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A Survey of Quality of Service in IEEE 802.11 Networks

A Survey of Quality of Service in IEEE 802.11 Networks






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    A Survey of Quality of Service in IEEE 802.11 Networks A Survey of Quality of Service in IEEE 802.11 Networks Presentation Transcript

    • A Survey of Quality of Service in IEEE 802.11 Networks
      • Hua Zhu, Ming Li, Imrich Chlamtac, B. Prabhakaran
      • The University of Texas at Dallas
    • Presentation Structure
      • IEEE 802.11 overview
      • QoS schemes for 802.11
      • Design challenges
      • Future Work
      • Conclusions
    • Introduction
      • IEEE 802.11 advantages
        • Broadband bandwidth capability
        • Low deployment cost
        • Internet services access anytime, anywhere
        • Mobility and connectivity
    • Introduction (cont’d)
      • IEEE 802.11 disadvantages
        • Best effort services
        • No build in QoS  modification of existing standards required
        • Shared medium
    • So what is the challenge?
      • End to end QoS
      • Protocol interoperability
      • Multihop scheduling
      • Full mobility support
      • Seamless vertical handoff among multiple mobile/wireless interfaces
    • 802.11 Family
      • 802.11a 54 Mbps 5GHz U-NII, OFDM
      • 802.11b 22 Mbps 2.4GHz ISM, HR/DSSS
      • 802.11c Bridge Operation Procedures
      • 802.11d Global Harmonization
      • 802.11e MAC enhancements for QoS
      • 802.11f Inter Access Point Protocol (roaming)
      • 802.11g 36/45 Mbps 2.4 GHz ISM, OFDM
      • 802.11h Dynamic Frequency Selection
      • 802.11i Security
    • 802.11 MAC – Medium access protocols
      • Contention-based distributed coordination function (DCF)
        • Based on CSMA/CA instead of CSMA/CD
      • Optional point coordination function (PCF)
        • Wireless channel is divided to superframes
        • Superframe consists of contention free period (CFP) and contention period (CP). At the beginning of CFP point coordinator (AP) contends for access of the wireless channel.
        • If an AP acquires the channel a polling / granting policy is applied for transmission.
    • 802.11 MAC Protocols – PCF problems
      • Substantial delay at low load (polling/granting policy, even in an idle system).
      • AP needs to contend for the channel at the beginning using DCF  Effective period of contention free polling may vary.
      • Difficult management of the polling for large number of interactive streams without harming the applications using DCF contention.
      • Central no distributed approach  Location – dependent errors.
    • 802.11 MAC Protocols – DCF
        • Based on CSMA/CA
        • Carrier sensing in both PHY & MAC layer (physical & virtual CS)
        • MAC Protocol Data Unit = 34 bytes MAC Header + Payload + 32 bit CRC
        • If MAC fr_length>RTS_threshold then RTS & CTS are used by stations to solve the hidden terminal and capture effect problems
    • CSMA/CA – RTS/CTS scheme
    • Common Tunnable Parameters
    • 802.11 QoS Mechanisms
        • Service differentiation
        • Admission control and bandwidth reservation
        • Link adaptation
    • Service differentiation mechanisms 802.11 DFC Priority-based Fair-scheduling-based Contention window EDFC 802.11e DDRR Distributed Deficit Round Robin DFS Distributed Fair Scheduling DWFQ Distributed Weighted Fair Queue P-DFC (Persistent Factor DFC) Interframe space AIFS Backoff algorithm
    • Service Differentiation
        • Priority based
          • Binds channel access to different traffic classes by prioritized contention parameters
        • Fair scheduling based
          • Partitions the channel bandwidth fairly by regulating wait times of traffic classes in proportion according to given weights
        • Tunable parameters :
          • Contention window (Cwmin/Cwmax), backoff algorithm, interframe space AIFS
    • Service Differentiation mechanisms - EDCF
      • Enhanced DCF (EDCF)
        • Part of upcoming 802.11e standard
          • Priority of traffic categories based on
            • AIFS (Arbitrary Interframe Space)
            • Max/min Contention Window
            • Multiplication factor for backoff window
        • Combination of above parameters is permitted according to the service provider needs
        • Although all traffic categories keep using the same DCF access method they have different probabilities of winning the channel contention by differentiating contention parameters.
