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

András Veres (IJ/ETH) andras.veres at ericsson.com
Wed Feb 21 00:52:46 PST 2007


The 10 minutes reference in GPRS never happens in reality. It is similar to RFC 793 where an upper limit of TTL = one minute is set for TCP packets. Neither 1 nor 10 minutes actually happen in real networks. 


-----Original Message-----
From: end2end-interest-bounces at postel.org [mailto:end2end-interest-bounces at postel.org] On Behalf Of Detlef Bosau
Sent: Tuesday, February 20, 2007 11:45 PM
To: e2e; Michael.kochte at gmx.net; frank.duerr
Subject: [e2e] Delays / service times / delivery times in wireless networks.

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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