The Asymmetrical Digital Subscriber Line (ADSL):
A New Transport Technology for Delivering Wideband Capabilities to the Residence
David L. Waring
Bellcore
445 South Street, Morristown, NJ 07960 USA
Abstract
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This paper provides an overview of the newly emerging Asymmetrical Digital Subscriber Line (ADSL). The ADSL is an access technology intended to provide one- way 1.5 Mbps digital transport from the network to the customer over the existing nonloaded copper loop plant. The ADSL is an attractive transport technology for economically delivering asymmetrical wideband services to the residence. The 1.5 Mbps signal is modulated above a conventional baseband analog voice channel, and a low-speed full-duplex digital control channel is also provided, all over one copper pair.A brief overview of ADSL technology is provided, describing important technology considerations, including noise impairments, spectral compatibility and "data over voice" design challenges. More detailed studies are left for subsequent papers in this session.
Next, potential service applications utilizing the unique advantages of the ADSL are discussed, including advanced videotext, compressed video and education applications.
Finally, this paper addresses network architecture and interface issues. Physical configuration at the customer premises and associated interfaces are discussed, along with a high level view of possible network switching and control architectures.
1. Evolution of DSL Technology in the Loop Plant The loop plant today is dominated by copper facilities. About 25 to 30% of this copper plant is "loaded," or contains load coils to equalize the 3 kHz voice frequency band. Loaded loops are not of much utility for providing higher bandwidth services. Of the nonloaded loop plant, most loops meet resistance design rules and are less than 18 kft in length.
Since a given customer's loop will typically include a variety of splices, gauge changes and bridged taps, transmission characteristics vary from loop to loop. These variations are not that significant for voice frequency service, but when attempting to provide high bandwidth digital transmission these slight variations have to be compensated for with high precision. Previous generations of loop transmission electronics required that equalizers and echo cancellers be adjusted by craft, using test sets to generate tones and determine settings. Digital Subscriber Line (DSL) technology uses self-adaptive digital filtering techniques to automatically adjust echo cancellers and equalizers to the particular cable pair being used. And DSL technology continuously adjusts itself to changes in channel characteristics that will occur due to environmental effects,
such as temperature changes, moisture changes in cables and aging of electronics.
DSL technology was the basis for the ISDN Basic Rate Access standard that provides ubiquitous digital access, delivering two 64 kbps "B" channels and a 16 kbps "D" channel over the majority of the nonloaded loop plant. Deployment of the DSL for ISDN is a significant milestone in the provision of digital services over the embedded copper plant. Although the theoretical basis for DSL technology has been well understood by scientists and engineers for decades, it wasn't until the 1980's that Very Large Scale Integration (VLSI) had advanced sufficiently to make the DSL economically practical on a per-circuit basis.
With continuing advances in VLSI, there is now active research to identify opportunities to apply DSL technology at higher rates. Research at 1.5 Mbps has led to the High bit-rate Digital Subscriber Line or HDSL for providing DS1 rate access. The HDSL is a technology replacement for existing T-carrier repeatered lines used in the loop plant today. Referred to as "Repeaterless Tl," HDSL will eliminate capital expenditures associated with placing line repeaters and will eliminate much of the complex engineering associated with T-carrier systems. The HDSL will operate over Carrier Serving Areas (CSAs), roughly equivalent to 9 kft of 26 gauge cable or 12 kft of 24 gauge cable, using two pairs in an architecture referred to as "dual duplex." Recent survey work shows that about half of the nonloaded loop plant conforms to CSA guidelines. Furthermore, data indicates that more than 80% of T-carrier lines currently deployed to provide DS1 rate access are within CSA range. This is because DS1 service is predominantly a business service, and by design most potential business customers are close to a wire center or within a CSA.
