High-Performance Networking Unleashed

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- 14 -

xDSL

by Louis Masters

This chapter introduces Digital Subscriber Line, or DSL, along with a brief history and background information on why is was invented. It explains the many derivatives of DSL (ADSL, SDSL, HDSL, VDSL), their advantages and disadvantages, and possible uses and capabilities. Several tables are provided to summarize speed/distance relationships and specific technological advantages. The chapter ends with a DSL cross reference table, contact information, and a few Internet resources.

Introduction to xDSL Technology

Current copper telephone lines carry voice-grade communications at speeds up to 28.8Kbps (53.3Kbps with software help). These lines use the existing telephone network and require no additional hardware or software to access almost any existing network location. This is copper's main advantage: instantaneous access to anywhere that the telephone network is. But, this is also a disadvantage: the entire network is wired for yesterday's speeds.

In a perfect environment, copper speeds would be limited only by the cable attenuation. But, in the existing telephone network, the bandwidth is largely limited by the filters in the network and the networks themselves. The upgrade of existing twisted copper wires would serve to enhance the entire network and allow for speeds virtually unheard of several years ago. But, the costs associated with this are extreme; another method would be needed.

This method would have to use existing cables and give notably increased performance. Bellcore, the research entity of several Bell operating companies, created the first Digital Subscriber technology and coined the term DSL (Digital Subscriber Line). Bellcore had two main reasons for creating this technology: increased performance and low cost. They aimed for a bandwidth of at least 2Mbps, but over the current copper network. This would make it both an easy and quick installation, but also a cheap alternative to other network upgrades.

The main reason DSL was invented was to provide high-speed digital access to local corporate and residential customers. DSL's main concept is to use older copper cables as high-speed digital pipelines. It does this entirely with electronics and software at each endpoint. DSL interfaces are installed at endpoints on the network, while the existing cabling is left untouched.

In a nutshell, DSL compensates for the copper wire's imperfections. As mentioned previously, attenuation by the cable is a major impedance to network bandwidth. DSL uses software and electronics to "fix" the problems with copper wire, creating an overall higher bandwidth data pipe.

What does DSL give us?

DSL provides for several more upgrades to the existing network infrastructure, including increased distances, decreased installation time, immunity to normal cable problems, and the like.

DSL can be used in a variety of environments. The local phone companies could use it for delivering video, audio, and increased bandwidth to both local residential and business customers. Businesses could also use it in a multiple building or even multiple floor setting, to provide increased network bandwidth or allow for newer interoffice network technologies (video on demand, conferencing, and more). The possibilities for this cheap network upgrade are as endless as the networks it can be applied to.

The Different Flavors of DSL

Several acronyms/technologies exist when speaking of the Digital Subscriber Line: DSL, SDSL, ADSL, HDSL, VDSL, VADSL, and BDSL. Table 14.1 summarizes the acronyms and their meanings. The two most common are ADSL and VDSL.

Table 14.1. DSL acronym table.

Type Definition
DSL Digital Subscriber Line (generic)
SDSL Single line/Symmetric Digital Subscriber Line
ADSL Asymmetric Digital Subscriber Line
HDSL High bit (data) rate Digital Subscriber Line
VDSL Very high data rate Digital Subscriber Line
VADSL Very high speed Digital Subscriber Line (another term for VDSL)

What Is ADSL?

ADSL, or Asymmetric Digital Subscriber Line, is an asymmetric version of DSL. The speed of this technology for use over existing mediums (phone lines) is astounding: up to 8.448Mbps to the customer and up to 800Kbps to the "network" (see Figure 14.1).

FIGURE 14.1. A basic ADSL.

The overall downstream rate depends on, among other things, the distance covered, the size of the wire, and interference. Upstream speeds range from 16Kbps to 64Kbps to 640Kbps. Table 14.2 gives the standard downstream speeds at certain distances.

Table 14.2. ADSL speed/distance.

