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Doc.: IEEE 802.11-02/065r0 Submission January 2001 Brockmann, Hoeben, Wentink (Intersil) 802.11g MAC Analysis Menzo Wentink Ron Brockmann.

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Presentation on theme: "Doc.: IEEE 802.11-02/065r0 Submission January 2001 Brockmann, Hoeben, Wentink (Intersil) 802.11g MAC Analysis Menzo Wentink Ron Brockmann."— Presentation transcript:

1 doc.: IEEE 802.11-02/065r0 Submission January 2001 Brockmann, Hoeben, Wentink (Intersil) 802.11g MAC Analysis Menzo Wentink mwentink@intersil.com Ron Brockmann rbrockma@intersil.com Maarten Hoeben mhoeben@intersil.com

2 doc.: IEEE 802.11-02/065r0 Submission January 2001 Brockmann, Hoeben, Wentink (Intersil) 802.11g MAC Related Settings The following parameters are used: 802.11b802.11a802.11g aSIFSTime10 usec16 usec10 usec aSlotTime20 usec9 usec20 usec aCWmin31 slots15 slots A 6 usec silence period is added to OFDM frames, to mitigate for the 16 usec OFDM SIFS ACK frames shall be sent at a Basic Rate or PHY mandatory rate The RTS Threshold can be dynamically set by a link optimization algorithm, or by an information element in the beacon

3 doc.: IEEE 802.11-02/065r0 Submission January 2001 Brockmann, Hoeben, Wentink (Intersil) Recommendation: SIFS 10 usec OFDM requires a 16, not 10 usec RX-TX turnaround This is solved in CCK-OFDM by adding a 6 usec postamble to the packet, effectively extending the SIFS for the receiver The transmitter is active longer than necessary, and the TX-RX turnaround time available is significantly reduced Recommendation: add a 6 usec silence period is added to each OFDM frame, with the same function as the CCK-OFDM postamble

4 doc.: IEEE 802.11-02/065r0 Submission January 2001 Brockmann, Hoeben, Wentink (Intersil) Recommendation: Slot Time 20 us When 802.11 DS was defined, a 20 us slot was equivalent to 5 bytes at the highest rate of 2 Mbit/s Today, 20 us can transfer 135 bytes at 54 Mbit/s ! Backoff slots are very expensive – this favors bursting techniques in PCF and TGe HCF Slot time is part of the definition of PIFS and DIFS affecting core MAC/TGe behaviours, and cannot be changed without significant coexistence issues

5 doc.: IEEE 802.11-02/065r0 Submission January 2001 Brockmann, Hoeben, Wentink (Intersil) Recommendation: CWmin 15 High cost of slot time calls for shorter backoff window 802.11a uses CWmin 15 Extensive simulations show CWmin 15 gives markedly higher overall performance in all typical scenarios than CWmin 31 802.11g nodes operating in full 802.11b backward compatibility mode (not using the 802.11g rates) should comply with 802.11b and use CWmin 31 For.11g+e products, CWmin can be overruled

6 doc.: IEEE 802.11-02/065r0 Submission January 2001 Brockmann, Hoeben, Wentink (Intersil) ACK Rates It is desired to transmit OFDM ACK frames in response to OFDM DATA frames because they are substantially more efficient Section 9.6 of 802.11-1999 and 802.11b contradict on whether this is required/forbidden when the Basic Rates do not include OFDM rates in a mixed environment Recommendation: clarify section 9.6 to support the use of OFDM Mandatory rates in response to OFDM frames even if they are not part of the Basic Rate Set as described in 02/xxx

7 doc.: IEEE 802.11-02/065r0 Submission January 2001 Brockmann, Hoeben, Wentink (Intersil) RTS Threshold RTS/CTS is used to protect OFDM frames in a mixed b/g environment Can either be enabled/disabled statically by MIB variable, or a dynamic link optimization algorithm can be used Perhaps, a Recommended Practice can be defined Legacy 802.11b STAs do not have to use RTS/CTS, unless required to optimize the link for hidden nodes or excessive collision scenarios

8 doc.: IEEE 802.11-02/065r0 Submission January 2001 Brockmann, Hoeben, Wentink (Intersil) Analysis of MAC Performance DCF Performance Mixed b/g – without RTS/CTS Mixed b/g – with RTS/CTS, Cwmin 31 Mixed b/g – with RTS/CTS, Cwmin 15 Migration rom Legacy to Pure OFDM Pure OFDM, TCP DCF Efficiency, CWmin 15/31 Pure OFDM, UDP DCF Efficiency, CWmin 15/31 TGe QoS Bursting TGe QoS Video Scenario

9 doc.: IEEE 802.11-02/065r0 Submission January 2001 Brockmann, Hoeben, Wentink (Intersil) DCF Performance

10 doc.: IEEE 802.11-02/065r0 Submission January 2001 Brockmann, Hoeben, Wentink (Intersil) Average Frame Tx Durations *) RTS CTS OFDM features cheap collisions (cost of one RTS) and built-in hidden node protection *

11 doc.: IEEE 802.11-02/065r0 Submission January 2001 Brockmann, Hoeben, Wentink (Intersil) Throughput Comparison for 24/22 Mbps

12 doc.: IEEE 802.11-02/065r0 Submission January 2001 Brockmann, Hoeben, Wentink (Intersil) Mixed b/g

13 doc.: IEEE 802.11-02/065r0 Submission January 2001 Brockmann, Hoeben, Wentink (Intersil) Mixed b/g – without RTS/CTS the throughput of the legacy nodes goes up the aggregate throughput goes down The throughput of OFDM nodes diminishes, because OFDM yields for CCK, but not v.v. 2 OFDM nodes without RTS/CTS + 2 legacy nodes 4 legacy nodes The unprotected OFDM packets collide with legacy CCK. The OFDM TCP flows are starved.

