The rapid development in the field of information technologies has led to the appearance of new services that require high-speed data transmission technologies. For example, services like voice-over-IP, video streaming, teleworking, telemedicine, telecommuting, broadcasting of TV programmes, high-speed fil sharing, user-generated video, online video gaming, online education, shopping etc, require high-speed Internet access for effective operation.

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Until now, existing high-speed services (coaxial cable, analogue modem, etc) were not well-suited to the real needs of these services due to several different reasons. The idea of using twisted-pair cabling seemed the best since throughout the world millions of connections of this type were already in place and it just needed equipment to be added to the telephone exchange along with a small installation at the user location to be able to access digital subscriber line (DSL) technology. Unshielded copper pairs used in the telephone network carry voice signals in the frequency range of 300 to 3400 Hz but are capable of transporting information at much higher rates. These cables have been used to transport data in LANs up to 10 MHz or more.

The performance of analogue modems is very poor in comparison to the DSL. Another solution lies in modem bonding, where two modems are close-coupled to one computer, which may theoretically double the performance of a single modem. Theoretically, this arrangement increases network performance by a factor of two, but many Internet users need much more improvement in their connectivity speeds to support features like video streaming, online gaming and large fil transfer at the home or workplace. For these purposes, DSL is a better option.

Cable modems support high-speed, ‘always-on’ Internet access using the cable television lines, which is comparable to that of DSL. But, the difference between DSL and cable line technology lies in the distribution of bandwidth. In DSL the bandwidth is dedicated locally to all the subscribers, i.e., speed won’t drop when others use it at the same time. On the other hand, cable modem service involves locally shared bandwidth. This means, the realised performance of a customer’s cable will depend on how many other customers in that local area subscribe to the same service.

Integrated services digital network (ISDN) technology provides data rates just twice that of the ordinary dial-up connection. But this much speed is not comparable to data rates of either cable modem technology or DSL technology. ISDN has been more widely available for several years from the telecom companies, but the very fast expansion of DSL networks superseded the ISDN.

For remote areas, which are out of reach of DSL service, satellite data service is a good option. But, here again, data rates are nearly one-third the data rates of DSL.

The advances in electronics have largely made DSL a successful, faster and cheaper technology, though digging trenches in the ground for new copper or fibre-opticcables remains expensive. All types of DSL utilise very complex digital signal processing to overcome the inherent limitations of the existing copper pair. Until the late 1990s, digital signal processors for DSL were very expensive. But the rapid development in very large-scale integration (VLSI) technology has signifiantly lowered the cost of signal processors that supported DSL as a commercially successful technology.

DSL technology
Digital subscriber line (DSL) is referred to as a broadband technology because it is an ‘always-on’ data connection that is able to support interactive services including Internet access. It supports the minimum download speed of 256 kbps to an individual subscriber from the point of presence (POP) of the service provider where multiple such individual broadband connections are aggregated. The subscriber is able to access these interactive services including the Internet through this POP.

Fig. 1: A typical xDSL set-up
Fig. 1: A typical xDSL set-up

DSL technology is a modem technology that uses existing twisted-pair telephone lines to transport high-bandwidth data, such as multimedia and video, to service subscribers. xDSL services, a family of technologies, are dedicated, point-to-point, public network access over twisted-pair copper wire on the local loop between an Internet service provider’s (ISP’s) central offce and the customer site, or on local loops created either intra-building or intra-campus.

In order to provide DSL connections, the existing telephone network is utilised and a network element called digital subscriber line access multiplexer (DSLAM) is installed at the central office/telephone exchange along with a modem placed in subscribers’ home or workplace. The DSLAM works like a concentrator. It provides multiple DSL connections for access to the bandwidth available to the DSLAM from the Internet backbone. The connection to the subscriber is then given from DSLAM via copper lines. The DSL modem performs the task of converting the DSL physical layer signal into a format that can be understood by a computer or any other equipment connected to it (Fig. 1).

The pair from the DSLAM is terminated into the main distribution frame (MDF), from where connections are provided to subscribers via twisted copper pair lines. These twisted pair lines terminate at DP box placed near a subscriber’s home/workplace. The connection from DP box is first run to the splitter that sits in the customer’s premises. The splitter, which is basically a low-pass filter,removes the plain old telephone system (POTS) signal (voice signal, 300-3400 Hz) from the incoming DSL signal. The high-frequency filtered DSL signal is then given to a DSL modem. Now the connection is taken out from Ethernet port of DSL modem and terminated to the subscriber’s computer (Fig. 2).

