1. doc.: IEEE 802.11-02/065r1 Submission January 2002 Brockmann, Hoeben, Wentink (Intersil)Slide 1 802.11g MAC Analysis and Recommendations Menzo Wentink (mwentink@intersil.com) Ron Brockmann (rbrockma@intersil.com) Maarten Hoeben (mhoeben@intersil.com) Tim Godfrey (tgodfrey@intersil.com) Mark Webster (mwebster@intersil.com) Steve Halford (shalford@intersil.com) Carl Andren (candren@intersil.com)

2. doc.: IEEE 802.11-02/065r1 Submission January 2002 Brockmann, Hoeben, Wentink (Intersil)Slide 2 802.11g 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 802.11-02/065r1 Submission January 2002 Brockmann, Hoeben, Wentink (Intersil)Slide 3 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 802.11-02/065r1 Submission January 2002 Brockmann, Hoeben, Wentink (Intersil)Slide 4 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 802.11-02/065r1 Submission January 2002 Brockmann, Hoeben, Wentink (Intersil)Slide 5 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 802.11-02/065r1 Submission January 2002 Brockmann, Hoeben, Wentink (Intersil)Slide 6 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 802.11-02/065r1 Submission January 2002 Brockmann, Hoeben, Wentink (Intersil)Slide 7 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 802.11-02/065r1 Submission January 2002 Brockmann, Hoeben, Wentink (Intersil)Slide 8 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 802.11-02/065r1 Submission January 2002 Brockmann, Hoeben, Wentink (Intersil)Slide 9 DCF Performance

10. doc.: IEEE 802.11-02/065r1 Submission January 2002 Brockmann, Hoeben, Wentink (Intersil)Slide 10 Average Frame Tx Durations *) RTS CTS OFDM features cheap collisions (cost of one RTS) and built-in hidden node protection *

11. doc.: IEEE 802.11-02/065r1 Submission January 2002 Brockmann, Hoeben, Wentink (Intersil)Slide 11 Throughput Comparison for 24/22 Mbps

12. doc.: IEEE 802.11-02/065r1 Submission January 2002 Brockmann, Hoeben, Wentink (Intersil)Slide 12 Mixed b/g

13. doc.: IEEE 802.11-02/065r1 Submission January 2002 Brockmann, Hoeben, Wentink (Intersil)Slide 13 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 802.11-02/065r1 Submission January 2002 Brockmann, Hoeben, Wentink (Intersil)Slide 14 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 802.11-02/065r1 Submission January 2002 Brockmann, Hoeben, Wentink (Intersil)Slide 15 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 802.11-02/065r1 Submission January 2002 Brockmann, Hoeben, Wentink (Intersil)Slide 16 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 802.11-02/065r1 Submission January 2002 Brockmann, Hoeben, Wentink (Intersil)Slide 17 Migration from Legacy to 802.11g

18. doc.: IEEE 802.11-02/065r1 Submission January 2002 Brockmann, Hoeben, Wentink (Intersil)Slide 18 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 802.11-02/065r1 Submission January 2002 Brockmann, Hoeben, Wentink (Intersil)Slide 19 Pure OFDM UDP Performance Comparison

20. doc.: IEEE 802.11-02/065r1 Submission January 2002 Brockmann, Hoeben, Wentink (Intersil)Slide 20 Performance in relation with CWmin (1) CWmin = 31 CWmin = 15

21. doc.: IEEE 802.11-02/065r1 Submission January 2002 Brockmann, Hoeben, Wentink (Intersil)Slide 21 Performance in relation with CWmin (3) CWmin = 31 CWmin = 15

22. doc.: IEEE 802.11-02/065r1 Submission January 2002 Brockmann, Hoeben, Wentink (Intersil)Slide 22 Pure OFDM TCP Performance Comparison

23. doc.: IEEE 802.11-02/065r1 Submission January 2002 Brockmann, Hoeben, Wentink (Intersil)Slide 23 Throughput comparison for TCP

24. doc.: IEEE 802.11-02/065r1 Submission January 2002 Brockmann, Hoeben, Wentink (Intersil)Slide 24 802.11e QoS Scenarios

25. doc.: IEEE 802.11-02/065r1 Submission January 2002 Brockmann, Hoeben, Wentink (Intersil)Slide 25 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 802.11-02/065r1 Submission January 2002 Brockmann, Hoeben, Wentink (Intersil)Slide 26 Streaming video with 802.11e/g aggregate throughput 2x 12 Mbps video no starvation of background

27. doc.: IEEE 802.11-02/065r1 Submission January 2002 Brockmann, Hoeben, Wentink (Intersil)Slide 27 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 802.11-02/065r1 Submission January 2002 Brockmann, Hoeben, Wentink (Intersil)Slide 28 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

29. doc.: IEEE 802.11-02/065r1 Submission January 2002 Brockmann, Hoeben, Wentink (Intersil)Slide 29 Element for Legacy Indication 802.11g introduces the need for a BSS to indicate the presence of legacy stations (either associated to, or in the vicinity of the BSS) so the 802.11g stations can make optimal decisions on whether RTS/CTS (or other protection mechanisms) are needed for OFDM frames.

