1. Some Concepts of Rate Management Testing
Mike Wilhoyte
Texas Instruments
November 16, 2004
Mike Wilhoyte, Texas Instruments.

2. Block Diagram of Rate 802.11 Management
Rate management comprised of rate adaptation (host) + retries with rate fallback (MAC)
Host
MAC
PHY
ACK
ACK
Metrics
Adaptation
Rate
Retry
Requested Tx Rate
Tx Rate
Tx Out
Retry on SIFS timeout
May retry at lower rate “Fallback”
Mike Wilhoyte, Texas Instruments.

3. Terminology
Rate management = Rate Adaptation (Host) + Rate Fallback during Retries (MAC)
Mike Wilhoyte, Texas Instruments.

4. General Goal of Rate Management
Select rate for best throughput under a given channel condition, e.g.:
Static AWGN channel
Static multipath
Time-varying multipath
Interference (static or time-varying)
Inner-BSS (collision environment)
Intra-BSS (ACI)
Mike Wilhoyte, Texas Instruments.

5. Metrics for Rate Management
Metrics for selecting rates with increasing channel impairments
Rate step down
Metrics for selecting rates with decreasing channel impairments
Rate step up
Mike Wilhoyte, Texas Instruments.

6. Example Rate Adaptation - Stepping down under a static AWGN channel
Can metrics defined for this example?
Can these metrics be generalized?
Mike Wilhoyte, Texas Instruments.

7. Test Configuration STA AP ATTN DUT Anechoic Chamber Anechoic Chamber
Mike Wilhoyte, Texas Instruments.

8. Fixed Rate STA->AP (Example 1)
Mike Wilhoyte, Texas Instruments.

9. Throughput w/ Rate Adaptation
Mike Wilhoyte, Texas Instruments.

10. Comparing Best Case w/ Actual under Rate Adaptation
Mike Wilhoyte, Texas Instruments.

11. Potential Metric (Efficiency)1 Or 2
Mike Wilhoyte, Texas Instruments.

12. Efficiency vs Rx Level
Mike Wilhoyte, Texas Instruments.

13. Average Efficiency In this case, average efficiency = 96%
Mike Wilhoyte, Texas Instruments.

14. Example 2
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15. Comparing Best Case w/ Actual under Rate Adaptation (Example 2)
Mike Wilhoyte, Texas Instruments.

16. Efficiency vs Rx Level (Example 2)
Mike Wilhoyte, Texas Instruments.

17. Average Efficiency In this case, average efficiency = 99.2%
Mike Wilhoyte, Texas Instruments.

18. Potential Generalization
I believe these metrics are valid for characterizing rate adaptation stepping down under any static channel condition including
Static AWGN (examples given)
Static multipath
Static interference
Mike Wilhoyte, Texas Instruments.

19. Not Addressed (stuff to think about)
Stepping up
Time metrics: reacting to sudden decreasing impairments
Time-varying channels
Statistical metrics?
Rate fallback under retries ?
Mike Wilhoyte, Texas Instruments.

20. Appendix: Some Analysis of how Rate Fallback During Retries Affects Throughput
Mike Wilhoyte, Texas Instruments.

21. Throughput Calculation With Packet Errors, Retries, and Rate Fallback
At the MAC layer
Throughput =
(PacketSz*8/TxTimeTot)*(FractionPacketsTx)2,
where
PacketSz = Number of bytes per packet (1000 bytes used),
TxTimeTot = Total time to transmit a packet with overhead taking into account retries due to packet errors and failed transmissions (next slide)
FractionPacketsTx = Fraction of packets successfully transmitted (explained in later slides)
Mike Wilhoyte, Texas Instruments.

22. Calculation of TxTimeTot
TxTimeTot = TxTime(1)*FracTx(1) + TxTime(2)*FracTx(2) TxTime(1)*FracTx(2)+ … + TxTime(N)*FracTx(N) TxTime(N-1)*FracTx(N)
TxTime(k) = PayloadTime(Rate(k)) + Overhead(k) at attempt k
Rate(k) = Tx rate at attempt k
Overhead(k) = DIFS+DCF(k)+Preamble+SIFS+ACK(k)
DCF(k) = ContentionWindow(k)*aSlotTime/2
ACK(k) = ACK time at kth attempt (may change due to rate fallback)
ContentionWindow(k) = 31,63,127,or 255 (11b/b+)
= 15,31,63, or127 (11g/a)
FracTx(k) = Fraction of packets successfully transmitted at attempt k taking into account PER at Rate(k)
TxTime(k-1)*FracTx(k) = Time penalty for failures at attempt k-1 that are retried at attempt k
Mike Wilhoyte, Texas Instruments.

23. Taking Into Account Failed Transmissions
The fraction of packets transmitted
FracPacketsTx = Sum(FracTx(k)) and lies in the interval [0,1]
Total time reflects the time to transmit successful packets only and should be normalized by FracPacketsTx to reflect the time it took to transmit a packet as if they were all successful
Normalized Time = (TxTimeTot/FracPacketsTx)
The throughput should be further scaled by FracPacketsTx to account for failed transmissions
Thus the term “(FracPacketsTx)2” in the calculation of throughput
Mike Wilhoyte, Texas Instruments.

24. Fractional Loss in Throughput Versus PER for OFDM 54 Mbps With Rate Fallback
0 retries no fallback
(dashed)
Blue Solid -10 retries at OFDM-54 before fallback
Red Solid – 1 retry at OFDM-54 before fallback
Black Solid – 0 retry at OFDM-54 before fallback
Mike Wilhoyte, Texas Instruments.

25. Estimated Throughput of OFDM Modes With Varying Retries in 25 ns Multipath
54 Mbps
48 Mbps
36 Mbps
24 Mbps
18 Mbps
Retries at
Highest
Rate before
Falling back 1 10
6 Mbps
Mike Wilhoyte, Texas Instruments.