14 doc.: IEEE 802.11-02/065r0 Submission January 2001 Brockmann, Hoeben, Wentink (Intersil) Mixed b/g – with RTS/CTS, CWmin 31 the aggregate throughput goes up The throughput of OFDM and legacy goes up by same amount due to fairness of DCF. RTS/CTS-protected 2 OFDM nodes with RTS/CTS 2 legacy nodes 4 legacy nodes Protected OFDM transmissions nicely mix with legacy

15 doc.: IEEE 802.11-02/065r0 Submission January 2001 Brockmann, Hoeben, Wentink (Intersil) DCF Fairness For equal CWmin, throughput increase is distributed over all nodes! –DCF gives each node equal number of transmit opportunities, regardless of their data rate –Legacy 802.11b frame transmissions are longer and they hog media time with their inefficient modulations –Aggregate throughput increases but less than expected By using a smaller CWmin, TGg nodes can get higher priority –Since their transmissions are shorter, total time spent on the media is comparable to legacy nodes

16 doc.: IEEE 802.11-02/065r0 Submission January 2001 Brockmann, Hoeben, Wentink (Intersil) Mixed b/g – with RTS/CTS, CWmin 15 the legacy throughput levels the throughput of OFDM nodes goes up, because of more efficient transmissions and smaller CWmin. 2 OFDM nodes with RTS/CTS + 2 legacy nodes 4 legacy nodes RTS/CTS-protected OFDM transmissions nicely mix with legacy the aggregate throughput goes up

17 doc.: IEEE 802.11-02/065r0 Submission January 2001 Brockmann, Hoeben, Wentink (Intersil) Migration from Legacy to 802.11g

18 doc.: IEEE 802.11-02/065r0 Submission January 2001 Brockmann, Hoeben, Wentink (Intersil) Migration to 802.11g from legacy 4 b 2 g-nodes 2 b-nodes 3 g-nodes 1 b-node 4 g-nodes w/o rts/cts Individual throughputs aggregate throughput OFDM and legacy CCK transmissions are mixed. 4 g-nodes

19 doc.: IEEE 802.11-02/065r0 Submission January 2001 Brockmann, Hoeben, Wentink (Intersil) Pure OFDM UDP Performance Comparison

20 doc.: IEEE 802.11-02/065r0 Submission January 2001 Brockmann, Hoeben, Wentink (Intersil) Performance in relation with CWmin (1) CWmin = 31 CWmin = 15

21 doc.: IEEE 802.11-02/065r0 Submission January 2001 Brockmann, Hoeben, Wentink (Intersil) Performance in relation with CWmin (3) CWmin = 31 CWmin = 15

22 doc.: IEEE 802.11-02/065r0 Submission January 2001 Brockmann, Hoeben, Wentink (Intersil) Pure OFDM TCP Performance Comparison

23 doc.: IEEE 802.11-02/065r0 Submission January 2001 Brockmann, Hoeben, Wentink (Intersil) Throughput comparison for TCP

24 doc.: IEEE 802.11-02/065r0 Submission January 2001 Brockmann, Hoeben, Wentink (Intersil) 802.11e QoS Scenarios

25 doc.: IEEE 802.11-02/065r0 Submission January 2001 Brockmann, Hoeben, Wentink (Intersil) Migration with 802.11e HCF Bursting 4 b-nodes 2 g-nodes (CFBs) 2 b-nodes 3 g-nodes (CFBs) 1 b-node Individual throughputs Aggregate throughput 4 g-nodes Throughput for g-nodes rises sharply Legacy throughput levels

26 doc.: IEEE 802.11-02/065r0 Submission January 2001 Brockmann, Hoeben, Wentink (Intersil) Streaming video with 802.11e/g aggregate throughput 2x 12 Mbps video no starvation of background

27 doc.: IEEE 802.11-02/065r0 Submission January 2001 Brockmann, Hoeben, Wentink (Intersil) Simulation Environment Network Simulator (NS) –from University of California –802.11 added by Carnegie Mellon –802.11e EDCF added by Atheros We added –802.11g PHY (next to 11b PHY) –Dynamic Rate selection and duration calculation –802.11e Contention Free Bursting Typical simulation setup –4 stations (b or g) and 1 AP (g)

28 doc.: IEEE 802.11-02/065r0 Submission January 2001 Brockmann, Hoeben, Wentink (Intersil) Conclusions Mixed 802.11b/g operation increases network throughput Pure 802.11g operation is efficient TGe enhancements work for mixed and pure g networks; provide greater MAC efficiency Recommendations to be adopted

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