[Ns-developers] Code review request for 802.11b physical model

[Pei, Guangyu]

Hi, Ruben

Thanks for reviewing. Please see my in line comments. 

> -----Original Message-----
> From: Ruben Merz [mailto:ruben at net.t-labs.tu-berlin.de] 
> Sent: Friday, May 29, 2009 1:15 AM
> To: Tom Henderson
> Cc: Mathieu Lacage; Pei, Guangyu; ns-developers
> Subject: Re: [Ns-developers] Code review request for 802.11b 
> physical model
> 
> Hi Tom, hi all,
> 
> This is nice work. In section 1.2.4, you wonder whether the 
> noise floor can be modulation dependent. I think it can, and 
> it probably does not only depend on the modulation but also 
> on the center frequency of the transmission (see the figure 
> on the bottom right of slide 8 of the talk I gave at the ns-3 
> workshop). This might actually be interesting as next
> step: to see how the RSSI values depend on the center 
> frequency. Also, is there for you the possibility to use 
> other 802.11 hardware for the validation?
> 

The short answer is yes. Figure 1.1 used Prism chipset. Since CMU
testbed currently uses Atheros chipset, we plan to do measurements with
ns-3 in emulation mode.

> Also, what kind of channel model was used for the analytical 
> results and the emulation results? Was there any fading or 
> multipath? Because, this would definitely change the results. 
> From what I read, I understand it is a free space model. I am 
> correct? For me, this also is an important issue to look at, 
> especially with most people using 802.11 link indoor.

Yes, it is a AWGN channel with the control of received signal strength.
The input signal to the receiver was the same as signal from transmitter
except that the signal strength is set at a predefined level. You can
view it as free space since no multipath, however, the path loss is no
longer function of distance and it is simply the difference between
transmitter power and received power. This is called "clear channel" in
CMU test bed.

I completely agree with you about importance of channel model and
multipath. In fact, Figure 13 in CMU paper illustrated the important
impact of dynamic equalizers under multipath. We are planning to attack
this issue in the near future. The validation steps we took so far are
initial steps. More results should follow.

BTW, CMU emulator does have log-distance model with configurable path
loss exponent and Ricean Fading Model. User can also specify multipath.
We plan to leverage these in our studies. Please let us know your
thoughts about how to build a ns-3 abstraction model with dynamic
equalizers under multipath.

> 
> Looking at Fig 1.6, I'm not sure I understand the meaning of 
> it. For unicast, you should look at the number of received 
> packets independently of the retransmissions. Then you would 
> probably get the same results than in 1.5.
> 

The y-axis in both Figure 1.5 and Figure 1.6 is the number of packets
received by network layer without errors. As RSS increases, the BER
decreases. The retransmissions in unicast make the slope steeper. 

For example, let RSS1 < RSS2, the packet error rate(PER) of broadcast is
90% for RSS1 and 50% for RSS2 respectively.
Thus, for broadcast, 10% success rate at RSS1 and 50% success rate at
RSS2. Let the number of retransmissions be 4 without RTS/CTS. The
success rate for unicast can be estimated as the following: 

At RSS1: 1-0.9^4= 34.39% (v.s. 10% for broadcast)
At RSS2: 1-0.5^4= 93.75% (v.s. 50% for broadcast)

Thus, for same RSS1 and RSS2 pair, the packet success rate difference is
40% for broadcast and 59.36% for unicast. That makes the slope steeper.

This example also showed that upper layer protocols may filter out the
impact of physical layer abstractions to some extend. In this case, MAC
layer retransmissions make the transition slope very steep. Under most
traffic loading settings, the small mismatch of slope between simulation
and hardware measurement may not be noticeable at network layer. 

Best,

Gary