrical It is generally accepted that residential broadband

service requirements are broadly perceived to be asymmetrical, whilst business broadband service requirements are perceived to be symmetrical. The residential services are likely to be dominated by Internet access, with streaming video, etc. However, home working may be more symmetrical (LAN access, etc.). Also, Internet access could require large upstream bandwidths from the home.

ADSL is inherently asymmetrical by design. Current VDSL requirements call for the capability to provision both asymmetrical and symmetrical service, to allow VDSL to meet the needs of differing market segments. Simultaneous operation of both on a network would have an impact on the achievable performance of both.

Latency Numerous factors contribute to the latency of the xDSL

channel, including the processing and functional implementation of the modulator, and interleaving. Invariably, the latency requirements for different broadband services range from those requiring low latency (delay-sensitive services, such as voice delivery, video conferencing, etc.) to those where latency is not a critical parameter (delay-insensitive services, such as file transfer).

For ADSL, the ANSI T1.413 standard provides dual latency channels to cater for these services. An interleaved channel gives good protection against burst noise, but the latency (around 20 ms) is too long for delay-sensitive services. A separate (non-interleaved) fast channel is also specified, with latency less than 2 ms.

Current VDSL requirements call for either dual latency or a programmable single latency within VDSL. The implementation of dual latency inevitably adds cost, and it is possible that buffering may make a single interleaved channel suitable for most services (streaming audio, video, etc.), but not telephony.

Lifeline service requirements Most incumbent telcos have an obligation to ensure that

narrowband telephony is available in the event of power failure at either the operator’s or customer’s site. For standard POTS, this is ensured by arrangements made by the telco for backup powering at the exchange, and the customer telephone is line-powered directly from the exchange.

Where xDSL is implemented as an overlay, this POTS lifeline service must be maintained. This requires the POTS splitter to be transparent to the narrowband service in the event of local power failure, either at the customer premises or at the cabinet.

6 The xDSL market

No overview of xDSLwould be complete without touching on market-related topics such as trials, deployment and the standards that ensure market adoption of the technologies.

Standards The specification of an international standard is

important from the perspective of ensuring multivendor interoperability and clearing the way for large-scale deployment by derisking the vendors’ development work. Standard development in the xDSL arena is pursued by ETSI, ANSI, the ITU-T and ADSLForum amongst others.

ADSL and HDSL: HDSL does not actually have a standard, but rather is specified by a core specification5. The recent work within ANSI in specifylng HDSL2 (single pair HDSL at 1-5 Mbit/s) may well become a standard.

ANSI standardised ADSL in 1995 (T1.4133). Issue 2.0 was completed in 1999.

The ITU-T, through the G.Lite activity, is standardising


splitterless ADSL-Lite (G.992.2) as well as full rate ADSL (G.992.1).

W S L : VDSL is not yet a mature technology, and although it is out of its infancy, it is still developing. The period from 1996 to 1998 involved principally the definition of system requirements, and the emphasis now is on the drafting of international standards.

ETSI is standardising VDSLthrough theTM6 group and published its ‘Functional requirements’ in early 1998. These do not address specific implementation. It is currently working on the Transceiver specification’, including line code and duplexing.

ANSI is also working on a standard through the T1.El committee. Definition of an international standard is expected in 2000 at the earliest.

A number of other groups have also considered VDSL. The ‘Gx’ group of telcos (see the next subsection) has

considered VDSL specifically through a subgroup, which has defined a set of key VDSL requirements. A good summary is given in Reference 6. The Gx group endorses the work of ANSI and ETSI and has communicated its findings to both groups. No line code or duplexing method is advocated, and this will probably be left to ETSI and/or ANSI. The Gx telcos’ position is that the Gx VDSL requirements should be regarded as a guide for manufacturers in the absence of a published standard.

The Gx initiative: The Gx telcos are a group of partnering telcos, working together towards a common specification for a full services access aetwork (FSAN). The group includes Bell Canada, Bell South, BT, CSELT, DT, France Telecom, GTE, NlT, SBC, Swiss FIT, Telecom Italia, Telefonica and Telstra. The importance of the Gx group should not be understated; the partners own between them approximately 330 million access lines world-wide, which means that they collectively represent nearly 50% of all lines.