30. doc.: IEEE 802.11-02/065r1 Submission January 2002 Brockmann, Hoeben, Wentink (Intersil)Slide 30 Recommendations In the form of Motions

31. doc.: IEEE 802.11-02/065r1 Submission January 2002 Brockmann, Hoeben, Wentink (Intersil)Slide 31 802.11g stations need to know if any legacy stations are associated in the BSS. If no legacy stations are associated, the 802.11g stations do not need to use protection mechanisms for OFDM frames. The AP keeps track of associated stations, and knows (by their capability information bits) whether they are 802.11g stations or legacy stations. Legacy stations will not understand this new element, and will ignore it. Need for a new element

32. doc.: IEEE 802.11-02/065r1 Submission January 2002 Brockmann, Hoeben, Wentink (Intersil)Slide 32 Element Definition A new element is defined, with one octet value. The octet contains two 1-bit fields. –B0 is set to 1 if any 802.11b stations are associated –B1 is optional. It is set to the same value as bit 0 unless optional, additional information is provided. This bit may be used by “smart” APs that implement techniques to provide additional information to stations. –“r” bits are reserved. Element ID Length =1 B0B1rrrrrr B0B7 One Octet

33. doc.: IEEE 802.11-02/065r1 Submission January 2002 Brockmann, Hoeben, Wentink (Intersil)Slide 33 Mandatory Functions An 802.11g conformant AP must generate this element. –The AP must set bit 0 to a “0” if no 802.11b stations are associated. The AP must set bit 0 to a “1” if any 802.11b stations are associated. –If the AP is not providing additional information, it must set bit 1 to the same value as bit 0. There is no mandatory behavior for a station. It may or may not make use of this element. –The recommended use of this information is to indicate the need to use protection mechanisms (such as RTS / CTS) for OFDM frames.

34. doc.: IEEE 802.11-02/065r1 Submission January 2002 Brockmann, Hoeben, Wentink (Intersil)Slide 34 Use of Bit 1 Bit 1 must be set to the same value as bit 0, unless additional information is conveyed through the following encoding: Bit 0Bit 1 Meaning 01 No 802.11b legacy stations are associated, but the AP recommends the use of protection mechanisms (possibly because legacy frames from another BSS have been received by the AP) 10 802.11b legacy stations are associated, but the AP suggests that protection mechanisms are not necessary currently, possibly because the legacy stations have all been “quiet” (perhaps in power save).

35. doc.: IEEE 802.11-02/065r1 Submission January 2002 Brockmann, Hoeben, Wentink (Intersil)Slide 35 Add a new clause to 7.3.2 (7.3.2.last+1) containing the following text: –The legacy indication element provides 802.11 stations with an indication of the presence of legacy stations in the BSS. See Figure xx. Stations may use this information to control their use of protection mechanisms (such as RTS / CTS) for OFDM frames. An Access Point shall generate this element in each Beacon Frame. The AP shall set bit 0 to a “0” if no 802.11b stations are associated. The AP shall set bit 0 to a “1” if any 802.11b stations are associated. The AP shall set bit 1 to the same value as bit 0 unless it is providing additional, optional information. If optional information is provided, it shall be according to this table: The editor is requested to assign a unique element ID. Bit 0Bit 1Meaning 00No 802.11b legacy stations are associated, and the AP suggests that protection mechanisms are not currently needed. 01No 802.11b legacy stations are associated, but the AP recommends the use of protection mechanisms 10802.11b legacy stations are associated, but the AP suggests that protection mechanisms are not currently needed. 11802.11b legacy stations are associated, and the AP recommends the use of protection mechanisms Element ID Length =1 b0b1rrrrrr B0B7 One Octet Figure xx: Legacy Indication Element

36. doc.: IEEE 802.11-02/065r1 Submission January 2002 Brockmann, Hoeben, Wentink (Intersil)Slide 36 Motion on RTS/CTS usage for OFDM Instruct the editor to incorporate the text in the previous slide into the draft.

