[Ns-developers] [ns-2.33] bug in WirelessPhyExt::sendUp()

[Mathieu Lacage]

hi,



On Tue, 2008-06-24 at 15:48 +0300, antoine.trux at nokia.com wrote:

> 1) Receiver sensitivity
>    --------------------
>    This is the minimum level at which a receiver must be able to
> detect a signal. The Standard(*) defines minimum requirements for
> these values. Excerpt from the Standard:
> 
> 	"17.3.10.1 Receiver minimum input sensitivity
> 	The packet error rate (PER) shall be less than 10% at a PSDU length
> of 1000 octets for rate-dependent input levels shall be the numbers
> listed in Table 17-13 or less. The minimum input levels are measured
> at the antenna connector (noise factor of 10 dB and 5 dB
> implementation margins are assumed)."
> 
>    For example, for 802.11a, 20 MHz channel spacing, and BPSK
> modulation with 1/2 coding rate, the Standard mandates that the
> receiver sensitivity must be no worse that -82 dBm (see Table 17-13).
> 
>    It should be stressed that this is only a minimal requirement. The
> receiver may well be able to decode an incoming signal even if the
> signal is weaker, even considerably weaker, than the specified minimum
> level.
> 
> 
> 2) Carrier-Sense Threshold
>    -----------------------
>    This is the reception level at which the channel is to be
> considered busy. Excerpt from the Standard:
> 
> 	"17.3.10.5 CCA sensitivity
> 	The start of a valid OFDM transmission at a receive level equal to or
> greater than the minimum modulation and coding rate sensitivity (-82
> dBm for 20 MHz channel spacing, -85 dBm for 10 MHz channel spacing,
> and -88 dBm for 5 MHz channel spacing) shall cause CCA to indicate
> busy with a probability > 90% within 4 μs for 20 MHz channel spacing,
> 8 μs for 10 MHz channel spacing, and 16 μs for 5 MHz channel spacing."
> 
>    For example, for 802.11a and 20 MHz channel spacing, the Standard
> states that the Carrier-Sense Threshold is -82 dBm (see Table 17-13).
> 
> 
> Now, it is true that the minimum receiver sensitivity and the
> carrier-sense threshold can happen to have equal values for a certain
> modulation, but that is not necessarily the case in general. For
> example, in the case of 2.4 GHz, the receiver's minimum sensitivity is
> -82 dBm, whereas the carrier-sense threshold is -76 dBm (see the
> Standard, clauses 19.4.6 and 19.5.1).

Although I agree with everything you said, and that I am fairly certain
that it is possible to build micro benchmarks where the relative value
of the CS sensitivity thresholds will have a dramatic impact on the
simulation output (For example, if CS threshold == sensitivity
threshold, every signal transmitted and potentially detected will
trigger a PHY CCA BUSY state, hence, minimizing potential collisions
since no signal transmitted goes undetected by the MAC DCF function),
the questions are:
  a) whether or not a real application will see a difference in
simulations (and in the real world)
  b) which values of the two thresholds would better modelize a real
device, do these values change a lot from one device to the other ?
  c) whether or not the new model will be really "better" (that is, its
predictions will more accurately reflect real-world experimental
measurements)

The current model and your proposed modification both seem to fall in
the category of the more or less arbitrary models which modelize little
to nothing of the real system and which are there because we don't know
how to do better (have no experimental data about the statistical
behavior of the synchronization process at the receiver and the
subsequent PER). So, is the proposed change worth it ? Maybe, but,  most
likely, not. I mean, this proposal seems to be about exchanging one kind
of arbitrary behavior against another kind: yes, your new behavior is
intuitively potentially more useful/accurate, but I don't trust my
intuition here. 

Mathieu