[Ns-developers] Importance of processing delays accounting.
nbaldo at cttc.es
Mon Aug 30 09:41:10 PDT 2010
sorry for the late reply. Please find a few comments below.
On 08/04/2010 02:30 PM, Elena Buchatskaya wrote:
> Generally speaking none of the ns-3 models account for processing
> delays. The necessity of adding random delay in Wi-Fi module has
> already been discussed (see bug 912), but I think the more general
> solution is desirable. We need to model non-deterministic processing
> delays at various OSI layers. I propose to add a adjustable delay:
> 1) between the IP and Wi-Fi MAC layers to accommodate the processing
> time at the IP layer. This is the time needed to to look up routes in
> a routing table, to recompute the checksum of the IP header as well as
> the time needed to prepare the packet for further transmission.
> 2) between MAC and PHY layers to accommodate the processing time at
> the MAC layer and time required to copy packet for kernel space to the
I agree with modeling the time required to copy packet from kernel space
to the device; I think the right point to do it is upon enqueueing the
packet onto DcaTxop.
However, I am not sure about adding a random delay between the MAC and
the PHY. In my experience, most of the delay at the MacLow level and
below is due to the processing done by the MAC (e.g., CRC calculation),
is tightly bounded by the standard (e.g., by SIFS timing) and in
commercial devices it is almost deterministic (some colleagues of mine
measured a standard deviation below one microsecond for the delay
between a DATA packet and its ACK). What would be the purpose of
introducing such a model in ns-3?
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