1 / 43

802.11g MAC Analysis and Recommendations

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)

csilla
Download Presentation

802.11g MAC Analysis and Recommendations

An Image/Link below is provided (as is) to download presentation Download Policy: Content on the Website is provided to you AS IS for your information and personal use and may not be sold / licensed / shared on other websites without getting consent from its author. Content is provided to you AS IS for your information and personal use only. Download presentation by click this link. While downloading, if for some reason you are not able to download a presentation, the publisher may have deleted the file from their server. During download, if you can't get a presentation, the file might be deleted by the publisher.

E N D

Presentation Transcript

  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) Brockmann, Hoeben, Wentink (Intersil)

  2. 802.11g MAC Related Settings • The following parameters are used: • 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 Brockmann, Hoeben, Wentink (Intersil)

  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 Brockmann, Hoeben, Wentink (Intersil)

  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 Brockmann, Hoeben, Wentink (Intersil)

  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 Brockmann, Hoeben, Wentink (Intersil)

  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 Brockmann, Hoeben, Wentink (Intersil)

  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 Brockmann, Hoeben, Wentink (Intersil)

  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 Brockmann, Hoeben, Wentink (Intersil)

  9. DCF Performance Brockmann, Hoeben, Wentink (Intersil)

  10. Average Frame Tx Durations * *) RTS CTS OFDM features cheap collisions (cost of one RTS) and built-in hidden node protection Brockmann, Hoeben, Wentink (Intersil)

  11. Throughput Comparison for 24/22 Mbps Brockmann, Hoeben, Wentink (Intersil)

  12. Mixed b/g Brockmann, Hoeben, Wentink (Intersil)

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

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

  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 Brockmann, Hoeben, Wentink (Intersil)

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

  17. Migration from Legacy to 802.11g Brockmann, Hoeben, Wentink (Intersil)

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

  19. Pure OFDM UDP Performance Comparison Brockmann, Hoeben, Wentink (Intersil)

  20. Performance in relation with CWmin (1) CWmin = 15 CWmin = 31 Brockmann, Hoeben, Wentink (Intersil)

  21. Performance in relation with CWmin (3) CWmin = 15 CWmin = 31 Brockmann, Hoeben, Wentink (Intersil)

  22. Pure OFDMTCP Performance Comparison Brockmann, Hoeben, Wentink (Intersil)

  23. Throughput comparison for TCP Brockmann, Hoeben, Wentink (Intersil)

  24. 802.11e QoS Scenarios Brockmann, Hoeben, Wentink (Intersil)

  25. Migration with 802.11e HCF Bursting Aggregate throughput 4 g-nodes 3 g-nodes (CFBs) 1 b-node 2 g-nodes (CFBs) 2 b-nodes 4 b-nodes Individual throughputs Legacy throughput levels Throughput for g-nodes rises sharply Brockmann, Hoeben, Wentink (Intersil)

  26. Streaming video with 802.11e/g aggregate throughput 2x 12 Mbps video no starvation of background Brockmann, Hoeben, Wentink (Intersil)

  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) Brockmann, Hoeben, Wentink (Intersil)

  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 Brockmann, Hoeben, Wentink (Intersil)

  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. Brockmann, Hoeben, Wentink (Intersil)

  30. Recommendations In the form of Motions Brockmann, Hoeben, Wentink (Intersil)

  31. Need for a new element • 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. Brockmann, Hoeben, Wentink (Intersil)

  32. One Octet One Octet One Octet B0 B7 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. Brockmann, Hoeben, Wentink (Intersil)

  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. Brockmann, Hoeben, Wentink (Intersil)

  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: Brockmann, Hoeben, Wentink (Intersil)

  35. One Octet One Octet One Octet B0 B7 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. Figure xx: Legacy Indication Element Brockmann, Hoeben, Wentink (Intersil)

  36. Motion on RTS/CTS usage for OFDM • Instruct the editor to incorporate the text in the previous slide into the draft. Brockmann, Hoeben, Wentink (Intersil)

  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 STAshall transmit its Control Response frame (either CTS or ACK) at the same rate as the immediately previousframe in the frame exchange sequence (as defined in 9.7), if this rate belongs to the PHY mandatory rates, orelse 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 inthe BSS.” • “To allow the transmitting STA to calculate the contents of the Duration/ID field, the respondingSTA shall transmit its Control Response and Management Response frames (either CTS or ACK) at thehighest rate in the BSS basic rate set that is less than or equal to the rate of at the same rate as the immedi-atelyprevious frame in the frame exchange sequence (as defined in 9.7). Inaddition, the Control Response frame shall be sent using the same PHY options as the received frame. “ Brockmann, Hoeben, Wentink (Intersil)

  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 inthe 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 respondingSTA shall transmit its Control Response and Management Response frames (either CTS or ACK) at thehighest rate in the BSS basic rate set that is less than or equal to the rate of at the same rate as the immediatelyprevious frame in the frame exchange sequence (as defined in 9.7). Inaddition, 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. “ Brockmann, Hoeben, Wentink (Intersil)

  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 Brockmann, Hoeben, Wentink (Intersil)

  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. Brockmann, Hoeben, Wentink (Intersil)

  41. Motion on the signal extension for CCK-OFDM • Change sub clause 19.6.2.4.1to 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. Brockmann, Hoeben, Wentink (Intersil)

  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. Brockmann, Hoeben, Wentink (Intersil)

  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. Brockmann, Hoeben, Wentink (Intersil)

More Related