[Ns-developers] Adding Wifi Phy models

[Ruben Merz]

Hi,

Comments below,

On 3/22/09 5:36 PM, Mathieu Lacage wrote:
> On Sun, 2009-03-22 at 17:14 +0100, Nicola Baldo wrote:
>
>> Such a hardware is for instance Calradio:
>> http://calradio.calit2.net/calradio1a.htm
>>
>> For those who are not familiar with it, Calradio is an experimental
>> platform with a 802.11b PHY and a fully customizable MAC layer written
>> in C code which runs on a DSP. The PHY in particular is based on two
>> components: the baseband module (the Intersil HFA3863 chipset I
>> mentioned earlier) and the RF module (based on the Maxim MAX2820 chipset).
>
> Not that I don't care about this kind of hardware but, really, the goal
> of the ns-3 PHY module is to modelize real hardware :) (more on this
> below)
>
>> In Calradio, the setting of the PHY mode belongs to the baseband module,
>> whereas the setting of the frequency to be used belongs to the RF
>> module. Both these settings are performed by writing to appropriate
>> configuration registers when desired. As a consequence, per-packet
>> frequency selection can be implemented by setting a new frequency just
>> prior to transmission.
>
> I would be curious to know how much time it takes to make the RF
> baseband actually perform the frequency switch. I suspect that this
> delay is sufficiently long that frequency switches on a per-packet basis
> are not practical and I suspect that this is also the case in real
> hardware and I also suspect that the cost of building hardware which can
> do this for 802.11 is sufficiently high that it would prohibit any real
> deployment.

Erm, I would not be so sure,

The following paper, "Understanding and mitigating the impact of RF 
interference on 802.11 networks" at
http://www.intel-research.net/Publications/Seattle/020520071626_397.pdf
claims a 250 us channel hopping time and it appears from Section 6.1 
that they actually implemented the scheme. So, I believe per-packet 
frequency switches are feasible.

Ruben

> But, well, the real issue here is that the goal of the current codebase
> is to modelize real 802.11 systems where per-packet frequency switches
> are simply not feasible so, I would tend to happily ignore this
> possibility for now.
 >
> Mathieu
>