Philippe Klein. avb-phkl qos-overview

Transcription

1 QoS Overview Philippe Klein IEEE Plenary Meeting Nov 08 Dallas, TX avb-phkl qos-overview

2 Disclaimer This presentation is not a highly detailed technical presentation but a crash course aiming to provide - prior to the joint meeting with the WG- a basic understanding of the QoS mechanisms currently defined in the IEEE specifications IEEE Plenary Meeting, Nov 08 Dallas TX 2

3 Abbreviations & Acronyms ADDTS AIFS AIFSN AP APSD BSS CSMA/CA CW DELTS DIFS DLS EDCA EDCAF EIFS HCCA HCF MAC MPDU NAV PIFS PS QoS S-APSD SIFS SP STA TXOP U-APSD WMM add traffic stream arbitration interframe space arbitration interframe space number access point automatic power-save delivery basic service set carrier sense multiple access with collision avoidance contention window delete traffic stream distributed (coordination function) interframe space direct-link setup enhanced distributed channel access enhanced distributed channel access function extended interframe space HCF controlled channel access hybrid coordination function medium access control MAC protocol data unit network allocation vector point (coordination function) interframe space power save (mode) quality of service scheduled automatic power-save delivery short interframe space service period station transmission opportunity unscheduled automatic power-save delivery WiFi Multimedia IEEE Plenary Meeting, Nov 08 Dallas TX 3

4 MAC Legacy Access Protocol IEEE Plenary Meeting, Nov 08 Dallas TX 4

5 MAC Access Protocol 1. Distributed Coordinated Function (DCF) Contention access Based on Carrier Sense Media Access /Collision Avoidance (PHY Carrier Sense & Network Allocation Vector) 2. Point Coordinated Function (PCF) Contention Free access: Access Point (AP) coordinates access for AP and STA transmissions Optional!!! IEEE Plenary Meeting, Nov 08 Dallas TX 5

6 DCF Contention Mechanism DIFS Backoff Time PIFS Busy Medium SIFS Contention Window Next Frame Slot time Deferred Access Decrement Backoff as long as medium is idle IEEE Plenary Meeting, Nov 08 Dallas TX 6

7 Interframe Spacing in SIFS Control Frame or next frame fragment may be sent here PIFS DIFS Beacon or New PCF Frame may be sent here New DCF Frame may be sent here ACK EIFS Slot time Slot time Frame Error Recovery done here (allow other frame exchanges to complete correctly before another transmission is allowed) time b a g SIFS 10 µs 16 µs 10 µs Slot time 20 µs 9 µs 9 µs PIFS (= SIFS + 1 x Slot time) 30 µs 28 µs 19 µs DIFS (= SIFS + 2 x Slot time) 50 µs 34 µs 28 µs IEEE Plenary Meeting, Nov 08 Dallas TX 7

8 Random Backoff Time Backoff Time = Random() * SlotTime Random() = Pseudo-random [0,CW] CWmin =< CW <= Cwmax CW: Incremented by 2n-1 on each retransmission Reset after successful Txm IEEE Plenary Meeting, Nov 08 Dallas TX 8

9 Distributed Coordinated Function DIFS DIFS DIFS DIFS Station A FRAME Station B Defer FRAME Station C Defer FRAME Station D Defer FRAME Station E Defer FRAME IEEE Plenary Meeting, Nov 08 Dallas TX 9

10 PCF: Point Coordinator Function Poll and response protocol to eliminate the possibility of contention for the medium. A Point Coordinator (PC) - always located in an Access Point - controls the PCF To operate in PCF, stations register to the PC PC regularly polls the registered stations for traffic while delivering traffic to theses stations. IEEE Plenary Meeting, Nov 08 Dallas TX 10

11 PCF Operation IEEE Plenary Meeting, Nov 08 Dallas TX 11

12 Co-existence of PCF and DCF CFP Repetition Interval CFP Repetition Interval Contention Free Period (CFP) Contention Period (CP) Contention Free Period (CFP) Contention Period (CP) B PCF DCF NAV B PCF DCF NAV Time Beacon Beacon CFP/C ratio is programmable in the AP Beacon marks the beginning of the Contention Free Period (CFP) CF-End ends the CFP NAV prevent outstanding DCF traffic to interfere with CFP IEEE Plenary Meeting, Nov 08 Dallas TX 12