    • Service Differentiation mechanisms –P-DCF
      • Persistent Factor DCF (P-DCF) .
        • Each traffic class is associated with a persistent factor P
        • In backoff stage a uniformly distributed number r is generated.
        • Each flow stops backoff and starts transmission only if r>P in the current slot time, given no transmission occurs in previous slot times.
        • Therefore the backoff interval is geometrically distributed random variable with parameter P
    • Service Differentiation mechanisms DWFQ Algorithm 1
      • Distributed Wighted Fair Queue (DWFQ) –algo 1
          • Backoff CW of any traffic flow is adjusted based on the difference between the actual and expected throughputs.
          • If actual_thru<exp_thru then decrease CW in order to increase flow’s priority and vice versa
    • Service Differentiation mechanisms DWFQ Algorithm 2
      • Distributed Wighted Fair Queue (DWFQ) –algo 2
        • Calculation of Li=Ri/Wi
          • Ri actual throughput, Wi corresponding weight of I station
        • Comparison of Li with all others and adjust CW for station I, ie if Li<all others  decrease CW of I
    • Service Differentiation mechanisms -DFS
      • Distributed Fair Scheduling (DFS)
        • Differentiate the backoff interval (BI) based on the packet length and traffic class and station with smaller BI transmits
        • BIi= Ρ i x scaling x factor x Li/ Φ i
          • Li packet length, Φ i weight, Pi random variable uniformly distributed in [0.9,1.1]
          • Pi is introduced to minimize the collision caused by multiple stations with the same BI
    • Service Differentiation mechanisms -DDRR
      • Distributed Deficit Round Robin (DDRR)
        • The i throughput class at the j station is assigned with a quantum rate ( Qi,j ) equal to the thoughput it requires and a deficit counter ( DCi,j ) that accumulated at the rate of Qi,j and is decreased by the packet length whenever a packet is transmitted.
        • DCi,j is used for Interframe Space ( IFSi,j ) calculation
        • IFSi,j is the waiting time before transmission or backoff starting
        • A larger DCi,j results in a smaller IFSi,j
    • Service Differentiation mechanisms – Conclusion and Comparison
      • Fair scheduling based
        • Fairly allocation of bandwidth among traffic classes.
        • Prevent starvation of specific class.
        • Often require a substantial modification of existing 802.11 standards.
      • Priority based
        • Require less modification of the existing DCF access method.
        • Provide better QoS support for real time applications.
      • Service differentiation does not perform well under high traffic loads due to the inefficiency of 802.11 MAC.
    • QoS mechanisms for Admission Control and Bandwidth Reservation (MAC)
      • Necessity in order to guarantee QoS in high traffic load.
        • A wireless node has no knowledge of the exact network condition.
        • With contention based CSMA/CA, bandwidth provisioning is almost impossible  only soft QoS guarantee.
      • In general admission control requires less modification than bandwidth reservation in 802.11 standards.
    • Admission Control and Bandwidth Reservation
    • Admission Control Approaches
      • Measurement-based (admission control based on measurements of existing network status)
        • Virtual MAC [Barry et al]: Channel is passively monitored by virtual MAC frames and local service (throughput/delay) is estimated by the measurement of virtual frames.
        • Probe Packets [Valaee & Li]: Admission procedure is based on a sequence of probe packets for ad hoc networks
        • Data Probe Packets:[Shah et al]: Data packets for measuring the network load
    • Calculation based approaches
      • Calculation-based (there are certain perfomance metrics or criteria for evaluating network status)
        • Permissible throughput: [Kazantzidis et al]: Admission decision criterion is permissible throughput. AODV routing protocol
        • Saturation-based[Zhu et al]: Prediction and prevention of saturation using piggybacked information (number of active stations, corresponding bit rates, average packet lengths) for each station. Admission control decisions are made dynamically at both source/destination in a fully distributed way.
