On Wed, 11 Sep 2002, Jim wrote:

>#1 ----
>
>Does anybody know about the compression that can be agreed
>upon during an LCP (Link Control Protocol) negotiation on
>a PPP link between two modems, compression like Van Jacobson
>Compressed TCP/IP?

There is VJ compression and BSD compression. The latter is seldom used. VJ
is used by default in most systems and involves compressing the IP
datagram header (which header contains a lot of redundant information).
The header is a small part of an IP packet, unless it's a ICMP or other
low protocol packet, so the gain is minimal, and mostly manifests itself
in packet assembly/disassembly speed afaik. Of course it affects ICMP
packets strongly (since they are short).

>May I guess that this compression is done my the main CPU
>in a Winmodem - and by the on-board processor on a full
>US-Robotics DOS-capable FAX-Modem?

No, the compression is done by the TCP/IP protocol stack implementation,
i.e. the main CPU, in both cases.

>#2 ----
>
>It has also been shown that PCI modem ping-times are like
>100 ms longer that the same modem but in an ISA slot -
>documentation of this item upon request ...

Most PCI modems are winmodems. The latency is due to the main CPU playing
DSP to send/receive data. This also shows up as frame count drop in a
highly demanding application (video, games, math). The modems with
controller use lookup tables (like caches) and DSP math tricks to get
bytes in and out through the codecs, fast, with low latency.

It pays to examine the modem datasheet to make sure that it does not
accept slow and uncompressed connections (MNP, Vxxx etc), and alter the
modem setup strings accordingly.

Peter

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