Overview

Digital Subscriber Line (DSL) is a copper-loop transmission technology, incorporating a number of variations, such as ADSL, SDSL, and VDSL, used for transmitting high-speed data over existing telephone lines. It is the most widely used high-speed Internet technology in the world accounting in 2005 for 65.2% of the total installations.  One of its major advantages is that it uses the existing legacy telephone system architecture and is carried on the same twisted-pair telephone line that provides basic telephony service. 

DSL technology takes advantage of the fact that voice calls and other traditional analogue telephony applications use only a small segment of the potential bandwidth available on a telephone line.  This technology creates an additional pool of channels that are split into two groups that are used for sending and receiving high-speed digital transmissions. It allows voice communication and high-speed data transmission on the same line at the same time, by transmitting the two on different frequencies.

DSL networks operate on a point-to-point architecture configuration with a DSL modem at the customer’s premises that connects to a designated port in the Digital Subscriber Line Access Multiplexer (DSLAM) normally installed in the telephone company’s local telephone exchange or Central Office.  The DSLAM is used to provide connectivity and routing of all data and voice signals between the subscriber’s premises and the intended destination.

There are various types of DSL services that are in use and the capabilities vary quite considerably in terms of the bandwidth and the distance that a signal can be transmitted. Although there are a number of factors that affect the performance of DSL, such as the underlying network infrastructure and the condition of the local loop, distance or more specifically, the length of the local loop between the customer’s premises and the Central Office is a major limiting factor. If the local loop exceeds a certain length, it is not possible to provide DSL service.  The loop length requirement also varies with the type of DSL technology being deployed. As an example with ADSL service, the maximum loop length is about 18,000 feet or 5.5 kilometres, whereas, VDSL has a maximum loop length of approximately 1.3 kilometres.

The type of DSL technology that can be deployed is important in assessing the competitive positioning of this technology, and the variations that exist in different countries.  As an example, most incumbent carriers in the European Community have typically shorter loop lengths than is the case in the United States.  As a result, European carriers have greater capability to deploy DSL technologies that have higher bandwidth than incumbent carriers in the United States, which enhances their ability to compete in the provision of bandwidth-intensive services using the existing network infrastructure.  For example, Korea Telecom uses VDSL to provide 13 and 20 Mbps capability to its Internet customers.

A brief summary of the major characteristics of these DSL services is as follows:

ADSL can provide data rates up to 8 Mbps from the network to the subscriber direction, and up to 1 Mbps from the subscriber to the network direction. The asymmetry of ADSL works well for today's home applications where the majority of bandwidth is consumed in the network to user direction. 

ADSL2 is a newer type of ADSL that provides downstream speeds op up to 12 Mbps at distances of less than 2.5 km (8000 feet). ADSL2+ is another variant that can provide 24 Mbps on the downstream and 1 Mbps on the upstream on distances of less than 1.5 km (5000 feet).  ADSL2+ has the added advantage of offering seamless bonding options enabling two lines to be bonded together to deliver approximately double the bandwidth to about 48 Mbps on the downstream. 

SDSL offers a competitive alternative to T1 and E1 leased lines. It runs over one pair of copper wires, with a maximum range of about 3 kilometers. The main difference between ADSL and SDSL is that SDSL has the same upstream data transfer rate as downstream, whereas ADSL always has smaller upstream bandwidth. SDSL was never properly standardized until the ITU established a standard for G.SHDSL. SDSL is often confused with G.SHDSL, which offers data rates from 192 kbps to 2.3 Mbps while providing a 30% longer reach than SDSL.

VDSL can support symmetrical or asymmetrical services. Asymmetrical VDSL is capable of providing data rates to the user of up to 52 Mbps, making it suitable for transporting high-speed applications such as real-time video streaming. The trade-off for this high speed is restricted reach. This requires that the customer be located close to the CO or that the infrastructure access gateway resides outside the CO (and closer to the customers) in a remote terminal (RT).

Advantages and Limitations

What are the capabilities and advantages of using DSL technologies?

• Marginal infrastructure investments needed: There is no need for extra access infrastructure - DSL technologies in general use the existing phone lines. DSL technologies are seen as an attractive opportunity especially for fixed line incumbents to re-use its earlier investments in copper infrastructure by upgrading it with DSL capabilities. If there is an existing infrastructure in place, DSL is often seen as fairly cost effective way to rollout a better infrastructure and offer more services to customers. In addition, where unbundling exists, DSL allows competitive operators to offer a variety of broadband services, without having to deploy their own infrastructure.
• DSL does not tie up the phone line while active (e.g. using the internet) and No Dial Up: there isn’t any need to 'dial-up' the Internet.

What are the constraints and disadvantages of using DSL technologies?

• Not universally available: DSL is not available everywhere, as the local telephone exchange must be ADSL enabled.  The cost of enabling an exchange, particularly when backhaul has to be provided can be uneconomic in sparsely populated areas and is a major issue in most advanced economies.  Even where an exchange has been enabled, the service may not be available to certain individuals because of distance from the exchange or Central Office.  In developing countries, where the deployment of basic telephony is still a major challenge, DSL is not an immediate option, particularly in rural areas.
• Cost Effectiveness. DSL Technologies are cost effective to the extent that a landline is already in service and there is sufficient market demand to justify the investment by the incumbent operator. Then, DSL can be enabled fairly easy through adding a DSL component at home and through upgrades at the exchanges.
• Trade Off between Reach and Quality of Services: For DSL technologies the bandwidth (hence, the speed), of the data transmission declines as the length of the local copper loop increases. Naturally, within countries, longer loops tend to be in rural areas. Thus, there is a universal problem in providing high bandwidth DSL services in rural areas, whether it is in emerging or advanced economies even when basic telephony service is available.        

DSL Technology and Local Open Access Networks

DSL technology has been adopted by the telephone industry, as it has given it the capability to provide high-speed Internet services on its existing copper based local loop.  A number of regulators have brought in unbundling requirements, whereby the incumbent operator is required to provide wholesale access to the local loop to other ISPs enabling them to compete for DSL customers with the incumbent carrier. 

Although this does increase the level of competition in the marketplace, it does not result in any fundamental shift towards telephone companies adopting a Local Open Access Network model for the conduct of its operations.  The business model adopted by telephone companies is to build and operate networks to provide services to the public and not just be providers of network access.

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