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Efficiency of VoIP on 802.11n: Capacity Analysis

Analyzing the efficiency of VoIP services on 802.11n networks, considering packet characteristics and delivery requirements. Includes capacity calculations, configurations, and enhancements like A-MPDU aggregation and MIMO technology.

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Efficiency of VoIP on 802.11n: Capacity Analysis

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  1. Efficiency of VoIP on 802.11n Authors: Date: 2007-11-12 Marvin Krym, Nortel Networks

  2. Abstract The 802.11n capacity features generally provide high efficiency for data services whose traffic is characterized by “long” packets and relatively time insensitive packet delivery (i.e. elastic). The efficiency gain drops considerably for services whose traffic is characterized by “small” packets and time sensitive packet delivery (i.e. inelastic). Such services include VoIP, location and RFID tracking. A hospital is an example environment which could have a high penetration of these services. This presentation characterizes the efficiency of VoIP on 802.11n. Marvin Krym, Nortel Networks

  3. Agenda • Reference Information • VoIP Reference Frame • Common Configuration Parameters • 802.11a Reference Capacity with VoIP • 802.11n Capacity with VoIP • 40MHz Channel Bandwidth • A-MPDU Aggregation • Reverse Direction TXOP Sharing • MIMO/Spatial Diversity with 4 streams • Capacity with Feature Mixes • Summary AP STA STA STA STA STA STA STA Analysis Configuration - 100% VoIP Marvin Krym, Nortel Networks

  4. Reference VoIP Frame MAC Hdr RTP Voice Samples IP Hdr UDP 40 20 8 12 160 -G711 Codec: -8000 samples/sec x 1 Byte/sample x 20 ms pktzation Octets • Voice Frame is relatively small: 240 Bytes (incl. Headers) • Packet timing is critical: 1 packet every packetization period. Delayed packets are discarded. • Capacity calculations in this presentation are based on “Theoretical Throughput Limits”, doc.: IEEE 802.11-06/0928r2 Marvin Krym, Nortel Networks

  5. Common Parameters • 802.11n PLCP Frame Header = 28 usec • Greenfield Mode: L-STF, HT-LTF1, HT-SIG, HT-LTF • Excludes multiple extension HT-LTF in MIMO scenario • 802.11a PLCP Frame Header = 20 usec • Mode: STF, LTF, SIG • Modulation: 16-QAM • Guard Interval: 800 nsec • Coding Rate: ¾ • RTS/CTS excluded • Slot Time: 9 usec • Min Contention Window: 15 slots • Avg Min Contention Window = 7.5 slots (67.5 usec) • SIFS, RIFS, DIFS: per IEEE P802.11n™/D3.00 • G711 codecs with 20 msec packetization • 80% air utilization Marvin Krym, Nortel Networks

  6. 802.11a VoIP Reference Capacity Contention Window PLCP Header Voice Frame Ack DIFS SIFS Time 20 us 34 us 67 us (min average) 16 us 24 us 1. Total bits/symbol = No of Carriers x bits per symbol x FEC rate 48 x 4 x 3/4 = 144 bits 2. Voice Frame = Symbol Time x Roundup [ bits/Byte x DataBytes / Total bit/symbol ] 4 x Roundup [ 8 x 240 / 144 ] = 56 usec 3. Air Capacity = Payload Size (bits) / Frame Start Interval (8 x 240) / (34+67+20+56+16+24)us = 9 Mbps; PHY rate = 36 Mbps 4. No of VoIP sessions = (Air Capacity x Air Utilization) / 2-WayCallThroughput 9M x 0.8 / 2(8 x 240 / 20ms) = 37 Marvin Krym, Nortel Networks

  7. 802.11n Capacity with 40MHz Channel BW(40MHz; Nss=1; MCS index=4) Frame size Reduction rel to .11a Contention Window PLCP Header Voice Frame SIFS DIFS Ack 34 us 67 us (min average) 28 us 16 32 us Time 1. Total bits/symbol = No of Carriers x bits per symbol x FEC rate 108 x 4 x 3/4 = 324 bits 2. Voice Frame = Symbol Time x Roundup [ bits/Byte x DataBytes / Total bit/symbol ] 4 x Roundup [ 8 x 240 / 324 ] = 24 usec 3. Air Capacity = Payload Size (bits) / Frame Start Interval (8 x 240) / (34+67+28+24+16+32)us = 9.5 Mbps; PHY rate = 81 Mbps 4. No of VoIP sessions = (Air Capacity x Air Utilization) / 2-WayCallThroughput 9.5M x 0.8 / 2(8 x 240 / 20ms) = 40 • - Increasing BW by 2x increases call capacity by 1.1 x 802.11a • Only reduces voice frame transmission time which is small to begin with • Does not reduce large transmission overhead Marvin Krym, Nortel Networks

