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IEEE802.11e Enhanced MAC for QoS and Efficiency

IEEE802.11e Enhanced MAC for QoS and Efficiency. What is QoS and why do we need it? Overview of 802.11e EDCA, TXOP, Traffic classes, burst ACKs Direct Link Protocol. QoS (Quality of Service). QoS parameters Delay/latency, available bandwidth, error correction, acknowledgement scheme

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IEEE802.11e Enhanced MAC for QoS and Efficiency

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  1. IEEE802.11e Enhanced MAC for QoS and Efficiency What is QoS and why do we need it? Overview of 802.11e EDCA, TXOP, Traffic classes, burst ACKs Direct Link Protocol WLAN EDCA

  2. QoS (Quality of Service) • QoS parameters • Delay/latency, available bandwidth, error correction, acknowledgement scheme • All 802 MAC schemes are Best Effort. But, • Voice traffic : rather loss than delay • Data traffic : no loss, less stringent delay • How do we provide QoS? • Categorize the traffic • Define a scheme to each category • Priority (e.g. CW, DIFS), specific transmission slots WLAN EDCA

  3. Features of IEEE802.11e • Fully backwards-compatible • Stations without 802.11e will be able to operate in an 802.11e environment. • Two means of QoS provisioning • Prioritizing traffic : CW, DIFS etc. • Allocating specific transmission times for traffic • Three optional means for increasing efficiency (throughput) of the network • Burst acknowledgement : many at a chance • Direct link protocol : between STAs without the AP • No acknowledgement : e.g.) temperature monitoring WLAN EDCA

  4. Hybrid CF (HCF) • Two features • Implemented at every station using 802.11e • In an Infrastructure BSS, a centralized scheduling function called Hybrid Coordinator (HC) that operates at the AP • Coexists with both DCF and PCF. WLAN EDCA

  5. Differentiated Traffic Classes e.g. 802.1D SAPs for 8 TSs SAPs for 8 TCs MAC PHY • Each packet is allocated either to • One of the Traffic Streams (TS) cf) per-flow • Or one of the Traffic Classes (TC) cf) per-class • Each station has, simultaneously in use, • Upto 8 Traffic Streams • And 8 Traffic Classes • Layers above the MAC specify through the MAC SAP of the TS or TC that each frame belongs to WLAN EDCA

  6. Default EDCA Parameter Sets for 802.11a and 802.11b WLAN EDCA

  7. Traffic Categories 802.1D(1993) WLAN EDCA

  8. User Priority and AC in IEEE 802.11e WLAN EDCA

  9. AIFS[j] Immediate access when Medium is free DIFS/AIFS[i] AIFS[i] DIFS Contention window DIFS/AIFS PIFS SIFS Backoff Window Busy medium Next frame Slot time Select slot and decrement backoff as long as medium is idle Defer access IEEE 802.11e -EDCA WLAN EDCA

  10. Traffic streams • Periodic traffic  TS  TXOP on HC • TXOP Transmission Opportunities • TS Specifications (Tspec) for negotiation even not guaranteed by HC • ACK policy (no ACK, ACK, Burst ACK) • Priority • Inter-arrival time of MSDUs • Min and mean data rate, maximum burst size • Delay and jitter (delay variation) bounds WLAN EDCA

  11. Enhanced Distributed Channel Access (EDCA) MAC SAP EDCF MAC AC 1 AC 2 AC 3 AC 4 • For packets not assigned to any TS • Different access categories (ACs) • AIFS Arbitration IFS (CWmin, CWmax) • Each Access Categories (ACs) runs the DCF protocol independently as a separate station. • Up to 4 ACs for a station WLAN EDCA

  12. IEEE 802.11e Access Category IEEE 802.11e station with four backoff entities Eight priorities 0-7 according to 802.1D are Mapped to four access actegories (Acs) 7 6 5 4 3 0 2 1 One priority Four access categories (Acs) representing four priority to four access actegories (Acs) Backoff entity High Priority Low Priority Backoff : DIFS 15 1023 Backoff : AIFS[AC_VO] CWmin[AC_VO] Cwmax[AC_VO] Backoff : AIFS[AC_VI] CWmin[AC_VI] Cwmax[AC_VI] Backoff : AIFS[AC_BE] CWmin[AC_BE] Cwmax[AC_BE] Backoff : AIFS[AC_BK] CWmin[AC_BK] Cwmax[AC_BK] Upon parallel access at the same slot, the higher-priority AC Backoff entity transmits; the other backoff entity entities act as if Collision occurred transmission transmission AIFS = 2,3 ….(for station AIFS= SIFS+aSlotTime x AIFSN WLAN EDCA

