Subject: [manet] Re: Effective capacity of 802.11?

From: "Mihail L. Sichitiu" <[email-masked]>
Date: Tue, 13 Jan 2004 13:14:07 -0500 (EST)

Hi,

The ns-2 models follow the standard that indeed has fixed SIFS/DIFS
periods and so on. However, 802.11b has an option for short preambles that
boosts the performance (by a bit); I think that Cisco cards use this short
preamble (I'm not aware of any other cards that do that).

We have a paper on the subject [1] that explains and quantifies all the
overheads encountered (some do change from technology to technology!).

We also performed measurements for 802.11b (with Iperf) and they match
closely the theoretical numbers.

Best regards,
Mihai

[1] Jangeun Jun, Pushkin Peddabachagari and Mihail Sichitiu, "Theoretical
  Maximum Throughput of IEEE 802.11 and its Applications", Proc. of the 
  2nd IEEE International Symposium on Network Computing and Applications
  (NCA'03), Cambridge, MA, April 16-18, 2003, pp. 249-256.
 - available at 
   http://www4.ncsu.edu/~mlsichit/Research/Publications/tmt_nca03.pdf 

> Message: 1
> Date: Tue, 13 Jan 2004 00:40:10 -0500 (EST)
> From: Nael Abu-Ghazaleh <[email-masked]>
> To: [email-masked]
> Subject: [manet] Effective capacity of 802.11?
> 
> 
> 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]

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From: Yang Xiao <[email-masked]>
Date: Tue, 13 Jan 2004 10:30:31 -0800 (PST)

Hi, 

The Theoretical Maximum Throughput was first introduced in

Yang Xiao and Jon Rosdahl, "Throughput Limit for IEEE 802.11,” IEEE 802
Interim Meeting, Wentworth Sydney, NSW, Australia, May 13-17, 2002,
document number: IEEE 802.11-02/291r0.  

Then, it appeared in 
Yang Xiao and Jon Rosdahl, “Throughput and Delay Limits of IEEE
802.11," IEEE Communications Letters, Vol. 6, No. 8, Aug. 2002, pp.
355-357.
http://www.cs.memphis.edu/~yxiao/IEEE_CL_8_2002_xiao.pdf

A follow up
Yang Xiao and Jon Rosdahl, “Performance Analysis and Enhancement for
the Current and Future IEEE 802.11 MAC Protocols,” ACM SIGMOBILE Mobile
Computing and Communications Review (MC2R), special issue on Wireless
Home Networks, Vol. 7, No. 2, Apr. 2003, pp. 6-19.
http://www.cs.memphis.edu/~yxiao/p6-xiao.pdf

Yang Xiao, “MAC Performance Analysis and Enhancement Over 100Mbps Data
Rates for IEEE 802.11," Proceedings of The IEEE Vehicular Technology
Conference, (IEEE VTC 2003 Fall), Oct. 6-9, 2003, Orlando, Florida,
USA,
 
Related work
Yang Xiao, “Concatenation and Piggyback Mechanisms for the IEEE 802.11
MAC,” Proceeding of IEEE Wireless Communications and Networking
Conference 2004, (IEEE WCNC 2004), accepted. 

Yang Xiao and Jon Rosdahl, “A Performance Analysis of IEEE 802.11a
Wireless LAN,” Proceedings of The 6th World Multi-Conference on
SYSTEMICS, CYBERNETICS AND INFORMATICS, (SCI 2002), July 14-18, 2002,
Orlando, Florida, U.S.A, pp. 243-248.

Regards

Yang

==
Yang Xiao
The University of Memphis
http://www.cs.memphis.edu/~yxiao/

> > Binghamton, NY 13902--6000

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