Speed (feet) Distance (Mbps)
<=9000 8.448
>9000 to <= 12000 6.312
>12000 to <= 16000 2.048
>16000 to <= 18000 1.544

ADSL was designed for two major functions: high speed data communications and interactive video. Data communication functions can be Internet access, corporate telecommuting, or other specialized network applications. Interactive video covers things such as video on demand, movies, games, or any other application that requires high-speed networked video. Presently, ADSL only supports interfaces to T-1/E-1; future enhancements are planned for ADSL to the desktop.

The biggest strength in ADSL lies in the fact that it uses existing telephone lines over which to communicate. This means that more than 700 million (existing) phone customers are already cabled for ADSL and require little or no upgrade. Technologies such as ISDN or cable modems can require expensive hardware and software upgrades in both the network and in the client site. But, ADSL is still in testing around most of the world, with an anticipated niche release slated for late 1997. This release will be available mainly for Internet access and other packet-based communication applications.

What Are CAP and DMT?

CAP and DMT are the two prevalent ADSL modulation systems out today. CAP was engineered by AT&T Paradyne, and DMT was developed by Amati Communications Corporation. The difference between the two is the way they send data down the pipe. Also, ANSI standard T1.413 is based on DMT.

Carrierless Amplitude/Phase Modulation CAP

Carrierless amplitude/phase modulation CAP is a proprietary modulation (line code) of AT&T Paradyne. Incoming data modulates a single carrier channel that is then sent down a telephone line. The carrier is suppressed before transmission and reconstructed at the receiving end.

Discrete Multi-Tone

It separates incoming data into many sub-carrier channels. Each channel is measured for quality, and then the channel is assigned a certain number of bits based on quality of the channel. DMT creates these channels using the technique of Discrete Fast-Fourier Transform.

DMT uses a mechanism we are familiar with to create a modem connection. When two DMT modems connect, they try the highest speed possible. Depending on line noise and attenuation, the modems will successfully connect at the highest rate or decrease the rate until a satisfactory connection speed can be reached.

How ADSL Works

ADSL uses its own proprietary modem hardware to connect each end of an existing twisted-pair connection. It creates a three-channel pipe (see Figure 14.2).

FIGURE 14.2. An ADSL three-channel pipe.

This pipe consists of one high-speed downstream channel (to the client), one medium-speed duplex channel, and one Plain Old Telephone Service (POTS) channel (4KHz) to ensure POTS, even if ADSL fails. Both the high-speed and medium-speed channels can be multiplexed to create multiple lower-rate pipes. Consult Table 14.3 for more speed information.

Table 14.3. ADSL average, minimum, and maximum speed.

Channel Average Minimum Maximum
High-speed downstream 6Mbps 1.5Mbps 9Mbps
Medium-speed duplex 64Kbps 16Kbps 640Kbps

Again, line speed is also affected by the length of the physical pipeline (as seen in Table 14.3), the thickness of the wire, and cross-coupled interference, just to name a few. The numbers in the Table 14.3 are estimates that can be used as a simple reference.

Technical ADSL

During the past several years, there have been numerous advances in telephone hardware technologies that permit ADSL to work. Yet, ADSL uses a very simple method to achieve its amazing speed results: squeezing. It uses very advanced DSP and algorithms to squeeze as much information as possible through the existing twisted-pair telephone lines.

ADSL creates a multiple-channel telephone line in one of two ways. It either uses Frequency Division Multiplexing (FDM) or echo cancellation. FDM uses one band of downstream data and one band of upstream data. It splits the downstream path by using a technique called time division multiplexing. The upstream pipe is also split into multiple low-speed channels. Echo cancellation overlaps the downstream pipe with the upstream and separates them by means of local echo cancellation (such as the V.34 line specification). Echo cancellation, while being more efficient, suffers from increased complexity and higher costs.

ADSL multiplexes the downstream, duplex, and maintenance channels into blocks; attaches an error code to that block; and sends the data. The receiver then corrects errors up to the limit implied by the code and the block length. Typically, 20ms of data is buffered, allowing for suitable error correction over ADSL's high-speed pipe. Initial tests indicate that ADSL modems can tolerate error rates suitable for MPEG2 and several other digital video schemes.

