|reply to Rokk |
Re: DSL and Cable Latency
quote:As Picaso so poignantly pointed out, blanket statements like this are simply wrong. Latency is the total result of interleaving, processing delays, and physical distance. Of course, the differences in physical distance delays whether we are talking about 1000 or 20000 feet, are both negligible. (The physical delay over a dsl connection of 20,000 feet is approximately 0.02ms.)
You can tell if interleaving is active on your line by measuring the first hop ping. If it is under 20ms, interleaving is disabled. If it is above 45ms, interleaving is enabled.
The physical delay is only really noticeable when you look at transcontinental, transoceanic or satellite link ping times. For example, going coast to coast in the US will add about 16ms of latency due to the physical distance alone. The up and down physical delay through a geosynchronous satellite will add a minimum of 242ms to the latency.
So, for all intensive purposes, the physical delay from your DSL or cable modem to the CO (central office) or DSLAM will have no bearing on the first hop latency. The real factors then, in your first hop latency are merely processing time, and interleaver depth.
Note of course that interleaver depths can add a lot of latency to your connection. Depending on the interleaver depths, the interleaving process can add anywhere from 0.15ms to 40ms or more to your latency. With HFC (hybrid fiber coaxial) cable internet, interleaving may add 0.22ms to 4.0ms of delay to your first hop latency. See this link for a chart from the DOCSIS RF specifications with exact interleaving depth vs. latency specifications for HFC cable internet systems: »[DSL] Changing Interleave to Fast-Track .
As far as interleaving depth vs. latency specifications go for ADSL, the exact delays are different in the various implementations (G.Lite, DMT, etc.), and for varying sizes of interleaver depth & block size. Typically when a carrier sets up a DSLAM, they set a parameter called 'maximum interleave delay' for each of the upstream and downstream paths. The modem and DSLAM then set the interleaver depth and block size according to other parameters, but in a way so that the maximum delay is not exceeded. IOW, the carrier sets the amount of delay. Likewise, the carrier may set the equipment up for 'fast' or 'fast-path' mode, meaning no interleaving is used for forward error correction. In general, carriers set the maximum delay somewhere between 20ms and 40ms. The interleaving induced delay, of course, is added to the variable processing latency to determine the total latency of the first-hop.
I found a good explanation of ADSL interleaving vs. latency from: »www.dslforum.org/aboutdsl/tech_faqs.html
quote:If latency is your primary concern, you can ask the carrier to turn off interleaving for your ADSL circuit. Sometimes the proper terminology to use is to put you on 'fast-path'. Realize however, by doing so you will lose most of your ability to correct any bit errors due to impulse noise. This will result in a lower data rate, and more packet re-transmits. Worst case, for example if you are at the extreme end of serviceable distance (18,000 feet or so), your bit-error-rate may be so high that your ADSL connection may not work at all. It is possible that your carrier will not accept your request if this is likely to be the case. If you are not a gamer where latency is crucial, there is nothing to gain by doing so. In fact you would be doing yourself a disservice, because you are will only lower your data rate.
The latency between the customers ADSL modem at the ADSL line card in the CO depends on the line coding technique and the interleaving depth of the error correction scheme. The interleaving depth can programmable up to latency of the order of 60 ms but is typically set to around 20 ms. This offers greater protection against impulsive noise and thus improved BER. Hence there is a latency/interactivity versus error performance trade-off. With the interleaver turned off the residual latency of standard ADSL is 2 ms. Some other modulation schemes can achieve less than this. Other parts of the system and ADSL above the physical layer must also be accounted for, e.g., ATM SAR function, router throughput, etc. ADSL delay time is variable and depends on the interleaving that is programmed. It varies from 2 - 20 msec each way. Hence worse case round trip delay added [due to interleaving] would be 2 x 20 msec = 40 msec.