IEEE e WLANs / WMM. S.Rajesh AU-KBC Research Centre, BroVis Wireless Networks, smartbridges Pte Ltd.

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1 IEEE e WLANs / WMM S.Rajesh AU-KBC Research Centre, BroVis Wireless Networks, smartbridges Pte Ltd.

2 Outline A short review of MAC Drawbacks of MAC Application requirement for QoS Key QoS supporting concepts in e WMM related points S.Rajesh, 2

3 Various operational modes of a b g Frequncy 2.4 GHz 5 GHz 2.4 GHz 2.4 GHz Data rate(s) (in Mbps) 1, 2 5, 9, 12, 18, 24, 36, 48, 54 1, 2, 5.5, 11 6, 9, 12, 15, 24,36, 48, 54 Modulation FHSS,DSSS OFDM DSSS OFDM Effective data throughput 1, 2 Mbps 32 Mbps 5 Mbps 32 Mbps Advertised range 300 feet 225 feet 300 feet 300 feet Encryption available? Yes Yes Yes Yes Encryption type 40 bit RC4 40 bit, 104 bit RC4 40 bit, 104 bit RC4 40 bit, 104 bit. RC4 Network support Ethernet (IEEE 802.3) Ethernet (IEEE 802.3) Ethernet (IEEE 802.3) Ethernet (IEEE 802.3) S.Rajesh, 3

4 Standard Set e (QoS) f (IAPP) h (TPC/DFS) i (Adv SEC) MAC PCF DCF Base band (BB) b (CCK) a (OFDM) g (both b & a) RF Spectrum 2.4 GHz 5 GHz 2.4 GHz S.Rajesh, 4

5 IBSS Independent BSS IBSS-1 Ad-hoc Network. STA-1 STA-2 Most basic type of LAN. IBSS Mode IBSS-2 STA-1 STA-2 IEEE STATION (STA) communicate directly Ad-hoc mode. Two IBSS can merge using Coalesce procedure. S.Rajesh, 5

6 BSS Infrastructure BSS BSS-1 BSS Basic Service set STA-1 AP STA-2 The basic building block of DS Distributed and centralized access are allowed. AP STA-1 STA-2 BSS-2 S.Rajesh, 6

7 MAC Modes Coordination functions control access to the wireless medium. Two types of services provided are: i) Contention based service - provided by DCF (Distributed Coordination Function)» basis of CSMA/CA, uses random backoff, RTS/CTS ii) Contention free service - provided by PCF (Point Coordination Function)» Used only in infrastructure networks» Point coordinators residing in access points used» Allows stations to transmit frames after a shorter interval to gain priority over contention based services S.Rajesh, 7

8 DCF Access Algorithm -Collision Avoidance Monitor channel activity Is channel idle for a DIFS? NO Monitor channel until it is idle for a DIFS YES Transmit packet S.Rajesh, 8

9 DCF Generate random back-off interval Decrement back off counter NO Is back off counter zero? Transmit RTS YES S.Rajesh, 9

10 DCF D S S S D N ST S S 1 R T S DATA NAV (RTS from 3) 2 C T S A C K C T S 3 NAV (RTS from 1) D S S S D BkOff S S time R T S DATA a parameters S SIFS (Short Inter Frame Space) 16 µs D DIFS (DCF Inter Frame Space) 34 µs ST Slot Time for each Back-off counter 9 µs S.Rajesh, 10

11 PCF Contention Free Repetition Interval Contention Free Period Contention period SIFS SIFS SIFS PIFS SIFS SIFS DIFS +BK SIFS SIFS SIFS PC Beacon Data & CF Poll to 1 CF Poll to 2 & Ack to 1 CF Poll to 3 CF End NAV Set by Beacon Cleared by CF End M1 Data from 1 &CF Ack NAV Set by Beacon Cleared by CF End M2 Dead NAV M3 CF Ack NAV Set by Beacon Cleared by CF End M4,5 IBSS Set by RTS NAV Time --> S.Rajesh, 11 R T S C T S Data A c k

