Date: Fri, 09 Feb 1996 14:23:54 -0800 From: Darryl Okahata <darrylo@hpnmhjw.sr.hp.com> To: freebsd-questions@freebsd.org Subject: Re: ISDN devices supported? Message-ID: <199602092223.AA146604635@hpnmhjw.sr.hp.com> In-Reply-To: Your message of "Fri, 09 Feb 1996 12:34:26 PST."
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> 128kbits/sec? Feh. With
> the cost of 28.8 modems and regular phone service, it's practically in
> "why bother" territory. Is there something I am missing about ISDN?
With modems, there's a potential "big" problem with packet latency.
Currently, the X-window protocol contains a lot of handshaking; an
X-window client (the program displaying the window) does a lot of
handshaking with the X-window server (where the window is displayed).
In order to do error correction and data compression, the transmitting
modem gathers data into fixed-sized "packets" before sending the data to
the receiving modem. However, to handle "interactive" sessions where a
user is typing on a keyboard, a partial packet is sent if data is not
received by a certain short timeout period. As the X-window protocol
can send packets smaller than the modem's packet size, the timeout must
occur before the data is sent to the receiving modem. Multiply this
timeout (50-80ms???) by many packets, and the "short timeout" can become
a substantial amount of time.
For example, if I ping a system connected via SLIP (28.8K modem
connected at 21.6K), the ping times are on the order of 160ms. If I
ping a system connected via ISDN, the ping times are on the order of
40ms.
[ Of course, sending 64 byte ping packets via a 28.8K modem does take
longer than a 64-128K ISDN connection, and all this does assume that
the packet size is smaller than the modem's buffer size. ]
If you multiply the 60ms difference ((160ms-40ms) divided by 2
packets/ping) by, say, 100 packets, you've got a six-second difference.
I'd say that's significant.
-- Darryl Okahata
Internet: darrylo@sr.hp.com
DISCLAIMER: this message is the author's personal opinion and does not
constitute the support, opinion, or policy of Hewlett-Packard, or of the
little green men that have been following him all day.
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