2. A New Wire-Pair Wideband Access Technology for the Residence
The HDSL may be somewhat restrictive for residential services. The need for two pairs and the CSA range limitation may prove prohibitive in certain areas where a significant portion of customers are beyond 12 kft or where the number of pairs per living unit are less than two. Thus the genesis for ADSL was to architect a new transport access capability to provide economic wideband services directly to the residential customer. Objectives are to cover most of the nonloaded loop plant, out to 18 kft as is the case for Basic Rate Access. In addition, it is highly desirable to reduce the number of pairs required, since pair-per-living unit ratios can run as low as 1.3 in some areas.
By using an asymmetrical architecture, the ADSL may be able to achieve all of these objectives. in full duplex systems like the DSL and HDSL, self near-end crosstalk or self-NEXT
Figure 1, other transmitters operating on other pairs within the same binder group will couple into the channel under consideration and interfere with its receive signal. By making the transmission asymmetric, self-NEXT is eliminated since all of the transmitters are located at the office end of the loops and all of the receivers are located at the customer ends. This removal of the self-NEXT impairment, at the expense of a one-way or simplex transmission capability, is the basic mechanism that will allow ADSL to operate on one loop over
an extended range.
-, -\
-
NEAfI END CROSSTAW
l" HDSL
Figure 1. Pictorial Description of Crosstalk Systems employing "data over voice" techniques have been developed for providing lower speed, full duplex data capabilities along with baseband voice service or POTS (Plain Ordinary Telephone Service). ADSL will use a similar approach, in which the 1.5 Mbps payload is placed above the voice band. Frequency division multiplexing, implemented with low and high pass filters in conjunction with the line coupling circuit, will be used to separate the two channels.
Initial studies of service applications and network control and maintenance architectures have identified the need for some type of low-speed reverse control channel, allowing the customer to signal the network and allowing the remote ADSL unit to pass maintenance information to a centralized operations center. This third, low-speed control channel could also be separated from the POTS channel and the 1.5 Mbps channel by using frequency division multiplexing techniques. The resultant ADSL architecture is depicted in block diagram form in Figure 2.
POTS SWITCH POTS
LOW SPEED CONTROL
3. ADSL Technical Challenges
The concept of a transport capability that can deliver 1.5 Mbps to a residential customer along with a low-speed digital control channel while preserving normal POTS service, all over the existing telephone line, is very exciting. To turn this concept into reality, in which economic ADSL systems can be widely deployed in the loop plant in a reliable fashion with minimal engineering restrictions, will require close examination of several technology issues. To date a complete ADSL system that satisfies all of the above objectives has not been prototyped. Plans for experiments in which a small number of ADSL lines are deployed in the field are being planned for next year. In the meantime, many technology issues can be examined analytically.
3.1 Receiver Sensitivity
Perhaps the most immediately obvious transmission challenge associated with the ADSL becomes apparent when one observes the high levels of signal loss that will be experienced at the extreme range of the nonloaded loop plant. Figure 3 shows a simulated ADSL receive signal at the end of an 18 kft, 24 gauge cable. Because the loop response characteristic drops off rapidly with increasing frequency, a 2.5 volt ADSL transmit signal will only be on the order of a millivolt by the time it reaches the receiver. A 16-QAM system in a white Gaussian noise environment needs a peak signal to rms noise ratio of 24 dB to achieve a bit-error ratio of lX10-7. When engineering practical systems, 6 to 12 dB of margin should be added to this theoretic SNR to account for a host of other impairments, including imperfect implementations of the receiver (for example, finite precision and finite length digital filtering), office and customer wiring, additional attenuation due to splices and temperature and moisture changes in the cable, and other noise impairments above and beyond the noise model used to calculate the original SNR.
