Qo S in IEEE 802 11 Networks Resources

Qo. S in IEEE 802. 11 Networks Resources

Introduction

IEEE 802. 11 Simple, Effective Designed for Best Effort Service Real Time Services: Throughput and Delay Sensitive End-to-End Qo. S Guarantees, IEEE 802. 11 e

IEEE 802. 11 Architecture ESS Existing Wired LAN AP STA BSS STA AP STA BSS Infrastructure Network STA Ad Hoc Network STA BSS STA STA

IEEE 802. 11 e - QAP = Qo. S AP - QSTA = Qo. S Station

Layers server Station TCP infrastructure network AP TCP IP LLC LLC 802. 11 MAC 802. 3 MAC 802. 11 PHY 802. 3 PHY

PHY Layer -HR/DSSS: High Rate Direct Sequence Spread Spectrum - FHSS : Frequency Hopping Spread Spectrum

MAC Sublayer PCF (Point Coordination Function) DCF (Distributed Coordination Function)

PCF -Point Coordinator (PC) -Only in infrastructure networks -Designed for delay-bounded services PIFS EIFS: Extended IFS

PCF - Centralized: location-dependent errors - Stations must wait for polling: Delay at low load - AP needs to contend for the channel using DCF to begin a CFP: variable CFP - Managing large number of stations using polling affects the applications that use DCF - No admission control

DCF • • • Distributed, Contention-based CSMA/CA Binary Exponential Back-Off (CW: Contention Window) Physical channel sensing Virtual channel sensing (NAV: Network Allocation Vector) Hidden Terminal problem: RTS/CTS Timers Retry limits Fragmentation

Fragmentation

NAV: Network Allocation Vector Priority DIFS: DCF Inter Frame Space SIFS: Short IFS PIFS: PCF IFS EIFS: Extended IFS DIFS medium busy DIFS PIFS SIFS contention next frame time

Tunable Parameters

Qo. S Mechanisms

Service Differentiation Priority: classification of traffic Fair Scheduling: partitioning the bandwidth fairly by regulating the wait times of traffic classes according to given weights

EDCF (Enhanced DCF) Priority AC For 1 0 Best Effort 2 0 Best Effort 0 0 Best Effort 3 1 Video Probe 4 2 Video 5 2 Video 6 3 Voice 7 3 Voice Virtual Collision Handler ~ Internal Collisions AC: Access Category AIFS: Arbitrary IFS

Persistent Factor DCF (P-DCF) - A persistent factor P is selected; small P means higher priority traffic class - A uniform random number r is generated every slot in back-off stages. - A flow stops the back-off and starts transmission only if r > p in the current slot given no transmission occurs in previous slots The back-off interval is a geometric distributed random variable with P Geometric random variable is the number of trials required to obtain the first failure

Distributed Weighted Fair Queue 2 schemes are proposed: • CW for a flow = Difference between actual and expected throughput A station decreases the CW to get higher priority Lower CW when the actual throughput is lower than the expected one • Li’ = Ri/Wi Ri = the actual throughput Wi = the weight Each station adjust its CW by comparing others Li’ Ø Selfishness Ø More stations will have small value of CW

Distributed Fair Scheduling (DFS) The back-off interval is based on the packet length and traffic class For flow i, BIi proportional to: 1. The weight (higher for higher throughput classes) 2. The packet length 3. A scaling factor (to min the probability of collisions in case different stations have same back off interval)

Distributed Deficit Round Robin (DDRR) 1. Each throughput class i at station j is given a service quantum rate (Qij) equal to its required throughput 2. A deficit counter (DCij) is advanced at the rate Qij in a round robin fashion 3. Once a DCij becomes positive, the ith queue is allowed to send one packet 4. After transmission, DCij will be decreased by packet length each time a packet is transmitted DCij is used to calculate IFSij (time before transmit or back-off): larger DCij, smaller IFSij

DDRR Queues of different throughput classes Polling in a round robin way

Admission Control and Bandwidth Reservation • Service differentiation does not perform well under high traffic loads • There is a need to protect existing streams • A wireless node has no knowledge of exact condition of the network • With CSMA/CA, bandwidth provision is quite difficult

Measurement-Based Admission Control - The decision is made on measurement of existing network status (delay, throughput, …) - Different methods used: - Virtual MAC: the use of virtual MAC frames, and using a virtual source algorithm to tune the virtual MAC. - Probe packet: the use of probe packet for ad hoc - Data probe: the use of data packets

Calculation-Based Admission Control • Performance metrics or criteria for evaluating the network status • Permissible throughput propagation • Saturation-based

Scheduling and Reservation-Based Schemes • ARME (ASSURED RATE MAC EXTENSION • ): - Extension of DCF - Uses token bucket-based algorithm to detect overloading condition - improvements mad by adjusting CW

Scheduling/Reservation • AACA: - RTS/CTS used for reservation - Mainly was for solving hidden terminal problem

Link Adaptation Dynamically change the transmit rate , specified in the PLCP header of the PHY layer, that depend on channel conditions

Link Adaptation • Received Signal Strength (RSS) – Each station maintains 12 RSS thresholds and corresponding transmission rate – Measure RSS and adjust the transmission rate • PER-Prediction – Decisions are based on Packet Error Rate-Prediction • MPDU-Based • Success/Fail Thresholds • Code Adapts To Enhance Reliability

Direct Link Protocol (DLP) • QSTA transmits directly to another QSTA • Set up with the QAP is needed • STAs cannot go into power saving mode for active duration of the direct stream. • DLP is not applied in Ad Hoc networks • DLP messages can include security information

Group ACK • Send a group of frames before any ACK to reduce overhead • Group. Ack. Req • Group. Ack frame with an ACK bitmap • Sender retry unacknowledged frames with a retry limit • Receiver should keep the state of burst data received (sender address, bit map, sequence numbers)

Challenges • • • IEEE 802. 11 e and Diff. Serv IEEE 802. 11 e and Int. Serv Integration of WLAN and MANET Integration of WLAN and Bluetooth Integration of WLAN and 3 G wireless networks

Resources 1. “A SURVEY OF QUALITY OF SERVICE IN IEEE 802. 11 NETWORKS” By: HUA ZHU, MING LI, IMRICH CHLAMTAC, AND B. PRABHAKARAN THE UNIVERSITY OF TEXAS AT DALLAS 2. www. eecs. berkeley. edu/~ergen/docs/IEEE-802. 11 overview. ppt 3. www. cs. ucla. edu/classes/ winter 04/cs 117/chap 7 wlan. Rvsd. ppt 4. http: //www. it. iitb. ac. in/~kirang/academic/MTP/Firststage/slides. pdf Up