My previous posts were related to different incidents that took place in different stages of my life. The readers may find it boring to read the autobiography of the blogger. Well, this one is not of the same type as the previous posts. It is about a topic related to my field --- “Asymmetric Digital Subscriber Line” Technology which is commonly known as ADSL.


ADSL is a data communication technology/modem technology that enables faster data transmission over copper telephone lines than any conventional modem can provide. That’s why the government has given much importance to this technology in the Broadband Policy.

It is transforming the existing public network from a network that was limited to voice, text and low resolution graphics to a more powerful network capable of bringing multimedia, including full motion video, to everyone’s home.

Features:

Asymmetry - The data flows faster in one direction than the other. More precisely, Data transmission is faster downstream (to the user) to the subscriber than upstream (from the user). That’s why while downloading and uploading files using this technology, the users will find that the download speed is 10-12 times greater than the upload speed.

Digital – No type of communication is transferred in analogue method. All data are purely digital, and only at the end, modulated to be carried over the line.

The most important feature of this technology is that it allows the Plain Old Telephone system (POTS) signal to co-exist with the ADSL data signal. The service is always on and is usually charged at a flat rate regardless of time on-line or data used.

Requirements:


Delivery of ADSL services requires a single copper pair configuration of a standard voice circuit with an ADSL modem at each end of the line splitting the telephone line into three information channels: a high speed downstream channel, a medium speed upstream channel, and a Plain Old Telephone Service (POTS) channel or an ISDN channel.

The main ADSL network components are –

1. The ADSL modem at the customer premises, called an ADSL transceiver unit-remote (ATU-R) which provides the local loop termination on the customer side.
2. The modem of the central office that is called an ADSL transmission Unit – central office (ATU-C). It terminates the ADSL local loop at the central office premises.
3. Digital Subscriber Line Access Multiplier (DSLAM). Many ATU-C units are inserted into the DSLAM. The unit can connect through an ATM or an ETHERNET access network to the internet.
4. Splitter: An electronic low-pass filter that separates the analogue voice or ISDN signal from ADSL data frequencies when they get to the subscriber premises. For outgoing traffic, when they are transmitted from the subscriber premises, it combines the voice and the data frequencies onto one line. This allows a POTS phone connection to operate at the same time as ADSL digital data is transmitted or received on the same line. One splitter is located at the central office and the other at the subscriber premises. The splitter at the central office can be a separate device or may be incorporated into the DSLAM.

Working Principle:

An ADSL link is basically a computer, connected to a modem, connected over the old PSTN (Public Switched Telephone Network) copper wire to a DLSAM at the telephone company main office. The PSTN is separated from the data traffic using high/low pass filters (splitters). The voice data is routed to the regular PSTN switching network, while the data is routed to the ISP. The multiple virtual circuits (ADSL connections) created, are then aggregated and transferred to the correct ISP using a T3/E3 line.

The ADSL specification allows the voice (PSTN) and data communication to co-exist on the same pair of twisted copper lines. This is accomplished by a set of low-pass and high-pass splitters installed at both the customers’ location and the central office. These splitters also protect the sensitive data communication from spikes originating in the PSTN network.


ADSL: Technology and Modulation

ADSL exploits the fact that all telephony signals are below 4 KHz in frequency although a typical copper pair line can transmit usable signals up to 1 MHz. ADSL uses the rest of the full copper pair line frequency spectrum, from above the voice frequencies up to 1.1 MHz. In ADSL modems, multiple channels are created by dividing the available bandwidth of a telephone line. On way to do this is by using Frequency Division Multiplexing (FDM). FDM transmits multiple signals simultaneously over a single transmission path. Each signal travels within its own unique frequency range (carrier), which is modulated by the data (text, voice, video etc).


FDM assigns one band for upstream data and another band for downstream data. In this way, ADSL splits off a 4KHz region for POTS at the end of the band. The ADSL modem combines the data stream from the computer, divides it into the available aggregate channels, interleave, attach an ECC and then finally modulate the data on the copper wires to be received by the demodulator at the other end. The receiving end will perform error checking, correct the errors or request to re-transmit if not possible. Interleaved blocks will be kept in the buffers until the entire blocks can be reconstructed from subsequent frames.

On the outside, ADSL looks simple and transparent, but on the inside there is a miracle of modern technology. In ADSL, downstream data rates can be between 1.5 Mbps and 8 Mbps, while upstream data rates are between 16 Kbps and 832 Kbps. The minimum configuration provides 1.5 or 2.0 Mbps downstream and a 16 Kbps duplex channel; others provide rates at 6.1 Mbps and 64 Kbps duplex. Products with downstream rates up to 8 Mbps and duplex rates up to 640 Kbps are also available.

The factors that can influence the downstream data rates are the length of the line, the gauge of the line, presence of bridged taps and crosstalk from other wires in the same time that cause noise. Line attenuation increases with line length and frequency, and decreases as wire diameter increases. Also, some copper loops use different gauge wires at different points and this can cause reflections in the signal, effectively attenuating some frequencies.

As customer demand for triple-play services – voice, data and video increases, so does the competition between carriers/DSL service providers and cable operators. While the later are already offering HDTV, VoD, VoIP and other high speed services, DSL providers cannot compete with the high data rates requirements as existing xDSL technology fail to achieve them. Hence we need some high speed fibre-like broadband capability and the sooner-the better.