Subject: Re: [TIME-LIST] RE: draft-ietf-pilc-asym-00.txt

From: Vernon Schryver <[email-masked]>
Date: Fri, 15 Oct 1999 08:54:27 -0600 (MDT)

> From: Goran Johansson <[email-masked]>

> ...
> We are mainly thinking of systems with a broadband broadcast downlink
> (Satellite or Digital Audio/Video Broadcast) and a narrow uplink (serial
> modem or cellular). In these systems you know your bottleneck link, and
> it is frequently placed close to the receiving host. Using 30 MBs DVB as
> downlink and 10 kbs GSM as uplink we have a raw bandwidth asymmetry
> of 3000. Considering 1500 byte data packets and 40 byte ACK packets
> the remaining effective bandwidth asymmetry is 80. If a simultanous
> uplink data flow is competing for bandwidth the situation gets worse.
> ...

Those numbers sound similar to the assymmetry ratios of the early cable-TV
systems of Hybrid Networks in about 1990, which had 10 MBit/sec toward
the subscriber and v.32 modems upstream.  They compressed ACKs apparently
successfully.  I think they did it in the most obvious fashion of looking
in the PPP transmit queue for the modem, and deleting adjacent ACKs,
without bothering to expand them beyond the modem.  The system was
installed and running in at least Cupertino, Calif.  I think they may have
some patents on their ACK compression, regardless that it seems obvious
to me.

They still seem to be at www.hybrid.com, and I see they still talk
about < http://www.hybrid.com/PRODUCTS/onewaycabl.htm >.  I guess
the reverse channel on cable TV plants is still a much bigger
problem in practice than it was supposed to be in theory.

Disclaimer:  I know several people who worked at Hybrid As far as
I know, all except one are now elsewhere.  I also wrote a very
little bit of code for them.

On rereading this, I see that Ericsson is talking about 3000:1 ratios,
not merely 10 Mbit/20 Kbit, or 500:1.  In practice as opposed to theory,
how will you get such high ratios?  What kind of TCP windows will the
mobile devices be advertising to allow 30 Mbit/sec streams, particularly
given the RTT's that Ericsson people have talked about for GSM data
services?  If the RTT is a fast, direct Internet 300 ms, and the mobile
device advertises a 1 MByte TCP-LW window, regardless of how much you
compress ACKs, you won't be able to reach 30 MBit/sec downstream.  Given
the far larger RTT's I've seen mentioned and the likely far smaller than
MByte per TCP connection buffering in the mobile phone, wouldn't it be
wise to use far smaller assymmetries as your design limit?

Vernon Schryver    [email-masked]