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 Client Wireless Distribution System Client Basic Service Set (BSS) Basic Service Set Wireless Access (BSS) Point 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.