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

From: Mineo Takai <[email-masked]>
Date: Tue, 25 Nov 2003 08:33:09 -0800

Hello Frank,

No, PMD_CS and PMD_ED do not correspond to CST and RXT in my paper. CCA 
returns busy if the radio successfully received a signal preamble whose 
power is more than or equal to PMD_CS. If it did not receive a signal 
preamble, the threshold is raised by 20 dB (PMD_ED) due to difficulty 
(or instability) of detecting energy on the channel without 
synchronization. So, PMD_CS and PMD_ED together correspond to CST.

We are making our 802.11 model more detailed by replacing CST with these 
two thresholds.

Thanks,

Mineo

Frank Li wrote:
> Hi Zhifei,
> 
> 
>>However, for the standard itself, the condition PMD_CS < PMD_ED should be true, at least 
>>for IEEE 802.11a where the PMD_ED is 20 DB greater than PMD_CS.
> 
> 
> Are you sure that PMD_ED is 20 dB greater than PMD_CS? Is the difference between PMD_CS and  PMD_ED the same for all datarates? In the paper posted to the same discussion by Mineo Takai last week, the two thresholds are set as CST = - 78 dBm and RXT = - 64 dBm for WaveLan card. But maybe they don't correspond to PMD_CS and PMD_ED? Any further comments here, Mineo?
> 
>   
> 
>>I think the receiver-sensitivity is used to calculate the transmission range, while the CCA >sensitivity (or CS threshold) is used to calculate the sensing range.
> 
> 
> No, I think receiver-sensivity (or CS threshold in ns2) should be used to calculate the sensing range, another threshold (PMD_ED?) which  is greater than receiver-sensivity, should  be used to calculate the tansmission range. 
> 
> List below are the sensing ranges for outdoor environment I calculated based on Agere Wireless LAN World PC Card (Extended) specification. Here Pt=15 dBm , receiver-sensitivity and transmission ranges are directly from the specification, and I used two-ray model 
> with Gt=Gr=1, ht=hr=1.5, L=1 to obtain the sensing ranges. 
> 
> Datarate                11 Mbps          5.5 Mbps            2 Mbps         1 Mbps
> 
> Transmission range      160 m              270 m              400 m           550 m
> 
> Receiver sensitivity    -82 dBm           -87 dBm            -91 dBm         -94 dBm
> 
> sensing range           399 m               532 m             670 m           796 m
> 
> 
> Any comments are welcome. 
> 
> Frank 
> 
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---

From: Frank Li <[email-masked]>
Date: Thu, 27 Nov 2003 14:22:54 +0100

Hi Mineo,

>
> No, PMD_CS and PMD_ED do not correspond to CST and RXT in my paper. 
> CCA returns busy if the radio successfully received a signal preamble 
> whose power is more than or equal to PMD_CS. 

This is quite clear now. But does PMD_CS correspond to 
receriver-sensitivity in the card specification?

> If it did not receive a signal preamble, the threshold 

The threshold for sensing or PLCP decoding?

> is raised by 20 dB (PMD_ED) due to difficulty (or instability) of 
> detecting energy on the channel without synchronization. So, PMD_CS 
> and PMD_ED together correspond to CST.

So you mean the PLCP header can be correctly decoded if the received 
signal is strong enough, even though the bit sequence is not synchronized? 

>
> We are making our 802.11 model more detailed by replacing CST with 
> these two thresholds.
>  

Are there any papers/reports on your model? What is the relationship 
between CST and these two thresholds? Do you express CST as x*PMD_CS + 
y*PMD_ED?

Frank

---

From: Mineo Takai <[email-masked]>
Date: Thu, 27 Nov 2003 11:27:37 -0800

Hello Frank,

> receriver-sensitivity in the card specification?

As someone else has already pointed out, the receiver-sensitivity in the 
card specification is typically the signal power level at which the card 
can achieve FER (Frame Error Rate) less than or equal to 10%. Since that 
FER is measured for data frames (I don't remember the exact size now, 
but it should be 1000 or 1500 bytes) PMD_CS can be lower than the 
receiver-sensitivity.

> The threshold for sensing or PLCP decoding?

This threshold is for CCA (ED).

> signal is strong enough, even though the bit sequence is not synchronized?

No, if the card misses the beginning of signal (preamble), it cannot 
decode the frame.

> between CST and these two thresholds? Do you express CST as x*PMD_CS + 

No and no. For the first question, I am working with several people to 
come up with a reasonable model with reasonable thresholds that exclude 
implementation dependent (proprietary) mechanisms. I don't know if that 
is possible, but I will try :-)

For the second question, if the card (CCA) uses CS only, CST = PMD_CS. 
If it does ED only, CST = PMD_ED. If it does CS and ED, CST = PMD_ED for 
most situations (PMD_CS < PMD_ED). If it does CS or ED, CST = {PMD_CS 
(with preamble) | PMD_ED (without preamble)}.

Mineo

---

From: "Standards Update" <[email-masked]>
Date: Thu, 27 Nov 2003 21:49:00 -0500

Hello,

I am new to this area and I have been reading up on ns2 for a while.

How/where can I get the source for the propagation model implementations in
ns2?

Any help would be highly appreciated.

Thanks,
--John

---

From: Amina Naimi <[email-masked]>
Date: Fri, 28 Nov 2003 09:51:51 +0100

>Any help would be highly appreciated.
Hello,
The common used propagation model is implemented in the file
tworayground.cc. Other models exist like shadowing (implemented in
shadowing.cc)

Regards

---

From: Frank Li <[email-masked]>
Date: Fri, 28 Nov 2003 11:17:03 +0100

Hi Mineo,  Zhifei,

So what I conclude on receiver-sensitivity is that it is a threshold for 
transmission range. Obviously the transmission ranges in any card 
specifications are not calculated from the two ray model. Which model do 
they use? Or the ranges are measured in real life?

Do you know any concrete values of PMD_ED and PMD_CS for any card? 
Moreover,  is the PMD_ED threshold mainly used in the presence of 
non-DSSS or non-802.11 equipments ?

> No, if the card misses the beginning of signal (preamble), it cannot 

I feel it is still possible to decode the PLCP header in this case. If 
the serial input bit sequence matches any predefined header pattern , 
the correction between them may result  in a high enough value, thus the 
receiver interprets that a specific type of header is decoded.

Frank

---

From: Mineo Takai <[email-masked]>
Date: Fri, 28 Nov 2003 16:19:00 -0800

Hi Frank,

My comments are inline.

Frank Li wrote:

> transmission range. 

I would not conclude that way. Receiver sensitivities on the data sheets 
show (probably) the best performance of specific products. Minimum 
requirements of PMD_ED and sensitivity are specified in the standard. 
Actual thresholds used to determine CCA status and reception of packets 
are somewhere in between and implementation dependent (not publicly 
available).

> Obviously the transmission ranges in any card 

I think they are based on measurements. But I don't know what typical 
indoor / open environments imply, and we cannot use these values to 
calculate the thresholds of our interest backwards.

> non-DSSS or non-802.11 equipments ?

The highest value of PMD_ED is specified in the standard. PMD_CS is not. 
If the receiver is synchronized with a particular signal, there is 
little difference between 802.11 signals and noise / (non-802.11) 
interference.

> receiver interprets that a specific type of header is decoded.

The contents of PLCP preamble is predefined and known to the receiver, 
but the PLCP header carries data (information) unknown to the receiver. 
So, once the receiver misses the preamble, it cannot decode the PLCP 
header or following data correctly.

Mineo