    • Bandwidth reservation
      • Flow reservation and priority allocation [Li and Prabhakaran]: Optimizing the usage of priority resources.
      • ARME [Banchs & Perez]: Based on an extension of DCF. A Token-bucket algorithm is used to detect overload and improve performance via adjustment of CW.
      • AACA[Liu et al]: ACCA adopted the RTS/CTS access method on a common channel solely for reservation purposes. After successful reservation a pair station transmit without interruption in the reserved channel.
    • QoS Mechanisms for Link Adaptation (PHY)
      • 802.11 specifies multiple transmission rates that are achieved by different modulation techniques in the PLCP header of PHY layer.
      • Rate adaptation and signaling are open.
      • Link adaptation mechanism has to maximize the throughput under dynamically changing channel conditions.
      • So an obvious solution is to focus on switching transmission rates specified at PLCP, without modification of existing standards.
    • QoS Mechanisms for Link Adaptation (PHY) –cont’d
      • Novel idea
      • Adjust the length of DSSS pseudo noise (PN) in 802.11b with slight modifications of 802.11b
    • Metrics used in Link Adaptation (PHY)
      • Channel signal to noise ratio / carrier to interference ratio (SNR/CIR).
      • Received power level.
      • Average payload length
      • Transmission acks
      • Combinations of above
    • Received Signal Strength RSS Pavon and Choi
      • Assumption:
        • Transmission power is fixed.
        • Linear relationship between average RSS and SNR.
      • Then:
        • A rate adaptation algorithm at every station maintain its own 12 RSS thresholds and corresponding rates. Based on the measured RSS a station dynamically switches to an appropriate transmission rate.
    • MPDU based Qiao et al
      • A combination of metrics is used including:
        • SNR
        • Average payload length
        • Frame retry count
      • The proposed algorithm pre-established a table of best transmission rate for decision making.
    • Success/Fail thresholds Cheville et al
      • Transmitted frames ACKs are used as a metric of channel condition.
      • If number of consecutive successful exceeds S, transmission rate is increased, otherwise transmission rate is decreased.
      • ACKs are used to indicate transmission success or fail.
    • Code Adapts To Enhance Reliability Mullin et al
      • CATER adaptive PN algorithm gives a throughput improvement under high bit error rate (BER) channel conditions at 802.11b.
      • But due to signaling overhead the throughput under low BER channel conditions is lower than the standard 802.11b
    • Challenges and future work Wireless Internet and Interoperability
      • IEEE 802.11 WLANs have been successfully applied as the last mile technology where there is a need for wireless/mobile users (Wireless Hotspots).
      • There is an urgent demand for e2e QoS guarantee to be provided in wire-cum-wireless heterogeneous networks.
      • Interoperability between IEEE 802.11 and DiffServ or IntServ.
    • Wireless Internet and Interoperability
    • 802.11 and DiffServ [Park & Kim]
      • A proposed architecture for e2e QoS across wired WAN, wired LAN and WLAN.
      • Protocols and drafts used
        • DiffServ (RFC 2475), IEEE802.1d, IEEE802.1Q, IEEE802.11. IEEE 802.3 .
      • 802.3 MAC frame carry the user priority via the 802.1Q VLAN tag. User priority is forwarded through 802.1D MAC bridge to 802.1e and used by EDCF to differentiate flows.
    • 802.11 and DiffServ [Park & Kim] cont’d
      • There is a necessity for mapping between DiffServ Code Point (DSCP) and Traffic Category Identification (TCID) defined in 802.1e.
      • Direct mapping
        • When IP packets are encapsulated in MAC frames they are placed in priority queues without preemption
      • Hierarchical mapping
        • IP packets are classified and shaped according to the priority of DSCP values before being forwarded to 802.1e priority queues  more accurate e2e QoS .
