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Considerations on short packet transmission overhead

Considerations on short packet transmission overhead. Date: September 18 , 20 1 1. Authors: . Objective. This contribution presents some considerations on small packet overhead in 802.11ah. 802.11ah Requirements.

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Considerations on short packet transmission overhead

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  1. Considerations on short packet transmission overhead Date:September 18, 2011 Authors:

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

  3. 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

  4. 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

  5. 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)

  6. 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

  7. Conclusions • The actual overhead is veryhigh 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

  8. 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

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