'
Thus, if a 30 to 36 dB peak signal to rms noise ratio is required at the detector, and the signal at the receiver for one channel is only 1 mV, the noise at the receiver must be less than 60 to 30 pVms. Across a bandwidth of 400 kHz and a termination of 100 ohms, this translates into a noise floor of -130 to -136 dBmMz. This presents a significant design challenge in terms of the receiver sensitivity. The noise voltage due simply to the electronics operating with a bandwidth sufficient to receive an ADSL signal will be several pV.1''As can be seen, at the extreme range limits of the nonloaded loop plant the ADSL system begins to stress the practical limitations of receiver sensitivity design. One simple way to ease this problem would be to use a more powerful transmit signal. But unfortunately, as will be discussed below, this may cause the ADSL to excessively interfere with other systems in the same binder group. A rough rule of thumb is
1. Typically the SNR is calculated using a theoretic model for crosstalk noise. Additional margin provides protection in the event that the actual noise environment found in practice is more severe than the modeled noise. For example, impulse noise, radio frequency interference and other noises generated in the customer's premises can occur, but are dimcult to predict and model.
Figure 2. ADSL System
Block
Diagram Depicting Threethat spectral compatibility can be preserved among different digital services sharing the same binder group if the transmit voltage level is limited to approximately 2.5 to 3.0 V. The DSL and HDSL will have 2.5 V transmit signals, and T- carrier uses a 3 V AMI line code. It may be possible to increase the ADSL transmit voltage to around 6 volts, raising the required noise floor to 120 to 60 pVm. But a further increase beyond 6 volts is not likely to be spectrally compatible.
"I" Channel 1 Volt Pulse Response Over 18 kft Of 24 AWG
0.5
I n
0 3 6 9 12 15 18 BAUD PERIODS (16-QAM 400 KBAUD)
Figure 3. An Example ADSL Receive Signal 3.2 Spectral Compatibility
In the discussion above we saw. that a noise floor somewhere in the neighborhood of 60 p V m on the longest loops must be accommodated to ensure reliable operation of the ADSL. One of the sources of this noise will be crosstalk from other systems sharing the same binder group. Since the ADSL transmits the 1.5 Mbps payload from the network to the customer only, there will be no self-NEXT to contend with. However, there will be far-end crosstalk or FEXT as transmit signals from other systems propagate in parallel with and couple into the ADSL. Conventional
FEXT
considers such crosstalk when all of the transmitters are co-located in the central office. At ADSL frequencies of operation conventional FEXT does not appear to be a significant impairment. Most of the FEXT couples into the ADSL channel within the first several kilofeet of the central office, where the ADSL signal is still relatively strong.Of much more concern are situations where the souFe of crosstalk is not in the central office, but rather in the distribution plant. Such crosstalk is referred to as unequal- level crosstalk, and unequal-level crosstalk will be a limiting impairment for ADSL.
There are several mechanisms that can create unequal-level crosstalk. Conventional DSLs used for ISDN Basic Access have most of their transmit energy concentrated below 40 kHz, but the lobes of the 2B1Q transmit spectrum, although decreasing with frequency, continue well up into the ADSL band. The current standard for the DSL calls for a modest 2nd order roll-off of transmit power above 40 kHz. If there is widespread deployment of ISDN in a neighborhood, the transmitters at the customer ends can enter a binder group relatively close to an ADSL receiver, and the cumulative
effect of the upper lobes of many DSLs can disturb the weak ADSL receive signal.
Another practical case will occur when HDSLs share the same binder group with ADSLs. Here the remote HDSL unit will create significant unequal-level crosstalk which will couple into the ADSL. Fortunately this interference will usually originate within a CSA, where the ADSL signal is stronger. Once an ADSL pair leaves the CSA, both the signal and the HDSL-induced crosstalk noise powers will be attenuated approximately equally. This, coupled with the fact that the HDSL and ADSL spectra will be designed to occupy separate frequency bands, should make the two systems compatible.
Detailed crosstalk simulation studies presented later in this session [21 show that the ADSL transmit spectrum must be carefully designed to enable ADSL systems to be widely deployed along with other systems. Unfortunately, these results also show that just one conventional T-carrier or HDSL system located near an ADSL receiver at the limits of the nonloaded loop plant will create unacceptable crosstalk. Since ADSL is a residential transport capability, one might not normally expect this to be a problem, since conventional DS1 service is not expected to be used by residential customers. However, quite often cables feed both residential and business customers. A long feeder cable might contain T-carrier, serving a small business park, as well as ADSLs serving housing developments. This problem is mitigated to some extent by the common practice to place T-carrier in separate binder groups, but this is not always the case. As depicted in Figure 4a, some type of engineering guidelines which restrict deployment of T-carrier and HDSL together with ADSL beyond a CSA now appear to be necessary.