12 Why QoS How IEEE Fails to Handle QoS? How IEEE e Handles QoS? EDCF / EDCA HCF / HCCA S.Rajesh, 12

13 Why QoS? Support multimedia traffic Traffic with different requirements Different throughput (bandwidth) requirements Different delay requirements Different delay-jitter requirements Different loss rate acceptance limits Dynamic setup and teardown of streams Immunity to channel impairments Circuit switched behaviour on packet networks S.Rajesh, 13

14 Typical Applications with QoS Requirements Traffic Type Voice Call Signaling Interactive Video Streaming Video Transactional Data Mission-Critical Data Routing Data Network-Management Best-Effort Data Bulk Data Scavenger Traffic Application VoIP Call Setup, Tear-down, dial-tone Video Conferencing Web Cast, Presentation, elearning Client/Server Application, SAP Robot Control IP Routing SNMP Browsing, (HTTP, SMTP) FTP, DB Backup/Sync/Replication Entertainment low band video Priority HIGHEST LEAST S.Rajesh, 14

15 VOIP VOIP phone VOIP phone S.Rajesh, 15

16 Video conferencing S.Rajesh, 16

17 Video streaming Broadcasting or multicasting S.Rajesh, 17

18 Webcast Wireless projector S.Rajesh, 18

19 Why QoS How IEEE Fails to Handle QoS? How IEEE e Handles QoS? Major Features EDCF / EDCA HCF / HCCA S.Rajesh, 19

20 How IEEE Fails to Handle QoS? Basic DCF is not QoS friendly Need to contend for every MSDU Contention treats all traffic similar. That is a voice frame contends with the data frame using the same parameters No concept of priority to important traffic even inside the STA. Once the sequence number is allotted, that MSDU must be transmitted before attempting the next MSDU S.Rajesh, 20

21 Contd (How IEEE handles it?) PCF is too complicated and incomplete, if not broken No control over transmission duration after poll Unpredictable Beacon Delays Interruption from station missing beacons S.Rajesh, 21

22 Why QoS How IEEE Fails to Handle QoS? How IEEE e Handles QoS? EDCF / EDCA HCF / HCCA S.Rajesh, 22

23 Major features of e EDCA CAP HCCA HCF IEEE e BLOCK ACK STREAM MANAGEMENT DLP S.Rajesh, 23

24 How IEEE e Handles QoS? Distributed Scheme Prioritize the Channel Access Centralized Scheme Preferentially Polled Channel Access QoS Resource Negotiations Direct Link between Stations Burst Handling Scheme S.Rajesh, 24

25 Major Features Hybrid coordination function Enhanced distributed channel access (EDCA) Distributed mechanism Provides a simple implementation for applications involving voice traffic HCF controlled channel access (HCCA) Centrally controlled mechanism Provides a reasonable guarantee on bandwidth and delay bounds S.Rajesh, 25

26 Contd (Major Features) Direct link protocol Makes the communication efficient by avoiding the middle-man, HC/ QAP Block- acknowledge procedure Improves the efficiency of ACK procedure for large data like video Stream creation and management Provides for dynamic creation, tear down and update of streams S.Rajesh, 26

27 Access Categories 4 Access Categories (ACs) are supported AC_BK: Background traffic AC_BE: Best-Effort traffic AC_VI: Video traffic AC_VO: Voice traffic S.Rajesh, 27

28 Why QoS How IEEE Fails to Handle QoS? How IEEE e Handles QoS? EDCF / EDCA HCF / HCCA S.Rajesh, 28

29 Enhancements in EDCF / EDCA Per Access Category parameters: AIFS CW min CW max TXOP Internal Collision Resolution S.Rajesh, 29

30 EDCF / EDCA S.Rajesh, 30

31 Contd (EDCF / EDCA) S.Rajesh, 31

32 Contd (EDCF / EDCA) AIFS Minimum AIFS used in EDCA is equal to DIFS used in DCF That is the best that can be done without imposing fine grain sub-slot concept into the standard However the contention window for EDCA is decided by the HC/ QAP and it is made significantly smaller than that for DCF In addition to this, the HC/ QAP in practice can limit the number of legacy STA that can get associated in the presence of QSTAs using EDCA S.Rajesh, 32

