Subject: RE: pilc minutes for IETF 48

From: "Michael Meyer (EED)" <[email-masked]>
Date: Fri, 22 Sep 2000 11:40:13 +0200

>I want to mention about some of our recent experimentations regarding
>the performance of wide-area TCP connections 
>in a cellular WCDMA environment. What we have observed is, 
>persistence on
>retransmission in the link layer is effective only under 
>certain channel
>load condition. In fact, in a WCDMA environment (which allows variable
>rate transmission at the link layer), increasing the number of 
>allowable
>retransmissions in the link layer improves TCP goodput when the channel
>load is neither too small nor too high. As the number of active TCP
>connections (passing through the BS) becomes large, link layer 
>retransmissions are no longer beneficial. 
>

I would like to know more about your assumptions on WCDMA. I can not see the reasons why retransmissions are not longer beneficial any more for many connections in a cell. Do you mean that the interference becomes too high and therefore also the block error rate? Which mechanisms do you consider to control the block eror rate?

/Michael

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From: Ekram Hossain <[email-masked]>
Date: Fri, 22 Sep 2000 02:54:03 -0700 (PDT)

> I would like to know more about your assumptions on WCDMA. I can not
> see the reasons why retransmissions are not longer beneficial any more
> for many connections in a cell. Do you mean that the interference
> becomes too high and therefore also the block error rate? Which
> mechanisms do you consider to control the block eror rate?

As the no. of active TCP connections increases, the link layer
transmission rate (which is chosen according to a sub-optimal rate
selection algorithm to have the minimum block error rate) decreases.
This increases the RTT value and presumably there are more timeouts.
As a result, for a fixed maximum window size, the steady-state TCP
throughput decreases.

In fact, TCP throughput can be very sensitive to the number of active TCP
connections in a WCDMA environment. Adaptive FEC at the link level might
be a better solution than ARQ in such an environment.  

Regards.
Ekram