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ELEC351 Assignment

Digital Subscriber Loop

By Geoff Knagge

NOTE : This summary is rather dated, and things have changed a fair bit since 2000. However, it may still serve as a useful introduction or background information.


  1. Introduction

  2. Overview of ADSL

  3. Traditional Bandwidth Limitations

  4. History of DSL

  5. How ADSL Works

  6. Conclusion

  7. References

  8. Graphics Sources

1. Introduction

The past decade has seen extensive growth of the telecommunications industry, with the increased popularity of the Internet and other data communication services. While offering the world many more services than were previously available, they are limited by the fact that they are being used on technology that was not designed for that purpose.

The majority of Internet users access their service via modems connects to the Plain Old Telephone System (POTS). In the early stages of the technology, modems were extremely slow by today's standards, but this was not a major issue. A POTS connection provided an adequate medium for the relatively small amounts of data that required transmission, and so was the existing system was the logical choice over special cabling.

Technological advances have seen these rates increase up to a point where the average Internet user can now download at rates approaching 50Kbps, and send at 33.6Kps. However, POTS was designed for voice transmission, at frequencies below 3kHz, and this severely limits the obtainable data rates of the system. To increase performance of new online services, such as steaming audio and video, and improve general access speed, the bandwidth hungry public must therefore consider other alternatives. Technologies, such as ISDN or cable connections, have been in development for sometime but require special cabling. This makes them expensive to set up, and therefore not have not been a viable alternative for most people.

Asynchronous Digital Subscriber Line technology, ADSL, is a more recent innovation which dramatically increases user bandwidth with potential for speed increase of more than a factor of 50. The major attraction is that it is implemented using existing twisted pair telephone lines, theoretically eliminating the need for installation of special cabling. This makes ADSL a more cost effective, and hence commercially viable, system for wide bandwidth data transmission.

2. Overview of ADSL

ADSL has the advantage that it was designed to suit the standard pattern of use of data networks, which is that significantly more data is downloaded than is uploaded. Hence, an ADSL implementation gives the incoming stream a large slice of bandwidth, and a smaller amount is dedicated to duplex communications. In addition, ADSL reserves bandwidth so that a POTS line can be simultaneously used with the data access service. This bandwidth division is illustrated by figure 1.

An ADSL channel is divided into three main sections:

  • POTS channel, 4 kHz wide

  • High bandwidth downstream channel, 1.5 - 8Mbps wide

  • Lower bandwidth duplex channel, 16kbps - 1Mbps wide
Figure 1: Division of ADSL Channel Bandwidth

The mechanism used for assigning the non-POTS channels can be one of two standards, Frequency Division Multiplexing (FDM) or Echo Cancellation. FDM assigns each channel its own section of the frequency spectrum, while Echo Cancellation overlaps the upstream and downstream sections to create a duplex channel which is separated by the local echo cancellation technique currently used in the V32 and V34 modem standards. These systems are demonstrated by figure 2.

Figure 2 : Standards for dividing the ADSL frequency spectrum

Even with this improved scheme of bandwidth use, the question still remains as to how the limitations of the POTS can be overcome in order to achieve such a high bandwidth.

3. Traditional Bandwidth Limitations

The main factors which limit conventional modem bandwidth to 33.6kbps are not actually due to the transmission medium itself, but are due to the way it is used. The telephone network was originally intended to carry voice data only, and early research revealed that most speech energy occurs below 3500Hz. Thus it seemed reasonable to only transmit data below this frequency and so interconnection equipment was designed to carry data in 4kHz channels. This is where the major bandwidth limitation has occurred, since all data must be transmitted in this narrow channel.

Traditionally, there is a modem at each end of a telecommunications data link, such as between an ISP and a user, between which the signal propagates in an analog format. With DSL, the telecommunications carrier also has a modem, allowing it to converts the data to a more appropriate digital format and transmit it across the network to the destination, possibly via another DSL link. Since modern technology has made high bandwidth connections across the telecommunications backbone possible, this eliminate losses and limitations which previously occurred within the network.

