Digital Subscriber Line Technology with a focus on Asynchronous Digital Subscriber Line

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Southern Methodist University

Digital Subscriber Line Technology

with a focus on

Asynchronous Digital Subscriber Line

by

Raymond McCurry

[Note: due to technical problems with use of a special graphics program, some of the figures in this version of the paper have a small image and excessive white space. This was not true of the original student paper. –RL]

Student No. xxx-xx-xxxx EE6302

Section 799 Fall 1997

[student contact information] [Address was here in original]

[telephone number]

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Abstract

Asymmetric digital subscriber line (ADSL) is a new modem technology

which converts existing twisted-pair telephone lines from your home, into access paths for multimedia and high speed data communications, at the same time allowing normal plain-old telephone service (POTS) to function independent of the data transmission channels offered by the technology. This is accomplished without any changes to the existing infra-structure of twisted-pair wiring between the subscriber and the Public Switched Telephone Network (PSTN).

ADSL is one “flavor” of the family of digital subscriber line (DSL) technologies, whose data rate is limited by the length of the twisted-pair from the PSTN to the

subscriber. The ability of ADSL to operate at data rates of more than 6 Mbit/s down-stream to the PSTN and up to 640 kbit/s updown-stream from the subscriber, relatively error free is due to several factors, such as advances in higher levels of integration in semi-conductor chips. This in turn allows real time Digital Signal Processing (DSP)

capabilities along with more efficient coding schemes, such as Carrier Amplitude Phase (CAP) and Discrete Multitone (DMT), which increase the bandwidth efficiency,

This technology has the potential to literally transform the existing Internet access from one largely limited by voice, text and low resolution graphics, using existing voice band modem technology, to a powerful, ubiquitous system capable of delivering a myriad of multimedia services. Some examples of these services that could be offered are real time video on demand (movies and TV), on-line video catalogs, remote CD-ROM access, and high-speed data and file transfer across corporate local area networks (LANs).

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Introduction

The huge explosion in Internet services and access, for both individual and business users has led to bottlenecks in data and information transfer on the World Wide Web. Surfing the Web is somewhat analogous to commuting during the rush hour, where travel is limited to short bursts of movement interleaved with waits until space opens up on the highway. Of course, on the Internet the cars and trucks are replaced with bits and bytes moving across the current infrastructure at data rates limited by the current

transmission media and technologies. In fact, accessing the Internet from home can be a frustrating and time consuming experience. Waiting for file, image and video downloads, making for long access times through the various Internet Service Providers via the PSTN makes for a slow, tedious process. Some impatient Internet users have facetiously referred to the World Wide Web as the “World Wide Wait”. However, there is light at the end of the tunnel for commuters on the Internet information highway. The Regional Bell operating companies (RBOCs) and telephone service providers have been working on ways to accelerate Internet access by speeding up transmission over the last few miles of copper wire, currently utilized in the POTS and standard voice band modems into the subscriber’s home.

Among the technologies attempting to break the Internet gridlock are cable modems, wireless cable, satellite feeds and DSL. Perhaps, the most promising of these is DSL because it utilizes your phone’s copper twisted-pair wiring to provide over 6 Mbit/s downstream from the PSTN to the subscriber. With no more than 18,000-ft. of

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twisted-pair now connecting approximately 650 million customers worldwide to the PSTN, a technology using the existing cable infrastructure offers a distinct advantage over the other technologies competing in this potentially very lucrative market.

The RBOCs are now providing Internet access to homes through POTS using customer provided 28.8 kbit/s and 33 kbit/s modems. However, the RBOCs want to get Internet users off the Central office (CO) switch, because they are tying it up at $20 a month or less, with connections that can last for hours. The RBOCs would like DSL service to have happened yesterday, but to implement it, they must upgrade their

hardware at the CO into the PSTN with appropriate DSL modems, and POTS splitters at both ends of the local loop. POTS splitters filter off the voice channel from the DSL data channels, to guarantee uninterrupted POTS, even if the DSL data transmission fails.

