[ns] Regarding txtime() in mac-802_11: Why PHY and MAC both data rates are used in txtime calculation?

[Basim Javed]

I think its the MAC rate(s) used for calculating the Tx time of a packet.
Yes the header is sent at basic rate always, to overcome the sync issue at
all.

Well for PHY, I think it should only be related for PHY calculations as per
the modulation used. So different PHY rates enable different MAC rates.

basim

On Fri, Nov 21, 2008 at 5:30 PM, Mayur <mayur at ee.iitd.ac.in> wrote:

>
> Dear Network and NS experts,
>
> Why MAC and PHY rates should be different for a given packet to be
> transmitted. I could not understand the calculation of the txtime of a
> packet in mac-802_11.cc.
>
> As observed in the txtime() calculation in mac-802_11.cc, the txtime of
> any packet is calculated as :
>       t = tp + tm,
>        where tp is calculated using the PLCPDatarate for the PLCP
> header length and tm is calculated using mac dataRate_ (or basicRate_.)
> for the MAC Header part. Now the doubt is as follows: The complete frame
> (PHY+MAC+MSDU) will go holistically or completely only. Then there
> should be only one data rate which decides the tx time. Why two rates
> working for each of its own layer? ie. phy data rate for phy part and
> mac data rate for mac part? I could not understand what happens in
> reality, and how is it tried to be put here in ns2 simulation.
>
> Pl clarify my doubts from both of these angles: Theoretically and from
> ns Simulation point of view too.
>
> regards,
> Mayur
>
>