9.12 kH
Figure 4a. ADSL Spectral Compatibility around a Wire Center
Although HDSL is limited to CSA range, HDSL systems will often begin where a fiber system ends, for example at a
remote electronics site. In this scenario it is likely that copper feeder cable is still in place and might be a candidate for
ADSL service. However, the HDSL crosstalk will be unacceptable to ADSL systems in that cable. One solution to this problem, as shown in Figure 4b, is to move the ADSL central office units out to the remote site, along with the HDSLs and other remote electronics. If the CSA administrative guidelines are being followed, all customers including ADSL customers should be within CSA limits. Customers beyond this range should fall into another, adjacent CSA.
GROUP
Figure 4b. ADSL Spectral Compatibility within a CSA
3.3 The Noise Environment a t the Residence
Perhaps the greatest uncertainty to confirming the viability of ADSL deployment revolves around the noise environment that will exist at the customer’s premises. The underlying viability of the ADSL hinges on the expectation that this noise environment will be relatively benign. Impulse noise, a common noise impairment found in the telephone network, is attributed to several mechanisms including high level
POTS
signaling voltages and currents which couple into pairs in close proximity on a distribution frame or in a binder group. Another source of impulse noise is heavy machinery operating in a central office or office building. These types of impulse noise are expected to diminish in a more residential setting, but this needs to be confirmed. In addition to impulse noise, there may be other forms of noise such as radio frequency interference which couple into drop and premises wiring. Much work has been done to understand the subscriber loop noise environment (See [31[~1 for example for a summary of, results), but the power levels and frequency of occurrence of impairments at ADSL frequencies and in a residential setting need to be better understood. To this end, Bellcore will conduct a small field experiment to record and analyze wideband low amplitude residential noise.[*]Publically available studies on impulse noise in North America are dominated by data taken at central offices and in large office buildings. Nevertheless, it is instructive to use this data to simulate the effects on the transmission rformance of ADSL. Survey data gathered in NYNEXr5~was used in computer simulation studies described in more detail later in this sessionJ6] in which over lo00 recorded impulses were digitized and injected into a simulated 16-QAM ADSL receiver. Unfortunately the study found that over 50% of
these impulses caused receive errors. The effects can be mitigated somewhat by increasing the transmitter maximum voltage level from 2.5 V to 6.0 V. But over 30% of the impulses still cause errors. The implication of these findings, subject to a better understanding of residential noise, may be that some type of coding will be necessary to combat impulse noise.
3.4 DOV Design Challenges
As has been discussed above, the relatively weak ADSL receive signal may be vulnerable to a variety of noise impairments coupled into the ADSL system from surrounding sources. In addition, within a single ADSL system itself there is the potential for interference between the three derived channels; the one-way 1.5 Mbps channel, the POTS channel and the low-speed control channel. As discussed, these channels will most probably be separated by using frequency division multiplexing, implemented by using appropriate filtering in the analog line interface. In particular, separating the POTS signaling voltages from the ADSL receiver will be challenging. Ringing voltage applied at the central office is 86
V ~ S superimposed on -48 Vdc with a 2 seconds on, 4 seconds off pattern. When ringing is interrupted due to a telephone handset going off-hook, referred to as ring trip, a considerable transient can be induced in the line. Other lower level transients are produced by dial pulsing and on-off hooks. Theoretically filtering will suppress high frequency components of these transients so that they will not interfere with the 1.5 Mbps and control channels. In practice, however, these high level voltage transients can create circuit leakage currents and nonlinearities that could affect receiver performance.
