Modulation

From BE Usergroup Technotes

In the world of ADSL, there are different types of modulation available...

Be* offer the best in the UK at time of writing, which is ADSL2+, providing up to 2.5Mbit upstream and 24Mbit downstream.

Here is a list of modulation types:

• G.DMT (G.992.1) - ADSL1
• G.992.3 - ADSL2
• G.992.5 - ADSL2+

Modulation determines how the data is placed onto the wire as frequencies. It may be more or less error-tolerant, it may be faster or slower. The basic ADSL is G.992.1, which is almost always rate adaptive (upstream tones are bitswapped to downstream to maintain a clear connection) and permits a maximum pre-ATM sync rate of 8192 kbps downstream. G.992.3 increases this to 12 Mbps, G.992.5 to 24 Mbps. The two ADSL2 standards do this by permitting the use of frequencies above 1.1 MHz (where ADSL1 stops) as far as 2.2 MHz. This is only any real use at very short distances from the exchange, since high frequencies suffer hugely more attenuation than low frequencies do.