1. 7/20/2005
Project: IEEE P 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:[ ], FAX: [ ], [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 P 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 P
James Gilb, SiBeam

2. Elements of frame duration
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doc.: IEEE <doc#>
7/20/2005
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
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3. Overhead depends on ACK Policy
7/20/2005
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
7/20/2005
Imm-ACK calculation
Time to send data
Time for frame
SIFS
ACK
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
7/20/2005
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
7/20/2005
Dly-ACK calculation
Time to send data
n*Time for frame
n-1*MIFS between n frames
SIFS
ACK
Throughput = n*(number bits/frame)/(n*Frame time + (n-1)*MIFS + 2*SIFS + ACK time)
James Gilb, SiBeam

7. Implied ACK calculation
7/20/2005
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 MIFS time PHY preamble
7/20/2005
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
7/20/2005
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
7/20/2005
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
7/20/2005
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 Guard intervals before CTAs
7/20/2005
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
7/20/2005
Theoretical Maximum Throughput
TMT is the best you can get if you had infinite data rate
has discussed TMT
CSMA/CA has serious issues with TMT
Pre .11e, TMT for was < 75 Mb/s
802.11e changes have improved this.
TDMA has different limits
No time for contention
James Gilb, SiBeam