| facility | reported by | min (ms) | avg (ms) | max (ms) | std. dev. |
|---|---|---|---|---|---|
| Road Runner, Motorola CyberSURFR | Phil Karn | 12.1 | 45.2 | 865.7 | |
| LanCity (non-DOCSIS), @Home, 1 pm | Sanjay Waghray | 2.321 | 5.501 | 205.094 | 13.546 |
| LanCity (non-DOCSIS), @Home, 9 pm | Sanjay Waghray (*) | 2.293 | 6.044 | 107.501 | 12.873 |
(*) "As soon as I started an upstream ftp session, the ping times went up to 200 ms per packet."
(Phil Karn, end-to-end mailing list, May 20, 1999)The CyberSURFR reverse link is demand polled. When you are idle, you send nothing. The cable router at the hub or head end periodically broadcasts poll invitations on the downstream channel. If you have nothing to send, you ignore it. If you do have traffic to send, you answer this invitation with a request to be polled. Poll requests are sent in a contention mode, which means they can collide with other modems. If your request is heard, you will soon be polled. If your request collides, you won't get polled. So you wait for the next invitation and repeat the process. These invitations go out on the order of 800ms or so, and they account for the long delay you often see on your first ping packet.
When you go active, you are added to a "fast poll" group. Modems in this group are round-robin polled as fast as possible, i.e., the rate at which you're polled depends on how many modems are in this group. It takes about 1ms to poll a modem that has no traffic. Like any polling scheme, the delay you see on any given packet depends on when you send it relative to the polling cycle.
If you've had nothing to send for about 5 sec, you are dropped to the "medium poll" group. Here you are polled about 4 times/sec. After something like 30-40 sec in this group, you drop back to the idle state.
channel rates channel BW 16 QAM 64 QAM 256 QAM 6 MHz (US) 20.9 31.3 41.7 7 MHz 24.3 36.5 48.7 8 MHz (Europe) 27.8 41.7 55.6 user data rates channel BW 16 QAM 64 QAM 256 QAM 6 MHz 19.2 28.8 38.5 7 MHz 22.4 33.7 44.9 8 MHz 25.6 38.5 51.3 Threshold C/N (dB, 10-8 BER) QAM 16 18.8 64 25.5 256 31.7
Last updated by Henning Schulzrinne