Can anyone describe the signalling methods used with ADSL modems between the home and the exchange. I thought that 56k was the thoeretical maximum for dial up modem modulated signalling. I also thought that pure digital signals degraded very rapidly when sent any distance through copper cables, so , how is it done?
56k Modems use the voice signals of a copper pair. These have very low bandwidths due to the nature of the use (speech). ADSL uses the data frequencies of the copper pair, thus having a higher bandwidth. As you are probably aware, your phone line has 2 wires between your socket and exchange. Only one is used when you are on a phone call or 56k modem. DIGITAL signals DO NOT Degrade. If you require more detail let me know. Allan
The Public Switched Telephone Network and supporting local access networks have been designed with voice in mind. Telephones, dial modems, fax modems and private line modems limit their transmissions over the local loop to the frequency spectrum of 300 to 3400 Hz. The highest achievable information carrying capacity using that spectrum is approximately 56kbps.
DSL technology eliminates the 3400Hz boundary, and like the traditional E1, uses a much broader range of frequencies than the voice channel.
As ADSL can only be provisioned over copper pairs, some of the existing Access technologies will impact on the ability to deliver an ADSL line. Things like WB900, FeatureNet 5000 services or ISDN do not have contiguous copper links to the local exchange, and so any customer utilising these technologies will have to be converted to an analogue PSTN line before ADSL can be connected.
Any copper pair will suffer from the problems of resistance, capacitance, induction and conductance. A generic circuit diagram can be drawn as below.
With electrical signals transmitted over a copper line, the use of higher frequencies to support higher speed services also results in shorted loop reach. This is because high frequency signals transmitted over metallic loops attenuate energy faster than the lower frequency signals (due to the inductive reactance of the line).
One way to minimise attenuation is to use lower resistance wire. Thick wires have less resistance than thin wires, which in turn means less signal attenuation and thus the signal can travel a longer distance.
Thanks mad_allan, finally getting there, so, are you saying that the data signals on these two lines are purely digital, more like an RS232 connection (this would certainly explain why the distance between the home and exchange is so limited).