802.11 max propagation delay and ACK timeout

[Bjørnar Libæk]

In the 802.11g implementation in the inetmanet package, the ACK
timeout is calculated as follows:

timeout = dataDuration + SIFS + ackDuration + MAX_PROPAGATION_DELAY *
2

where MAX_PROPAGATION_DELAY is a constant equal to 2 microseconds,
which corresponds to a 600m distance at the speed of light.

In my simulations, I encountered a situation where the ACK timeout
fired before the reception of the ACK, when the communicating nodes
are 700m appart, which is no surprise according to how the timeout is
calculated. I'm using the Ieee80211gNIC with default ned parameters
(11mbps).

Why is the value of 2 microseconds chosen? The standard document (IEEE
Std 802.1-2007), section 9.2.8, sais:

[]
After transmitting an MPDU that requires an ACK frame as a response
(see 9.12), the STA shall wait for an ACKTimeout interval, with a
value of aSIFSTime + aSlotTime + aPHY-RX-START-Delay, 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.
[/]

aPHY-RX-START-Delay is 128microsec (!)

[Alfonso Ariza Quintana]

I have just actualized the code of Ieee80211NewMac in inetmanet to compute
the time like the standard. If you want to use this modification you must
use Ieee80211NewMac instead of Ieee80211gMac and set the parameter
useModulationParameters to true

-----Mensaje original-----
From: Bj�rnar Lib�k
Sent: Tuesday, May 10, 2011 12:15 PM
To: omnetpp
Subject: [Omnetpp-l] 802.11 max propagation delay and ACK timeout

aPHY-RX-START-Delay is 128microsec (!)

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