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Digital Subscriber Lines: XDSL, ADSL, SDSL

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xDSL uses digital encoding to provide more bandwidth over existing twisted-pair telephone lines. Many iterations of xDSL allow the phone to be used for data communication at the same time it's being used to transmit data. In this article, Kyle Cassidy explores the different digital subscriber lines and Frame Relay.
This article is excerpted from The Concise Guide to Enterprise Internetworking and Security.


xDSL ranges from 6.1Mbps to 155Mbps incoming, and from 600Kbps to 15Mbps outgoing. The "x" is a wildcard that can be ADSL (asynchronous) or SDSL (synchronous). xDSL uses digital encoding to provide more bandwidth over existing twisted-pair telephone lines (POTS). Many iterations of xDSL allow the phone to be used for data communication at the same time it's being used to transmit data. This is because phone conversations use frequencies below 4KHz, above which xDSL tends to operate. Several types of xDSL modems come with "splitters" for using voice and data concurrently.

xDSL connections use frequencies of more than 4000KHz to achieve their great bandwidth. This comes at the expense of attenuation. The two most popular types of line coding, CAP and DMT, use lower frequencies and therefore are able to support longer loops between the user and the phone company. You can see a breakdown of their capacity in Figure 1.

Figure 1 Bit rates of the various xDSLs.


Asymmetric Digital Subscriber Line (ADSL) is asymmetric because of its relatively high capacity to download data when compared to its lower upload capacity. ADSL allows you an 18,000-foot loop from the phone company and is capable of transmitting at speeds of up to 8Mbps over ordinary twisted copper pairs. ADSL allows for a splitter box that lets users talk on the telephone at the same time data is being transmitted. The asymmetric speed of ADSL is appropriate for home users who typically draw more from the Internet than they send out to it. ADSL uses carrierless amplitude phase modulation (CAP) or discrete multitone (DMT).


The speed of a Rate Adaptive Digital Subscriber Line (R-ADSL) is dependent on the capacity of the twisted pair it's running over. R-ADSL allows for on-the-fly determinations of proper transmit and receive speeds based upon the quality of the connection, length of the loop, and the type of wire being used in the loop. It should be used in situations in which the quality of the line connection is variable or affected by weather. R-ADSL also allows for a splitter. It transmits data using CAP.


HDSL (High-Speed Digital Subscriber Line) is the result of early 1990s research into approaching T1 and E1 speeds (1.5Mbps and 2.0Mbps, respectively) over POTS. HDSL uses the same encoding methods employed by ISDN and employs two sets of phone lines. It also employs a higher number of bits per baud. The incoming and outgoing speeds of HDSL are identical.


ISDN Digital Subscriber Line (IDSL) technology ports ISDN functionality to DSL. It permits data speeds of 128Kbps over ordinary twisted-pair phone lines in loops of 18,000 feet. IDSL is capable of using the same hardware as ISDN. IDSL has the advantage of being able to use any transport protocol that ISDN can, such as PPP or Frame Relay. IDSL uses the same 2B1Q line coding that ISDN does. IDSL does not support voice transmission.


Very High Digital Subscriber Line (VDSL) suffers from extremely high attenuation, resulting in loop lengths of only about 3,000 feet.


Symmetric Digital Subscriber Line (SDSL) is symmetrical in that the incoming and outgoing bandwidth is the same. SDSL can duplicate T1 or E1 speeds over normal twisted-pair (copper) phone cable for distances up to 11,000 feet (looped, so you must be within about 1 wire mile of your telephone company). SDSL uses carrierless amplitude phase modulation (CAP).

Splitterless DSL or DSL-Lite

DSL-Lite has no splitter box and is less expensive than other forms of DSL. It provides data-only communication and requires no interventive hardware setup on the user end. Because it isn't cost-effective to replace the copper wire infrastructure with fiber, there has been a great leap to trying to send more data across the existing wire. Both carrierless amplitude phase modulation (CAP) and discrete multitone (DMT) use the unused high frequencies of the phone line to encode multiple bits of data in each signal.

Carrierless Amplitude Phase Modulation (CAP)

CAP is a variation on quadrature amplitude modulation (QAM), which has been used in analog modems for several years. CAP divides the frequency spectrum into the POTS spectrum (0–4KHz), downstream data, and upstream data.

QAM is a method for encoding digital data on an analog signaling system. QAM uses a combination of amplitude and phase variations to provide more than 1 bit per signal, or baud. Generally QAM is represented by a map that lays out a bit pattern on a coordinate axis (see Figure 2). The combination of amplitude and phase will always map to a certain number of points.

Figure 2 Changes in amplitude modulation (left) and phase shift (right) allow for one baud (signal) to have multiple meanings. This allows for significantly faster data transmission.

Take, for example, the simple QAM of two possible amplitudes and four possible phase shifts. Combining the two possible amplitudes and the four possible phase shifts gives you a total of eight possible wave forms. This can be mapped out in a table rather easily (see Figure 3).

Figure 3 Bit values of phase and amplitude shift.

It is quite convenient that this example has eight possible wave forms because that can easily be represented by 3 bits. Using Figure 3, say that you wanted to transmit the following data:


You would first break it down into 3-bit representations. You can see how each 3-bit value maps to a specific wave combination:


The whole point is to transfer more than 1 bit of data per signal.

There is no carrier frequency transmitted in CAP. Three channels are allocated: POTS, upstream data, and a high-speed downstream data. Each channel is separated in the frequency domain.

Data modulates a single carrier and is transmitted down a phone line. The carrier is removed before transmission and added again at the receiving end.

Discrete Multitone (DMT)

Copper lines distort heavily at the high frequencies used by ADSL circuits. DMT is particularly good at overcoming this. DMT was developed by Amati Communications in collusion with Stanford University. Originally, it was intended for sending video over copper. Actually a form of frequency-division multiplexing (FDM), Discrete Multitone is a multicarrier modulation that divides the available channel into individual subchannels called tones. Each of these tones has the same bandwidth but is modulated on a separate carrier. There are 256 different tones that range from 20KHz to 1.024MHz at 4KHz intervals.

DMT dynamically adjusts to compensate for line noise, allocating subcarriers with a high signal-to-noise ratio to the highest spectrum.

Loading Coils

Some telephone lines are equipped with induction devices called loading coils. The loading coil is designed to improve voice transmission over POTS when the local loop is more than 18,000 feet (the last 3 copper miles). It does this by compensating for the capacitance of the wire and raising the frequency. This is nice for voice transmission; however, a side effect is the distortion of high frequency signals. This distortion will annoy xDSL. Ask your telephone company about its procedure for removing loading coils from xDSL connections.

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