What Standards Are in Place for ADSL?

The American National Standards Institute (ANSI) has an approved ADSL standard with rates up to 6.1Mbps. This is denoted as ANSI Standard T1.413. The European Technical Standards Institute (ETSI) added an Annex to T1.413 that describes the European requirements. Issue II, now under study by T1E1.4, will expand the standard to include a multiplexed interface at the premise end, protocols for configuration and network management, and other improvements.

ADSL is a recognized physical layer transmission protocol by both the ATM forum and DAVIC.

What Is the Future of ADSL, Technically Speaking?

What ADSL will become may not be radically different from what it is today. ADSL is currently implemented in two ways: CAP and DMT, the latter being the standard. Although CAP is not the standard, it was developed by AT&T Paradyne and has been widely implemented by the telecommunications industry. It is difficult to say with any certainty what will happen to ADSL. However, we do know that there is a bottleneck that the vendors and telephone companies need to overcome to develop networks that can be used quickly today, and still used tomorrow.

What Is VDSL?

Simply put, VDSL is a very fast version of ADSL. With VDSL, the maximum downstream rate for short lengths of cable can reach almost 55Mbps (see Table 14.4). Upstream rates can reach speeds of up to 2.3Mbps (future projections reach 19.2Mbps or higher). Both the upstream and downstream data channels can be separated (by frequency) and overlaid on existing POTS or ISDN services, making VDSL a very appetizing solution to high-speed, cheap networking. Later upgrades may need to switch to echo cancellation or some other method to manage the pipeline. Like ADSL, VDSL will be used mainly for real-time video transmissions and high-speed data access.

Table 14.4. VDSL speed/distance (estimates).

Speed (Feet) Distance (Mbps)
>1000 51.84
>1000 to <= 3000 25.82
>3000 to <= 4500 12.96

Technical VDSL

Architecturally, VDSL is like a high-speed ADSL. VDSL uses both upstream and downstream multiplexing to achieve the very fast transfer rates. It also uses inline error correcting to compensate for line noise and other interference. VDSL is meant for shorter reaches of line, with far less transmission restrictions (see Figure 14.3).

FIGURE 14.3. A basic VDSL.

To date, four line codes have been proposed for VDSL: CAP, DMT (both are discussed in the section "What Is ADSL?"), DWMT, and SLC.

What Is DWMT?

DWMT, or Discrete Wavelet Multitone, is a multiple carrier modulation system that uses wavelet transforms for the individual carriers.

What Is SLC?

SLC, or Simple Line Code, is a version of baseband signaling that filters the base band and restores it on the receiver.

VDSL separates its channels using frequency division multiplexing. In the future, if VDSL supports symmetric data rates, this may have to change to echo cancellation. The normal configuration is to have the downstream channel above the upstream, but the DAVIC specification reverses this to enable the distribution of VDSL signals over coaxial cable.

Forward error control is another characteristic of VDSL. Very similar to ADSL (T1.413), it uses a form of Reed Solomon encoding coupled with interleaving to correct for line noise.

The Current Condition of VDSL

VDSL is still in the embryonic stage. It has yet to be tested over long distances at all of its data rates. Most data about line characteristics are speculative at best, and this may be detrimental to VDSL performance on the whole. Also, VDSL signals are in the range of amateur radio and may be impacted severely by this over both long and short line reaches. Equipment costs at each point (customer and premises) may also be substantial and interfere with a widespread rollout.

ADSL, unlike VDSL, has the advantage of having a better established product base. A strategy of coupling VDSL with ADSL might be wiser--use ADSL for the longer stretches and a combination of ADSL and VDSL for the short stretches of pipeline. Both ADSL and VDSL offer more "pipe for the buck" and may better serve to meet the future's data needs today.

What Is HDSL?

