IEEE 802.11e QoS on WLANs Overview  802.11 background information DCF PCF Qos limitations of 802.11  802.11e (QoS support) EDCA HCCA  Simulation result IEEE 802.11 background information

 WLAN( Wireless Local Area Network ) A LAN to which mobile users (clients) can connect and communicate by means of high- frequency radio waves rather than wires.  WLAN Standard  IEEE 802.11 (IEEE)  HiperLAN (European Telecommunications Standards Institute, ETSI) IEEE 802.11 IEEE 802.11 Topology

 Basic Service set networks (BSS)  Independent BSS networks  Infrastructure BSS  Extended Service Set (ESS) networks The Basic Service Set (BSS)

 The BSS consists of a group of any number of stations.  The basic building block of IEEE 802.11 LAN Independent BSS (IBSS) network

 stations communicate directly  no connection to a wired network  Ad hoc network Infrastructure BSS (BSS)

 Include an access point (AP)  The AP may have connection to an wired network  All stations communicate with the AP  Communication between stations must go through AP  consume twice the bandwidth The Extended Service Set(ESS)

Wireless Wireless Client Distribution System Client Basic Service Set (BSS) Access Basic Service Set Point (BSS) Wireless Access Client Point Wireless Client  Distribution system (DS) provides logical services necessary to handle address-to-destination mapping and seamless integration of multiple BSSs  An AP is a STA that provides access to the DS Terminology- Interframe Space

 Time interval between frames. SIFS – Short Interframe Space PIFS – PCF Interframe Space DIFS – DCF Interframe Space DIFS > PIFS > SIFS  Fixed for each PHY  Provide priority levels IEEE 802.11

 Two coordination functions are defined  the mandatory Distributed Coordination Function ( DCF) based on CSMA/CA optional Point Coordination Function (PCF) based on poll-and-response mechanism.  Most of today’s 802.11 devices operate in the DCF mode only. Distributed Coordination Function ( DCF)

 Possible in both Infrastructure and Ad hoc mode  Known as Carrier Sense Multiple Access with Collision Avoidance (CSMA/CA)  Used During Contention period (CP)  DCF can support best-effort services , not any QoS guarantees. Basic Access Mechanism-CSMA/CA

 Sense the medium before transmitting.  If the medium is not busy, the transmission may proceed.  If the medium is busy, invoke backoff procedure.  Transmit after the backoff procedure  If the transmission is not successful, invoke backoff procedure. Virtual Carrier Sense Mechanism

 Network Allocation Vector (NAV)  Set NAV based on duration information (available in MAC header, RTS, CTS)  NAV counts down to zero at a uniform rate.  IF NAV=0, medium is idle; otherwise, it is busy. Basic Access Mechanism-CSMA/CA Example of backoff intervals IEEE 802.11 DCF (cont.)

 BackoffTime = Random() × aSlotTime where  Random( ) = [0, CW]  CWmin≤ CW ≤CWmax.  aSlotTime fixed for each PHY  CWnew= (CWold+ 1) ×PF –1 (where PF=2)  PF : Persistence Factor A example of CW Other Collision Avoidance Mechanism – RTS/CTS

 Hidden Terminal Problem:  Hidden Terminal are STAs that the receiver can hear but that cannot be detected by other senders.  In order to solve the problem,a optional RTS/CTS is introduced.  The source sends a short RTS frame before each data transmission,and the receiver replies with a CTS frame if it is ready to receive. Basic access scheme v.s RTS/CTS access scheme Point Coordination Function (PCF) [Optional in Standard]

 Used during Contention-Free Period (CFP)  A single AP controls access to the medium, and a Point Coordinator (PC) Agent resides in the AP.  AP polls each station for data, and after a given time interval moves to the next station.  No stations are allowed to transmit unless it is polled.  AP could have a priority scheme for stations.  PCF is useful for time-sensitive applications. PCF(cont.) IEEE Legacy 802.11 MAC

 SuperFrame consists of Contention Period (CP) and Contention Free Period (CFP).  PCF used during CFP and DCF used during CP. QoS Limitations of 802.11

 DCF (Distributed Coordination Function)  Only support best-effort services  No guarantee in bandwidth, packet delay and jitter  Throughput degradation in the heavy load

 PCF (Point Coordination Function)  Inefficient central polling scheme  Unpredictable beacon frame delay due to incompatible cooperation between CP and CFP modes  Transmission time of the polled stations is difficult to control IEEE 802.11e – Overview