    • 802.11 and IntServ [Liu & Zhu]
      • Integration of RSVP and WRESV (WRESV is a proposed MAC layer flow reservation and admission control protocol in IEEE 802.11WLAN).
      • Message mapping at Access Point are implemented by cross-layer interaction and user priorities are mapped to 802.11 MAC with 802.1p.
      • WRESV is working with most of the existing MAC schedulers (DCF, EDCF, DFS).
      • This scheme also considers support of both node mobility and QoS in handoff.
    • Support of Full Mobility
      • Mobility is supported through extended service set (ESS) for roaming among multiple AP. This roaming capability is achieved through Mobile Station (MS) beacon scanning in a channel sweep.
      • 802.11 WLAN service is only available for low mobility devices in isolated hot spots.
      • Recent efforts have been made to extend 802.11 WLANs into outdoor cellular networks to provide fully mobile broadband service with ubiquitous coverage and high speed connectivity.
    • Support of Full Mobility - Examples
      • [Leung et al] claim that without standard modification the DCF access method with RTS/CTS is feasible for large outdoor cellular coverage (service area 6 Km).
      • Beam transmission instead of in all directions extends the coverage of 802.11 [Vivato Inc]. Cisco Aironet, Motorola Inc Canopy Radio and Proxim may reach up to 10 Km.
      • But a cell with large outdoor coverage does not guarantee high speed connectivity due to unavoidable channel contention  throughput may degrade in overcrowded cell.
    • QoS and Mobility Management in Hybrid Wireless Networks
      • Seamless horizontal handoff and roaming among 802.11 WLAN supporting QoS anytime anywhere.
      • Vertical handoff between WLAN, mobile and ad hoc networks (MANET), Bluetooth, Universal Mobile Telecommunications System (UMTS) and Wideband Code Division Multiple Access (WCDMA)
    • WLAN – MANET Integration
      • [Lamont & Wang]:
        • Routing within MANET is handled by the Optimised Link State Protocol (OLSP).
        • Handoff between MANETs & WLANs is supported through automatic node detection and node switching capabilities of the mobiles.
        • Functionalities of OLSP are extended to support Mobile IPv6
    • WLAN – Bluetooth Integration
      • [Conti & Dardari]:
        • Analytical model for evaluation of the interference between IEEE 802.11 & Bluetooth.
        • In the proposed model PHY and MAC layers are considered, and the model can be easily implemented.
        • Performance is evaluated by packet error probability in terms of the relative distanced between the two systems for different conditions.
    • WLAN – 3G Integration
      • [Jaseemudin]:
        • A mobile node is maintaining two connections in parallel :
          • Data connection through WLAN.
          • Voice connection through UMTS.
      • [Park &Yoon]:
        • Vertical handoff between WLANs and CDMA
        • Real time traffic takes into account handoff delay
        • Best effort traffic takes into account throughput.
    • WLAN – 3G Integration cont’d
      • [Buddhicot & Chandranmenon]:
        • Combination of the features of high rate small-coverage WLAN and wide-coverage low rate 3G to improve the QoS and flexibility of wireless service.
        • A loose integration approach is realized with an IOTA gateway and a new client software in order to support seamless mobility, OoS guarantees and multiprovider roaming agreements.
    • WLAN – 3G Integration IOTA gateway
    • WLAN – 3G Integration cont’d
      • Integration of WLANs and 3G/4G requires a low call dropping probability in the 3G/4G networks.
      • [Lou & Li]:
        • Adaptive allocation scheme termed measurement based preassignment in order to prevent handoff failure in wireless cellular networks. A periodic measurement of traffic status within a cell help to adjust the number of reserved channels for handoff.
    • Summary
      • Classification of QoS schemes
        • Link adaptation in the PHY layer.
        • Channel access coordination in the MAC layer.
        • Admission control strategies in MAC & higher layers.