EDCA Enhancement to Improve Link Reliability for Multicast Streams November 2007 1/16/2019May 2008 doc.: IEEE 802.11-07/2752r1 EDCA Enhancement to Improve Link Reliability for Multicast Streams Date: 2008-7-11 Authors: Hang Liu, et. al, Thomson Inc. Peter Ecclesine, Cisco Systems

November 2007 1/16/2019May 2008 doc.: IEEE 802.11-07/2752r1 Abstract This contribution proposes EDCA enhancements to improve link reliability for multicast/broadcast stream transmissions Hang Liu, et. al, Thomson Inc. Peter Ecclesine, Cisco Systems

Outlines Problems for multicast in 802.11 November 2007 1/16/2019May 2008 doc.: IEEE 802.11-07/2752r1 Outlines Problems for multicast in 802.11 RTS/CTS enhancement for multicast protection Bitmap-based multicast/broadcast RTS/CTS Contention-based multicast/broadcast RTS/CTS A mechanism for sending multicast/broadcast CTS Hang Liu, et. al, Thomson Inc. Peter Ecclesine, Cisco Systems

Problems for Multicast/Broadcast in 802.11 1/16/2019May 2008 Problems for Multicast/Broadcast in 802.11 Collision in the BSS (intra-BSS Collision) The associated STAs transmit when an AP is transmitting multicast/broadcast frames Overlapping BSS (OBSS) Collision (Inter-BSS Collision) The STAs/APs in OBSSes transmit when an AP is transmitting multicast/broadcast frames Hidden node problem No ACK Collision S2 S1 S1 Collision S7 AP1 AP2 S5 S4 S3 S6 S3 S2 AP2 is a hidden node to AP1 Hang Liu, et. al, Thomson Inc.

RTS/CTS only for Unicast RTS/CTS can alleviate the hidden node and OBSS problem by contending and reserve a TXOP before data frame transmissions in EDCA However RTS/CTS is currently only for unicast in 802.11 For multicast, there is no RTS/CTS protection from hidden node and OBSS TXOP AIFS Busy Backoff RTS Data Data Data Block- AckReq Source SIFS SIFS CTS Block- Ack Destination Contention Window AIFS Other NAV (RTS) NAV (CTS) Hang Liu, et. al, Thomson Inc.

Multicast/Broadcast RTS/CTS A multicast/broadcast transmitter/AP may send multicast/broadcast RTS (MB-RTS) to contend for a MB TXOP. MB STAs may reply with MB-CTS: clear to send MB-NCTS: Not clear to send (optional) Two approaches for MB-RTS/CTS/NCTS exchanges In MB-RTS, contains a bitmap, each bit corresponding to a MB receiver The MB receivers send MB-CTS in the order that they are listed in the MB-RTS bitmap field. In MB-RTS, specify # of MB-CTS slots and # of MB-NCTS slots A MB receiver sends MB-CTS in a MB-CTS slot with a probability A MB receiver sends MB-NCTS in a MB-NCTS slot with a probability Special cases: # of MB-CTS or MB-NCTS slots = 0, 1 Hang Liu, et. al, Thomson Inc.

Bitmap-Based MB-RTS/CTS Frame Control Duration RA TA Bitmap control Partial virtual bitmap FCS MB-RTS MB-CTS/NCTS Frame Control Duration RA FCS Each bit in the bitmap identifies a STA by its association ID MB-TXOP CSMA Data Data Block AckReq AP MB- RTS Data STA 1 MB- CTS1 STA 2 MB- CTS2 BAck MB- CTS3 MB receivers send MB-CTS in the order listed in MB-RTS bitmap STA 3 MB-CTS Transmission Period STA 4 MB- CTS3 AP assumes obtaining MB TXOP and sending pending MB data frames only if Received all MB-CTS from the STAs in the bitmap (or use other local policy) If no AP Tx after MB-CTS transmission period (AP decides not using this TXOP) STAs may reset their NAV STAs that sent MB-CTS may send out cancel-MB-CTS (optional) Hang Liu, et. al, Thomson Inc.

Contention Slot-Based MB-RTS/CTS Frame Control Duration RA TA # of MB-CTS MB-NCTS FCS MB-RTS MB-CTS/NCTS Frame Control Duration RA FCS MB-TXOP CSMA AP MB- RTS Data Data Data MB receiver 1 MB- CTS1 MB receiver 2 MB- CTS2 selects a slot in CTS Contention Period (CCP) to transmit a MB-CTS MB- CTS3 MB receiver 3 CTS Contention Period NCTS Contention Period MB receiver 4 selects a slot in NCTS Contention Period (NCCP) to transmit a MB-NCTS MB receiver 5 AP assumes obtaining MB TXOP and sending MB data frames only if No MB-NCTS At least one MB-CTS Hang Liu, et. al, Thomson Inc.

An Advanced MB-CTS Transmission Slot Selection Algorithm CCP/NCCP is divided into an integral number of slots as specified in the MB-RTS Each slot is split into 2 phases – a Signaling Phase and an Acknowledgement Phase Signaling phase is used by STA(s) to transmit the MB-CTS/NCTS Acknowledgement phase is used by the AP (which originally transmitted the RTS) to Ack the MB-CTS/NCTS CCP: MB-CTS Contention Period NCCP: MB-NCTS Contention Period Hang Liu, et. al, Thomson Inc.

An Advanced MB-CTS Tx Slot Selection Algorithm (Cont.) Each STA maintains a Overlap Degree (OD) which is the number of unique beacons it receives from different APs. Associated with each OD is a CTS/NCTS transmission probability PT(OD) Each STA that receives the RTS and desires to send a CTS contends for a slot in the CCP All STAs that have picked a given slot #, defer from transmitting a CTS in the signaling phase of that slot with probability (1 - PT(OD)) Hang Liu, et. al, Thomson Inc.

An Advance MB-CTS Tx Slot Selection Algorithm (cont.) In case of a collision free CTS transmission in the signaling phase, the AP reciprocates with a CTS Ack in the acknowledgement phase The STA which transmitted the CTS, along with all STAs with OD = 1, which “hear” the CTS Ack, stop competing for the remaining slots All other STAs in the BSS, compete to transmit in the following slot, till all possible slots in CCP have been exhausted Tx probabilities are assigned in such a way that a STA with a high overlap degree is much more likely to transmit than one with a low overlap degree thus ensuring that the transmitted CTS is “heard” by as many neighboring APs as possible In case of a collision with the CTS transmission, all STAs within the BSS remain in contention for the following slot, and the process continues till all possible slots in the CCP have been exhausted The same algorithm can be envisaged for the NCTS transmission in the NCCP Hang Liu, et. al, Thomson Inc.

References [1] IEEE 802.11-07/1972r13, “Draft PAR and 5 Criteria for Video Transport Stream (VTS) SG” [2] IEEE 802.11-08/303r2 “VTS Robust Broadcast/Multicast” [3] IEEE 802.11-08/0249r0 “Achieving Video Streaming Capability Using Synchronous Collision Resolution” [4] IEEE 802.11-08/0263r1, “Synchronous Collision Resolution Follow-up Questions & Answers” [3] IEEE P802.11n Draft Standard D3.00 Hang Liu, et. al, Thomson Inc.

Thank You! Questions? Hang Liu, et. al, Thomson Inc.