 New terminology  QAP – QoS Access Point  QSTA – QoS Station  HC – Hybrid Coordinator  In order to support QoS in 802.11 WLAN , 802.11e has defined a new mechanism , namely, Hybrid Coordination Function (HCF).  HCF is implemented by all QAPs and QSTAs  HCF has two access mechanisms  Contention based  Enhanced distributed channel access (EDCA)  Controlled channel access  HCF Controlled Channel Access (HCCA) IEEE 802.11e – MAC frame format IEEE 802.11e Overview - Enhanced distributed channel access (EDCA)

 EDCA defines four Access Categories (AC)  Background  Best Effort  Voice  Video  EDCA supports 8 User Priority (UP) values  Priority values (0 to 7) identical to the IEEE 802.1D priorities  Rules  One UP belongs to one AC  Each AC may contains more than one UP  Traffic of higher UP will be transmitted first in one AC IEEE 802.11e Overview- User Priority (UP)

 8 User Priority  Identical to IEEE 802.1D priority tags. Basic concepts – Transmission opportunity (TXOP)

 Time interval permitted for a particular STA to transmit packets.  During the TXOP, there can be a series of frames transmitted by an STA separated by SIFS.  TXOP types  EDCA TXOP initiation

 Obtained by winning a successful EDCF contention  Polled TXOP (HCCA TXOP)  Obtained by receiving a QoS CF-poll frame from the QAP 802.11e EDCF – Access Category

 EDCF defines access category (AC) mechanism to support the priority mechanism at the non-AP QSTAs.  Each QSTA has four ACs.  An AC is an enhanced variant of the DCF which contends for transmission opportunity (TXOP) using the set of parameters such as CWmin[AC], CWmax[AC], AIFS[AC], etc.  Each AC queue works as an independent DCF STA and uses its backoff parameter.  In EDCA, the size of Contention-Window (CW) and Inter- frame space (IFS) is dependent on AC EDCA – Accessing the medium

 EDCA use different IFSs  SIFS – ACKs, between multiple frames with the continuation EDCA TXOP  DIFS / AIFS – Used by DCF and EDCA for different access categories EDCF - Arbitration Interframe Space (AIFS)

 QSTA use AIFS to defer the contention window or transmission for each AC  AIFS[AC] = AIFSN[AC]x aSlotTime+ aSIFTime AIFSN for each AC is broadcast via beacon frame containing ‘EDCA Parameter Set’element  DIFS = 2 x aSlotTime+ aSIFTime EDCF – Some elements of the Beacon frame Default EDCF Parameter Set EDCA details

 Each AC has own  Interframe space – AIFS  Backoff Counter (BO)  CWmin, CWmax, CW  TXOP limit  QSTA listens beacon frames to receive this information  Each QSTA implements own queues for each AC traffic  From the queues the frame with the highest priority is sent if internal collision happens Legacy 802.11 station and 802.11e station with four ACs within one station. 802.11e HCCA Overview - Hybrid Coordination Function(HCF)

 Designed to increase efficiency by reducing the contention on the medium  Uses ”polling”  Like PCF  Thus, HCCA can send ”polling” both under CFP and CP  PCF – Only polling in CP  Specifically assigned transmit times for every frame  Enables QoS guarantees 802.11e HCCA (cont.)

 Has higher priority than EDCA .Under HCCA, HC(Hybrid Coordinator) has full controll over the wireless medium  If HC needs it, it could take over the controll of the medium by sending a QoS CF-Poll 802.11e HCCA (cont.)

 Differences between hybrid coordinator (HC) and point coordinator (PC): HC can poll QSTAs in both CP and CFP HC grants a polled TXOP to one QSTA, which restricts the duration of the QSTA’s access to the medium. An example of an 802.11e superframe Improve Efficiency – 802.11e

 Block Acknowledgment  Send multiple MSDUs without a bunch of ACKs  Group ACK  Direct Link Protocol (LDP)  No support for DLP in legacy MAC – Needs to talk through the AP  Less use of the channel  The STAs is enable to talk directly to each other  Sounds easy, lots of things to think about:  Power saving  Security Simulation-QoS , does it work?

 EDCF Parameters for three queues Throughput and delay performance for EDCF

 EDCF maintains the throughput of high-priority audio and video flows by punishing the background traffic. Throughput(KB/s) Comparison of total throughput between EDCF and DCF

 The total throughput of EDCF is lower than that of DCF when the traffic load is larger than 48%:EDCF reduces the throughput of low-priority flows considerable and therefore results in decreasing the total throughput. Mean delay of audio,CBR video versus channel load for EDCF and HCF

 The result show that the HCF controlled channel access mechanism can guarantee the minimum delay requierment(50ms) for the admitted flows in different load rate  EDCF works very well under low load conditions but suffers from delay degradation in high-load condition