The primary aim of Gx is to enable operators to gain earlier, more reliable and lower price access to technology it believes its partners need. The philosophy is to derisk the vendors’ developments ahead of standardisation by converging operators’ needs and achieving the widest possible consensus on operators’ requirements.

Trials Trialing new xDSL technologies represents an

important step towards deployment for the telcos. Technology trials give the operator important information about the performance and key issues related to the deployment of the xDSL technology on their particular network, with their particular service mix. Additionally, market trials give valuable indications as to the likely penetrations, usage and ultimate success of the technology, and the information is often used to determine the service content and the pricing strategy to subscribers.

To date, ADSL has been widely trialed, whilst VDSL is only just entering the trial phase. Trials (both technology

and market trials) typically last 3 to 12 months with anywhere between 10 and 2000 users. However, trials with less than 100 homes connected are normal, and these are often ‘friendly’ customers (e.g. employees of the service company).

With the exception of a few early trials, trials have been driven by Internet and LAN access with (usually) telecommuting. Some trials have been full-speed (up to 7 Mbit/s downstream), but most have been limited to below 1.5 Mbit/s. Early trials for ADSL started in 1995, and although new trials are regularly starting up, we are now in a phase where established telcos (ILECs) are commencing serious ADSL deployment (at least in selected markets).

In Europe, trials have mostly been conducted by established operators (the unbundling deregulation is not as advanced as in North America). Trials typically started later than in North America. Different telcos in Europe have different requirements specific to their local markets. For example, the Belgacom 1998 trial in Antwerp, Brussels (and other locations) is geared for VoD, whereas Telecom Finland’s forthcoming trial is aimed at Internet and LAN access. In the UK, BT is currently trialing ADSL in its West London Interactive Services Network trial, whilst Kingston Communications has recently completed a trial involving 250 homes around Hull.

G.Lite trials have already been conducted in Oregon in late 1998 and in Singapore in 1999.

Whilst trials have proved important in smoothing the way for ADSL deployment, trialing will be even more important for VDSL, answering some of the as yet unanswered questions and in fine-tuning VDSL, thereby opening the way for an operator to deploy VDSL systems on a larger scale.

Deployment Deployment of ADSL started on a serious scale in North

America during 1998, but this was very much in selected targeted markets, in large cities, and on key exchanges. All the RBOC (Regional Bell Operating Companies) have now either started deployment, or have announced and are about to start deployment.

Some customers are already finding that they are being offered ADSL by both an ILEC and a CLEC, with up to 13 service providers competing in New York. This kind of competition will ultimately drive prices down and may lead to a widespread adoption of ADSL.

In Europe, stated deployment intentions vary according to country. However, some PlTs are already offering service, and it is likely that other major PITS will announce deployments soon. For example, Deutsche Telekom in Germany recently started a major roll-out of 100 000 lines in 1999.

A total of 70 000 ADSLlines were deployed world-wide at the end of March. Forecasts for xDSL deployment vary, but most agree that xDSL systemswill be deployed in large numbers. The forecast from TeleChoice7 is typical. It predicts an installed base of 500 000 xDSLlines in 1999 and 1 million in 2000.


Igor Czajkowski gained a BSc in Physics from the University of Surrey in 1987, and a PhD in 1990 in experimental and theor- etical physics of semiconductor optoelectronic devices, also from the University of Surrey. He joined Nortel Networks in 1991, contributing in the design of high-speed laser and modulator optoelectronic devices. Since 1994 he has been interested in broadband access, most recently with VDSL. He is currently with the Broadband Access team at the Nortel Networks Harlow Laboratories. Igor was a presenter for the 1993/1994 IEE Faraday Lecture tour, titled ‘Anyone, anywhere, anytime: the magic of telecommunications’.

Address: Broadband Access, Nortel Networks, London Road, Harlow, Essex CM17 9NA, UK. E-mail: [email protected]

Unbundling of the local loop In North America, the regulatory environment has led to

unbundling of the local loop. This enables competitive operators to gain access to customers using copper loops that they rent from the incumbent. A large number of CLECs and ISPs, such as Covad, Northpoint and Concentric Networks, offer ADSL service today using unbundled rented loops.

