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ADSL overview

Zbynek Soban, 16. March 2004 - 0:00

Asymmetric digital subscriber line (ADSL) uses existing twisted pair telephone lines to create access paths for high-speed data communications and transmits at speeds up to 8.1Mbps to a subscriber. This article explains the basic function of ADSL transceivers and their modulation scheme.

Introduction

Asymmetric digital subscriber line (ADSL) uses existing twisted pair telephone lines to create access paths for high-speed data communications and transmits at speeds up to 8.1Mbps to a subscriber.

Delivery of ADSL services requires a single copper pair configuration of a standard voice circuitwith an ADSL modem at each end of the line, creating three information channels â a high speed downstream channel, a medium speed upstream channel, and a plain old telephone service (POTS) channel for voice.

ADSL refers to a modulation scheme used to deliver network traffic to a customer's residence using the same copper twisted-pair wiring used for voice and ISDN service. It coexists with both services, while offering 6-8Mbps speeds downstream and up to 640kbps upstream. Although this asymmetry sounds unusual for a data transmission scheme, it is actually well suited to typical client network traffic, where the downstream traffic is about ten times more voluminous than upstream traffic.

ADSL Architecture

Overall Network

The overall network diagram below describes the network elements incorporated in multimedia communications, and suggests a group of transport configurations ADSL will encounter as networks migrate from Synchronous Transfer Mode (STM) to Asynchronous Transfer Mode (ATM).

overall network

ADSL Asymmetric Digital Subscriber Line STM Synchronous Transfer Mode
ATM Asynchronous Transfer Mode TE Terminal Equipment
OS Operations System See System Reference Model for reference
PDN Premises Distribution Network point definitions
SM Service Module

System Reference Model

DSL System reference model, which is shown in figure, describes the basic blocks of an ADSL-system.

System Reference Model

Definition

The decomposed and routed data from the access module, see figure , is connected to an ATU-C (ADSL Transceiver Unit - Central Office) in which the data will be converted into analog signals. The analog signals are then carried with POTS signals to remote end. ATU-C also receives and decodes data coming from customers premises send by ATU-R (remote).

ADSL Signal Encoding

I) Copper loop quality

Several factors may affect the throughput of a twisted-pair copper loop:

  • Loop length--The length of the copper loop between the central station and the residence is the most prominent factor in available throughput. Signals are attenuated by an amount proportional to the loop length. In addition, the attenuation is a function of the frequency, such that higher frequencies are attenuated more than lower frequencies.
  • Bridged taps--Lengths of unterminated twisted-pair cable connected in parallel to the primary pair.
  • Ham and AM radio--These radio transmissions fall within the spectrum used by ADSL and can be a severe disruption to the signal.
  • Crosstalk--Interference from adjacent wires in the feeder trunks running to the neighborhood. This distortion cannot be compensated for by increasing the transmission power, since the noise from the higher-powered adjacent lines would also grow in proportion.
  • Wire gauge--The effective range and throughput can be shortened by higher-gauge (smaller) wire. Some copper loops user different gauge wires at different points--this can cause reflections in the signal, effectively attenuating some frequencies.

In addition, loading coils--inductors placed every 6000 feet in series with the wire--must be removed before ADSL modems can be used. These were used to filter noise on analog voice signals by acting as a low-pass filter, blocking the high frequencies that ADSL requires to function. Fortunately, most phone lines are not equipped with inductors.

For neighborhoods which are located too far from the central office to obtain useful ADSL connection speeds, an optical network unit can be placed closer to the area. Data is transmitted over fiber to the optical network unit, which then distributes the signals to ADSL modems for connection to the residences.

II) Frequency bands

In ADSL DMT-systems the downstream channels are divided into 256 4-kHz-wide tones. The upstream channels are divided into 32 subchannels.


Figure: Frequency bands

Upstream data transfer frequencies range from 25 kHz to 138 kHz, whereas downstream data transfer frequencies range from 139 kHz to 1.1 MHz

Note:

Guardbands divide the three frequency bands. Each tone has a bandwidth of 4 kHz and a spacing of 4.3 kHz; each supporting a maximum number of 15 bits, as limited by its signal-tonoise ratio. Since the tones in higher frequencies are more susceptible to attenuation and noise, higher frequencies

III) Modulation

DMT

Telecommunication Union (ITU) chose discrete multitone (DMT) modulation as the standard line code for ADSL. DMT, as its name implies, divides the data bandwidth into 256 sub-channels, or tones, ranging from 25 kHz to 1.1 MHz.

DMT is able to allocate data so that the throughput of every single subchannel is maximized. If some subchannel can not carry any data, it can be turned off and the use of available bandwidth is optimized. The examples in figure give an idea about of the functionality of DMT.


Figure: Example of DMT

First an equal number per tone is transmitted to measure the characteristics of the line. The processing of the signal takes place in ATU-R, and the optimized bit distribution information will be delivered for ATU-C by using the same phone-line at a secure low speed.

The first example describes a segment of 24-gauge twisted pair phone-line. Low frequencies are eliminated by the transformer coupling. The attenuation at the higher frequencies depends on the length of the phone-line.

The second example includes the notch in spectrum that is illustrative of bridge taps and also the interference of an AM radio station.

A third example shows that DMT is also an interesting possibility for other transmission channels, such as coaxial cable-TV networks, as well.

QAM - Quadrature amplitude modulation

Quadrature Amplitude Modulation refers to a combination of phase-shifting the carrier and varying its amplitude. By doing this, it is possible to transmit more than one bit for every change in the carrier signal.

ADSL uses quadrature amplitude modulation (QAM) to achieve the 15-bitmaximum that any single tone can carry. This technique employs a combination of amplitude modulation and phase shift keying.

For example, a signal that transmits at three bits per baud requires eight binary combinations to represent the signal. This example assumes two possible measures of amplitude and four possible phase shifts, which allow for eight possible waves. Table shows the correspondence between each binary combination and amplitude and phase shift. Using the above technique, a large bit stream can be broken down into three-bit words, as shown in the following example:

001-010-100-011-101-000-011-110

Figure illustrates QAM-encoded signals of the above bit stream with each wave shifted in relation to the wave that immediately precedes it.

 

IV) Connection

Data transmission

Framing

Data is transmitted in superframes containing 68 ADSL frames. Each superframe also contains a synch symbol which occupies one additional frame. Thus, the total number of frames transmitted is 69. Each ADSL frame consists of two parts, one from each of two buffers: the fast buffer and the interleaved buffer. The fast buffer, in addition to user data, may contain CRC error checking bits, and forward error correcting bits. The fast byte in frame 1, 34, and 35 contain indicator bits used for administrative functions. The interleaved buffer contains purely data.

 

 

 

Links

 

na clanek na tema: ADSL Applications
 http://www.dslforum.org/aboutdsl/app_notes.htm

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