xDSL

xDSL

The xDSL technologies including High-Bit-Rate Digital Subscriber Line (HDSL), ISDN Digital Subscriber Line (IDSL), Single-Line Digital Subscriber Line (SDSL), Multirate Symmetrical Digital Subscriber Line (M/SDSL), Asymmetrical Digital Subscriber Line (ADSL), Rate-Adaptive Digital Subscriber Line (RADSL), and Very-High-Rate Digital Subscriber Line (VDSL) offer many home users their first taste of broadband, and most users find that once they've tried broadband, they'll never go back. It's exciting how much it can improve your performance, despite it's not being as fast as the data rates we will see in the coming years. According to Cahners In-Stat Group (Business Communications Review, July 2001), the worldwide DSL subscriber base is expected to include some 12 million in 2002, 17 million in 2003, and 23 million in 2005.

Bellcore (which is now Telcordia) created DSL as a technique to filter out the incessant background noise on copper wires and to allow clearer connections through the use of electronic intelligence in the form of DSL modems at either end of the twisted-pair line. DSL modems are limited in transmission distance. They generally have a range of up to 3.5 miles (5.5 kilometers), although new specifications are constantly being created to increase the permitted distances. The general rule of thumb with DSL is that the greater the distance, the lower the performance, and the shorter the distance, the greater the data rate you can experience. Another characteristic of DSL to keep in mind is that it is a point-to-point connection that is, it is always on. So, when you have access into your Internet service provider (ISP) through a DSL line and you've powered up your computer, the connection is on throughout the day. This has security implications it's very important that you consider some form of firewall and security software to prevent the potential activities of a curious hacker.

DSL provides high-bandwidth transmission over copper twisted-pair. It uses efficient modulation, or line-coding, techniques that enable it to carry more bits in a single cycle (that is, Hz) than older twisted-pair. It uses echo cancellation, which enables full-duplex transmission to occur over a single electrical path, and it relies on frequency splitting to enable you to derive separate voice and data channels from one wire. DSL also retains power in the event of a power failure; if the electricity goes out, you'll lose your high-speed data services, but you will retain your voice services.

Factors that can affect the viability of DSL for a subscriber include the following:

         Attenuation Attenuation is signal loss, and it's a function of frequency. As the frequency increases, the distance the signal can travel decreases, by the square root of the frequency. Higher frequencies lose power more rapidly, thereby limiting the loop length; we have to use the full-frequency spectrum available on the twisted pair to carry the promised data rates; thus, the higher frequencies must be used.

         Resistance As signals are transmitted through wires at very high frequencies, a phenomenon called the skin effect occurs. As electricity migrates to the medium's skin, resistance increases because less of the wire is used. This increased resistance weakens the signals. The skin effect is why there are currently no services above 1GHz over wired media.

         Crosstalk When two adjacent wires carry signals, signals from one wire might be able to enter the other wire as a result of electromagnetic radiation this is called crosstalk. Crosstalk increases with increasing frequency, a principal cause of signal degradation at the frequencies required by high-speed services. This affects how many pairs within a cable can be used to deliver DSL service.

         Phase error Phase error introduces bit errors where modulation techniques depend on phase modulation.

         Loads Loaded pairs which means there are loading coils placed on twisted-pairs for purposes of improving performance over 3.5 miles (5.5 kilometers), when the subscriber is greater than 3.5 miles (5.5 kilometers) away from the access point cannot be used for DSL.

         Taps Taps are open points on the cable bundle that are left so that technicians will be able to easily splice off a pair to bring additional service to a home or bring service to a new home. These open points cause too much distortion to be used with DSL.

         Loop carriers Loop carriers, or remote concentrators, are not compatible with most of the DSL family. xDSL, therefore, must work around the existing loop carrier systems, or the network operators have to replace older-generation loop carriers with next-generation loop carriers that are designed to work with the DSL modems. Currently only HDSL and IDSL work with existing digital loop carriers. Approximately 30% to 40% of the U.S. population is served by such digital loop carriers, and around the world, remote and rural locations are generally served with digital loop carriers. As a result, certain market territories do not have quick and easy access to DSL. DSL access in those areas will depend on the operator's ability and desire to upgrade the plant.