13 Quality of Service Limitations Both PHY s DCF and PCF methods do not differentiate between traffic types or sources and do not provide QoS PCF provides only iso-synchronous synchronous services worse latency access only is deterministic access latency is function of the number of registered stations, the CF traffic and the CFP/CP ratio IEEE Plenary Meeting, Nov 08 Dallas TX 13

14 Medium Access Control Quality of Service Enhancements (previously e amendment, now part of ) 2007) IEEE Plenary Meeting, Nov 08 Dallas TX 14

15 Major Enhancements in e Basic Elements for QoS Traffic Differentiation Concept of Transmission Opportunity (TXOP) New Contention Channel Access Enhanced Distributed Channel Access (EDCA) New Contention Free Channel Access HCF Controlled Channel Access (HCCA) IEEE Plenary Meeting, Nov 08 Dallas TX 15

16 Major Enhancements in e Automatic Power-Save Delivery (APSD) New mechanisms for higher h throughputh t Block Acknowledgement (Block Ack) Direct Link Setup (DLS) IEEE Plenary Meeting, Nov 08 Dallas TX 16

17 Enhanced Distributed Channel Access (EDCA) Same Contention Based that EDC but 1. 8 priority i levels l per station (identical i to IEEE 802.1D priority tags) mapped to 4 access categories (AC) 2. Different Arbitration InterFrame Space (AIFS) per access categories 3. TxOP Burst Limits IEEE Plenary Meeting, Nov 08 Dallas TX 17

18 EDCA Frame Priorities IEEE Plenary Meeting, Nov 08 Dallas TX 18

19 EDCA Implementation Model 802.1D tagged MSDU Mapping to Access Category Voice Data Video Data Best Efforts Background Txm AC_VO AC_VI AC_BE AC_BK queues Resolve internal collision AIFSN, CWmin, CWmax AIFSN, CWmin, CWmax AIFSN, CWmin, CWmax AIFSN, CWmin, CWmax Txm attempt IEEE Plenary Meeting, Nov 08 Dallas TX 19

20 Traffic Prioritization & Collision Resolution Depends on 2 timing parameters a that vary for each AC: 1. AIFSN (Arbitrary Inter-Frame Space Number) 2. CW (Contention Window) (and TxOP Limit for AC_VI and AC_VO) IEEE Plenary Meeting, Nov 08 Dallas TX 20

21 Arbitration InterFrame Space AIFS (AC i-1) AIFS (AC i) CW DIFS PIFS CW Busy Medium SIFS Contention Window Next Frame Deferred Access IEEE Plenary Meeting, Nov 08 Dallas TX 21

22 EDCA + Admission Control Admitted flows AP Rejected Video stream AC3 (Voice) AC2 (video) STA1 AC2 (video) AC1 AC2 (video) STA3 STA2 Capability Negotiation of Admission Control per Access Category (AC) occurs during association: STA sends an Add Traffic Stream (ADDTS) request with Txm Specifications (TSPEC) prior to sending packet on a particular AC. TSPEC can be for upstream, downstream, or bi-directional traffic. AP has proprietary algorithm to accept/deny ADDTS request. IEEE Plenary Meeting, Nov 08 Dallas TX 22

23 TSPEC Element IEEE Plenary Meeting, Nov 08 Dallas TX 23

24 TSPEC Main Parameters User Priority Maximum MSDU size Maximum Burst Size Minimum PHY rate Peak data Rate 0 (lowest) to 7 (highest) maximum size of the packets (octets) maximum size of the data burst that can be transmitted at the peak data rate (octets) physical bit rate assumed by the scheduler for transmit time and admission control calculations (b/s) maximum bit rate allowed for transfer of the packets Mean data rate average bit rate for transfer of the packets (b/s) Delay bound Nominal MSDU size Maximum Service Interval maximum delay allowed to transport a packet across the wireless interface (including queuing delay, in ms) nominal size of the packets (octets) interval required by TS in this TSPEC between the start of two successive TXOPs IEEE Plenary Meeting, Nov 08 Dallas TX 24