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The underlying technology of transport across DSL facilities is a high-frequency sinusoidal carrier modulation, which is an analogue-based signal transmission. Each end of a DSL circuit has a modem that modulates patterns of bits into certain high-frequency tones, representing that bit pattern, for transmission across the length of the facility. Tones received from the far-end modem are demodulated back into a corresponding bit pattern that the near-end modem retransmits, in true digital form as pulses of voltage, to its interfaced equipment (such as a computer, router and switch).

Unlike traditional dial-up modems, which modulate bits into carrier that can fall only in the 300-3400Hz base-band (voice service), DSL modems modulate frequencies from 4 kHz to as high as 4 MHz. This frequency band separation enables DSL service and POTS to coexist on the same copper pair facility.

DSL technology divides the frequencies used in a single phone-line into two primary bands. The high-frequency band is used for data services and lower-frequency band (below 4 kHz) is utilised for voice. Ideally, DSL service provides ‘always-on’ connection to the customers to access the Internet. DSL service implemented with point-to-point over Ethernet (PPoE) does not support ‘always-on’ connection, but even in this case a DSL router/modem can automate the connection process.

Fig. 2: A typical connection at subscriber’s premises
Fig. 2: A typical connection at subscriber’s premises

Different xDSL technologies
The DSL technology is based on discrete multi-tone modulation technique and covers a number of similar yet competing forms of DSL (collectively termed as xDSL) including IDSL, HDSL, SHDSL, ADSL, RADSL, UDSL, Etherloop, VDSL and GDSL. xDSL is drawing significant attention from implementers and service providers because it promises to deliver high-bandwidth data rates to dispersed locations with relatively small changes to the existing telco infrastructure.

These technologies are differentiated by:
1. Speed of transmission
2. Maximum distance of signal transmission
3. Variation in speed between upstream and downstream
4. Symmetric or asymmetric character of the connection

Currently, the primary focus in xDSL is on the development and deployment of ADSL and VDSL technologies and architectures.

ISDN digital subscriber line (IDSL). This is an integrated services digital network (ISDN)-based technology that provides data flowrates of 144 kbps, which is slightly higher than the dual-channel ISDN data rate of 128 kbps. The goodness of IDSL lies in its ‘always-on’ connectivity which transmits data via a data network rather than the carrier’s voice network. Thus it gives freedom from overloading of voice switches by data users.

IDSL uses a 2B1Q (two-binary, one-quaternary) line code on a single copper pair line to transmit information through the ISDN ‘U’ interface. The major limitation of IDSL is that customers cannot access ISDN signaling or voice services. But if the requirement is Internet browsing at higher speed, IDSL is a better option than ISDN.

High-bit rate digital subscriber line (HDSL). HDSL is a symmetric technology that provides the same amount of bandwidth for upstream and downstream traffic. It offers spee of 2.048 Mbps over two copper pairs with operating distances of 3.6 km to 4.6 km, and is ideal for connecting PBX systems, digital local loops, point of presence, Internet servers and campus-based networks.

HDSL-II. HDSL-II, another technology proposed within the American National Standard Institute (ANSI) and the European Telecommunication Standard Institute (ETSI), offers the performance of HDSL but over a single pair.

HDSL was originally developed in USA, as a better technology for high-speed local exchange carrier systems which carry high-speed data links and voice channels using T1 lines. T-carrier circuits operate at 1.544Mbps and were carried using alternate mark inversion (AMI) line code. Due to limited range of AMI, the line code B8ZS (bipolar 8-zero substitution) has been used. Later, 2B1Q line code was used, which allowed a 784kbps data rate over a single twisted-pair cable and 1.544Mbps with two twisted-pair cables. But, the problem still continued due to the differences between the T1 (1.544Mbps) and E1 (2.048 Mbps) standards.

A new standard for HDSL has been developed using the carrierless amplitude phase modulation (CAP) line code, which reached the maximum bandwidth of 2.048 Mbps using two pairs of copper. HDSL can be used either at the T1 rate or the E1 rate. Multiple of 64kbps channels inside the T1/E1 frame can be used to provide slower speeds to customers but the line rate is still the full T1/E1 rate.