One way around this type of problem would be to encode the ringing and regenerate it in the ADSL unit near the customer’s premises. However, it is very desirable for the ADSL to be transparent to POTS service, including dial pulsing, touch tones, ringing, meter reading signals, calling number identification signaling, and any other low speed telemetry or signaling that may be in use today. This will eliminate a spate of compatibility, provisioning and powering problems.
4. Potential ADSL Technologies
There are at least three different classes of transmission technologies that can be considered for ADSL. And within each of these classes, additional techniques such as coding can be applied. Although the use of baseband line codes might appear to be a misapplication for ADSL, they might be considered, with a performance penalty that will be dependent on the desired location of the 1.5 Mbps signal. A baseband line code transmitted over a loop using transformer coupled line interface circuits actually looks somewhat passband in nature, because the line coupling transformers cause a null at DC. This shaping could be accentuated with filtering to produce a bandpass signal. The lost low frequency energy would degrade performance somewhat. The extent of this degradation will depend on where the lower band edge of the 1.5 Mbps begins. For the ADSL, frequencies in the range of 20 kHz to 100 kHz are being considered for the lower band edge. This would require significant shaping of a baseband spectrum, and probably makes other transmission methods more appropriate for this application.
Quadrature Amplitude Modulation, or QAM, uses a carrier to modulate the transmit signal into a passband. This modulation format is well suited to the ADSL application, where the 1.5 Mbps spectrum needs to "fit" above the POTS and the low-speed digital control channel spectra. From a spectral compatibility point of view. the 1.5 Mbps transmit spectrum also needs to fall between the HDSL spectrum, a baseband system using the 2B1Q format operating at 392 kbaud, and T-carrier which uses an AMI line code with transmit power spectral density centered at 772 kHz. A "camerless" implementation of QAM, referred to as Carrierless AM/PM,T]I reduces hardware complexity while preserving the flexibility of placing the signal in a desired portion of the spectrum.
QAM is a well understood technology, used extensively in modem and radio applications. The major difference for the subscriber loop application is that the channel is not flat and noise is colored. Since these characteristics vary from loop to loop, the optimal baud rate and signaling constellation will
also vary. However, preliminary studies have shown that either 16-QAM or 64-QAM are almost always optimal on longer loops, and the loss in performance using 16 QAM exclusively is minimal.
Another family of approaches, in which the channel is divided U into subchannels, has received much recent study.[*] A paper in this session will describe the multicarrier approach applied to ADSL.r'ol In a multicarrier system, each of the subchannels is individually optimized for maximum throughput. The lower frequency subchannels can be intentionally nulled, so that the ADSL system will place no energy below a selected frequency. Similarly, an upper frequency cutoff could be established. Given sufficient precision hardware and enough subchannels, this scheme makes optimal use of the available bandwidth. A multitone system will require feedback from the receiver to the transmitter during training.
When optimized against a given channel, and assuming idealized implementations, QAM and multicarrier systems provide similar performance.["] Both schemes can be enhanced with coding. Trellis codes have been widely studied and successfully applied in modem and radio applications, where the channel and noise spectra are both relatively flat and the noise is approximately Gaussian. Trellis codes are attractive because they can achieve performance gains without consuming additional bandwidth. But the coding gains achieved in a flat AWGN (Additive White Gaussian Noise) channel, where Trellis codes have been optimized, will not be realized in the subscriber loop application where both the channel response and the crosstalk noise vary substantially with frequency across the transmission band.
Techniques for overcomin this problem have received much recent [13] [141q151 and could be applied to a QAM system. Techniques such as precoding move the feedback filters of a conventional decision feedback equalizer from the receiver to the transmitter. The channel is essentially pre-equalized, and then a Trellis code is effectively applied to the equalized channel. Training of the overall system now requires feedback from the receiver to the transmitter.
For the multitone system, coding can be applied to each of the subchannels. If the number of subchannels is large, each individual subchannel looks approximately flat and AWGN, and Trellis codes will achieve good gains.