All seven RBOCs and most telephone service providers worldwide are involved in ADSL tests and trials, with some are planning commercial deployment by early 1998. To avoid the pitfalls encountered by the RBOCs in marketing and deploying Integrated Services digital network (ISDN), which has not become pervasive, ADSL will be

marketed differently than ISDN. To quote one RBOC representative “We will not market another four letter acronym, such as ISDN to the public. The customer wants fast Internet service and doesn’t really care about the underlying technology”. So it is likely it will be marketed as a “no-wait” Internet access at ‘x’ dollars per month.

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required to achieve the multi-megabit data delivery capability over the POTS.

Chapter 1 Digital Subscriber Line Technologies

DSL technology has evolved into a hodge-podge of acronyms which, has proven to be very confusing for the consumer and even for non-technical Telecommunications professionals. For example, there is Asymmetric DSL (ADSL); High-bit rate DSL (HDSL); Symmetric DSL (SDSL); 384DSL; Rate adaptive DSL (RADSL); Very high data rate DSL (VDSL); This confusion is partially due to how these technologies have been developed, over time, with different research organizations and different companies, each hoping their version would be adopted as a de facto standard for digital data

transmission over the local loop. Table 1 summarizes the various data rate capabilities, maximum cable distance, and standards applied to the DSL family of technologies.

HDSL is the oldest technology in the DSL family. It uses two twisted-pair wires to provide T1 and E1 (1.544 and 2.048 Mbit/s) equivalent service. It is preferable to analog T1, because it does not require providers to condition the twisted-pair lines by removing taps and inductor coils, as analog T1 requires to achieve higher data rates. HDSL works over 2 miles without the three repeaters that analog T1 would require for this distance. The coding scheme used by HDSL is a multilevel encoding approach in which pulses represent blocks of bits rather than single bits. This scheme is called “two binary one quaternary” (2B1Q) is similar to that used by ISDN. This encoding technique achieves 2 bit/s per MHz efficiency by grouping 2 bits and assigning a line voltage of + or – 1V and + or – 3V to each of four possible combinations. However, the 2B1Q coding

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Technology Standards Group Data Rate / Mode Distance (ft.) HDSL ANSI T1, ETSI DTR/DM-3017 1.544Mbit/s /duplex 2.048Mbit/s /duplex 12,000

SDSL None 1.544Mbit/s /duplex

2.048Mbit/s duplex

10,000

384DSL/symm. None 384 kbit/s duplex 18,000

RADSL None 0.032 to 9 Mbit/s to downstream, 0.032 to 1.5 Mbit/s upstream Depends on Data Rate VDSL Proposed ANSI, Proposed Digital Audio Visual Council

12.96 Mbit/s downstream, 25.92 Mbit/s downstream, 51.84 Mbit/s downstream Upstream ranges from 2 to 20 Mbit/s symmetric

to downstream

4500, 3000, 1000,

CAP ADSL Proposed ANSI 6.144Mbit/s downstream 640 kbit/s upstream

12,000

DMT ADSL ANSI T1.413-1995 6.144Mbit/s downstream 640 kbit/s upstream

12,000

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duplex mode and significant errors in transmission can occur, due to cable attenuation. This is in spite of the HDSL transceiver using DSP to perform electrical echo

cancellation by subtracting the transceiver’s own transmitted signal from the received signal. HDSL also, has an inherent interoperability problem, because the standard is not specific enough for different vendors’ parts to work together. The 2B1Q coding scheme is illustrated in Figure 1.

Another technology in the family is SDSL, which generically refers to modems that have equal bit rates downstream and upstream from the PSTN. SDSL reduces to utilizing one of the two twisted pairs used by HDSL, thus doubling the potential throughput for cable bundles from a CO. The It employs the same coding scheme as HDSL and therefore has the same limitations in data rate capability, except it cannot operate over as long as a cable distance since it only uses one single twisted pair wire. It has a greater interoperability problem than HDSL because there have been no clearly defined standards.

Another technology, 384DSL was proposed by Brooktree the semiconductor manufacturer. Their proposal was designed to implement a very low cost alternative to provide interactive Internet service on the POTS. The BT 8960 Zipwire™ modem chip permits data to travel greater distance over the same grade of cable, but at a lower data rate than ADSL. It uses a symmetric data rate upstream and downstream ( 384 kbit/s) and the 21BQ coding scheme. Brooktree uses a lower data rate based on the assumption that downstream and upstream Internet access have a near 4 to 1 ratio and not the 10 to 1 ratio as ADSL groups claim. The company also reasons that once the DSL bottleneck

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-1V -3V 00 3V 1V 01 10 11 BITS 1 0 SYMBOL DATA 3 1 0 2 -1 -2 -3 VOLTS

HDSL achieves 2bit/s per MHz efficiency by grouping 2 bits and assigning a line voltage of + or -1V and + or -3V to each of four possible combinations.