  8. 802.11n Capacity with A-MPDU Aggregation(20MHz; Nss=1; MCS index=4) Contention Window Voice Frame PLCP Header Voice Frame Ack DIFS SIFS 34 us 67 us (min average) 28 us 16 32 us Time 1. Total bits/symbol = No of Carriers x bits per symbol x FEC rate 52 x 4 x 3/4 = 156 bits 2. Voice Frame = Symbol Time x Roundup [ bits/Byte x DataBytes / Total bit/symbol ] 4 x Roundup [ 8 x 240 / 156 ] = 52 usec 3. Air Capacity = Payload Size (bits) / Frame Start Interval 2(8 x 240) / (34+67+28+52+52+16+32)us = 13.6 Mbps; PHY rate = 39Mbps 4. No of VoIP sessions = (Air Capacity x Air Utilization) / 2-WayCallThroughput 13.6M x 0.8 / 2(8 x 240 / 20ms) = 57 • Aggregating two voice frames increases call capacity by 1.5 x 802.11a • Provides good improvement in capacity but it increases packet delay limiting its use • An aggregation of two voice frames will result in a delay of 40ms. Marvin Krym, Nortel Networks

  9. 802.11n Capacity with Reverse Direction TXOP Sharing (20MHz; Nss=1; MCS index=4) Contention Window PLCP Header PLCP Header Voice Frame Voice Frame Ack Ack DIFS SIFS SIFS 34 us 67 us (min average) 28 us 16 32 us 28 us 16 32 us Time 1. Total bits/symbol = No of Carriers x bits per symbol x FEC rate 52 x 4 x 3/4 = 156 bits 2. Voice Frame = Symbol Time x Roundup [ bits/Byte x DataBytes / Total bit/symbol ] 4 x Roundup [ 8 x 240 / 156 ] = 52 usec 3. Air Capacity = Payload Size (bits) / Frame Start Interval 2(8 x 240) / (34+67+2(28+52+16+32))us = 10.7 Mbps PHY rate = 39Mbps 4. No of VoIP sessions = (Air Capacity x Air Utilization) / 2-WayCallThroughput 10.7M x 0.8 / 2(8 x 240 / 20ms) = 45 • - Reverse Direction TXOP increases call capacity by 1.2 x 802.11a • Saves one contention window time & DIFS which provides modest capacity gain • Limited to one packet in each direction Marvin Krym, Nortel Networks

  10. 802.11n Capacity with 4 Stream MIMO(20MHz; Nss=4; MCS index=28) Assume voice frame divided across 4 streams (i.e. spatial multiplexing only) Frame size Reduction rel to 802.11a Contention Window Voice Frame PLCP Header Ack DIFS SIFS 34 us 67 us (min average) 28 us 16 32 us Time Frame Sequence – one per MIMO stream 1. Total bits/symbol = No of Carriers x bits per symbol x FEC rate 4 streams x (52 x 4 x 3/4) = 624 bits 2. Voice Frame = Symbol Time x Roundup [ bits/Byte x DataBytes / Total bit/symbol ] 4 x Roundup [ 8 x 240 / 624 ] = 16 usec 3. Air Capacity = Payload Size (bits) / Frame Start Interval (8 x 240) / (34+67+28+16+16+32)us ] = 9.9 Mbps; PHY rate = 156 Mbps 4. No of VoIP sessions = (Air Capacity x Air Utilization) / 2-WayCallThroughput 9.9M x 0.8 / 2 x (8 x 240 / 20ms) = 41 • - 4 stream MIMO increases call capacity by 1.1 x 802.11a • Reduces voice frame transmission time which is small to begin with • Does not reduce large transmission overhead Marvin Krym, Nortel Networks

  11. Capacity with Feature Combinations* * Notes: - See backup charts for calculations - RD TXOP Sharing includes one voice packet in each direction Marvin Krym, Nortel Networks

  12. Summary • Applications whose traffic profiles are characterized by “short” packets (relative to the frame size), with sensitive delivery times do not benefit adequately from the 802.11n capacity enhancement features • The contributing factors are: • 40MHz Channel: Only reduces voice frame transmission time which is small; Does not impact large transmission overhead • Forward Aggregation: Provides good improvement in capacity but it increases packet delay limiting its use • RD TXOP Sharing: - Saves one Contention Window Time to provide modest capacity gain; Limited to one packet in each direction • MIMO – Spatial Diversity: Only reduces voice frame transmission time which is small; Does not reduce large transmission overhead Marvin Krym, Nortel Networks