B
Trellis codes will combat Gaussian noise such as crosstalk. To combat impulse noise, interleaved codes or forward error correcting codes can be applied. This is a well studied problem in magnetic recording, where the channel is subject to drop-outs, and these same techniques may be effective in combating large amplitude but short duration impulses (typically less than 125 psec ) which are common in wire pair systems. The application of forward error correcting codes to the ADSL is discussed in detail in this session.['61
Trellis codes achieve their gains without using additional bandwidth, at the expense of delay. Interleaved codes also require additional system delay. Delays below ten milliseconds would accommodate powerful Trellis codes at the 1.5 Mbps rate, and would also allow sufficient interleaving to handle most impulses. For asymmetric applications, this delay is of little concern.
Advanced adaptive digital filtering and coding techniques will be required for the ADSL, o rating at a baud rate in the describes in detail one architecture for implementing the adaptive Until recently, the necessary real-time signal processing to operate at these bit rates was not economical on a per-circuit basis. Today, the latest generation single chip digital signal processors can begin to implement these functions, and there is ample evidence that dedicated VLSI for the ADSL a lication is well within the scope of range of 200 to 400 kbaud!'
R"
A paper in this sessiontechnical fea~ibility."~]
v
O1 5. Potential ApplicationsAn access technology that can work over existing nonloaded phone lines, providing high speed asymmetric digital delivery without disrupting normal voice service, and that is economically attractive through the use of sophisticated but highly integrated signal processing hardware presents a powerful vehicle for making new services available to residential customers. Several asymmetric services have been envisioned for the ADSL. Bulletin boards and videotext services of various types are being offered today,[211 using dial up modems with access rates well below 64 kbps. Home catalog shopping, banking and stock market quotes, weather, games and "yellow pages" are the most common application examples. ISDN will significantly enhance the capabilities of such services, but the ADSL will go even further to make high speed graphics and video of very high quality viable. In these applications the user only needs a low-speed link through the network to the service database to transmit keystrokes or mouse movements, and the service provider would respond virtually instantly at 1.5 Mbps. Significant effort in the personal computer industry is now being devoted to CD ROM-based software and applications databases. As the amount of software and application specialization increases, users may find a time-sharing arrangement attractive, in which an access and usage fee are paid rather than an outright purchase of a particular software application or database. Since the CD ROM access rate is less than 1.5 Mbps. the ADSL would provide performance across the network indistinguishable from a local copy.
Over the last several years there has been exciting progress in video compression. The CCIlT H.261 recommendation and the more recent IS0 Moving Picture Experts Group (MPEG) recommendation both provide full motion video at rates of around 1.3 Mbps. The video, an audio channel and
associated overhead can be transmitted within a 1.5 Mbps signal. These algorithms have brought 1.5 Mbps video to the threshold of acceptability in entertainment applications.[221 This has led to the notion of delivering full motion entertainment video to a viewer over a phone line. Users could signal the network, scan archives of programming and receive video "on demand." This possibility has sparked the imagination of programmers, network providers and consumers alike. However, it is unlikely that such a service would displace current methods of distribution. Rather, it would help fill some of the needs of an expanding marketplace, in which viewers will have increasing access to larger selections of programming content in the timeframe that they want it. Commercial broadcast television, cable television, direct broadcast satellite and purchased and rented recordings are all evolving to fill this need. ADSL can enable the LECs to meet part of this demand,
Education may be a significant new service driver. For example, there may not be enough demand at one High School to hire a specialized teacher, say in foreign language or
advanced calculus. But if one specialist teaches students throughout several counties by way of 1.5 Mbps video, the economies are much more attractive and the quality of education is improved.
Another potential ADSL application is CD format audio. Again, the access rates for most versions of CD format audio are less than 1.5 Mbps. A service provider could offer "audio on demand" to customers over the public network. A large inventory coupled with creative selection and programming features might fill needs not currently satisfied by commercial radio or recorded albums.