Table 13.2. xDSL Characteristics
Characteristic	HDSL	IDSL	SDSL	M/SDSL	ADSL	RADSL	VDSL
Maximum deployment distance	2.2 miles (3.6 km)	3.5 miles (5.5 km)	3.5 miles (5.5 km)	5.5 miles (8.9 km)	3.5 miles (5.5 km)	3.5 miles (5.5 km)	1 mile (1.5 km)
Symmetrical/Asymmetrical	Symmetrical	Symmetrical	Symmetrical	Symmetrical	Asymmetrical	Symmetrical or asymmetrical	Asymmetrical
Capacity	Up to 2Mbps	Up to 128Kbps	Up to 2Mbps	Up to 2Mbps	ADSL-1: 1.5Mbps to 2Mbps downstream, 16Kbps to 64Kbps upstream; ADSL-2: 6Mbps to 8Mbps downstream, 640Kbps to 840Kbps upstream	600Kbps to 7Mbps downstream, 128Kbps to 1Mbps upstream	Up to 52Mbps downstream, up to 1.5Mbps to 2.3Mbps upstream
Number of twisted-pairs	Two	One	One	One	One	One	One

         Other external impairments Leakage, impulse noise, narrowband interference, and the general quality of the copper pair can all have an effect on the quality of DSL service.

All these factors together determine whether you will get the kind of service that DSL promises.

The following sections look at each of the DSL family members in turn. Table 13.2 summarizes some of the characteristics of xDSL. Keep in mind that the rates shown in Table 13.2 vary, depending on the loop conditions, crosstalk, impairments, and so on.

HDSL

HDSL is the oldest of the DSL techniques. It has been in full use for over a decade, and it is most commonly used by telcos to provision T-1 or E-1 services. HDSL enables carriers to provision T-1 or E-1 services at a reduced cost because it does not require special repeaters, loop conditioning, or pair selection in order to deliver that service.

HDSL is a symmetrical service, meaning that it gives you equal bandwidth in both directions. Because it is full-duplex, it allows communication in both directions simultaneously. The allocation of bandwidth on HDSL depends on whether you are operating on T-1 or E-1 capacities; in the T-1 environment, it offers 784Kbps in each direction, and in the E-1 environment, it offers 1.168Mbps in each direction. HDSL is largely used to provision digital services to business premises. It is standardized under ITU G.991.1 and ADSI T1E1.4, Tech Report 28.

HDSL reduces the cost of provisioning T-1/E-1 because of the way that the bandwidth is delivered (see Figure 13.1). A traditional T-1/E-1 environment makes use of two twisted-pairs. Each pair carries the full data rate, which is 1.5Mbps with T-1 and 2.048Mbps with E-1. Because each pair is carrying such a high data rate, higher frequencies need to be used; as a result, repeaters need to be spaced roughly every 0.5 to 1 mile (900 to 1,800 meters). Furthermore, no bridge taps are allowed in the traditional T-1/E-1 environment.

Figure 13.1. Traditional T-1/E-1 versus HDSL provisioning

HDSL modems contain some added intelligence in the form of inverse multiplexers. Because of these multiplexers, each pair carries only half of the data rate. As a result, those bits can ride in the lower range of frequencies, thus extending the distance over which they can flow without the need for a repeater. With HDSL, you need a repeater only at about 2.2 miles (3.6 kilometers). In addition, bridge taps are allowed with HDSL. These factors reduce the cost of provisioning services to customers and allow more customers who are outside the range of the traditional T-1/E-1 environment to enjoy the privileges of this high-bandwidth option. Because taps can be used with HDSL, provisioning can occur rather quickly. Also, HDSL is a good solution for increasing the number of access lines via the digital-loop carrier transport because it is compatible with the existing loop carriers. Key applications of HDSL include replacement of local repeater T-1/E-1 trunks, use as a local Frame Relay option, use in PBX interconnection, and use in general traffic aggregation.

The HDSL2 specification was developed to provide the capacities and symmetry of HDSL to residences. HDSL2 involves the use of a single twisted copper pair for distances up to 2.2 miles (3.6 kilometers). HDSL2 is a symmetrical, full-duplex service that offers up to 768Kbps in each direction. The HDSL2 specification has not yet been fully ratified, and so for the time being various companies (such as ADC Telecommunications, PairGain Technologies, and ADTRAN) are producing products that are proprietary in nature.

Estimates suggest that HDSL will not be an extremely popular environment in the future, but in the short-term, it will continue to be used to provision T-1/E-1 services to businesses at relatively low cost.

