RIFS mode in 11ac Date: 2011-01-17 Authors: Joshua Zhao, Atheros

Background RIFS mode is added in 11n in hope to improve MAC efficiency over using SIFS RIFS interval (2us) is much shorter than SIFS interval (16us) RIFS and A-MPDU were proposed by two different groups and both adopted into 11n RIFS was proposed w/o A-MPDU in mind A-MPDU turns out to be more efficient and is more widely adopted RIFS is only allowed during bursting between two STAs when no SIFS-separated response is expected Only delayed BA can be used RIFS can only be used between HT PPDUs In 11n RIFS support is mandatory for receiving but can be prohibited by BSS (See 9.2.3.0b in 802.11n-2009) Two frames separated by a RIFS shall both be HT PPDUs. Atheros Communications

Low value of RIFS mode On top of A-MPDU, RIFS gives very marginal improvement on MAC efficiency Can only be applied when bursting is allowed Bursting is beneficial only if there are big amount of data. In that case we expect average packet length would be >1ms. Saving 14us over SIFS only saves < 1.4% overhead “No Immediate BA” seriously restricts whether a RIFS burst could happen or how long it can last No RIFS burst if Block Ack window is full A-MPDU w/ SIFS bursting could be more efficient Very limited benefits can be seen in very limited scenarios RIFS imposes high complexity on receiver implementation Baseband/MAC processing latency can easily be longer than 2us High HW burden (complexity/testing) to make sure new packets can be received correctly before previous packet is finished Very limited (if not none) RIFS adoption has been seen so far Atheros Communications

Pre-Motion 1 Do you support that VHT STAs shall not transmit any HT or VHT frames separated by RIFS? Atheros Communications

Pre-Motion 2 Do you support that VHT AP shall set the RIFS mode bit to 0 in the HT operation element? Atheros Communications