Subject: about ns-2

From: "Li-Ping Tung" <[email-masked]>
Date: Wed, 16 Jan 2002 17:30:49 +0800

hello,

In the ns-2 simulator with CMU wireless extension,
the document said that the implementation is based on IEEE 802.11.
It's clearly to know that the PHYSICAL layer is FHSS, but in this module,
the PHY layer is DSSS.
We know that the PHY layer will infect the transmission rate like
IEEE 802.11   -> FHSS ->  2Mbps
IEEE 802.11b -> DSSS -> 11Mbps
So, why in the CMU wireless extension, the implementation is DSSS, but
the transmission rate is still limited in 2Mbps ?
In other way, can we change the rate to 11Mbps..
Please help  me to answer my questions.

thanks.

sincerely,
ice

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From: Joe Macker <[email-masked]>
Date: Wed, 16 Jan 2002 10:52:35 -0500

>It's clearly to know that the PHYSICAL layer is FHSS, but in this module,

802.11 Physical layers can be FHSS or DSSS.  Most manufacturers implement pure DSSS with alternate modulations (DPSK, DQPSK,etc) for achieving different signal rates (1,2, 5.5, 11 Mbps). So there is nothing wrong with DSSS at 2 Mbps as you seem to indicate. In fact, FHSS systems are less common in implementation.  

It would be interested to have both modeled and to have the multi-rate feature.  If you are interested in 11 Mbps, you should be able to go into the ns2 code and make appropriate modifications to change the raw rate and to estimate achievable ranges (transmit range will be smaller), related parameters,etc at higher modem rates.  If you just want to look at 11mbps fixed rate this shouldn't be too hard, dynamic rate changes would be harder to model.

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From: Mineo Takai <[email-masked]>
Date: Wed, 16 Jan 2002 13:28:31 -0800

Hello,

I have a list of comments from my modeling & simulation experience:

- Parameters in the ns-2 802.11 model set by default are based on a several
year ago WaveLAN card, which was made before the IEEE 802.11 standard.
Therefore, the channel frequency in ns-2 is set differently from the standard
(ns-2 802.11: 914 MHz, IEEE 802.11 DSSS PHY: 2.4 GHz) along with other PHY
parameters.

- The IEEE 802.11 DSSS PHY sets the PHY preamble and header size in
microseconds (192us; 802.11b has a short preamble option of 96us) while the
ns-2 802.11 does it in bits (192 bits). Compared to the MAC header size, the
PHY preamble is much longer and has significant impacts on the overall network
performance. When the data rate is set to 11Mbps in ns-2, the PHY preamble
size needs to be multiplied by 11 (or 5.5 assuming the short preamble option,
which seems common in real 802.11b implementations).

- The ns-2 802.11 model holds a power value of previously received packet, and
uses it as the baseline to determine successful reception of incoming packets.
This results in very optimistic simulation results as there is no
consideration of noise or accumulated interference from multiple neighbors,
particularly when there are several ongoing transmissions in the network. I
believe other simulators like QualNet, GloMoSim and OPNET consider noise and
accumulated interference for successful packet reception by calculating Signal
to Noise Ratio, or SNR.

- When the data rate of 11Mbps (with CCK modulation) is used, the required SNR
to achieve the same BER (Bit Error Rate) increases. This may result in smaller
communication range as Joe Macker says in presence of interference, but it may
not be true if the channel is in good condition. As ns-2 802.11 PHY model does
not simulate interference properly, it will be hard to observe real effects of
high data rate communication in ns-2. Just setting the data rate to 11Mbps
will give you even more optimistic simulation results!

Our recent publication (ftp://pcl.cs.ucla.edu/pub/papers/mobihoc.ps) includes
some simulation results from the comparison of ns-2 and GloMoSim, which may
help understand what I wrote here.

Mineo

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From: "Li-Ping Tung" <[email-masked]>
Date: Tue, 22 Jan 2002 15:58:27 +0800

Hello, Mineo :

So if I want to use ns-2 simulator with 11 Mbps, should I change any ns-2
source source, or that I just set the parameter to let the transmission rate
to become 11 Mbps ?

Li-Ping

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From: Mineo Takai <[email-masked]>
Date: Wed, 23 Jan 2002 00:49:50 -0800

Hi Li-Ping,

If you use ns-2 and set 11 Mbps as the data rate, I would recommend the
following changes to the ns-2 802.11 PHY source:

(1) Change the transmission duration of PHY preamble and header to 96us
(or 192us).

(2) Set PHY parameters (carrier frequency, tx power, cs sensitivity etc.)
based on the IEEE 802.11 standard and one of its implementations.

(3) Implement a model that calculates BER based on instantaneous SNR and
CCK (complementary code keying) modem performance, and use it instead of
the capture threshold to determine successful frame reception.

These changes should make the ns-2 802.11 PHY model much better. Please
note that doing (3) requires rather extensive effort, so you might want
to ask for volunteers on the ns-2 user mailing list. (Sorry, I am not an
active ns-2 user!) Other simulators such as GloMoSim, QualNet and OPNET
already have a frame reception model similar to (3).

Mineo

--
Mineo Takai, Ph.D.
Principal Development Engineer
UCLA Computer Science Department