[Ns-developers] questions and patch

[mathieu lacage]

Francesco Gringoli wrote:

> I must admit the DRAFT is not very plain about the time between two  
> consecutive TXOPs in CFP if CFPOLL+ACK is not supported. Starting  
> from the phrase before the last one in clause 9.9.2.1.2 "CAP  
> generation" I think you are right.
>
> "The HC may sense the channel and reclaim the channel after a  
> duration of PIFS after the TXOP, if the channel remains idle". Well I  
> would say "reclaim" could not be interpreted as "switch to the next  
> TXOP" but since I don't find another place where this is stated  
> better, I will interpret "reclaim" in that way.


Maybe it woul be worth asking the 802.11 commitee for a clarification. I 
don't care that much though.

> To summarize, here is the scenario I think should happen where N1  is the
>
>> number of packets in TXOP1:
>>
>> TXOP1 transmission:
>> CFPOLL1+SIFS+ACK+PKT1+SIFS+ACK1+SIFS+...+PKTN1+SIFS+ACKN1+SIFS
>
>
> Uhm, what is the first ACK? Do you mean (ACK+PKT1) is a single packet?


no, it is the ACK for CFPOLL1. If you don't send it, the QAP thinks the 
TXOP is not used and switches to the next TXOP.

> I would remove the last SIFS and if N1 is the maximum number of  
> "nominal" packet QSTA1 can send inside its TXOP, I will say that
>
> TXOP1 must contain [ CFPOLL+SIFS+(N1-1)*(PKT+SIFS+ACK+SIFS) +PKT_N1 
> +SIFS+ACK ]
>
> Hence we can fix
>
> TXOP1=duration(CFPOLL)+N1*duration(PKT)+N1*duration(ACK)+2*N1*SIFS
>
> However (this is the key point) to calculate the CFP duration we have  
> to add PIFS between every consecutive TXOPs plus a PIFS at the end of  
> the CFP. Only if we calculate CFP duration in this way everything is  
> ok. Imagine what happens if we don't count the PIFS: if we have a lot  
> of backlogged traffic in all stations, every one will not send  
> QOSNULL since there is no time. The last station however will never  
> be polled because the m_txopTimer always added up PIFS to the TXOP  
> and if the CFP=TXOP1+TXOP2+TXOP3+... all the added PIFSs have  
> consumed time that should be granted to last station. Then it has no  
> time to complete the TXOP inside the CFP.


yes, hence my suggestion to add PIFS somewhere :)

regards,
Mathieu