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Wed, 26 Dec 2007

CDMA (EVDO) vs GPRS.

Good people gave me SkyLink CDMA modem (model USB CNU-550 with EVDO support) to test internet conection in Linux and compare it against GPRS.

Well, here are my conclusions:

  • CDMA works always while GPRS really sucks in the middle of the day: well, it has to be proven tomorrow, but I tested CDMA modem at about 19-00 and it worked ok, while MTS (Mobile TeleSystems in Russia) at about 12-00 worked very bad (I connected quickly, but ssh login took enormously long time).
  • CDMA speed is usually about 10-16 kb/s, while GPRS is usually can not be higher than 1-2 kb/sec. More on this: I believe CDMA session can have higher speeds (pppd session requests about 900 kbit/sec), behaviour of initial login (I saw quite a few of them on different initial speeds during usual work and userspace network stack testing) shows there is some limitation on server or hardware side (i.e. SkyLink either because of special tariff scale or driver limitations, I was told there is no 16 kb/sec limitation with given card though) side (note on testing different congestion control algorithms and develop own if needed: speed downgrades to less than 10kb/sec frequently during download of the big file. SkyLink is a very interesting source of data for such development: it looks like its RTT is quite high for default (new CUBIC) congestion control, at least low-traffic but very-small-latency-wanted source (mutt over ssh on remote host without serious traffic shaping) works quite bad in this setup. Very likely it is just an empty speculation and problem is in hardware on the server (i.e. it has support for bulk streaming access at high speeds (16 kb/sec), but fails to work with low-latency applications, which work in small packets ping-pong environment)).
  • CDMA USB CNU-550 modem works ok in Linux (modulo above issues) with this peers/pap-secrets files without any problems.
Anyway, CDMA SkyLink is much faster and more smooth than MTS GPRS, so decision about what is better is quite obvious...

/other :: Link / Comments (3)

a. wrote at 2007-12-27 19:13:

ICMP Roundtrip time ~100ms for the packet-switched mobile network is considered a good achievement.

Zbr wrote at 2007-12-27 21:44:

I have about 170 ms average RTT with minimum about 125 ms, and it is not comparable to other non-wired connections.

a. wrote at 2008-01-09 20:14:

What I meant was the inherent latency of the GPRS/CDMA networks. Especially CDMA (not EVDO mode) and WCDMA (not sure about hsdpa). There are so many delays on the "air interface" network (SGSN/PDSN->BSC->BTS->mobile->back) that even plugging directly into ggsn router, the best you'll get is 80-100msec. Speed "downgrades" in CDMA are frequently due to the "shared" nature of the spectrum allocation at default QoS. Even another powerful (literally) mobile station using cdma data mode may half your bandwidth on the same BTS. Although, GPRS is using dedicated channel during data transfer, you'll never get many of them to reach even remotely comparable performance of CDMA.

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