IDSL

IDSL is basically ISDN without the telephone switch. It makes use of a DSL access multiplexer (DSLAM), which shunts traffic to a data network, away from the circuit-switched network. IDSL is a full-duplex, symmetrical service, and it offers 128Kbps in each direction. Unlike traditional ISDN, it cannot be channelized. Because it uses the same transmission technology as Basic Rate Interface (BRI) ISDN (that is, 2B1Q line coding), it is compatible with existing loop carriers. The distance limitation on IDSL is 3.5 miles (5.5 kilometers).

Because IDSL offers throughput of only 128Kbps (compared, for example, to the 1.5Mbps throughput that ADSL provides), there's little interest in it, and it is unlikely to have much of a future.

SDSL

SDSL involves a single twisted copper pair that can be up to 3.5 miles (5.5 kilometers) long. It is a symmetrical, full-duplex service. Symmetry can sometimes be very important, depending on the application. If your only goal is to surf the Internet and browse Web sites, then most of the bandwidth you will need is in the downstream direction from the network to you in which case solutions such as ADSL are appropriate. But if you're telecommuting or operating in a small office/home office (SOHO) and you need to do large file or image transfers, or need to engage in videoconferencing, then you need a great deal of bandwidth in the upstream direction as well as in the downstream direction, in which event, symmetrical services are best. So, if your major purpose for wanting broadband access is beyond Internet surfing for example, to download and upload a lot of data for schoolwork and professional work then SDSL is probably a better option than ADSL. It is more costly than asymmetrical options, but it gives you a better performance guarantee.

SDSL also supports multiple data rates up to T-1 or E-1 rates so you can subscribe to varying bandwidths, up to 1.5Mbps or 2Mbps. Applications of SDSL include replacement of local repeater T-1/E-1 trunks, use as fractional T-1/E-1, interconnection of PBXs, support of multirate ISDN, support for switched 384Kbps service (and therefore appropriate bandwidth for lower-level videoconferencing), support for local Frame Relay options, traffic aggregation, and high-speed residential service.

M/SDSL

M/SDSL is a descendent of SDSL; it supports changing operating line rates of the transceiver, and thus, the operating distance of the transceiver.

It involves a single twisted copper pair, which can be run up to 5.5 miles (8.9 kilometers), and it provides symmetrical, full-duplex service. M/SDSL offers eight variable line rates, ranging from 64Kbps to 2Mbps. At 5.5 miles (8.9 kilometers), the data rate supported is 64Kbps or 128Kbps; 2Mbps can be enjoyed at distances of 2.8 miles (4.5 kilometers). M/SDSL is designed to provide an autorate plug-and-play configuration, which means it adjusts automatically to the operating distance and line conditions.

ADSL

ADSL was initially introduced in 1993, with the principal driver being the much-anticipated deployment of video-on-demand. However, because of some early issues with video servers, including storage capacity and processing power, video-on-demand was largely abandoned. ADSL has now been identified as the perfect solution for Internet access.

There are two ADSL standards: ADSL-1 and ADSL-2. In the North American and T-carrier countries, ADSL-1 provides 1.5Mbps downstream, and in countries that follow E-carrier, it provides 2Mbps downstream and an upstream channel of up to 64Kbps in both North American and European standards. The distance limitations of ADSL-1 vary depending on the gauge of the wire used, and the range is 2.8 to 3.5 miles (4.5 to 5.5 kilometers).

ADSL-2 is what most of us would really like to get, but there's very little of it in commercial deployment. In T-carrier countries, ADSL-2 provides 6Mbps downstream, and for countries that observe the ITU standards, it provides 8Mbps downstream. ADSL-2 is bidirectional, and the upstream channel provides a range of 640Kbps to 800Kbps. Despite the fact that ADSL-2 is asymmetrical, it would provide sufficient bandwidth in the return channel to support videoconferencing.

As with ADSL-1, the distance ADSL-2 can cover also depends on the gauge of the wire, but it is roughly 1.7 to 2 miles (2.8 to 3.5 kilometers).

ADSL is standardized under ITU G.992.1 and ANSI T1.413, Issue 2. ADSL allows for simultaneous voice and Internet traffic on the same twisted-pair that used to be your phone line. It reserves the bottom 4KHz of spectrum for the voice traffic; filters (known as splitters) are used at each end of the copper pair to split the frequency bands. The lower frequencies are sent to the local exchange to switch the voice traffic. The higher frequencies are sent to the DSL modems, and you are generally connected over a packet-switched backbone to your ISP.