The personal computer industry is aggressively pursuing "multimedia," in which all of the best attributes of graphics, video and compressed high fidelity audio are used simultaneously to enhance interaction with the application. Many applications are being envisioned, such as news, travel, shopping and real estate services.[231 As multimedia capabilities become more sophisticated and of higher fidelity, the advertising industry is becoming more interested in the medium. Advertising coupled with next-generation applications could help to defray costs of offering new services, bringing them within reach of the mass market.
6. Network Architecture and Interface Issues
Once it has been verified that ADSL will be able to widely operate over the nonloaded loop plant without excessive susceptibility to crosstalk, impulse or other noise, many networking issues need to be worked before service rollaut can begin.
Under present rules, LECs can provide transport of services like the ones discussed above. The telephone company connects a customer to an Information Service Provider, but does not provide, manipulate or control the content of the information. Restrictions on services offered using LEC- provided transport were spelled out by the MFJ Court in a
1987 Order. Such services can only be offered within a LATA. A telephone company can provide billing management services to the Information Service Provider. The telephone company can provide an introductory screen to a number of different Infomation Service Providers, and subsequent address translation to connect a customer to one of
these. Telephone company blocking features are also allowed,
in which users can choose to restrict access to certain providers. Once connected. the customer interacts directly with the Information Service Provider.
The service interface provided to the customer needs to be defined and will be discussed next in the context of the reference model shown in Figure 5. The remote ADSL unit interfaces to the single loop pair on one side and demultiplexes the three channels previously described. The POTS channel is routed to the customer telephones, answering or facsimile machines, analog modems, etc., in the conventional fashion. The 1.5 Mbps channel and the low- speed control channel connect to a service module. The nature of the service module will vary from application to application. For example, it may include an MPEG video decoder, fast packet hardware, or CD-audio hardware. The output of the service module might be a baseband video signal, a packet protocol or an audio signal.
I
1.5 MBPS CHANNEL SERVICE MODULE + ADSL + CONTROL CHANNELI
ADSL SERVICE SERVICE
LOOP MODULE SPECIFIC
INTERFACE INTERFACE INTERFACE
Figure 5. Reference Model for ADSL Remote Equipment
The location of the network interface, and what equipment is network provided and what is customer provided will depend on a number of factors. The telephone companies want to be flexible to best serve the needs of their customers. This might lead to one scenario where the ADSL and Service Module are both located in a pedestal in the outside plant. Arrangements for providing power from commercial 120 Vac would be made. This architecture would facilitate upgrade of network transport, for example from the copper-based ADSL to a fiber-based optical network unit in the future, while preserving the customer's service interface and minimizing displacement of customer owned equipment.
In other cases the customer may already own or prefer to own equipment. For example, in computer-based applications the Service Module functionality may reside on a card that plugs into a PC. This might lead to a scenario where the ADSL is network provided and the Service Module is customer provided. The ADSL equipment would separate the POTS wiring from the Service Module Interface wiring, with the most likely location for the ADSL being either in a pedestal or close to the POTS protector inside the residence. This separation of wiring may provide important advantages in shielding the digital channels from the noise environment in
the premises. If placed inside, the equipment is protected from the environment and power is more readily available. But an installer visit may be necessary.
TO
OTHER SWITCH COMMON
cos CONTROL
I
-
:
OS1 SWITCH e * FABRIC 2 -w I aFigure 6a. A Possible Network Architecture for a Service Delivered with ADSL
Scenarios in which all of the equipment is located well inside the premises are also being examined. This could have considerable advantages in installation and provisioning. For example, the user might purchase a complete unit consisting of ADSL and Service Module electronics, take it home, place it on top of a
TV
or near a PC and connect it to the telephone line at the nearest available jack. Unfortunately, this approach appears to require installation of special filters to isolate telephones from the ADSL. Otherwise the telephone instruments will bridge directly across the ADSL receiver and may attenuate the ADSL signal. Small, easily installable filters might be used, but this approach has drawbacks. Wall phones, commonly found in kitchens, would be difficult to isolate. If a filter was misplaced or not installed, maintenance problems could occur.At the network end of the ADSL system, an ADSL unit accepts the 1.5 Mbps signal input and provides the low-speed control channel and the POTS channel. The POTS channel will terminate on a conventional local switch. Initially the 1.5 Mbps stream is expected to be handled by emerging fabrics that provide switched DS1 capability. The low-speed control channel will terminate on a packet handler, which in turn communicates with the DS1 switch and the Information Service Provider as depicted in Figure 6a. Later, if demand grows sufficiently, switch fabrics tailored and optimized for asymmetric applications may emerge.