HDSL is mainly aimed at a better and more efficient way of transmitting T-1/E-1 over traditional copper lines (using no repeaters and being more efficient). While ADSL uses one wire to transmit both upstream and downstream, HDSL implements a second wire to transmit data in both directions (see Figure 14.4). HDSL transmits at speeds of 1.544Mbps to more than 2Mbps. Typical applications inlcude, but are not limited to, Internet service providers, private networks, PBX networks, and more.

FIGURE 14.4. A basic HDSL.

What Is SDSL?

SDSL is a single line version of HDSL. It is, however, limited to shorter distances, but at speeds up to 6Mbps (in the future). Current speeds range from 160Kbps to 400Kbps to 2.048Mbps at distances up to 9,000 feet. Common applications are users with limited access to only a single communication line and who require only symmetric access.

DSL Cross Reference Table

Table 14.4 shows the more common DSL technologies, their standard mode of operation, and the relative speeds of each.

Table 14.4. DSL cross reference.

Name Description Rate Mode
DSL Digital Subscriber Line 192Kbps Duplex
HDSL High Data/Bit Rate Digital 1.544Mbps Duplex
Subscriber Line 2.048Mbps Duplex
SDSL Single Data Line Digital 1.544Mbps Duplex
Subscriber Line 2.048Mbps Duplex
ADSL Asymmetric Digital 1.5 to 9Mbps Down
Subscriber Line 16 to 640Kbps Up
VDSL Very High Data Rate Digital 13 to 52Mbps Down
Subscriber Line 1.5 to 2.3Mbps Up

Where to Find Information on ADSL?

The following sections provide a list of ADSL resources on the Web. Keep in mind that xDSL as a technology is growing in popularity every day. Therfore this list is but a sampling of some of the companies and links involved with xDSL development.

ADSL Involved Companies

For more information on xDSL products, contact the following companies:

3M ADC Telecommunications
Advanced Micro Devices AG Communication Systems
Alcatel Telecom Amati
Ameritech Analog Devices
Aptis Communications Atlantech Technologies
Aware BellSouth Telecommunications
British Telecom Broadcom Corporation
Burr-Brown Ltd. Cabletron Systems
Cellware GmbH Cincinnati Bell
Compaq Computer Digital Technology MA
DSC Communications DSL Networks
ECI Telecom EPL Ltd.
Ericsson Fluke Corporation
Global Village Communication GlopeSpan Technologies
Harris Semiconductor Hi/fn
Integrated Device Technology Integrated Network Systems
Integrated Technology Express Intel
Interphase Kingston Communications
Metalink Motorola Semiconductor
NEC Australia NEC Electronics
Orckit Communications Paradyne
Performance Telecom Promatory Communications
Promptus Communications RNS
Rockwell Semiconductor Systems SGS-Thomson Microelectronics
SpellCaster Telecommunications Sprint
Telecom Finland Teltrend
TERACOM Svensk Rundradio AB TTC
U.S. Robotics US West
Westell Inc. Westell INternational

A Few ADSL Internet Resources

The following is a sampling of several of the best Web sites for xDSL information. Please be aware that most are companies involved in not only promoting the technology, but also in selling it. For more links, simply use your favorite search engine. New sites are being added all the time.

http://www.adsl.com/

http://www.mot.com/SPS/MCTG/MDAD/adsl/index.html

http://poseidon.aware.com/product_info/adsl

http://www.zdnet.com/intweek/print/960408/infra/doc5.html

http://www.gte.com/Adsl/

http://www.westell.com/

http://www.telechoice.com/ http://www.pairgain.com/

Summary

This chapter should have given you a thorough introduction into the realm of DSL. You should be able to differentiate between the different styles of DSL, which to use for specific instances, and how to implement them in a network. The tables provided can and should be used for a reference into DSL.

DSL is a relatively young technology that is based on a relatively old infrastructure. Yet, this should not be looked upon as only a hindrance. DSL can be implemented with relatively lower startup costs and in a very short period of time. Smaller companies or organizations that do not wish to spend a great deal of money, or don't have the time to implement a full-scale network redesign (both hardware and software), should look to DSL as a viable solution.


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