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opens up on the local loop, then the digital switch at the CO, becomes the limiting factor. Anecdotal evidence from Brooktree points out that, as the data delivery increases for users on a switch, the Internet response time levels off because of the bottlenecks in data provision [Billington, 95]. In other words why buy a car that goes 120 mph, when the only highway to travel on is so congested with traffic that you can only go 50 mph?

Another approach is RADSL, which takes advantage of an ADSL feature that some Internet providers refer to as “training”. Training allows modems at both ends of the DSL to find and operate at the highest error-free data rate that a cable setup and noise environment can support. This approach thus identifies and uses the best data rate for a specific cable. However, RADSL does pose a tariff problem for the provider because so many possible bit-rate services exist. To solve this problem, a standard for RADSL will probably limit the number of variances of the bit-rate that providers can make to the adaptive rates. Such a standard would greatly simplify tariff structures.

Standards bodies are working on the newest proposal, VDSL, to get maximum bandwidth from cable plant’s shorter installed twisted-pair cables. VDSL technology will resemble to a large degree ADSL, although ADSL must face a larger dynamic range and is considerably more complex to implement as a result. Essentially VDSL takes the ADSL concept to higher data rates over much shorter distances, which might prove be easier to implement than at ADSL data rates. In addition to the coding schemes of CAP and DMT, two other coding schemes have been proposed for VDSL. These are Discrete Wavelet Multitone (DWMT) and Simple line code (SLC). DWMT is a multi-Carrier system using Wavelet transforms to create and demodulate individual carriers. It also uses Frequency Division Multiplexing (FDM) for upstream multiplexing.

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SLC is a version of four-level baseband signaling that filters the baseband and restores it at the receiver.

ADSL, RADSL, and VDSL all need more complex coding schemes to achieve the higher data rates of up to 8 Mbit/s and more. This in turn requires greater DSP power in the modem devices. Increasing the number of DSP operations and the clock rate significantly increases power consumption. ADSL market trials are using two

coding schemes; 1) Carrier Amplitude Phase; 2) Discrete Multitone. Both types of coding schemes consume more power as the data rate increases. Proponents of both CAP and DMT almost fanatically support their technologies as a coding scheme for ADSL technology. Obviously, the one that is accepted as the standard for ADSL has much to gain, hence the fierce competition between the two. Thus the prize in the battle goes to the one that can best satisfy the RBOCs requirements of reliable performance at a

reasonable cost. Because the evolutionary predecessors of CAP have operated in modems for almost 20 years, the technology is available at relatively low cost. However CAP’s noise performance has not met ADSL objectives. On the other hand, DMT is relatively new and untested with ADSL developers still working on integration issues and

improving the algorithm efficiency. A more detailed description of CAP and DMT coding schemes will be given in Chapter 3, explaining their implementation with ADSL.

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Chapter 2 Basic Operation of ADSL

The copper twisted-pair cable that delivers POTS to every home is now carrying signals with 1000 times more information content than it did 20 years ago. The new technologies enable 1 Mbit/s to travel up to 20,000 ft. on 24 AWG wire with a receiver at the other end recovering the data. To reach this data rate, high-speed DSP silicon performs complex coding and error correction algorithms, and analog silicon performs spectral waveshaping.

Cable attenuation limits the maximum operating frequency on 12,000 ft. of cable to a little more than 1 MHz. Bandwidth efficiency, measured in bits per second per MHz which is the ratio between the bit rate and the used bandwidth. Using a coding scheme such as Nonreturn to zero (NRZ) on 12,000-ft. cable, limits the bit rate to approximately 1 Mbit/s. Therefore NRZ encoding bandwidth efficiency is 1 bit/s per MHz. This is not high enough for the high data rates that ADSL requires. In order to obtain data rates of more than 6 Mbit/s required by ADSL, coding techniques that employ multiple bits per symbol, error correction and echo cancellation are needed.

