The Network Interconnect Consultative Committee seems to have given UK broadband users a massive moral boost yesterday, with the news that is has agreed to support the use of ADSL2+ in the UK copper local loop. Until now the ANFP (Analogue Network Frequency Plan) has favoured VDSL for ultra fast DSL connections, but now it seems a way has been found to allow ADSL2+ and VDSL to co-exist. Most other countries have generally taken a choice over which standard is deployed, so in theory the UK should now be able to use which ever solution it desires. Of course this is assuming the support by the NICC does translate into an ANFP that all the other interested parties do agree too.
The change in heart appears to be because Bulldog (a subsidiary of the much larger firm Cable & Wireless) have found a way to shape the transmissions so that the VDSL and ADSL2+ can co-exist. No precise details are available, but we would expect to know more once the NICC release their full report. There are no indications of whether there are any reach or speed impacts due to this shaping.
One thing we would like to point out, is that while ADSL2+ does offer potential for much faster broadband connections, it is not a magical 35 times faster than existing ADSL which some reports have suggested. The current G.DMT ADSL deployed can reach a maximum speed of 8Mbps, ADSL2+ on a single line tops out at around 26Mbps, though this is on a line that is only about 300m long. At 1.5km you can expect around 18Mbps, at around 3.5km perhaps 4Mbps. A key factor that will see people getting greater than 10Mbps, is a procedure called sub loop unbundling, whereby telecos place DSLAMs in street cabinets to reduce the length of copper the DSL signal travels over. For those at the furthest reaches of ADSL currently, the ADSL2 standard which is a pre-cursor to ADSL2+ saw an estimated extension in reach of around 180m.
The observant reader will have noticed that 4Mbps at 3.5km is not much different from the existing ADSL deployment. While BT Wholesale limits users at around this length to 2Mbps, if you are willing to risk a higher number of line faults, 3 to 4Mbps is likely to be achievable. The problem is that the performance pf ADSL2+ drops off around the 2.5km mark to be only 50kbps faster than existing ADSL. One trick ADSL2 and ADSL2+ share is bonding of lines to increase speeds, though this requires renting another copper pair. Additionally rate adaption should work a lot smoother as this can be done on the fly rather than having to drop the line to re-adapt to a new speed.
ADSL2+ and its ability to co-exist with VDSL, is a great step forward, and hopefully we will see it deployed during 2005. The important thing is to have realistic expectations, currently if the usual UK pattern of installing the DSLAM at the exchange is followed then only those with less than 2.5km of copper cabling to the exchange will see significant speed boost.
For some background reading on ADSL2 and ADSL2+ see:
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