Considerations on short packet transmission overhead Month Year doc.: IEEE 802.11-yy/xxxxr0 Considerations on short packet transmission overhead Date: September 18, 2011 Authors: Name Company Address Phone email George Calcev Huawei Technologies Co., Ltd. 3601 Algonquin Road, Suite 1000 Rolling Meadows, IL 60008,  USA George.Calcev@huawei.com Bin Chen Bin.Chen@huawei.com Lin Cai Lin.Cai.@huawei.com John Doe, Some Company

Objective This contribution presents some considerations on small packet overhead in 802.11ah

802.11ah Requirements Operate under 1GHz bands; in US it will operate at 902-928 MHz with output power limit of 30dBm for less than 6dBi antenna gain and uniformly spread over more than 500KHz bandwidth Provide a mode of operation with low rate and high range (100kbps, 1Km) Allow for more than 2007 STAs to be associated with an AP Operate in indoor as well as in outdoor environments

Short Transmissions Overhead In sensor networks the packet length is mostly between 100-200 bytes Overhead PHY overhead – 8 symbols MAC overhead –MAC header with at least 3 addresses (Table 8-18) , FC(2)+Duration(2)+RA(6)+TA(6)+DA(6)+SC(2) + FCS(4) = 28 octets=224 bits Protocol overhead – ACK Channel access overhead – at least SIFS for polled access

SIFS, CCA durations (for 1.25 MHz) aRxTxSwitchTime << 1 μs aCCATime ≤ 64 μs (scaled up from 4us for 20 MHz to 1.25 MHz, see Table 17-17) aRxTxTurnaroundTime ≤ 2 μs aAirPropagationTime < 4 us aMACProcessingDelay ≤ 2 μs aSlotTime =: aCCATime + aRxTxTurnaroundTime + aAirPropagationTime+ aMACProcessingDelay=72 us (scaled) aSIFSTime =: aRxRFDelay + aRxPLCPDelay + aMACProcessingDelay + aRxTxTurnaroundTime = (16 us in 20 MHz, scales to 256us, see Table 17-7)

Short Transmissions Overhead Each symbol can carry 24 bits in 52 subcarriers , with BPSK, ½ (Table 17-4) Data 100-200 bytes about 33.3-66.7 symbols necessary MAC overhead =9.33 symbols PHY overhead =8 symbols (Greenfield HT), 10 symbols in 11ac, and 10 symbols in 11n mixed (Figure 19-1) SIFS overhead = 4 symbols (16us scales up based Table 17-7) ACK overhead =13 symbols (14 bytes MAC, 14*8/24=4.6 symbols +8 PHY) The overhead is (8+9.33+4+13)/33.3= 103% or 34.33/66.7 =51.5% If the extra SIFS before the transmission is considered the overhead increases This overhead gets potentially higher for higher data rates when data takes less symbols

Conclusions The actual overhead is very high for short packet communications The actual aggregation is not useful in UL transmissions for smart sensors networks (it could add large delays) 802.11ah needs better solutions for handling short packets

References https://mentor.ieee.org/802.11/dcn/11/11-11-0905-03-00ah-tgah-functional-requirements-and-evaluation-methodology.doc 802.11REVmb_D8.0 P802.11ah_D1.0