What does xDSL and ADSL mean?

The term DSL or xDSL means Digital Subscriber Line and covers all technologies installed for digital transport of information over a single telephone line connection. xDSL technologies are divided into two main types, those using symmetric transmission and those using asymmetric transmission. These two types are described further on in this document.

The term ADSL means Asymmetric Digital Subscriber Line. This system enables the coexistence of a high speed downstream channel, an average speed upstream channel and a telephone channel (called POTS in telecommunications, which means: Plain Old Telephone Service).

The use of xDSL and ADSL technologies

The rapid development of information technologies has led to the appearance of new services hungry for transmission capacity. High speed Internet access, videoconferencing, interconnecting networks, teleworking, broadcasting of TV programs, etc. all form part of the new multimedia services that the user wants at home or in the office.

Until now, existing high speed services (coaxial cable, fibre optic) were not well suited to real needs (cabling too expensive to replace with fibre optics or unstable connection with coaxial cable). The idea of using twisted pair cabling seemed the best suited since throughout the world more than 800 million 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 ADSL.

Characteristics of ADSL technologies

The term DSL or xDSL can be broken down into several groups: HDSL, SDSL, ADSL, RADSL, VDSL. Each of these groups relates to a use and has particular characteristics.

These technologies are differentiated by:

  • The speed of transmission
  • The maximum distance of transmission
  • The variation in speed between upstream and downstream
  • The symmetric or asymmetric character of the connection

The point to point connection is conducted via a telephone line between two pieces of hardware, the NT (Network Termination) installed at the user's location and the LT (Line Termination) installed in the connection exchange.

Symmetric solutions

The connection is carried out across twisted pairs with an identical upstream and downstream speed.


HDSL (High bit rate DSL) was the first DSL technology and was developed at the beginning of the 1990s.

This technology consists of dividing the digital core of the network, T1 in America and E1 in Europe over 2 twisted cables for T1 and 3 twisted cables for E1.

With this technology, it is possible to achieve a speed of 2Mbps in both directions over three twisted pairs and 1.5 Mbps in both directions over two twisted pairs. It is possible that the speed, if it is at 2 Mbps may fall to 384 kbps due for example to the line quality and the distance of the line over the last kilometre (between 3 and 7 km depending on the wire diameter, between 0.4mm and 0.8mm respectively).

The connection may be permanent but there is no telephone channel available during an HDSL connection.

The current problem with this technology is that its standardization is not yet perfect.


SDSL (Single pair DSL, or symmetric DSL) is the forerunner to HDSL2 (this technology, derived from HDSL should offer the same performance but over a single twisted pair).

This technology is designed for a shorter distance than HDSL (see table below). SDSL technology will certainly disappear in favor of HDSL2.

Downstream: [Kbit/s] Upstream: [Kbit/s] Distance: [km]
Distances and speeds of an SDSL connection

Asymmetric solutions

By studying different scenarios, it was realized that it was possible to transmit data more quickly from an exchange to a user but that when the user sent information to the exchange, it was more sensitive to noise caused by electromagnetic disturbances (the nearer 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.


In the same way as HDSL, ADSL (Asymmetric Digital Subscriber Line) has existed for around ten years and was firstly developed to receive television via the standard telephone network. But the development of the Internet found another use for this technology, that of being able to surf the net quickly without occupying the telephone line.

ADSL is also currently one of the only technologies available on the market which offers the transport of TV/video in digital format (MPEG1 or MPEG2) using a telephone connection.

Notably, ADSL allows the transport of TCP/IP, ATM and X.25 data.

The ADSL standard was finalised in 1995 and provided:

  • A telephone channel with analogue or ISDN connection
  • An upstream channel with a maximum capacity of 800 kbit/s
  • A downstream channel with a maximum speed of 8192 kbit/s

As for all DSL technologies, the loop distance between the exchange and the user must not exceed certain scales so as to guarantee good data speed (see table).

