Introduction

The term xDSL is used interchangeably with the acronym DSL to describe the variations of the Digital Subscriber Loop (DSL) technology. Some of the most common variations are: ADSL, SDSL, and VDSL. They all fall into one of two categories: asymmetric DSL and symmetric DSL with the term symmetric referring to the upstream vs. downstream data rates.

ADSL Technology

The acronym ADSL stands for Asymmetrical Digital Subscriber Loop and this technology uses your existing telephone line to transmit data between the Central Office and your computer. The major features of ADSL are:

• It doesn't tie up the phone line. Both analog phone conversations and high-speed data transmission can occur simultaneously.

• The data path is "always on", no dial-up is required.

• The copper wire infrastructure is very reliable.

• The Telco service provider has experience maintaining a high reliability network.

There are two types of ADSL systems in use a "full rate" system and a "lite" system. Generally, the ADSL lite system is used for residential service and Figure 1 shows the major components for this type of system.

Figure 1. Residential Service ADSL system

In the ADSL lite system, sharing of the same wire for voice and data is possible because the system uses different frequencies for each service. The lower frequencies, used for telephone service are below 4,000 Hz. and all the phones need to have a filters so that these phones don't cause interference with the higher frequencies being used by the ADSL modem.

The opposite end of the wire appears at the telephone Central Office. There is a frequency splitter that separates the voice signal and sends it the voice switch in the normal way. The data signal is sent to a unit called a DSL Access Multiplexer (DSLAM) that combines data from many ADSL modems and forwards them to the Internet.

A standard voice signal on your telephone line uses frequencies between 300 Hz. to 4,000 Hz. The phone line can actually handle higher frequencies than that and these and these higher frequencies are used to handle the upstream and downstream data between the DSLAM and the ADSL modem. This is shown graphically in Figure 2.

Figure 2. Frequency usage for an ADSL system

The ADSL Technology

ADSL is built upon the Discrete Multi-Tone (DMT) technology. DMT divides the frequency spectrum available on the copper wire into 256 sub-frequencies from 0 Hz to 1.1 MHz. The first 5 sub-channels are not used and reserved for the voice signal.

Each of the sub-frequencies is independently modulated using a QAM modulator. Depending on the quality of the signal in each sub-frequency, a different number of bits are able to be sent. If the Signal to Noise Ratio (SNR), a measure of the channel quality, was very good, many bits could be sent in that channel every second.. Should the SNR be very poor, fewer bits would be sent.

Figure 3. The frequency range is broken into 256 sub-frequencies

The flexibility of the technology allows for very good optimization of the transmission medium. Should a strong signal interference occur (radio station), then that frequency band might be total dropped from usage. It is also constantly optimizing its transmission to a changing environment.

Limitations of ADSL

ADSL service is limited to about 15,000 ft. In many cities, most residences are within that distance from the telephone central office. It is easy to offer ADSL to this type of customer since the DSLAM can be housed in the same building as the Voice Switch. Power, cooling, and maintenance are also provided by the same facility.

In many suburban and rural areas, telephone service is often beyond the 15,000-foot limitation. Often times these areas are served by something called a Digital Loop Carrier (DLC). The DLC is a box located in an area that brings together many telephone lines and combines them into a high speed digital channel. That channel can be over an optical fiber or high capacity copper wire. These DLC boxes have their own local power and emergency power.

The presence of the DLC box have caused major problems in the deployment of the ADSL service.

• The ADSL signal cannot pass through the DLC conversion. An ADSL termination device similar to the DSLAM in the Central Office must be put into the DLC cabinet.

• Many DLC cabinets are full or nearly full of equipment for telephone service. Often times this requires another box be installed at the same location. This causes delays in providing the service.

• If only a few people in that area of the DLC cabinet take the ADSL service, the cost per subscriber is very high.

Figure 4. A typical DLC cabinet

Other DSL Technologies

There are a number of DSL technologies. A quick summary of these technologies is shown in the following table:

Table 1. List of the Major xDSL Technologies

A short description of each of these technologies is provided below.

HDSL High Bit-Rate Digital Subscriber Line

HDSL is the most established of the DSL technologies. It is symmetric, with a maximum 1.5 megabits per second traveling both ways over two copper phone lines, or 2 Mbps over three phone lines. It is often utilized as an alternative to T1 connections. (A T1 connection is a high-speed, dedicated telephone line offering 1.54 megabits per second of data transfer.) HDSL is limited to a distance of 12,000 - 15,000 feet. This range can be extended with the use of signal repeaters.

HDSL II - High Bit-Rate Digital Subscriber Line II offers the same performance as HDSL, but over a single phone line.

SDSL - Symmetric Digital Subscriber Line

SDSL offers a symmetric transmission of data at the same speed as HDSL, with two important differences: it can be done using only one phone line and the user must be no more than 10,000 feet from the phone company's central office. SDSL is the forerunner to HDSL II.

VDSL - Very High Bit-Rate Digital Subscriber Line

VDSL is the fastest DSL technology, with rates from 13 to 52 megabits per second downstream and 1.5 to 2.3 megabits per second upstream. The tradeoff for this speed is that the maximum distance from the central office to the user must be between 1,000 and 4,500 feet. The provider can extend this distance by setting up a network interface within 4,500 feet of the user's location and connecting it to the central office with fiber optic cable.

IDSL - ISDN Digital Subscriber Line

IDSL is a hybrid of DSL and ISDN technologies. It uses the same data encoding technique of ISDN devices and delivers up to 144 kilobits per second bandwidth. The difference between the two is that IDSL bypasses the congested phone network and uses the data network instead. Also, there is no call setup delay like you experience with ISDN connections.

RADSL - Rate Adaptive Asymmetric Digital Subscriber Line

RADSL operates at the same bandwidths as ADSL - up to 7 megabits per second downstream and up to 1.5 megabits per second upstream, with the additional capability of adjusting bandwidth to the quality of the phone line during the data transmission, instead of just once at the start of the connection.

More Information

The DSL Forum, an alliance of companies advancing ADSL technology, provides a useful site at: www.adsl.com .

Related seminars:

Cable Modems vs. ADSL, Technology, Applications and Myths - Learn about these two types of broadband delivery systems. This seminar provides an overview of the technologies, where they shine and the the truth behind the myths being spread by these fierce competitors.

Cable Modem - The Cable Modem standard for the US is DOCSIS (Data Over Cable System Interface Specification). This series looks provides an overview of the workings of a cable TV system, the messages that control the system and the modulation technology.

In Summary:

• ADSL provides a reliable way to provide broadband services to customers..

• ADSL automatically adjusts its frequencies to avoid interference from other signals.

• Digital Loop Carrier cabinets have slowed deployment of the ADSL technology.

• There are many forms of DSL technology.