ADSL2+ reaches a maximum bandwidth of 25 MBit/s downstream and 1 MBit/s upstream, whereas current ADSL delivers a maximum downstream of 8 MBit/s only and an upstream of 1 MBit/s, too. This triplication of speed was realized by doubling the used frequency spectrum from formerly 25-1104 kHz to 25-2200 kHz and some other advancements of the signal modulation.
By the way, if you have an ISDN telephone line maximum downstream with ADSL2+ will be cropped to 16 MBit/s, as ISDN requires more frequency spectrum (up to 138 kHz) than a regular phoneline and therefore you'll need to use special "Annex B" ADSL-hardware with ISDN (see this article).
Anyway those figures are pure theory - the bandwidth ADSL2+ will serve on your line is very much dependant on signal attenuation, that in turn depends on the length of the telephone line from your ADSL modem to the DSLAM located at the local telephone exchange.
So, as you can see from the chart below, the farther you're away from the telephone exchange, the lower your expectations of ADSL2+ you should be.
source: TPG Internet Pty Ltd
Users with a long line can't get much extra bandwidth by ADSL2+, but anyway it will improve line attenuation due to the wider frequency spectrum used. On the one hand that means a longer range (of about 500 m) bringing broadband to customers, who couldn't get ADSL before. On the other hand connections with ADSL2+ will become more stable and will compensate the increasing interferences by so-called "cross-talking", an effect occuring in the cable harnesses, which connect your neighbourhood to the telephone exchange. The more neighbours subscribe to ADSL, the stronger cross-talking will become and that's by the way the reason why some people suddenly start experiencing problems with their ADSL line.
Personally I have seen signal attenuation decreasing from 32 to 22 dB and from 61 to 47 dB and equivalent improvements in signal-noise-ratio after switching to ADSL2+.
In order to benefit from ADSL2+ your ISP needs to upgrade your local DSLAM and your modem must be ADSL2+-capable, too. (overview of ADSL2+ capable or upgradable DSLAM chipsets)
Before becoming too euphoric about ADSL2+, you should also bear in mind, that ADSL2+ is only about the connection from your home to the telephone switch. In order to actually take advantage of those higher bandwidths, the rest of your ISP's infrastructure must be capable of those rates, too. And not enough, all networks involved in the connection to a certain server, need to have sufficient free capacity, so you can download a file with 25 MBit/s. In reality we are far away from such speeds and must rather be content to see the maximum downloadrate of current ADSL lines.
In Germany, where ADSL2+ was rolled out last year, people rarely enjoy download rates of more than 4-6 MBit/s - either because of not enough bandwidth within ISPs' networks or on the transit routes or it's the capacity of the destination servers.
Even high-bandwidth IPTV, which is streamed from within ISPs' networks, has come out to be problematic, as existing ATM-backbones obviously can't handle the steady load of too much IPTV-viewers. That's why Deutsche Telekom currently builds up a completely new Gigabit-Ethernet backbone in Germany.
In view of Telecom's problems with backbone capacity with current ADSL I fear ADSL2+ may disappoint most users, to whom it is made available at all.
Other related posts:
Internet performance in NZ
Comment by barf, on 29-May-2007 15:51
Great article Inquisitor, it is interesting to read that Deutsche Telekom is retiring the ATM network, hopefully Telecom NZ will do the same.
Comment by Gareth, on 16-Oct-2009 11:12
I'm with Xtra in Christchurch. I get speeds of 13.015Mbps 1631.25 KB/sec download speed maximum. But I usually get 1.5 MB/sec download ;D. Around 700 KB/sec upload. I'm pretty happy for $60 a month with that.
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