Downstream: [Kbit/s] Upstream: [Kbit/s] cable diameter: [Mm] Distance: [km]
Speeds according to distance and cable diameter

For transmitting data, two modulation techniques have been used by ADSL hardware manufacturers:

  • CAP (Carrierless Amplitude and Phase Modulation) which is a variant of the QAM technology (Quadratic Amplitude Modulation). Widely used at the beginning of the 1st ADSL, this type of modulation has never correctly been standardized, and because of this there is no interoperability possible between hardware from different manufacturers.
  • DMT (Discrete Multi Tone) is a more recent modulation technique. Its principle relies on the use of a large number of sub-carriers shared over the frequency band used by the system (see "ADSL modulation techniques").

This diagram presents the various functional blocks which make up an ADSL connection.

functional blocks which make up an ADSL connection

The two service categories are separated on the network and at the client's location by a splitter (see chapter 4.2).

At the end of 1998 the ITU (International Telecommunication Union) normalised a new standard: ADSL-Lite which is in fact a lighter version of ADSL. ADSL-Lite has a lower speed than its senior (around 1.5 Mbit/s) and does not require a splitter.


RADSL (Rate Adaptive DSL) technology is based on ADSL. Transmission is fixed automatically and dynamically by looking for the maximum possible speed on the connection line and continually readapting it without disconnecting.

RADSL should allow upstream speeds of 128 kbps to 1 Mbps and downstream speeds of 600kbps to 7 Mbps, for a maximum loop of 5.4 km.

RADSL uses DMT modulation (as is mostly the case for ADSL). It is in the process of being standardized by ANSI.


VDSL (Very High Bit Rate DSL) is the fastest of the DSL technologies and is based on RADSL. It is capable of supporting, over a single twisted pair, speeds of 13 to 55.2 Mbps downstream and 1.5 to 6 Mbps upstream, or if a symmetric connection is required, a speed of 34 Mbps in both directions. So VDSL can be used in symmetric or asymmetric connection.

VDSL was principally developed for the transport of ATM (Asynchronous Transfer Mode) at high speed over a short distance (up to 1.5 km).

The standard is currently being normalised. QAM, CAP, DMT, DWMT (Discrete Wavelet MultiTone) and SLC (Simple Line Code) modulations are being considered.

For data transport, VDSL hardware is linked to the connection exchange by fibre optics forming SDH loops at 155 Mbps, 622 Mbps or 2.5 Gbps. The transport of voice between VDSL hardware and the exchange can also be provided by copper loops.

How to reconcile an analogue and ADSL network on the same line

Description of a copper cable

The twisted pair is made up of two copper conductors of a diameter between 0.4mm and 0.8mm inclusive (rarely 1mm). The conductors are isolated and paired in order to reduce crosstalk. In the majority of cases, twisted pairs are grouped together in fours in a cable protected by a plastic sheath. The cables used on the telephone network are made up from 2 to 2,400 pairs and are not sheathed.

structure of a copper cable

Traditional telephone services require a band width of 3.1kHz (the bandwidth is between 300 Hz and 3,400 Hz), however the cables connecting the telephone exchanges to users all have a higher bandwidth in the region of several hundred kHz. It is over this cabled access network that the xDSL technologies have been developed.

At high frequencies problems related to distance are the most restrictive (fading, crosstalk, phase distortion). At low frequencies, there are difficulties related to impulse noise which dominates without too much difficulty up to 1Mhz. Furthermore, their use becomes tricky and requires very high performance transmission systems.

Limitations of the analogue network

The maximum possible speed over the analogue network is from 33,600 bit/s upstream and 56,000 (in theory) downstream.

This includes the use of a technology going beyond the bandwidth of 3.1 kHz.

The use of an ISDN connection in fact already calls on xDSL technology because it covers a frequency spectrum up to 80 kHz.

As explained in chapter 2.3.1, CAP modulation technology has been replaced by DMT technology which was accepted for the ANSI T1.413-1995 standard.

DMT (Discrete Multi Tone) is a form of multi carrier modulation. For its application to ADSL, the frequency spectrum made up of between 0 Hz and 1,104 MHz is divided into 256 distinct sub-channels separated by 4,3125 kHz. The lower sub-channels are generally reserved for POTS, so sub-channels 1 to 6 (up to 25,875 kHz) are in principle unused and left for analogue telephony.

