[e2e] Delays / service times / delivery times in wireless networks.

Detlef Bosau detlef.bosau at web.de
Tue Feb 20 14:45:18 PST 2007

Hi to all,

I go in circles here and perhaps, someone can help me out.

In networks like GPRS, IP packets can experience extremely large 
delivery times. The ETSI standard accepts up to 10 minutes.

The effects of large and varying latencies on TCP are widely discussed. 
However, I still do not really understand where these delays come from.
When delivery times vary from 1 ms to 10 minutes, we talk about 6 orders 
of magnitude. My question is: What is the reason for this?

Possible delay sources (except processing, serialization, queieng, 
propagation) are:

- recovery / retransmission,
- roaming
- scheduling.

I hardly belive that recovery latencies are that large because I can 
hardly imagine that a packet in the whole or in pieces is repeated up to 
1 million times. And even if this would be the case, it is interesting 
to see the variation of such latencies.

In addition, I hardly believe in roaming latencies. Years ago, I was 
told extremely large latencies would result from roaming. However, if 
e.g. a mobile roams from one cell to another, to my understanding it 
keeps its SGSN in many cases and only the antenna station changes. So, 
this situation is basically not different from roaming with a normal 
voice stream. And that works transparent to the user.

The only source where I ever read latencies and delay spikes of several 
seconds is the Globecom 2004 paper by Thierry Klein, which reported 
delay spikes of up to 2 seconds. And when I think about proportional 
fair scheduling, I think this _can_ be a source of large delay spikes.
However, I do not have access to any reliable material here.

The problem appears to be quite technical and not very much TCP related. 
However, I´m admittedly somewhat tired to think about dely variations 
and delay spikes and possible adverse consequences upon TCP without 
having really understood the reasons for large delays and
delay spikes.

I don´t know whether this topic is of intereset here, but I would 
greatly appreciate any discussion of this matter. If someone can help me 
on this one, he is welcome to contact me off list, if this topic is not 
of general interest here.

However, I´m somewhat helpless here.



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