Hello all,

Although I searched the ns-users list on this subject, I could not find
a satisfactory answer. 

I am wondering why ns 802.11 MAC code guesses RTS/CTS and Data
retransmission timeouts as are? 

For example retransmission timeout for RTS packets is calculated as:


timeout = txtime(phymib_.getRTSlen(), basicRate_) +
DSSS_MaxPropagationDelay + sifs_ + txtime(phymib_.getCTSlen(),
basicRate_) + DSSS_MaxPropagationDelay; 


If we assume RTS packets have been in collision, the transmitter may
figure out a collision has occurred if no other transmission starts at
the slot after SIFS (CTS transmission should have been started). So
timeout can be (this approach can save considerable bandwidth in some
scenarios):

timeout = txtime(phymib_.getRTSlen(), basicRate_) +
DSSS_MaxPropagationDelay + sifs_ + 1 slot.

A similar discussion holds for data timeout. So, I wonder about the
reasoning behind the guess of the 802.11 ns MAC code. I'll appreciate
any help/answer. I'm including a paragraph from 802.11 document if it
may help. But Ack-timeout value is not specified anywhere in the
document. 


Best regards,

Inanc Inan



------------------------------------------------------------------------
----
After transmitting an MPDU that requires an ACK frame as a response (see
9.7), the STA shall wait for an ACKTimeout interval, starting at the
PHY- TXEND.confirm. If a PHY-RXSTART.indication does not occur during
the ACKTimeout interval, the STA concludes that the transmission of the
MPDU has failed, and this STA shall invoke its backoff procedure upon
expiration of the ACKTimeout interval. If a PHY-RXSTART.indication does
occur during the ACKTimeout interval, the STA shall wait for the
corresponding PHY-RXEND.indication to determine whether the MPDU
transmission was successful. The recognition of a valid ACK frame sent
by the recipient of the MPDU requiring acknowledgment, corresponding to
this PHY-RXEND.indication, shall be interpreted as successful
acknowledgment, permitting the frame sequence to continue, or to end
without retries, as appropriate for the particular frame sequence in
progress. The recognition of anything else, including any other valid
frame, shall be interpreted as failure of the MPDU transmission. In this
instance, the STA shall invoke its backoff procedure at the
PHY-RXEND.indication and may process the received frame. The sole
exception is that recognition of a valid data frame sent by the
recipient of a PS-Poll frame shall also be accepted as successful
acknowledgment of the PS-Poll frame.

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