ADSL uses these advanced error correction and echo cancellation techniques in the two coding schemes currently competing to be the ADSL standard, i.e. CAP and DMT. ADSL depends upon the advanced digital signal processing and creative algorithms to increase the bits/s per MHz through twisted-pair telephone lines on the local loop. In addition, many advances have been required in transformers, analog filters, and A/D converters. Long telephone lines may attenuate signals at 1 MHz (the outer edge

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of the ADSL band) by as much as 90 dB, forcing analog sections of modems to work very hard to realize full dynamic ranges and channel separation along with maintaining low noise figures. The bandwidth used on the twisted pair is 1.2 MHz and 1 MHz

respectively depending on whether DMT or CAP modulation is employed to implement ADSL.

To create multiple channels, ADSL modems divide the available bandwidth of a telephone line in one of two ways; Frequency division multiplexing (FDM) or Echo cancellation. FDM assigns one band for upstream data and another band for downstream data. The downstream data path is then divided by time division multiplexing (TDM) into one or more high speed channels and an upstream band to one or low speed channels. Echo cancellation assigns the upstream band to overlap the downstream band, and separates the two by means of local echo cancellation, a technique well known in V.32 and V.34 voice band modems. With either technique, ADSL splits a 4 kHz region for POTS at the direct current (DC) end of the band by means of a passive filter. This bandwidth allocation for CAP is shown in Fig.2 and in Fig. 3 for DMT.

An ADSL modem organizes the aggregate data stream created by multiplexing downstream channels, duplex channels, and maintenance channels together into blocks, and attaches an error correction code to each block. The receiver then corrects errors that occur during transmission up to the limits implied by the code and the block length. The ADSL transceiver may, at the user’s option, create super-blocks by interleaving

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100 120 FIGURE 2 40 4 1000 FREQUENCY (kHz) ISDN

CAP UPSTREAM CAP DOWNSTREAM POTS

Frequency Division Multiplexing (FDM) can be used to carry POTS, CAP and ISDN on upstream channels and CAP downstream channels,

all on the same twisted-pair cable.

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impulse noise on video signals, introduces a delay of up to 20 ms. Whereas this is acceptable for video signals, it is not acceptable for LAN and Internet Protocol (IP) based data communications applications. Therefore, ADSL must be able to distinguish which type of signal is being transmitted, in order to decide whether to apply error control or not. This creates a complicated protocol which needs to be realized in the ADSL modem, which is well beyond the functions of simple data transmission and reception.

ADSL’s ability to work on the twisted-pair phone cable without interrupting the POTS is a key feature of the technology. POTS occupies the spectrum band up to 3.4 kHz and ADSL’s upstream signals begin about 25 kHz, providing a guardband between the two signals. POTS splitters combine the signals going onto the twisted pair and separates the POTS signals from the ADSL signals leaving the twisted pair. The splitter is a cross-over filter with significant design considerations for signal attenuation, testing, power dissipation, transient performance, noise immunity, and telephone interference. The splitters help protect ADSL receivers from POTS related signals, such

as dial pulses, ringing voltage, and ring trip. In addition the splitter must perform these functions without interrupting telephone service, if the power is turned off to the ADSL modem or it fails of its own accord.

Consideration should be given to using active impedance matching circuits to relax non-overlapping passband requirements and improve telephony performance

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FREQUENCY (kHz)

3.4 30 138 1142

FIGURE 3

Tones with noise interference carry fewer bits. POTS occupies the lowest available spectrum, and echo cancellation (EC) allows for

overlap of upstream and downstream.

POTS FDM

UPSTREAM DOWNSTREAM

EC

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the premise. This approach protects high frequency ADSL from the telephone wire tangle in the home. From the single splitter, new twisted pair should carry the ADSL signal to the modem. A lowpass filter at each telephone ensures that the ADSL does not interfere with the POTS.