37. doc.: IEEE 802.11-02/065r1 Submission January 2002 Brockmann, Hoeben, Wentink (Intersil)Slide 37 Background on Rate for ACK frames IEEE 802.11-1999 Section 9.6: –“All Control frames shall be transmitted at one of the rates in the BSSBasicRateSet (see 10.3.10.1), or at one of the rates in the PHY mandatory rate set so they will be understood by all STAs.” –“In order to allow the transmitting STA to calculate the contents of the Duration/ID field, the responding STA shall transmit its Control Response frame (either CTS or ACK) at the same rate as the immediately previous frame in the frame exchange sequence (as defined in 9.7), if this rate belongs to the PHY mandatory rates, or else at the highest possible rate belonging to the PHY rates in the BSSBasicRateSet.” IEEE 802.11b modified this section to read: –“All Control frames shall be transmitted at one of the rates in the BSS basic rate set so that they will be understood by all STAs in the BSS.” –“To allow the transmitting STA to calculate the contents of the Duration/ID field, the responding STA shall transmit its Control Response and Management Response frames (either CTS or ACK) at the highest rate in the BSS basic rate set that is less than or equal to the rate of at the same rate as the immedi-ately previous frame in the frame exchange sequence (as defined in 9.7). In addition, the Control Response frame shall be sent using the same PHY options as the received frame. “

38. doc.: IEEE 802.11-02/065r1 Submission January 2002 Brockmann, Hoeben, Wentink (Intersil)Slide 38 Motion to instruct the editor to add text to section 9.6 as follows: “All Control frames shall be transmitted at one of the rates in the BSS basic rate set so that they will be understood by all STAs in the BSS. For the IEEE 802.11g PHY, Control Response frames shall be sent at one of the Extended Rate PHY (ERP) mandatory rates in response to an OFDM frame as described below. “To allow the transmitting STA to calculate the contents of the Duration/ID field, the responding STA shall transmit its Control Response and Management Response frames (either CTS or ACK) at the highest rate in the BSS basic rate set that is less than or equal to the rate of at the same rate as the immediately previous frame in the frame exchange sequence (as defined in 9.7). In addition, the Control Response frame shall be sent using the same PHY options as the received frame. For the IEEE 802.11g PHY, if the received frame was sent at an OFDM rate, the Control Response frame shall be sent at the highest mandatory ERP rate that is less than or equal to the rate of the received frame. “

39. doc.: IEEE 802.11-02/065r1 Submission January 2002 Brockmann, Hoeben, Wentink (Intersil)Slide 39 Motion on aCWmin Instruct the editor to add a sub clause 19.4.3.8.5 specifying to use the table in sub clause 18.3.3 for the MAC timing calculation, with the following changes: –Use an aCWmin value of 15 unless in a 11b legacy network which uses the value in 18.3.3 –aMACProcessingDelay is < 2us

40. doc.: IEEE 802.11-02/065r1 Submission January 2002 Brockmann, Hoeben, Wentink (Intersil)Slide 40 Motion on the signal extension for ERP/OFDM Add a sub clause 19.4.3.8.6 to state that the packet is followed by a Signal Extension Field which is quiet time (no carrier) of 6 microseconds.

41. doc.: IEEE 802.11-02/065r1 Submission January 2002 Brockmann, Hoeben, Wentink (Intersil)Slide 41 Motion on the signal extension for CCK-OFDM Change sub clause 19.6.2.4.1 to state that the Signal Extension is quiet time (no carrier). Change figure 19.6.2.4.1 to indicate that the Signal Extension is quiet time Change sub clause 19.6.2.4.5 to specify that the Signal Extension is quiet time.

42. doc.: IEEE 802.11-02/065r1 Submission January 2002 Brockmann, Hoeben, Wentink (Intersil)Slide 42 Motion to instruct the editor to change the TXtime equation for ERP/OFDM Change the Txtime equation in 19.4.4.1 (which is currently a copy of the.11a definition) to add the 6 us Signal extension. The new equation would be: TXTIME =T PREAMBLE +T SIGNAL +T SYM *Ceiling((16 + 8*LENGTH + 6 )/ N DBPS )+Signal Extension Where Signal Extension is defined as 6 microseconds.

43. doc.: IEEE 802.11-02/065r1 Submission January 2002 Brockmann, Hoeben, Wentink (Intersil)Slide 43 Motion on Adjacent channel rejection Instruct the editor to add the following text to Section 19.4.3.10.1: –While receiving legacy 802.11b signals (1, 2, 5.5, 11 Mbps), the adjacent channel rejection should conform to the specifications of Subclause 18.4.8.3. While receiving OFDM signals (6, 9, 12, 18, 24, 36, 48, and 54 Mbps), the adjacent channel rejection shall conform to Subclause 17.3.10.2 with a +/- 25 MHz spacing.