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

From: Jan Kullander <[email-masked]>
Date: Tue, 19 Oct 1999 09:23:07 +0200

From: Vernon Schryver    [email-masked]

>> 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).
>> ...
>
>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.
> ...

Our focus in ack compression is the decompression. We try to recreate 
the compressed acknowledgements as close to the originals as possible 
to reduce the interference from the reverse bottleneck link.

>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?

> It is true that normal maximum window sizes of today limits the top speed 
> of *individual* connections with RTTs of 300ms. However, there may be 
> competing traffic over the reverse bottleneck link, effectively increasing
> the asymmetry and pushing the limits.
> 
Furthermore, proxies adapted to large bw*delay accesses could be 
deployed to enable single TCP connections to utilise more of the 
downstream bit rate in your example.

///Göran/Haider/Jan