  13. References • Green, Balmy, Emmelmann; Theoretical Throughput Limits, doc.: IEEE 802.11-06/0928r2 • Yuxia Lin & Vincent W.S. Wong; Frame Aggregation and Optimal Frame Size Adaptation for IEEE 802.11n WLANs; GLOBECOM '06, November 2006 • Wireless LAN Medium Access Control (MAC) and Physical Layer (PHY) specifications; IEEE P802.11n™/D3.00 Marvin Krym, Nortel Networks

  14. Backup Charts Marvin Krym, Nortel Networks

  15. Mixed Features: VoIP on PC 2BW+RD+2Agg+2MIMO Contention Window PLCP Header PLCP Header Voice Frames Voice Frames Ack Ack DIFS SIFS SIFS 34 us 67 us (min average) 28 us 16 32 us 28 us 16 32 us Time 1. Total bits/symbol = No of Carriers x bits per symbol x FEC rate 2 x (108 x 4 x 3/4) = 648 bits 2. Voice Frame = Symbol Time x Roundup [ bits/Byte x DataBytes / Total bit/symbol ] 4 x Roundup [ 8 x 240 / 648 ] = 12 usec 3. Air Capacity = Payload Size (bits) / Frame Start Interval 2x2x(8 x 240) / (34+67+2x(28+12+12+16+32))us = 25.5 Mbps; PHY rate = 162Mbps 4. No of VoIP sessions = (Air Capacity x Air Utilization) / 2-WayCallThroughput 25.5M x 0.8 / 2(8 x 240 / 20ms) = 106 Relative Improvement = 106 / 37 = 2.9 Marvin Krym, Nortel Networks

  16. Mixed Features: Highend Handset 2BW+RD+2Agg Contention Window PLCP Header PLCP Header Voice Frames Voice Frames Ack Ack DIFS SIFS SIFS 34 us 67 us (min average) 28 us 16 32 us 28 us 16 32 us Time 1. Total bits/symbol = No of Carriers x bits per symbol x FEC rate (108 x 4 x 3/4) = 324 bits 2. Voice Frame = Symbol Time x Roundup [ bits/Byte x DataBytes / Total bit/symbol ] 4 x Roundup [ 8 x 240 / 324 ] = 24 usec 3. Air Capacity = Payload Size (bits) / Frame Start Interval 2x2x(8 x 240) / (34+67+2x(28+24+24+16+32))us = 22 Mbps; PHY rate = 81Mbps 4. No of VoIP sessions = (Air Capacity x Air Utilization) / 2-WayCallThroughput 22M x 0.8 / 2(8 x 240 / 20ms) = 92 Relative Improvement = 92 / 37 = 2.5 Marvin Krym, Nortel Networks

  17. Mixed Features: Basic Handset 2BW+RD Contention Window PLCP Header PLCP Header Voice Frames Voice Frames Ack Ack DIFS SIFS SIFS 34 us 67 us (min average) 28 us 16 32 us 28 us 16 32 us Time 1. Total bits/symbol = No of Carriers x bits per symbol x FEC rate (108 x 4 x 3/4) = 324 bits 2. Voice Frame = Symbol Time x Roundup [ bits/Byte x DataBytes / Total bit/symbol ] 4 x Roundup [ 8 x 240 / 324 ] = 24 usec 3. Capacity = Payload Size (bits) / Frame Start Interval 2x(8 x 240) / (34+67+2x(28+24+16+32))us = 12.7 Mbps; PHY rate = 81Mbps 4. No of VoIP sessions = (Air Capacity x Air Utilization) / 2-WayCallThroughput 12.7M x 0.8 / 2(8 x 240 / 20ms) = 53 Relative Improvement = 53 / 37 = 1.4 Marvin Krym, Nortel Networks

  18. Data Capacity: 500B Packets 802.11a: 1. Total bits/symbol = No of Carriers x bits per symbol x FEC rate (48 x 4 x 3/4) = 144 bits 2. Data Frame = Symbol Time x Roundup [ bits/Byte x DataBytes / Total bit/symbol ] 4 x Roundup [ 8 x 500 / 144 ] = 112 usec 3. Air Capacity = Payload Size (bits) / Frame Start Interval (8 x 500) / (34+67+20+112+16+24)us = 14.6 Mbps; PHY rate = 36Mbps 802.11n: 2BW+RD+4Agg+4MIMO 1. Total bits/symbol = No of Carriers x bits per symbol x FEC rate 4 x (108 x 4 x 3/4) = 1296 bits 2. Data Frame = Symbol Time x Roundup [ bits/Byte x DataBytes / Total bit/symbol ] 4 x Roundup [ 8 x 500 / 1296 ] = 16 usec 3. Air Capacity = Payload Size (bits) / Frame Start Interval 2x4x(8 x 500) / (34+67+2x(28+4x16+16+32))us = 84 Mbps; PHY rate = 36Mbps Relative Improvement = 84/14.6 = 5.7 Marvin Krym, Nortel Networks

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