While much of the limitation occurs due to the network implementation, the transmission medium is not ideal either. There are several factors which must be dealt with to improve the bandwidth achieved over a twisted pair network :

  • Losses due to resistance and other cable characteristics

  • Near End Crosstalk, NEXT, is caused by a strong, nearby transmission source interfering with a receiver.

  • Far End Crosstalk, FEXT, is the result of signals coupling into a cable pair at the far end of the link, but is not a major issue for ADSL systems.

  • When a service is connected to the network, this is done by "bridging" existing lines together to create the connection. If these bridges are not removed from previous connections, then the result is a transmission line with redundant branches at various points. For voice transmission this was not a problem, but at high frequencies these branches can cause reflections that degenerate system performance.

  • Similarly, the telecommunications network is made up of cables of varying gauges and characteristic impedances. These can also introduce undesired effects for high frequency applications

  • Longer lengths of cable used to be loaded to improve performance of voice transmission. DSL technology cannot cope with the effects of this, and so it can only be used on unloaded cable. In most circumstances, this is not a problem.

4. History of DSL

Several variants of DSL have evolved over the years :
  • HDSL is the pioneering high speed format, but is not a commercially viable option due to its need for two twisted pairs and does not have support for normal telephone services.

  • SDSL is symmetric DSL, and operates over a single twisted pair with support for standard voice transmission. The problem with this system is that it is limited to relatively short distances and suffers NEXT limitation due to the use of the same frequencies for transmitting and receiving.

  • IDSL stands for ISDN DSL, and is in many ways similar to ISDN technology. It's disadvantages are the lack of support for analog voice, and that its 128kbps rate is not much greater than that offered by standard 56kbps V90 modems.

  • VDSL provides very high bit rate DSL, up to 52Mbps, but requires shorter connections lengths than are generally practical. It has been used in conjunction with an experimental project, FTTC (Fiber to the Curb), but development in this area has slowed due to commercial viability issues.

  • ADSL is the most promising DSL technology, proving suitable for personal broadband requirements and allowing for the same channel to still act as a traditional POTS service.

  • Rate Adaptive DSL, RADSL, is a further advancement which is able to automatically optimise the ADSL data rate to suit the conditions of the line being used.

5. How ADSL Works

ADSL is a passband system, meaning that it carries its information in one or more channels above the baseband region of the frequency spectrum. This frees the 4kHz baseband section for use by standard voice transmission. In order to transmit data at high rates over a relatively lossy medium, the hardware must implement similar techniques to those employed by standard modems. This involves sending "symbols" rather than individual bits, a process known as quadrature amplitude modulation (QAM). ADSL has two competing standards for implementing this.

The accepted standard, Discrete Multi-Tone (DMT), divides the spectrum into 256 4kHz channels called bins. Some of these channels are used bidirectionally and require echo cancelling to prevent corruption, and the rest of the channels are used for downstream data only. The baseband voice channel is well separated from these, and can be accessed by use of a simple splitter device. The effect is similar to a standard voice line and a bank of standard modems operating in parallel to deliver the high bandwidth service, but over a single twisted pair. This is depicted by figure 3.

Figure 3: Division of DMT bandwidth
Since many impairments of the twisted pair cable access are frequency sensitive, the hardware can optimise transfer rates by monitoring the performance of each individual channel. If the quality degrades, then the bit rate can be reduced on that channel, and possible reassigned to another channel with a better signal to noise characteristic. Bandwidth within individual channels can be optimally used with modulation techniques similar to those employed by traditional modems.

6. Conclusion

For the user, ADSL provides significant advantages. Relative to other technologies, is a cost effective means of obtaining a high bandwidth data connection. No special cabling is required and no extra lines are required, since ADSL allows POTS access over the one twisted pair. The improved bandwidth makes fast Internet access a reality, and allows for services such as high quality streaming of audio and video, or any other data access application that may be developed as a result of the technology's availability.

7. References

8. Graphics Sources

(C)opyright, Geoff Knagge.
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