33 Contd (EDCF / EDCA) CW min CW max Each Access category contends with the same back-off mechanism with different parameters High AC can be configured to choose from smaller Contention Window and vice-versa CW min [AC] Start size of the contention window of that AC CW max [AC] Largest size of the contention window of that AC S.Rajesh, 33

34 Contd (EDCF / EDCA) TXOP The right to access the channel won through contention is not limited to one MPDU+ ACK or frag- burst. The length of time that a QSTA can retain the channel access is EDCA- TXOP- LIMIT (implemented using medium- occupancytimer). The EDCA- TXOP- LIMIT for each AC is decided by HC/ QAP and advertised in its beacons and probe responses When EDCA- TXOP- LIMIT for an AC is zero, then the QSTA is allowed to send one complete MSDU or MMPDU (incl ack) when it wins the channel access for that AC S.Rajesh, 34

35 Contd (EDCF / EDCA) Internal Collision Resolution Since there are four independent channel access functions, one for each AC, it is possible that there will be collisions among them. These are called internal collisions. During internal collisions the following are performed similar to external collisions Short/ long retry counters for MSDU/ MMPDU are incremented A new backoff value is selected within the new CW Contention window (CW) size is not doubled Absence of expected response for a transmitted frame results in loss of EDCA-TXOP and hence new backoff being generated S.Rajesh, 35

36 Why QoS How IEEE Fails to Handle QoS? How IEEE e Handles QoS? EDCF / EDCA HCF / HCCA S.Rajesh, 36

37 HCF / HCCA S.Rajesh, 37

38 Block Ack ADDBA Request ACK ADDBA Response Set up ACK QoS Data MPDU Block ACK Request Data & Block ACK (Multiple times) Block ACK DELBA Request ACK Tear Down Originator Recipient S.Rajesh, 38

39 Stream Setup & Management Three types of traffic under HCCA/HCF Uplink From QSTA to QAP Downlink From QAP to QSTA Side link QSTA to QSTA A Traffic Stream (TS) can avail parameterized QoS based on Traffic Specification (TSPEC) Admission Control at QAP Each QSTA can have uptp 8 up/side link and 8 downlink streams ADDTS To set up stream between QSTA and QAP DELTS To Tear down Stream between QSTA and QAP QAP allow stream setup only if its QoS requirements can be full filed S.Rajesh, 39

40 Direct Link Protocol QAP DLP Request Tear down Request DLP Response Tear down Response QSTA Data Exchange QSTA S.Rajesh, 40

41 QoS Partitioning Traffic streams Reservation request from upper layer Initiation always from WSTA Access Classes As per TOS field in IP header Class mapping as required VLAN partitioning S.Rajesh, 41

42 Major features of e EDCA CAP HCCA HCF IEEE e BLOCK ACK STREAM MANAGEMENT DLP S.Rajesh, 42

43 WMM - Default Parameters AC CWmin (slots) CWmax (slots) AIFS (slots) TxOp Limit STA AP STA AP AC_BK AC_BE AC_VI ms AC_VO ms S.Rajesh, 43

44 more on WMM Ack Policy: support for no-ack (mandatory) Admission Control ACM bit: restrict STAs from using specific traffic class (mandatory) TSPEC (traffic specification): STA send requests to AP for bandwidth to support a particular type of traffic (optional) Triggered Power Save (APSD/UPSD) (optional) Uplink polling for buffered packets AdHoc support (optional) S.Rajesh, 44

45 WSM WMM (Prioritized Access) WMM Scheduled Access statistical >> scalable deterministic >> power efficient distributed >> hidden node Application: video centralized >> long range app. Application: voice S.Rajesh, 45

46 Implementation Tweaks Scheduler Schemes Reordering based on channel behavior Buffering Algorithms RED Admission Control Techniques Bandwidth management Traffic Shaping & Policing Burst handling S.Rajesh, 46

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