Two different modulation schemes are used in ADSL modems Carrierless Amplitude and Phase Modulation (CAP) and Discrete Multitone Technology (DMT) which contribute to interoperability problems. CAP relies on the Quadrature Amplitude Modulation (QAM) technique. Its adapters are less expensive than DMT adapters, but CAP is more susceptible to interference. CAP is a single-carrier modulation scheme whereby the bits are spread across the entire frequency spectrum of the twisted-pair. Other devices in the environment (such as CB radios and ham radios) can cause interference, so if those devices are operating while you're transporting information over a CAP-based DSL line, you might experience static on your voice call or corruption in your data bits. The better technique is DMT, which is standardized by ANSI, ETSI, and the ITU. DMT is a multicarrier technique, and its spectrum is divided into 256 4KHz carriers. Variable numbers of bits are put on each carrier, and the portions of the frequency band that suffer interference from other devices don't have any bits put onto them. The result is improved performance. Compared to CAP, DMT is less prone to interference, can carry data over a longer distance, and is marginally more expensive. DMT is the preferred modulation scheme for DSL, but depending on when commitments were made and when rollouts began, some network operators still rely on CAP (for example, in the United States, Qwest ADSL services are CAP based).

Figure 13.2 shows an example of an ADSL environment. At the residence is a splitter that is splitting off the plain old telephone service (POTS) to the telephone instrument, using the bottom 4KHz of spectrum on the twisted-pair; the remainder of the line is left for the ADSL modem and the data communications. At the top of the figure is a business environment that may also be making use of a DSL line to carry the voice and data traffic, on an integrated basis.

Figure 13.2. An ADSL configuration

In Figure 13.2, numerous DSL lines come in from residential and business premises. Their first point of termination is the DSLAM, which then splits the voice and data traffic, sending the voice traffic through traditional local exchanges onto the PSTN and sending the data traffic through packet-switched backbones onto the appropriate ISP or corporate network. The DSLAMs also convert the DSL traffic into ATM cells, to pass over a backbone that has ATM deployed in the core. DSLAMs are designed to concentrate hundreds of DSL access lines onto ATM or IP trunks and then route them to the ISP. The DSLAMs aggregate dedicated DSL pipes up to the routers or multiservice edge switches. They combine ADSL bit coding and ATM cell switching, and they allow ATM demarcation points to be at the local exchange or at the customer premises.

ADSL Lite and ADSL Heavy

There are variations of ADSL called ADSL Lite and ADSL Heavy.

The Universal ADSL Working Group was established to develop a lower-speed, lower-cost consumer version of ADSL, referred to as splitterless DSL. A range of equipment manufacturers and carriers banded together to provide more strength to the DSL standard, and this standard began to be known as G.Lite, or ADSL Lite. ADSL Lite supports downstream rates of up to 1.5Mbps and upstream rates up to 512Kbps. It can be deployed over distances up to 4.5 miles (7.5 kilometers), depending on the quality of the copper plant.

A benefit associated with ADSL Lite is that no "truck rolls" that is, physical visits to the home (which cost about US$200 per dispatch) are required to install the splitters. In reality, however, oftentimes a truck roll is still required for actions such as installing microfilters, which are used between wall jacks and the phones to equalize impedance. As current flows through a wire, a resisting force, called impedance, slows it down. In a stable circuit, impedance stays constant and can be dealt with easily. However, because home telephone systems are dynamic phones continually go on and off hook the impedance is constantly changing. Shifting impedance disrupts data, and in the absence of a splitter, data is on the same wire as the phone traffic. Basically, installing microfilters helps to facilitate better performance of data.

Reduced powering and computer processing needs are additional benefits of ADSL Lite. Some ADSL Lite modems borrow a portion of their processing power from the host PC's CPU, with the aim of reducing the modem cost. Because ADSL Lite operates at lower rates, the chipsets use less power, which makes it easier to add these modems into other types of equipment and into local exchange racks. ADSL Lite equipment achieves densities four to eight times greater than those of full-rate ADSL, which could become a big issue in achieving high penetration rates in the residential market. The industry is trying to standardize on a single line-coding scheme, G.DMT. However, ADSL Lite may never quite materialize because there is now an equivalent standard, G.Heavy, which is called ADSL Heavy.

ADSL Heavy is a full-rate splitterless system that is now becoming available. ADSL Heavy can support 8Mbps downstream and up to 1Mbps upstream. ADSL Heavy platforms are either available or in development from Alcatel, 3Com, 3Wire, Orckit Communications, and Westell Technologies, and systems are being tested or deployed by Belgacom, Bell South, SBC Communications, Verizon, and Telecom Italia.