HDSL further gave birth to two new technologies, called HDSL2 and SDSL. HDSL2 offers the same data rate over a single pair of copper and can work up to longer distances over a low-quality or lower-gauge copper. On the other hand, single-line rate digital subscriber line (SDSL) is a multi-rate technology offering speeds ranging from 192 kbps to 2.3 Mbps using a single pair of copper.

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Single-pair high-speed digital subscriber line (SHDSL). Single-pair high-speed DSL technology supports symmetrical data rates. It is best suited for PBX, VPN, Web hosting and other data services that do not need the service guarantees of frame relay or the higher performance of a leased line. It cannot support voice service on the same pair as it takes over the entire bandwidth.

The ITU-T recommendation G.991.2 defines the standards for SHDSL. With one pair of copper line, the SHDSL having multiple of 64kbps payload provides symmetrical download and upload data rates ranging from 192 kbps to 2.304 Mbps. Moreover, the SHDSL provides symmetrical data rates from 384 kbps to 4.608 Mbps in 128kbps increments for two pair applications.

The distance covered is about 3 km and depends on the loop rate and noise conditions. One option to increase the coverage area is to decrease the data rates. Higher data rates can be achieved using two or four copper pairs, and one such extension of SHDSL provides data rates up to 5.696 Mbps.

The payload may be either unstructured, T1, E1, multiple ISDN basic rate access (BRA), asynchronous transfer mode (ATM) cells or Ethernet packet transfer mode (PTM). In order to share the SHDSL bandwidth, a dual bearer mode can be used, which allows a combination of two types of payloads.

Asymmetric digital subscriber line (ADSL). By studying different scenarios, it was realised that it was possible to transmit data more quickly from an exchange to a user. But when the user sent information to the exchange, it was more sensitive to the noise caused by electromagnetic disturbances (the nearer the subscriber to the exchange, the greater the concentration of cables, generating more crosstalk). So the idea was to use an asymmetric system, imposing a lower speed from the subscriber to the exchange. This idea gave birth to the asymmetric digital subscriber line technology, which was originally developed at Bellcore (now Telcordia Technologies) in 1988.

ADSL caters specifially to connections between ISPs and customers. The Internet is used largely for downloading fies, HTML and graphical content. Processes like uploading filesor other content to servers are limited to very few users. Hence the bandwidth required for downstream data (from ISP to client) is more than that required for upstream data (from client to ISP).

This DSL-based technology enables transmission and reception of data at speeds higher than legacy copper media. The modulation technique used allows several bits to be represented by one transmission symbol.

In ADSL, bit rate allocation for a channel within the available band-width is not the same as for the other channels, and hence the term ‘asymmetric.’ In other words, the upstream bandwidth is smaller than the downstream bandwidth. ADSL offers an upstream data rate of 500 kbps and a downstream data rate of up to 8 Mbps.

ADSL Lite, another variant of the ADSL standard, offers upstream speeds up to 500 kbps and downstream speeds up to 1.5 Mbps. Further, ADSL has many variants like ADSL2, splitterless ADSL2, ADSL2+ and ADSL++.

ADSL2/G.DMT.bis is defined in ITU G.992.3 and is an improved version of ADSL with data rates of 12 Mbps in downstream and 3.5 Mbps in upstream. Splitterless ADSL2/G.lite.bis is define in ITU G.992.4 and is capable of providing 1.536Mbps down-stream and 512kbps upstream.

ADSL2+ defined in ITU G.992.5 can provide up to 24Mbps theoretical downstream speed, which is double of the ADSL2 speed. The upstream speed is up to 3.5 Mbps. Thus ADSL2+ doubles the frequency band of typical ADSL from 1.1 MHz to 2.2 MHz. More importantly, ADSL2+ provides port bonding known as G.998.x or G.Bond. This is a very attractive feature of ADSL2+ in which the download and upload speeds are the sum of individual speed of all provisioned ports to the end user. It means if two lines with 24Mbps were bonded, the net result would be a speed of 48 Mbps.

ADSL++, another variant of ADSL, developed by Centillium Communications, is capable of providing download speeds up to 50Mbps, and uses the frequency band up to 3.75 Mhz.

Rate-adaptive digital subscriber line (R-ADSL). R-ADSL operates with the same transmission rates as ADSL, but the modem adjusts dynamically to varying lengths and quantities of the twisted-pair local lines. It makes possible to connect over different lines at varying speeds. Connection speed is negotiated by the end-points when the line synchronises up or as a result of a signal from the central office.

