Subject: RE: How asymmatric are paths?

From: Göran L Johansson <[email-masked]>
Date: Thu, 2 Dec 1999 11:30:59 +0100

We have been involved in developing a wireless system consisting of
Digital Audio Broadcast (~1.2 Mbps) downlink and GSM uplink (9.6 kbps).
This work has been within the MEMO project (http://memo.lboro.ac.uk/),
which is part of the ACT project in EU's fourth frame work. This system
has been thouroughly tested and is in small-scale commercial use
(Teracom http://www.rd.teracom.se/). The asymmetry is about 1:125.

Since this system has large bandwidth-delay product it is vital to include
the SACK and Timestamp options. Since these options are not compressed
in standard (Van Jacobson) TCP/IP header compression we end up with
relatively large ACK packets. Thus we see that the return path limits the
throughput, even though the asymmetry is just 1:100.

At IFA 99 (Internationale Funkausstellung in Berlin) there was a
demonstration
of IP access over GSM and terrestial DVB. They managed to utilise ~8 Mbps 
out of ~30. The demonstration was made by Teracom and was a deliverable in
the MEMO project. To achieve this speed we believe they maintained a queue
to the GSM link and selected which acks to drop in it.

Our conclusion was that we need a compression method capable of handling
Timestamp and SACK options. Since it is a wireless system we believe
that the compression method should also be more robust against losing
compressed packets compared to VJ. This is part of the motivation behind
our suggestion of ACK Compression.

Göran Johansson