IEEE 802 11 e Enhanced MAC for Qo

IEEE 802. 11 e Enhanced MAC for Qo. S and Efficiency u What is Qo. S and why do we need it? u Overview of 802. 11 e ØEDCA, TXOP, Traffic classes, burst ACKs ØDirect Link Protocol WLAN EDCA

Qo. S (Quality of Service) Qo. S 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 Qo. S? 1. Categorize the traffic 2. Define a scheme to each category Priority (e. g. CW, DIFS), specific transmission slots WLAN EDCA

Features of IEEE 802. 11 e Fully backwards-compatible Stations without 802. 11 e will be able to operate in an 802. 11 e environment. Two means of Qo. S 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 802. 11 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) Or one of the Traffic Classes (TC) cf) per-flow 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 e. g. 802. 1 D SAPs for 8 TSs SAPs for 8 TCs MAC PHY WLAN EDCA

Default EDCA Parameter Sets for 802. 11 a and 802. 11 b AC 802. 11 a 802. 11 b (a. CWmin=15, a. CWmax=1023) Non-AP Station AP TXOP Limit CW min CW max AIF SN BK 15 1023 BE 15 VI VO CW min CW max AIF SN 7 15 1023 7 1023 3 15 63 7 15 2 7 3 7 2 3 Limit CW min CW max AIF SN 0 31 1023 3 0 31 15 1 3. 008 ms 7 1 1. 504 ms min CW max AIF SN 7 31 1023 7 0 1023 3 31 127 3 0 15 31 2 15 31 1 6. 016 ms 3 15 2 7 15 1 3. 264 ms WLAN EDCA CW

Traffic Categories TC 1 2 0 (default) Acronym Types BK Background - Spare BE Best Effort Prior to 1 & 2 3 EE 4 CL Controlled load 6 7 Traffic types BE 1 Excellent effort 5 # of que ues 802. 1 D(1993) 2 BE 3 VO BE CL 4 BK BE 5 BK BE VI Video 6 VO Voice 7 NC Network control 8 WLAN EDCA VO CL VI VO BK BE EE CL VI VO BE BE EE CL VI VO BK - N C

User Priority and AC in IEEE 802. 11 e Priority User Priority (UP) Access Category (AC) Designation (informative) Lowest 1 AC_BK Background . 2 AC_BK Background 0 AC_BE Best Effort 3 AC_VI Video 4 AC_VI Video 5 AC_VI Video 6 AC_V 0 Voice 7 AC_V 0 Voice . . Highest WLAN EDCA

IEEE 802. 11 e -EDCA AIFS[j] Immediate access when AIFS[i] Medium is free DIFS/AIFS[i] DIFS/AIFS DIFS Contention window PIFS SIFS Backoff Window Busy medium Next frame Slot time Select slot and decrement backoff Defer access as long as medium is idle AC_VO[0] AC_VI[1] AIFS 2 2 3 7 CWmin 3 7 15 15 CWmax 7 15 1023 WLAN EDCA AC_BE[2] AC_BK[3]

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, 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 MAC SAP EDCF MAC AC 1 AC 2 WLAN EDCA AC 3 AC 4

IEEE 802. 11 e Access Category IEEE 802. 11 e station with four backoff entities Eight priorities 0 -7 according to 802. 1 D are Mapped to four access actegories (Acs) 7 6 5 4 3 0 2 1 One Four access categories (Acs) representing four priority to four access actegories (Acs) High Priority Low Priority priority Backoff entity 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 AIFS = 2, 3 …. (for station AIFS= SIFS+a. Slot. Time x AIFSN WLAN EDCA transmission

Transmission Opportunities (1) Acquired in two ways Qo. S-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 data SIFS Block AC K reques t CK Block A Originator Recipient WLAN EDCA

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 Normal path AP DLP STA BSS STA WLAN EDCA

Use-case : video conferencing and data traffic over 802. 11 g WLAN using DCF and EDCA Video conferencing stations AP Wired Network (e. g. The Internet) Web browsing File transfer IEEE 802. 11 Network Source : Sony Shimakawa and Stanford Tobagi WLAN EDCA

Issues Good Qo. S : voice, video, lip sync (<133 ms) 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 : 384 kbps Image quality : PSNR>20 d. B Frame rate : > 5 fps (encoded at 15 fps) Voice : 64 kbps Mean opinion score (MOS)>3. 6 Playout deadline of 150 ms Voice/video synchronization Video may lag voice by < 133 ms TCP condition RTT 1~60 ms, RWin=16~64 k. Bytes WLAN EDCA

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

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

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

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