[ns] Regarding txtime() in mac-802_11: Why PHY and MAC both data rates are used in txtime calculation?
mayur at ee.iitd.ac.in
Fri Nov 21 08:30:15 PST 2008
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.
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