25 EDCA Limitations Still no guaranty of service provided but.. Probabilistic bili priority it mechanism only to allocate bandwidth based on traffic categories IEEE Plenary Meeting, Nov 08 Dallas TX 25

26 HCF Controlled Channel Access (HCCA) HCF = Hybrid Coordination Function Hybrid Coordinator (HC) operates at QoS-AP Controls the iteration of Contention Free Period (CFP) and Contention Period (CP) Schedule Transmit Opportunities for contention free accesses IEEE Plenary Meeting, Nov 08 Dallas 26

27 HCF Controlled Channel Access (HCCA) Similar to PCF Same polling mechanism to assign Transmission Opportunity (TXOP) to QoS-STA but Polling can be issued in both CFP and CP (in Controlled Access Periods) HC Polling is scheduled according to Transmit specifications (TSPECs) IEEE Plenary Meeting, Nov 08 Dallas TX 27

28 PCF vs HCF Comparison IEEE Plenary Meeting, Nov 08 Dallas TX 28

29 802.11e Media Access Summary CAP Controlled Access Period CFP Contention Free Period CF-Poll Contention Free Poll CF-End Contention Free End CP Contention Period EDCF Enhanced Distributed Coord. Function QAP QoS Access Point QSTA Qos Station TBTT Target Beacon Transmission Time TXO Transmission Opportunity IEEE Plenary Meeting, Nov 08 Dallas TX 29

30 802.11e Automatic Power-Save Delivery IEEE Plenary Meeting, Nov 08 Dallas TX 30

31 Automatic Power-Save Delivery y( (APSD) Enhancing Power Saving mode in QoS Basic Service Set QoS-AP automatically delivers downlink frames, which belong to some specified Access Category, to Power-Saving stations Two types of delivery mechanism Unscheduled APSD (U-APSD) Scheduled APSD (S-APSD) IEEE Plenary Meeting, Nov 08 Dallas TX 31

32 Unscheduled APSD (U-APSD) 1. Power-saving QoS-STA STA wakes up and send a trigger data t frame belonging to trigger-enabled AC to QoS-AP 2. After receiving trigger frame, a Service Period (SP) is started 3. QoS-AP send frames belonging to delivery-enabled AC to QoS-STA S IEEE Plenary Meeting, Nov 08 Dallas TX 32

33 Scheduled APSD (S-APSD) B Poll AckQ-APVoice Voice Packet Packet Poll Ack B Q-STA Voice Packet Ack Voice Packet Ack Voice Frame 20 ms Voice Frame Service Start Time Service Start Time 1. QoS-STA negotiates a APSD Schedule with QoS-AP 2. QoS-AP start transmitting the frames of the specified Traffic Stream at Service Start t Time and the following periods 3. QoS-STA STA must wake up at Service Start Time and the following periods to receive frames IEEE Plenary Meeting, Nov 08 Dallas TX 33

34 802.11e Throughput Enhancements IEEE Plenary Meeting, Nov 08 Dallas TX 34

35 Block ACK Improves channel efficiency by aggregating several acknowledgments into one frame. 2 types of Block Ack mechanisms: immediate and delayed Immediate ACK Delayed ACK IEEE Plenary Meeting, Nov 08 Dallas TX 35

36 Direct Link Protocol Ability to exchange data directly between two stations in the network, without traversing the AP. QoS AP QoS STA1 5: DLS Data QoS STA2 IEEE Plenary Meeting, Nov 08 Dallas TX 36

37 WMM (Wi-Fi Multimedia ) Certification program initiated by the Wi-Fi Alliance ( wi Specification document: Wi-Fi Alliance Technical Committee Quality of Service (QoS) Task Group, WMM (including WMM Power Save) Specification Version 1.1 Subset of IEEE e : EDCA & U-APSD only IEEE Plenary Meeting, Nov 08 Dallas TX 37

38 Thank you IEEE Plenary Meeting, Nov 08 Dallas TX 38