From http://www.dslreports.com/
''For internet telephony or interactive applications such as remote administration over a telnet session, latency is a key (and much discussed) attribute of the connection.

At the moment, the class of users most obsessed with latency would have to be the online gaming community. They are often buy entry level residential ADSL lines, with high download speeds, and are dismayed to read that friends on slower SDSL or IDSL lines are getting better ping times!

The first reaction of a disappointed gamer is to blame the ISP for routing inefficiencies or congestion, and although that is often the root cause, there is more to latency than just many hops and/or long distances.

ADSL modems commonly employ data Interleaving, which is a technique to increase resistance to noise bursts on a line. Interleaving "smears" out micro bits of data (interleaves them over time) so that a short burst of signal destroying noise can only remove part of any given larger block. Data blocks reserve some space for error-correction data.. which can salvage a partially damaged block. Interleaving increases the chance that noise on the line will only cause partial damage, not complete loss. Thats the good news.

The down-side of Interleaving is that it increases latency! this is because your little (say) quake movement packet is smeared out over several packets before it can be fully sent or fully received.

ADSL modems with typical Interleaving defaults can be 10-30ms behind in latency over equivalent speed SDSL modems... this means latency to any point for some ADSL modems can be at best 50ms! On the same setup, the aforementioned SDSL modems that typically add only 10ms. So for use of a nearby server, ADSL Interleaving can be the biggest single source of latency that you have.''

SO how do i disable interleaving?
This can only be done by contacting your ADSL service provider and telling them what you want. they will have to modify your ADSL profile to disable the interleaving function. This may have an adverse effect on your download speed though.

You might be able to check if your line is interleaved or not by looking at your ADSL line data, usually on the admin page of your router. BT homehubs have this information on the status pages, other routers may vary.

The trick is to get them to understand what it is you want. It is surprising how many ADSL 'support' people dont know what interleaving is.