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Fig. 3: Comparison of various DSL technologies in terms of downstream data rate
Fig. 3: Comparison of various DSL technologies in terms of downstream data rate

R-ADSL is designed to increase range and noise tolerance by sacrificing upstream speed. The modem automatically creates a greater frequency band for the downstream than the upstream band. If line noise or signal degradation is large, the upstream bandwidth is decreased and may fall up to 64 kbps, which is equal to the speed of ISDN.

Uni-digital subscriber line (UDSL). Uni-DSL technology was originally developed by Texas Instruments and is meant for one DSL for universal service. Thus all discrete multi-tone services can be provided from one line card or home gateway, which yields in ease of deployment and more affordability. Further, Uni-DSL is backward-compatible to ADSL, ADSL2, ADSL2+, VDSL and VDSL2. The aggregate downstream and upstream speed provided by UDSL is at least 200 Mbps.

Etherloop. Ethernet local loop is the next-generation DSL technology that incorporates the features of Ethernet and DSL. It is capable of delivering speeds up to 6 Mbps over a reach of 6.5 km on a moderate-quality copper line. Etherloop uses half-duplex transmission and is almost unaffected by interference caused by poor line quality. So it is possible to provide Internet services up to long distances. Etherloop modems can also be used as a LAN extension in a situation where direct Ethernet is not possible due to distance limitation.

Very high-speed digital subscriber line (VDSL). VDSL is the fastest xDSL technology over a single copper-pair wire, supporting downstream rates of 13 to 52 Mbps and upstream rates of 1.5 to 2.3 Mbps. It was standardised by ITU-T recommendation G.993.1 in November 2001. The standard VDSL specified both quadrature amplitude modulation (QAM) and discrete multi-tone (DMT) modulation systems. Seven different frequency bands are used in VDSL, which enable customisation of upstream and downstream data rates. Further, this technology provides a cost-effective alternative to fibre-to-the-home (FTTH). This asymmetric technology operates over 300 metres to 1.4 km only from the central office.

One method of VDSL deployment is to run a fibreup to an optical network unit in the neighbourhood and then run copper pair to the customer’s premises within 1.2 km of the optical network unit. The high bit rate support makes VDSL ideal for services like high-definition televiion (HDTV), as well as voice-over-Internet protocol (VoIP) and general Internet access.

In February 2006, ITU-T recommended discrete multi-tone for an advanced version of VDSL called ‘VDSL2’ and the recommendation was defined in G.993.2. This new and advanced version is capable of supporting triple-play services such as data, voice, video, HDTV and online gaming. Data rates of more than 100 Mbps simultaneously in both the downstream and upstream are achievable up to about 300 metres.

VDSL2 utilises bandwidth of up to 30 MHz. The performance greatly depends on the loop attenuation and degrades quickly from 250 Mbps at source to 100 Mbps at 0.5 km and 50 Mbps at 1 km. Its performance is equal to ADSL2+ for distances more than 1.6 km. ADSL-like long-reach performance is one of the key features of VDSL2.

Gigabit digital subscriber line (GDSL). Gigabit digital subscriber line technology is based on binder multiple-input and multiple-output (MIMO) technology. Channel matrices generated from a binder MIMO channel model use transmission methods that can yield more then 1Gbps symmetric data rates over four twisted pairs of copper wire for a 300m range. In practice, extra copper pairs exist (usually two to six pairs) in the final drop segment to connect a subscriber, but there is almost no extra copper pair (from drop point near the subscriber to central office)to connect all these pairs for each subscriber. So the information-carrying capacity of a fibrecan be exploited to connect back to central offie. Using this arrangement, all the unused copper pairs can be properly vectored and bonded to be treated as a single transmission path and can be utilised to provide high-speed data rates.

To sum up
DSL is drawing significantattention from service providers because it has the ability to deliver high-speed data transmission over existing infrastructure with relatively small changes. xDSL family of technologies provides data and voice services at the same time and on the same copper line, as it uses high-frequency bands for data services and low-frequency band for regular voice services. A comparison of downstream data rate offered by different DSL technologies is shown in Fig. 3.

The author is working with Bharat Sanchar Nigam Limited as a junior telecom officer and is currently posted at Ludhiana, Punjab. He holds PhD degree in electronics engineering from Indian Institute of Technology-BHU, Varanasi, India, and has authored and co-authored more than 25 research papers in peer-reviewed national/international journals including IEEE and conference proceedings