Initially, one switch fabric with interfaces to service providers may serve a large area. Customers served out of another central office might be connected to this switch by way of digital cross-connect and fiber transmission systems.
Figure 6b shows how the ADSL can be used with remote electronics. The POTS service terminates on a conventional plug-in in a digital loop carrier remote terminal. The 1.5
Mbps signal is carried over a spare DS1 channel on the fiber multiplexer that feeds the remote electronics site. The low- speed data channel terminates on a data channel unit in the remote terminal, or perhaps a number of ADSL low-speed
control channels are concentrated first before being transported over a 56 kbps data channel.
To allow use of existing data protocols, it is desirable for the low- speed control channel to be full duplex. At least two data formats are being discussed. An X.25 protocol could run at relatively low-speeds, e.g. 9.6 kbps, and network transport is widely available. An advantage of the lower bit rate is that it simplifies multiplexing of the control channel along with the POTS and 1.5 Mbps channels because the modulated signal will require less bandwidth. If technically possible from a transmission point of view, it may be desirable to use a 16 kbps channel that could operate 4.931 and LAPD, like the ISDN Basic Rate "D" channel. This would provide better throughput for user interaction with the network, and the rollout of ISDN means that offices will increasingly be able to terminate such channels. Some have even suggested higher rates, up to 64 kbps, in order to facilitate transfer of large customer generated files that may be created by scanners or video cameras. This would of course exacerbate transmission issues and it is not yet clear what rates are feasible.
Figure 6 includes an ADSL common controller, which communicates with a number of ADSL office units. The office units in turn communicate with the remote units by way of an overhead channel. This overhead will facilitate system synchronization and maintenance. Transmission performance and alarm conditions can be collected and forwarded to an operations system (OS). Eventually, many of the functions shown in Figure 6 may be integrated into one system, in which the ADSL office units become "line cards" in an advanced switch or remote terminal.
COMMON CONTROL
FIBER
TERMINAL ADSL Kxx>=K
Figure 6b. A Possible Architecture at a Remote Electronics Site
7. Conclusions
The ADSL concept has been described for providing economic one-way 1.5 Mbps transport to the residence. Such a capability would make possible a variety of exciting new services. But before the ADSL becomes reality, complex transmission issues must be resolved. The nature of the noise environment at the residence needs to be better understood, to determine if advanced signal processing techniques can provide reliable digital transmission. Although design of a
robust but economical ADSL system is a challenging task, the rewards for success will be high. A successful ADSL technology will mean that over 70 million subscriber loops are potential candidates for delivery of new wideband services. 8. Acknowledgements
This paper represents a synthesis of thinking and studies of many experts. In addition to the public references sited, I would also like to acknowledge the many private contributions of Bellcore and Regional company colleagues who have examined ADSL from a technology, services, operational and regulatory point of view.
REFERENCES
1. Original analysis attributable to Dr. H. Samueli, Integrated Circuits and Systems Laboratory, UCLA, IEEE HDSL Workshop '91, June 19-20, 1991.
2. K. Sistanizadeh, 'Spectral Compatibility of Asymmetrical Digital Subscriber Lines (ADSL) with Basic Rate DSL, High-bit-rate DSL, and T1 Lines," Bellcore, Globecom '91 Proceedings, November 199 1
3. J. J. Werner, 'The HDSL Environment," AT&T Bell Laboratories, IEEE J. on Selected Areas in Commun., August 1991
4. C. F. Valenti, "Residence Impulse Noise Measurement Program," Bellcore, IEEE HDSL Workshop '91, June 19- 20,1991
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