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

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

Features of IEEE802.11e Fully backwards-compatible Stations without e will be able to operate in an e 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

Hybrid CF (HCF) Two features Implemented at every station using e 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

Differentiated Traffic Classes 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 PHY e.g D MAC SAPs for 8 TSs SAPs for 8 TCs

Default EDCA Parameter Sets for a and b AC a (aCWmin=15, aCWmax=1023) b (aCWmin=15, aCWmax=1023) Non-AP StationAP TXOP Limit Non-AP StationAP TXOP Limit CW min CW max AIF SN CW min CW max AIF SN CW min CW max AIF SN CW min CW max AIF SN BK BE VI ms ms VO ms ms WLAN EDCA

Traffic Categories  802.1D(1993) TCA cronym Types 1BK Background 2- Spare 0 (default) BE Best Effort Prior to 1 & 2 3EE Excellent effort 4CL Controlled load 5VI Video 6VO Voice 7NC Network control # of que ues Traffic types 1 BE 2 VO 3 BECLVO 4 BKBECLVO 5 BKBECLVIVO 6 BK BEBEE CLVIVO 7 BK BEBEE CLVI VOVO BEBE 8 BKBK -BEBEE CLVIVOVO NCNC WLAN EDCA

User Priority and AC in IEEE e Priority User Priority (UP) Access Category (AC) Designation (informative) Lowest.. Highest 1AC_BKBackground 2AC_BKBackground 0AC_BEBest Effort 3AC_VIVideo 4AC_VIVideo 5AC_VIVideo 6AC_V0Voice 7AC_V0Voice WLAN EDCA

IEEE e -EDCA AC_VO[0]AC_VI[1]AC_BE[2]AC_BK[3] AIFS2237 CWmin3715 CWmax WLAN EDCA AIFS[j] AIFS[i] DIFS PIFS SIFS Backoff Window Busy medium Contention window Next frame DIFS/AIFS Slot time Defer access Select slot and decrement backoff as long as medium is idle Immediate access when Medium is free DIFS/AIFS[i]

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

Enhanced Distributed Channel Access (EDCA) 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 MAC SAP AC 1AC 2AC 3AC 4 EDCF MAC

IEEE e Access Category WLAN EDCA IEEE e station with four backoff entities Eight priorities 0-7 according to 802.1D are Mapped to four access actegories (Acs) Four access categories (Acs) representing four priority to four access actegories (Acs) High Priority Low Priority 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 Backoff : DIFS One priority Backoff entity transmission AIFS = 2,3 ….(for station AIFS= SIFS+aSlotTime x AIFSN

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

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

Block ACKs Acknowledge multiple MPDUs by a Block ACKs to reduce the overhead WLAN EDCA OriginatorRecipient data Block ACK request Block ACK SIFS

Direct Link Protocol 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 WLAN EDCA AP STA BSS DLP Normal path

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

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

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

Capacity of V/C w.r.t. Cell Size The larger cell, the poorer channel Limiting factor is voice delay. WLAN EDCA m 20m25m 30m Video conf. capacity 54Mbps 12Mbps 6Mbps

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

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

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. WLAN EDCA Video conf. capacity EDCA DCF EDCA DCF FTP usersVideo conf. users Average FTP bitrate [Mbps] 2 FTPs 4 FTPs Small cell (r=10m) 24Mbps data rate

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