Subject: RE: [manet] Simulating 802.11b and g

From: "Frank Li" <[email-masked]>
Date: Tue, 18 Nov 2003 13:10:15 +0100

Hi Claude, 

I guess you are using two-ray model with 'receiver sensitivity' -83 dBm and Tx power 15 dBm to get the following CS threshold.

Phy/WirelessPhy set CSThresh_ 5.011872e-12

My question is - is it sensible to use 'receiver sensitivity' to calculate CS threshold? (Using the same calculation for -94 dBm at 1 Mbps, the sensing range will be 796 m).
Furthermore, does the CS threshold mean that the PHY header can be correctly decoded if received power is above the threshold, or the channel is sensed as busy but the PHY header can NOT be decoded? 

BTW, CPThresh is the capture threshold, not required SNR to 'understand' the frame. When multiple frames are detected, the frame with strongest power (CPThresh>10) will be decoded, otherwise, all frames are discarded. 

BR
Frank

-----Original Message-----
From: Claude Chaudet [mailto:[email-masked]]
Sent: 18. november 2003 10:59
To: Emmanuel Agu
Cc: [email-masked]
Subject: Re: [manet] Simulating 802.11b and g

Hello,

That's true, NS (at least version 2.26) simulates 2Mbps 802.11.

Nevertheless, you can change this by simply putting in your script the 
following lines :
	Mac/802_11 set bandwidth_   11e6
	Mac/802_11 set dataRate_      11e6
	Mac/802_11 set basicRate_     2e6

This only changes the rate at which data is sent. This can be useful 
for simulating
802.11b protocol, for which only the physical layer changes. But 
simulating 802.11a
will need at least some timings modifications in the mac/mac-802_11.* 
files.

Concerning the physical layer parameters, I tried to update some 
parameters based on the
specification of an Avaya card. Nevertheless, being no expert in 
physical layers, I cannot
guarantee this will be the exact parameters.
Here they are :
	Phy/WirelessPhy set CPThresh_ 10.0
	Phy/WirelessPhy set CSThresh_ 5.011872e-12
	Phy/WirelessPhy set RXThresh_ 1.15209e-10
	Phy/WirelessPhy set Rb_ 11e6
	Phy/WirelessPhy set bandwidth_ 11e6
	Phy/WirelessPhy set Pt_ 0.031622777
	Phy/WirelessPhy set freq_ 2457e+6
	Phy/WirelessPhy set L_ 1.0

Concerning this, I wondered if anyone on this list could provide good 
values once
and for all, so that we can use ns, opnet and any other simulator on 
the same basis.
I am especially interested in the CPThresh parameter, i.e. the signal 
over noise
ratio (in dB) required to understand a frame. I found no value in any 
NIC specification.
I suppose it depends on the physical layer. 10 dB was used for early 
802.11, is it still
the good value for newer ones ?

Claude Chaudet.

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---

From: Claude Chaudet <[email-masked]>
Date: Thu, 20 Nov 2003 15:34:41 +0100

Hi Frank,

In fact I hesitated about using the "receiver sensivity" parameter of 
the card as the carrier sense
threshold or as the minimum power needed to understand a frame (Rx 
threshold). I chose to
use it as a carrier sense threshold as the range in ns, using these 
values, almost corresponded
to the range  advertised for "open office" in the Avaya specification.

For the capture threshold, that's true, that is not the SNR. Sorry for 
this confusion.
But apart from NS or a particular simulator, I am still looking for 
realistic values for
these parameters.

I don't know what is needed in a simulator to model the physical layer 
accurately enough.
The parameters used in NS are important, but are they enough ? For 
example,  a
different set of values would be needed for each transmission rate / 
physical layer coding,
especially  considering that different parts of 802.11 frames are 
transmitted at different rates.

Anyway, to get back to the original question, do these PHY layer 
parameters have a real influence on
the MAC layer behavior or do they just change the communication and 
interference ranges ?

Claude.

Le 18 nov. 03, à 13:10, Frank Li a écrit :

> Hi Claude,
>
> I guess you are using two-ray model with 'receiver sensitivity' -83 
> dBm and Tx power 15 dBm to get the following CS threshold.
>
> My question is - is it sensible to use 'receiver sensitivity' to 
> calculate CS threshold? (Using the same calculation for -94 dBm at 1 
> Mbps, the sensing range will be 796 m).
> Furthermore, does the CS threshold mean that the PHY header can be 
> correctly decoded if received power is above the threshold, or the 
> channel is sensed as busy but the PHY header can NOT be decoded?
>
> BTW, CPThresh is the capture threshold, not required SNR to 
> 'understand' the frame. When multiple frames are detected, the frame 
> with strongest power (CPThresh>10) will be decoded, otherwise, all 
> frames are discarded.
>
> BR
> Frank
Claude.

