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Project: IEEE P802.15 Working Group for Wireless Personal Area Networks (WPANs)

Project: IEEE P802.15 Working Group for Wireless Personal Area Networks (WPANs) Submission Title: [Throughput calculation discussion] Date Submitted: [ 20 July, 2005 Source: [ James Gilb ] Company [ SiBeam ] Address [ 840 W. California, Suite 110, Sunnyvale, CA 94086 ]

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Project: IEEE P802.15 Working Group for Wireless Personal Area Networks (WPANs)

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  1. Project: IEEE P802.15 Working Group for Wireless Personal Area Networks (WPANs) Submission Title:[Throughput calculation discussion] Date Submitted: [20 July, 2005 Source: [James Gilb] Company [SiBeam] Address [840 W. California, Suite 110, Sunnyvale, CA 94086] Voice:[858-484-4339], FAX: [858-484-4339], E-Mail:[last name at ieee dot org] Re: [TG3c requirements document] Abstract: [Calculating the theoretical throughput of the system is important in comparing PHY proposals. this document provides a simple method for calculating the throughput. ] Purpose: [Provide a method for calculating throughput at the PHY SAP.] Notice: This document has been prepared to assist the IEEE P802.15. It is offered as a basis for discussion and is not binding on the contributing individual(s) or organization(s). The material in this document is subject to change in form and content after further study. The contributor(s) reserve(s) the right to add, amend or withdraw material contained herein. Release: The contributor acknowledges and accepts that this contribution becomes the property of IEEE and may be made publicly available by P802.15. James Gilb, SiBeam

  2. Elements of frame duration • Frame duration consists of: • TP: Duration of the preamble • TPheader: PHY header duration • TMheader: MAC header duration • THCS: Header check sequence • Tdata: Data duration, including any coding • TFCS: FCS duration • Each frame is separated from the preceding frame by an inter-frame spacing (IFS) • SIFS: One of the DEVs is changing TX to RX or RX to TX • MIFS: None of the DEVs need to change TX or RX state James Gilb, SiBeam

  3. Overhead depends on ACK Policy Imm-ACK Policy Frame SIFS ACK SIFS Frame SIFS ACK SIFS Frame SIFS ACK SIFS Frame SIFS ACK No-ACK Policy Frame MIFS Frame MIFS Frame MIFS Frame MIFS Frame MIFS Dly-ACK Policy Frame MIFS Frame MIFS Frame MIFS Frame MIFS Frame SIFS Dly-ACK SIFS Implied-ACK Policy Frame SIFS Response + ACK SIFS Frame Response + ACK SIFS Frame SIFS James Gilb, SiBeam

  4. Imm-ACK calculation • Time to send data • Time for frame • SIFS • ACK • SIFS Throughput = (number bits/frame)/(Frame time + 2*SIFS + ACK time) James Gilb, SiBeam

  5. No-ACK calculation • Time to send data • Time for frame • SIFS Throughput = (number bits/frame)/(Frame time + SIFS) James Gilb, SiBeam

  6. Dly-ACK calculation • Time to send data • n*Time for frame • n-1*MIFS between n frames • SIFS • ACK • SIFS Throughput = n*(number bits/frame)/(n*Frame time + (n-1)*MIFS + 2*SIFS + ACK time) James Gilb, SiBeam

  7. Implied ACK calculation • Assume TX and RX frames are same size • Time to send data • Time for frame • SIFS Throughput = (number bits/frame)/[2*(Frame time + SIFS)] Bi-directional throughput = (number bits/frame)/(Frame time + SIFS) James Gilb, SiBeam

  8. Key issues in overhead • SIFS time • Switch circuits, wideband is fast • Is the frame OK? Demodulation and FEC shall have completed. • MIFS time • Don’t necessarily have to complete frame demodulation and FEC. • PHY preamble • How long to sync, equalize, AGC, frequency offset, phase sync, etc. James Gilb, SiBeam

  9. Not an issue for high rates • Assume 500 Mb/s data rate • PHY header: 4*8/5e8 = 64 ns • MAC header: 10*8/5e8 = 160 ns • MAC HCS: 2*8/5e8 = 32 ns • MAC FCS: 4*8/5e8 = 64 ns • Frame size? • In practice it will be hard to get large enough frames. • 20 k octets: 20*1024*8/5e8 = 327 us • Requires aggregation, which is application dependent. James Gilb, SiBeam

  10. Lost frames decrease throughput • Not every frame makes it. • PER = 1-(1-BER)^(# of bits) • For low BER, ~ (# of bits) * BER • Real throughput (TP) reduced, depends on ACK policy • No-ACK: TPactual = TP/(1+PER) • Imm-ACK: TPactual = TP*(1-PER) • See next slide • Dly-ACK: Similar to Imm-ACK • Implied-ACK: Similar to Imm-ACK James Gilb, SiBeam

  11. Typical links have low PER • Typically the link needs to have PER < 10 % • No-ACK: TPactual = 1/(1+0.1) = 91% TPNo-ACK • Imm-ACK: TPactual = (1-0.1) = 90% TPImm-ACK • This will be roughly the same for all PHYs • Recommend ignore PER for throughput calculations James Gilb, SiBeam

  12. What else is left out • Beacon time • CAP time • Variable, can be zero for a given PHY • Guard intervals before CTAs • Only integer number of frames in a supeframe • Not all space filled • Allocations for other devices • Many applications require a reverse data channel James Gilb, SiBeam

  13. Theoretical Maximum Throughput • TMT is the best you can get if you had infinite data rate • 802.11 has discussed TMT • CSMA/CA has serious issues with TMT • Pre .11e, TMT for 802.11 was < 75 Mb/s • 802.11e changes have improved this. • TDMA has different limits • No time for contention James Gilb, SiBeam

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