  13. Transmission Opportunities (1) • Acquired in two ways • QoS-Polled TXOP by the HC • Or, an AC can successfully contend on the medium. • A specified period time is allowed to a station or AC. • All frames within a TXOP are separated by SIFS. • Multiple MPDUs may be transmitted within a TXOP. • It may fragment MSDU or MMPDUs. WLAN EDCA

  14. Transmission Opportunities (2) • TXOP can start during either the CFP or CP, but must finish within that period. • Controlled Access Period (CAP) • For the HC, to satisfy TSPECs and deliver data it has been queued, • A CAP may be used by the HC to transmit data or to allocate TXOPs to other stations WLAN EDCA

  15. Block ACKs data SIFS Block ACK request Block ACK Originator Recipient • Acknowledge multiple MPDUs by a Block ACKs to reduce the overhead WLAN EDCA

  16. Direct Link Protocol AP STA STA STA • Within an Infrastructure BSS • Within transmitting range of the source • Not in power save mode • Before DLP handshake via the AP • Exchange capability (security) • Tear down via the AP Normal path DLP BSS WLAN EDCA

  17. Use-case : video conferencing and data traffic over 802.11g WLAN using DCF and EDCA Video conferencing stations AP Web browsing Wired Network (e.g. The Internet) File transfer IEEE 802.11 Network Source : Sony Shimakawa and Stanford Tobagi WLAN EDCA

  18. Issues • Good QoS : voice, video, lip sync (<133ms) • Video conferencing is a demanding application. • High bandwidth • User-perceived quality sensitive to loss and delay • Impact of delay • DCF vs EDCA with prioritized packets • Realistic simulation • Protocols and wireless channel (path loss, fading) • Realistic traffic and quality metrics WLAN EDCA

  19. V/C Quality Requirements • User-perceived quality requirements • Video : 384kbps • Image quality : PSNR>20dB • Frame rate : > 5fps (encoded at 15fps) • Voice : 64kbps • Mean opinion score (MOS)>3.6 • Playout deadline of 150ms • Voice/video synchronization • Video may lag voice by < 133ms • TCP condition RTT 1~60ms, RWin=16~64kBytes WLAN EDCA

  20. Capacity of V/C w.r.t. Cell Size • The larger cell, the poorer channel • Limiting factor is voice delay. Video conf. capacity 20 54Mbps 10 12Mbps 6Mbps 4 15m 30m 20m 25m WLAN EDCA

  21. EDCA MAC protocol • EDCA : Prioritized MAC protocol • Each device includes 4 Channel Access Functions (CAF). Voice Video Web FTP All traffic CAF-VO CAF-VO CAF-VO CAF-VO DCF DCF MAC EDCA MAC WLAN EDCA

  22. EDCA vs. DCF • Low priority • CWmin=15, AIFS>DIFS • High priority • CWmin=3, AIFS=DIFS  more collisions, less overhead • Contention-free bursts • reduces overhead • may increase delay for low priority traffic WLAN EDCA

  23. EDCA vs DCF with FTP Traffic • DCF allows higher capacity when 0 or 1 FTP users • EDCA improves FTP Performance • Extra delay due to CAF is not an issue. Video conf. capacity Average FTP bitrate [Mbps] 15 7 EDCA EDCA DCF 10 4 2 FTPs DCF 4 FTPs 5 2 Small cell (r=10m) 24Mbps data rate 2 8 4 6 2 8 4 6 FTP users Video conf. users WLAN EDCA

  24. Conclusions • In good channel conditions, up to 19 simultaneous video conferencing sessions can be supported by a single AP. • TCP-based traffic (FTP, Web) reduces V/C capacity when DCF is used. • EDCA effectively priorities V/C over TCP while improving TCP application performance. • EDCA supports fewer V/C calls when few or no TCP applications present, due to high collision rate WLAN EDCA

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