According to T1.413 only sub-channels 1 to 31 can be used for upstream speed.

The upstream and downstream speeds are separated, either by EC (Echo Cancelling) which allows the lower sub-channels (from 1 to 31) to be used for the downstream and upstream, or by FDM (Frequency Division Multiplexing) which is the most used because of its simplicity and low cost, which separates the upstream/downstream sub-channels via a passive filter.

Distribution of DMT channels on POTS with EC

Sub-channels 1 to 6 are used for telephone, sub-channels 7 to 31 for upstream, sub-channel 32 is reserved, sub-channels 33 to 256 are used for downstream.
Note that sub-channels 16 and 64 are used to transport a pilot signal and channels 250 to 256 are only useable over low bandwidth connection lines. Above 1 MHz, disturbances are too great to allow a stable flow.

Distribution of the DMT channels on POTS with EC

In this case, DMT uses the echo cancelling technique on these sub-channels which results in dual flow on sub-channels 7 to 31. If DMT had applied FDM, only the higher sub-channels (33 to 256) would be used for the downstream.

Distribution of DMT channels on ISDN with FDM

As we have seen previously, IDSN uses the lower bandwidth up to 80 KHz (for IDSN with 2B1Q - 2 Binary 1 Quaternary; encoding of 2 binary elements in one moment of quaternary modulation). To enable simultaneous use of ISDN and ADSL on the same telephone line, sub-channels 1 to 28 are free.

Distribution of the DMT channels on ISDN with FDM

The lower channels are used for the upstream speed because the user hardware has a weaker transmission strength than the hardware installed at the exchange so by transmitting in lower frequencies, the signal will undergo less attenuation.

The higher channels are used for downstream speed because the hardware located at the exchange are strongly disrupted by transmission devices with higher frequencies so it appears to be more effective to transmit on higher channels in order to benefit from a better signal/noise ratio.

ADSL hardware


DSLAM (Digital Subscriber Line Access Multiplexer) is a piece of hardware generally installed in the telephone exchange providing multiplexing for ATM flows to the transport network.

This element does not only host the ADSL cards but can also host different DSL services such as SDSL or HDSL by inserting the corresponding multiplexing cards. Each card supports several ADSL modems.

The elements grouped together in the DSLAM are called ATU-C (ADSL Transceiver Unit, Central office end).

In fact, all services available on the network (Internet, LAN-MAN-WAN, Teleshopping, Video MPEG) arrive by broadband to a DSLAM station to then be redistributed to the users.

Maintenance and configuration of the DSLAM and ADSL hardware is done remotely.

ADSL modems and routers

In the previous chapter we saw how data is sent to the user. But now the user must decode the data, this is the job of the modem which is called ATU-R (ADSL Transceiver Unit, Remote terminal end).

To date, there are three types of modem depending on the user's requirements:

With 10/100 baseT interface, for PCs equipped with an Ethernet card

ATMD 25 for PCs equipped with an ATM card or for redistributing ADSL over an ATM network

With USB interface for PCs equipped with USB interface

If the user wants to redistribute ADSL over his IT network, the user is best to use a router with ADSL interface.

The splitter and microfilter

The splitter is installed in the telephone exchange, downstream from the DSLAM and audio switch.

Then, if the user has an ISDN connection, he must install a splitter at home upstream of his modem and his ISDN NT.

ADSL splitter

If the user has a traditional analogue connection, he does not need to install a splitter at home, but a microfilter in front of each telephone.

The splitter's role: The splitter is a switching filter which separates bandwidth reserved for the telephone service from bandwidth used for ADSL transmissions. It provides sufficient separation to avoid the signals transmitted on one frequency band disturbing the operation of the other. Note that the installation of a splitter is compulsory to have ADSL with an ISDN connection.

The microfilter's role: the microfilter is a low pass filter and is installed on analogue connections. There is therefore no reason to install a splitter.

The splitter and microfilter

Using standard digital broadcast by the terrestrial hertzian network DVB-T (Digital Video Broadcasting), it becomes possible to receive digital TV in MPEG format over a decoder connected to a TV station.

Bibliography and Internet links

Julien Perriard

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