---

From: Mineo Takai <[email-masked]>
Date: Thu, 20 Nov 2003 07:57:28 -0800

Hello Claude,

You might be interested in one of our papers that discusses issues with 
PHY parameters and the overall manet performance:

Mineo Takai, Jay Martin and Rajive Bagrodia, "Effects of Wireless 
Physical Layer Modeling in Mobile Ad Hoc Networks," MobiHoc 2001, pp. 87-94.

Mineo

---

From: Frank Li <[email-masked]>
Date: Thu, 20 Nov 2003 17:10:23 +0100

Hi Claude

> to the range  advertised for "open office" in the Avaya specification. 

So what you use as the sensing range is still 550 m, the transmission 
range for  1 Mbps in the card specification which is also
the default value in ns2. But I feel the sensing range should be even 
larger. For example, in Xu, Gerla and Bae Globecom02 paper ' How 
effective is the IEEE 802.11 RTS/CTS handshake in ad hoc neteworks?', 
the sensing range for 2 Mbps is calculated as 670 m (I guess they used 
the 'receiver sensitivity' to obtain this value).  As 1 Mbps has longer 
transmission range, the corresponding sensing range should be larger.

Let me repeat the same question in my previous mail. Assume the received 
power is just above the CS threshold and the channel is sensed as busy. 
Does this mean that the PHY header (PMD/PLCP) can be correctly decoded 
but not the MPDU, or  even the PHY header itself can NOT be decoded? Can 
anyone give me an 'authoritative' answer to this point? Thanks.

> the MAC layer behavior or do they just change the communication and 

I do believe they have impact on the MAC layer performance. But the 
current version of ns2 obviously does not provide us with enough info on 
physical layer, e.g. the phsical header is not readable, all data rates 
have the same transmission range, the modulation schemes are not really 
implmented.

Frank

---

From: Vikram Kanodia <[email-masked]>
Date: Thu, 20 Nov 2003 11:05:01 -0600 (CST)

Hi,

To evaluate our protocol, Opportunistic Auto rate (OAR) for multi rate
802.11 (MobiCom 2002) we have implemented multi-rate 802.11 in
ns-2.1b7a with the following salient features:

- PHY headers are transmitted at the base rate of 1 Mbps.

- Possible data transmission rates (for the MPDU part of a packet) are 2
Mbps, 5.5 Mbps and 11 Mbps.

- The range for each data rate (corresponding to a different received
power threshold) was set using data from atheros web site of data rate
versus range values and also from experiments performed using an ORINOCCO
wireless NIC).

The range (with no channel fading and only two ray ground path loss
model) were as follows:

0-100m ---> 11Mbps
100m-200m --> 5.5Mbps
200m-250m --> 2Mbps

The code (with our implementation of OAR) and related papers can be found
at:
http://www.ece.rice.edu/networks/

and by following the link to software and OAR.

Comments, feedback, anecdotes (preferably humorous ones) about your
experiences with the code are appreciated.

Also if you do decide to use the code (for your research or for fun) we
would appreciate it if you drop
an email to Bahareh ([email-masked]) or Vikram ([email-masked]) to let
us know of the fact - this will help us keep a track of people who use the
code and also keep you informed of any updates to the code/bug fixes etc.

Thanks,
-Vikram

> have the same transmission range, the modulation schemes are not really

Department of ECE, Rice University | 713 348 3786 (O)| 713 664 5650(H)
WWW: http://www.ece.rice.edu/~kanodia

---

From: Vikram Kanodia <[email-masked]>
Date: Thu, 20 Nov 2003 11:42:13 -0600 (CST)

Also to add to my previous email - we have not tied the possible data
transmission rates to physical distance - rather we have tied the possible
data transmission rate to received power thresholds which correspond to
the distances given earlier in the presence of a \emph{non-fading
channel} only.

In the presence of a fading channel (Ricean for example) the data
rate-distance chart does not hold true and it may happen that for severe
fading (Ricean parameter, K = 0) maximum possible data rate is 2Mbps at
distances as low as 75 m.

Hope this clarifies matters a bit more.

-Vikram