Figures 4 shows the ADSL Reference model developed by the ADSL forum, which was formed in 1994 to promote the ADSL concept and facilitate development of ADSL system architectures, protocols, and interfaces for major ADSL applications. The Forum has more than 200 members representing service providers, equipment manufacturers, and semiconductor companies throughout the world [ADSL Forum,1997]

As shown in Figure 4, two ADSL modems have slightly different requirements because of the asymmetry of bit rates and the digital interface. The Va interface between the ADSL transceiver unit (ATU-C) at the CO and access node depends on the digital switch protocol to which the interface connects. This interface could be synchronous transfer mode (STM), asynchronous transfer mode (ATM), or both. A combination may be necessary in applications supporting digital broadcast and Internet services.

The T and T-SM interfaces between the ADSL transceiver (ATU-R) at the remote site and service modules are probably the least standard part of the system, although the ADSL Forum is now focusing on this area.

Design considerations for the Va interface are specific to the intended application. DSL “Access Multiplexer” (DSLAM) has become a generic term for this interface.

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ATU_R ACCESS NODE PREMISES DISTRIBUTION NETWORK ATU-C ATU-C ATU-C ATU-C ATU-C PSTN PHONES Digital Broadcast Broadband Network Narrowband Network Management Operations PC I/O PC I/O SET TOP ISDN ISDN ISDN TV

Reference Model from ADSL Forum shows standard defined interfaces Subscriber Loop Va Service Module Terminal Equipment Vc T T-SM T Service Modules U-C U-R U-C2 U-R2 Pots Splitters 17

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application, video on demand, ANSI ADSL includes “channelization,” which means that ADSL can simultaneously carry as many as seven independent channels: four high-speed simplex downstream-only channels, and three duplex symmetrical-bit-rate channels (Figure 5). Because these channels are simply bit pumps, they can individually carry any kind of protocol, such as IP traffic. In an application simultaneously supporting multiple video-stream and Internet accesses to many ATU-Cs, it would be difficult to meet the real time considerations of the video stream and simultaneously support high bandwith for downloading a large Internet file over the same multidrop bus. A high bandwidth standard bus such as personal computer interface (PCI) may be able to handle such a system, but the bus arbiter would need to limit arbitration latency and provide fairness. A design using ADSL channelization would require a PCI arbiter to work with priority requirements for individual streams to multiple channels on each ATU-C.

Another important factor in large scale deployment of ADSL modems in a central office is power dissipation, because the central office has limited space and cooling capabilities. Realizing the simple retrofit of ADSL modems to loops requires a modem design using insignificantly more power at the central office, than is currently available. at the central office. The ADSL modem design, then must consider trade-offs for power conservation.

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FIGURE 5 ACCESS NODE ATU-C PREMISES DISTRIBUTION NETWORK AS0 AS1 AS2 AS3 LS0 LS1 LS2 AS0 AS1 AS2 AS3 LS0 LS0 LS1 LS2 OPERATIONS MAINTENANCE AND CONTROL MAINTENANCE INDICATORS

ANSI ADSL can simultaneously carry as many as seven independent channels: four high-speed channels, simplex, downstream only channels

and three duplex, symmetrical bit rate channels NOTES:

LS1 AND LS2 ARE OPTIONAL DUPLEX CHANNELS FOR UPSTREAM TRAFFIC. LS0 IS THE CONTROL CHANNEL.

ATU-R

AS0, AS1, AS2, AND AS3 ARE SIMPLEX DOWNSTREAM CHANNELS.

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Chapter 3 Coding Schemes for ADSL

Manufacturers and users align themselves behind one or other of these technologies. The American National Standards Institute (ANSI) ADSL working group has adopted DMT as the standard coding. However, CAP appears to have a lead in actual modems in trials worldwide. CAP promoters continue to re-submit the technology at ANSI working group meetings in an attempt to get those groups to ratify it as a standard. Meanwhile, end-users are benefiting from the competition between CAP and DMT. The battle to become the de facto standard is driving improvements in power consumption, integration, error

correction, and performance.

CAP uses a multilevel 2-D encoding scheme to represent groups of 3 to 8 bits with 2^3 or 2^8 symbols. A unique combination of signal amplitude and phase shift represents each symbol. CAP takes the 2B1Q multilevel coding to a more complex level. CAP evolved from quadrature Amplitude modulation (QAM), a proven technology that standard voice-band modems use. CAP combines QAM with efficient spectral shaping, which smoothes the transmission line pulses and enables each pulse to represent more data bits accurately. The smoothing also reduces the high-frequency components of the signal, thereby reducing bandwidth requirements and improving bandwidth efficiency.

