[This is *way* off-topic, but since he asked...]
Svein Ove Aas <svein+usenet01@brage.info> wrote:
+---------------
| Rob Warnock wrote:
| > [1] AT&T Wireless's GSM/GPRS/EDGE cellular data service, which,
| > while it has pretty good bandwidth [3-7 times dialup modem],
| > has *terrible* round-trip latency:
| > [high latency ping]
|
| Hang on, isn't that the same link you use to talk with?
+---------------
Yes. But voice calls pay the call-setup delay [see below] only once
per call, then they are billed continuously for the minutes that the
call stays connected.
+---------------
| Where's the latency, then?
| I'd be most interested in a traceroute.
+---------------
Almost all of the latency is between the cellular modem and the first
"service processor" [which is still behind the NAT gateway, before the
data even gets to the Internet]. It appears that, being a "best-efforts"
service, data is carried on an "as-available" basis, which in practice
seems to mean that they're using "fast circuit-switching" -- that is,
a circuit-switched call from the modem to the first IP service processor
is set up and torn down for *each* burst of IP packets. That means that
once-a-second things like "ping" (ICMP ECHO_REQUEST) and "traceroute"
(UDP to a random port) suffer a full call-setup delay per packet.
Traceroute from the cellular modem side [first few only]:
traceroute to fast.rpw3.org (66.93.131.53), 64 hops max, 44 byte packets
1 10.251.1.26 (10.251.1.26) 974.123 ms 609.204 ms 629.911 ms
2 10.251.0.1 (10.251.0.1) 599.648 ms 609.683 ms 629.716 ms
3 10.250.1.2 (10.250.1.2) 619.403 ms 609.467 ms 629.447 ms
4 allntxgdmzrt01-f4-0-0.network.attwireless.net (209.183.48.2) 599.935 ms 618.991 ms 599.909 ms
5 allntxgisprt01-f1-1-0.network.attwireless.net (172.16.1.102) 599.503 ms 769.224 ms 639.480 ms
6 allntxgisprt02-f4-1-0.network.attwireless.net (172.16.1.226) 409.877 ms 619.285 ms 599.623 ms
7 se-2-0-1.hsa1.Dallas1.Level3.net (209.245.240.21) 499.499 ms 619.134 ms 599.913 ms
...
Ping from the other side (my server) back to the host listed in #7 above:
PING se-2-0-1.hsa1.Dallas1.Level3.net (209.245.240.21): 56 data bytes
64 bytes from 209.245.240.21: icmp_seq=0 ttl=250 time=54.419 ms
64 bytes from 209.245.240.21: icmp_seq=1 ttl=250 time=52.418 ms
64 bytes from 209.245.240.21: icmp_seq=2 ttl=250 time=53.107 ms
...
As I said, almost *all* of the delay is between the cellular modem
and the first ATTWS-internal IP service processor...
-Rob
-----
Rob Warnock <rpw3@rpw3.org>
627 26th Avenue <URL:http://rpw3.org/>
San Mateo, CA 94403 (650)572-2607