Subject: [manet] Effective capacity of 802.11?

From: Nael Abu-Ghazaleh <[email-masked]>
Date: Tue, 13 Jan 2004 00:40:10 -0500 (EST)

In the NS-2 models for 802.11, the PLCP header as well as the inter-frame
separation periods (IFS and DIFS) take a fixed time regardless of the
channel's raw transmission rate (i.e., 1Mbps, 2Mbps, etc...)

So, as the channel capacity increases, they start to take up an
increasing percentage of the available transmission time, and we see
effective throughput numbers that are much smaller than the raw
channel capacity.  

For example, with 11 Mbps channel (PLCP at 1Mbps), the
throughput of a single hop connection with no inteference is roughly
3.7Mbps.  With a 54 Mbps channel, this rises only to a little over 4 Mbps!

My questions: Is this effect observed experimentally?  Should/could the
SIFS/DIFS period be scaled down with the channel throughput?  If not, is
the reason why they are fixed fundamental (e.g.,  they are related to
propagation delay), or is the reason interoperation with connections whose
channel state is different (in which case I assume  we can scale them down
if we assume fixed transmission rate)?

best,
-nael
---
Dr. Nael Abu-Ghazaleh
Assistant Professor -- CS Dept. 
State University of New York
Binghamton, NY 13902--6000
(607) 777 4748 (Phone)
(607) 777 4729 (Fax)
[email-masked]

_______________________________________________
manet mailing list
[email-masked]
https://www1.ietf.org/mailman/listinfo/manet

---

From: Theo Pagtzis <[email-masked]>
Date: Tue, 13 Jan 2004 12:13:29 +0000

Hello Nael,

This is not the case in reality (i.e. not observed experimentally). In 
previous work we have conducted on 802.11in MobiSys group

http://www.cs.ucl.ac.uk/staff/t.pagtzis/papers/papers/ICC01opIssues802.11b.pdf 
and in presentation
http://www.cs.ucl.ac.uk/staff/t.pagtzis/papers/slides/ICC01.ps.gz

we show how the raw bandwidth differentiates from the data bandwidth 
considering interframe separation.

Interframe separation to our knowledge if scaled down per signalling 
rate modulation scheme would complicate the calculation of the NAV 
(network allocation vector) and probably lead to unfairness in 
contention with other STAs and the AP. The later is the case in the 
event where your exponential backoff starts counting down before 
attempting to capture the medium (after it waits for proportionally 
differentiated DIFS). If I am an 11Mbps STA and my DIFS is tuned for 11 
MBps I would always win the contention with a 5.5 Mbps STA. This creates 
unfairness (although proportional) on medium access which is not good if 
you support a low data rate (say instead of 54 Mbps for .11g you only 
support 5.5 Mbps).

Also remember that if DIFS expires for an STA too soon chances are also 
that it may inflate exponentially its backoff time. This would be more 
the case under increased load conditions.

Also remember that with dynamic rate shifting, havoc is just around the 
corner if you decide to change your SIFS/DIFS/PIFS since you will be 
osscilating at the modulation scheme  Tx range boundaries between rates 
FASTER than the calculation of the NAV can represent you as a waiting 
delay for the STAs that do NOT support your rate. So IMHO such 
optimization will shoot itself on the foot as you move around the cell 
(remember that structural obstructions can change the Tx rate without 
you moving really AWAY from the AP).

hope this helps
theo
UCL/Mobile Systems Group