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frequency interference and distortion. Reed-Solomon and eight state trellis coding is used to accomplish error correction on the downstream channel [Ungerboeck, 1987].

The upstream channel uses only trellis coding. FDM is used to carry POTS, ISDN, and CAP upstream channels and CAP downstream channels all on the same twisted-pair cable. The spectral assignment for CAP is shown in Figure 2.

CAP RADSL enables a modem to adapt to the signal conditions from impedance discontinuities and noise on the loop. During a cold start, the transceiver automatically determines the loop conditions by sending and receiving measurement signals

This information is then fed to the host processor, which then adjusts the error-free data rate to the highest possible rate that can be tolerated by the loop conditions. The

transceiver can independently adjust downstream and upstream rates by varying the bits per symbol from 3 to 8 bits and the baud rate to 360, 656, or 952 kbit/s. (The upstream baud rate is only 136 kbit/s.) This approach yields line rates of 680 kbit/s to 7.616 Mbit/s in 320 kbit/s steps in the downstream direction and 136 kbit/s to 1.088 Mbit/s in the upstream direction. During operation, the transceiver can again perform the rate-adaptive procedure if measurements of the eye pattern scatter exceed preset limits, indicating a hazardous noise profile on the subscriber loop.

Few semiconductor suppliers have developed silicon for CAP implementations of ADSL. This has in turn, limited the adoption of the technology. To help promote the technology, Globespan, a New Jersey Company has published a specification for line side characteristics that should help other CAP vendors solve interoperability issues.

In contrast to CAP, DMT divides the available spectrum into 256 subcarriers, each having a frequency spacing of 4.3 kHz and carrying a sub-channel, called a “bin”,

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or “tone” (Figure 6). DMT assigns as many as 16 bits to each bin, depending on its capacity for error free transmission, and depending on cable loop conditions, such as attenuation, impedance discontinuities, and noise, from power supply and Radio Frequency (RF) characteristics. DMT encodes the bits for each tone using signal amplitude and phase shift to produce a burst for that tone. This approach is similar to to that of CAP QAM coding, only for many discrete tones. An inverse Fast Fourier transform (FFT) operation in the DSP section of the modem changes the discrete tones in the frequency domain to a waveform in the time domain. So, essentially, the transmission line pulses are the sum of all the tone bursts. At the receiving end, an FFT function is performed in the modem, breaks the transmission line pulse into discrete tones and recovers the original digital information. The performance can be enhanced by adding optional complexity with forward echo correction (FEC), trellis coding, and echo cancellation.

DMT uses cyclic redundancy check (CRC), scrambling, and FEC to ensure accurate data transmissions. It adds the CRC bytes to the data stream as an overhead after channel multiplexing. DMT then uses Reed-Solomon coding and interleaves and

processes the data according to the order of the tones.

The ANSI ADSL standard for DMT specifies categories 1 and 2 for performance. To comply with the standard , a design must meet Category 1, or the “basic” feature set. Category 2 enables higher bit rates, longer distances, and better performance under poor

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4.3kHz FREQUENCY FREQUENCY BITS/SUBCHANNEL CABLE PASS ABILITY FIGURE 6 (A) (B) A + B = C DMT divides the spectrum into 256 subcarriers, each having a frequency spacing of 4.3 kHz and carrying a sub-channel called a "bin", or "tone". Tones with noise interference carry fewer bits.

FREQUENCY

(C)

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Viterbi decoding at the receiver provide a maximum likelihood sequence estimation of received bits, a bandwidth efficient way to add redundancy to the data. This approach is useful for enhancing performance on long cables. Trellis code works against crosstalk, which is more significant problem in long cables. Echo cancellation increases the bandwidth by overlapping the upstream and downstream frequency “bins”. Developers based the technology on the principle that overlapping frequency “bins” know the outgoing signal and, therefore, can subtract a delayed and reduced amplitude copy of the outgoing signal from the incoming signal to get the true signal. This technique permits an increase in either the upstream or downstream bit rates, without sacrificing one stream’s bit rate for the other’s [Ruiz, June 1992].

