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Latency and Interleaving
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.

Unfortunately, there is little clear information supplied with, or available online, about what latency a given DSL modem or ISP connection has built-in.

Some ADSL modems allow the user to turn off Interleaving, or turn it down to a narrow range, at the expense of possible data-loss on noisy lines. The Cisco 675, for example, has a full operating system inside it, and one of the attributes of the ether interface is Interleaving. See Randy Lutton's US West page). Some ISPs may be delivering this unit with Interleaving on, and some off. In other cases, it is the DSLAM (central office equipment) that has the Interleaving set, and this cannot be changed.

If you want to read more about Latency, then we can recommend this paper by Stuart Cheshire, which was published back in 1996 but is totally relevant now.
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Anon

Re: Latency and Interleaving

Nice to see something so un-mentioned getting some coverage. Would like to see more about the differences between interleaving or fast path switching.

Karl Bode
News Guy
join:2000-03-02

Karl Bode

News Guy

So wait, after reading this little blurb again, it states that you cannot change the interleave setting at the CO? I thought you could swap this to "Fast-Path" mode??

justin
..needs sleep
Mod
join:1999-05-28
2031
Billion BiPAC 7800N
Apple AirPort Extreme (2011)

justin

Mod

Re: Latency and Interleaving

The information available is vague. I'm hoping some tech types might contribute. Fast-path vs interleaving are options on the Cisco 675 ADSL home router ..
»www.cisco.com/univercd/c ··· ap04.htm indicates this, but there is no sign interleaving can be disabled, or checked for, on other brands of home ADSL equipment.
Since closed boxes rely on the remote DSLAM for their settings, Interleaving may be a rollout choice of the DSL network and not user selectable.

TenYardFight
join:1999-12-08
Sudbury, MA

TenYardFight to Karl Bode

Member

to Karl Bode
Defanitely(sp?) very interesting. What about us Covad customers that signed up for ADSL service and recieved SDSL modems because this was all they were set up for? I signed up for 384/128 but during nice conditions I have seen it send at just under 300Kb/s, averaging around 250Kb/s.

I get 30ms to my first hop. This is not that bad considering my first hop is physically located in Wash DC and I am in Boston. Of course 10ms would be better .

DeputyDog007
@cadretechnology.com.

DeputyDog007

Anon

This stuff does exist... I have had a lot to do with it in NZ. Most of the time it will drop the first hop latency to around 10 - 15 ms. Awesome for online gaming....

The only time I have seen it have undesired results is when someone had it enabled on a rural line that probably should never have had DSL in the first place. Electric fences seriously disrespect ADSL. =)

Unfortunately Telstra aren't helpful toward this at all... I am going to get to the bottom of this though... Going to try setup my Router for fastpath tonight. Will post the results.