Unbundling in Europe is at an earlier stage, but already some countries such as Germany and Holland have moved to allow competitive service providers to rent existing copper. Other countries, such as the UK, are in the process of consultation and decision to define their unbundling strategies.

Unbundling poses some difficult questions which as yet have no clear answers. For example, how will cabinet- based DSL services be provided by independent CLECs when room is sparse in cabinets? Will there be a whole plethora of different coloured cabinets in close proximity, each with a different company’s modems? Furthermore, in the unlikely event of unforeseen interference from xDSL on the unbundled copper, will the ILEC have the right to ‘switch-off the CLEC service to protect its own services, and if so what are the legal implications? As has been demonstrated in this paper, the interference effects largely dictate the specification of the xDSL standards. Control of these spectrum issues will be vital for successful roll-out, and this suggests regulation of the spectral usage in the cable.

7 Summary

xDSL technologies are now well established as a route for telcos to maximise the reuse of their installed copper network in their drive towards a broadband future. HDSL is widely deployed to provide leased-lines and pair-gain, with some residential connections in the USA. ADSL is rapidly gaining a foothold, with operators moving from trial to deployment. Additionally, many observers believe that ADSL-Lite will bring mass deployment of high-speed

access for Internet. VDSL requirements are largely well understood, and standards are progressing. VDSL will provide the telcos with a high-speed copper access technology capable of extracting the most value from their copper as they migrate fibre closer to the customer.

Maximising the reuse of the copper network with high- speed copper access presents ever increasing challenges in understanding the network and its characteristics, modem coding and implementation, and spectrum management. The stage is now set for a breadth of customer choice as they move towards broadband. The success of xDSL technologies will to a large part be determined by the customers.

Acknowledgments

The author would like to thank many colleagues at Nortel Networks for invaluable support and helpful discussions. He is particularly indebted to Les Humphrey, Chris Tate, Roger Williamson, Mike Grant and Dave Phillips.

References

1 ‘Click till you drop’, Time Magazine, 3rd August 1998, pp. 40-45 2 Eckhard Pfeiffer, President and CEO, Compaq, Comdex

Keynote Presentation, December 1997 3 ANSI T1.413 Issue 1.0 (1995): ‘Network and customer

installation interfaces - asymmetric digital subscriber line (ADSL) metallic interface’. Issue 2.0 recently completed.

4 ETSI TS 101 27@1 vl.1,l (1998): Transmission and multiplexing (IN ); access transmission systems on metallic access cables; very high speed digital subscriber line (VDSL); Part 1: Functional requirements’

5 ETSI ETR 152 RTR/TM-06002 (December 1996): Transmission and multiplexing CM) ; HDSL transmission system on metallic local lines; HDSL core specification and application for 2048 kbit/s based access digital sections’

6 W S L copper transport system’, co-authored by the Gx/VDSL Telco partners, IEEE 8th International Workshop on Optical/Hybrid Access Networks, Atlanta, March 1997

7 TeleChoice, from Internetweek, 1st May 1998, p.18 (referenced by ADSL Forum)

Useful W W sites

http://www.adsl.com/ (ADSL Forum) http://www.etsi.fr/ (ETSI) http://www.ansi.org/ (ANSI) http://www.itu.ch/ (ITU) http://www.labs.bt.com/profsoc/access/ (Gx FSAN (full service access network)) http://china.si.umich.edu/telecom/telecom-info.h~l (links to an impressive wealth of telecoms information (operators, manufacturers, standards, news, etc.)) http://www.xdsl.com background, trials, modems, vendors and news)

(a raft of information on xDSL

OIEE:1999 Received 1st October 1998 and in final form 5th May 1999.


mailto:[email protected]

http://www.adsl.com

http://www.etsi.fr

http://www.ansi.org

http://www.itu.ch

http://www.labs.bt.com/profsoc/access

http://www.xdsl.com

high-speed copper access: a tutorial overview

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