POTS occupies the lowest end of the available spectrum. The upstream signals occupy the mid-range, and the downstream channel uses the upper part of the spectrum. The ANSI standard requires FDM, and optional echo cancellation can increase the usable spectrum and, therefore, the bit rate, for either the upstream or downstream channel.

Like any other technology, both DMT and CAP each have many pros and cons. For DMT, cons include its complex, time consuming and power consuming initialization and RADSL procedures. On the other hand, CAP’s RADSL procedure is simpler and slightly more efficient than that of DMT. In addition DMT’s use of FFT calculations causes latency delays that could cause problems with time sensitive services, such as telephony and video. On the pro side, however, these are the same features that enable

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The coding complexity of DMT complicates such techniques as FEC and echo cancellation. The spiraling increase in DSP power results in more watts per bit, and power consumption becomes important in the CO switch facility, which has to integrate mass deployment of ADSL technology. The increased power usage results in space and cooling considerations, which, in turn, lead to maintenance and reliability concerns. The simple upgrade to ADSL then has side effects that are only appreciated when they reach the total system level. On the other hand, DMT developers could counter this drawback by using higher integration and improved processes in silicon, such as complementary metal oxide semiconductor (CMOS), which requires less power to operate devices in ADSL modem designs.

Because the subcarriers for DMT have a relatively narrow frequency range, equalization is not such a concern as it is with CAP. Additionally, DMT can use shorter loops to provide higher bit rates, because it can adapt to more usable bandwidth. The flexibility of DMT appears to give it an advantage with online reconfiguration and bit swapping to avoid electro-magnetic interference (EMI).

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Conclusions

The competing technologies for fast Internet access services such as digital satellite broadcast, cable modems, and wireless cable will not be standing still, while the ADSL battles are shaking out a clear winner. In order for DSL services to become a real success story, and pervasive, economic factors as well as technical issues will have to be strongly considered. For example, what kind of services will be offered to individual subscribers and businesses, and what will they cost? Possible services are video on demand (movies and TV), “No-wait” Internet service, large data file transfer,

and imaging access for industry and medicine. The cost of these services will have to be comparable to other similar services offered now, such as cable TV and dedicated broadband cable access. The marketing of services to users may be offered incrementally in terms of cost of the service. For example, the customer could just pay for the specific data rates needed for any given application, therefore, they would only pay for the specific data rates that they need, rather than one price for the full data rate available. As some of the technical issues are resolved the ADSL silicon chip sets will become more highly integrated which will cause the ADSL equipment manufactures to produce cheaper hardware solutions. This evolution in hardware development should translate into more cost-effective implementations for the RBOCs, and hence to competitive

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as an excellent opportunity to get in at the ground floor with the new DSL technology and gain significant market share, to sell their devices to modem manufacturers, who in turn will sell these to the RBOCs and subscribers. Companies such as Motorola, Analog devices, and Texas Instruments are forging ahead with their chip set implementations of ADSL. However, because of the fact that there are still competing coding schemes for the implementation of ADSL, there is still a great risk for incompatibility across the vendors and RBOCs. Nevertheless, it appears that the majority of vendors of ADSL modem chips sets are leaning towards DMT implementations.

ADSL has already demonstrated blazing Internet access while maintaining the POTS on the same twisted-pair cable. Two questions cloud the crystal-ball look ahead into the future. Which of the brawling contenders, CAP or DMT, will control the

subscriber loop, and how will a technology simultaneously deliver multimegabit streams to millions of nodes? The interoperability question of DMT is a concern now, but, when that problem is solved, the competition between the various vendors and the fact of deployment, on a large scale could drastically cut costs of hardware implementation. CAP has slightly less to worry about, with respect to interoperability, but must demonstrate that it can perform as well as DMT in the presence of real-world noise. Delivering multiple streams means adapting the ADSL interface to suit the best switching method available, which is probably ATM. The ADSL Forum understands this problem and is working with T1E1.4, ETSI, and the ATM Forum to develop standard system interfaces to help the data streams flow. The home browser is sure to see who will become the dominant force and become the multimegabit master of the twisted pair

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Digital Subscriber Line Technology with a focus on Asynchronous Digital Subscriber Line