Applications of ADSL

The main applications of ADSL are in support of Internet access and remote LAN access. However, Voice over DSL (VoDSL) presents potential opportunities, as does DSL bonding, which is an elegant way to increase bandwidth. The jump from T-1/T-3 to E-1/E-3 from 1.5Mbps to 45Mbps is rather large. DSL bonding enables you to link together several DSL lines to configure bandwidth between the T-1/T-3 and E-1/E-3 rates. The newest emerging application of ADSL is video-on-demand.

RADSL

RADSL adapts the data rates dynamically, based on changes in line conditions. It can therefore operate over a wide range of loop lengths and conditions, up to a maximum of 3.5 miles (5.5 kilometers). It can also operate with either symmetrical or asymmetrical send and receive channels. The downstream rates range from 600Kbps to 7Mbps, and the upstream rates range from 128Kbps to 1Mbps. Most RADSL devices rely on DMT encoding.

VDSL

VDSL is everyone's dream medium. It relies on a single twisted copper pair, but it operates over extremely short distances. The loop length range is just 1,000 to 5,000 feet (300 to 1,500 meters). At the higher distance, VDSL provides 13Mbps downstream and maybe 1.5Mbps upstream, and at the lower distance, it might be capable of providing up to 52Mbps downstream and around 2.3Mbps upstream.

VoDSL and Video over DSL

VoDSL is being positioned as the next great thing, particularly for small businesses. One of the most lucrative business lines for a telco is leased lines the T-1s and E-1s (remember that T-1s offer 1.5Mbps and E-1s offer 2Mbps) which normally have high pricetags. In the United States, a customer easily pays in the range of US$1,000 to US$1,500 per month for a T-1 access line, and outside the United States the costs of leased lines are even higher. The idea with DSL services is to provide the service to the residential users at a price point that's palatable, which is somewhere in the range of US$50 to US$80 per month. So the telcos had to stop and think. They were afraid they were going to cannibalize their leased-lines business, which is extremely lucrative, by providing the same service offering and same data rates for a fraction of the cost. They backed off to reevaluate their business plans, and they decided that perhaps the best application and marketing for DSL would be for business applications. They decided to let cable modems take the residences because they figured that where the money lies is with the businesses, and so they positioned VoDSL for the business market. Using a single VoDSL connection, service providers would be able to deliver high-speed data access and up to 16 telephone lines. The best target group for this is businesses with 20 to 49 employees. The customer needs an integrated access device that enables the combination of voice and data traffic onto the single DSL line.

Video over DSL is now starting to receive quite a bit of attention as well. DSL was originally developed as a way for the telcos to take on the cable companies, by transporting digitized video over copper wires. The early plans were originally abandoned because of difficulties with server technology, because it was the early days, and because of cost. Telcos are again on a quest to compete with cable companies, by delivering multichannel video to customers and offering bundled packages. Content is the most critical element of success here, but it is leading toward a trend of using Video over DSL, currently ADSL in particular. (VDSL is the preferred technique, but it is still a very early technology and extremely limited in terms of distance. So ADSL is a more appropriate platform for accessing more subscribers.) Early Video over DSL providers include a venture between Blockbuster-Enron, Intertainer, MeTV Network, Pace MicroTechnology, Thirdspace (which is a joint venture between Alcatel and Oracle), and uniView Technologies Corporation. Watch for developments in Video over DSL in the coming year or so, as there is bound to be quite a bit of attention focused there.

VDSL is a very high-capacity technology, and its performance degrades rapidly over distances, so it's really meant to be almost a sister technology to fiber-to-the-curb (FTTC), which is discussed later in this chapter, going the very short distance from the curb to the home. (Or in the case of fiber-to-the-building [FTTB] with a multiunit dwelling, VDSL could be run over twisted-pair from the building to each apartment.)

The key applications for VDSL are the next generation of TV high-definition TV (HDTV), digital TV (DTV), and forms of interactive multimedia Internet access. (See Chapter 10 for more on TV standards.)

VDSL is standardized under the working title ITU G.vdsl, and numerous companies are developing it. Standards efforts are under way in various bodies, including ANSI, the ADSL Forum, the ATM Forum, and the Digital Audio Video Council (DAVIC). Ultimately, the goal of VDSL is to provide less power dissipation, lower costs, and much higher data rates than ADSL provides. VDSL should begin playing a major role within the next three years.