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ISSUES IN WIRELESS MAC PROTOCOLS. Mohit Virendra Peng Lin Vidhya Seran. OUTLINE. MAC Fairness in Wireless Ad-Hoc Networks MAC Fairness in Wireless Cellular Networks Power Controlled Multiple Access Protocol for Wireless Ad-Hoc Networks. Why Mac Fairness?.
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ISSUES IN WIRELESS MAC PROTOCOLS Mohit Virendra Peng Lin Vidhya Seran
OUTLINE • MAC Fairness in Wireless Ad-Hoc Networks • MAC Fairness in Wireless Cellular Networks • Power Controlled Multiple Access Protocol for Wireless Ad-Hoc Networks
Why Mac Fairness? • Mobile Stations share a common broadcast channel. • Existing protocols cannot prevent the “Capture Effects” • Hidden Terminal Problem and Exposed Terminal Problem. • Tradeoff between fairness and channel utilization
Fairness:Various Approaches • 1.DFWMAC (IEEE 802.11 std.) [1] • 2.MACAW :Improvement over MACA (Multiple Access Collision Avoidance) [2] • Distributed Fair Scheduling • Flow-Graph Based Approach etc. • Estimation Based Fair Medium Access: Improvement over earlier approaches. (based on MACAW and DFWMAC)
Brief Overview of MACAW • Uses modified RTS-CTS-DS-DATA-ACK message exchange. • Uses modified BEB algorithm(milder): Collision:Finc(x)=MIN[1.5x,BOmax] Success:Fdec(x)=MAX[x-1,BOmin] • Per Stream fairness not Per Station (allocates bandwidth equally to streams and not stations) • Results in 37% throughput improvement with 6% overhead addition over MACA.
Problems with MACAW: • In above configuration when load increases to a certain degree,st3 captures channel and st2 suffers degradation in throughput • Backoff Copy scheme works only when congestion is homogeneous
Estimation based Fair Medium Access: Notations: • Ø(i) :A predefined fairshare that station i should receive • W(i) :The actual throughput achieved by station i. • L(i) :Station i’s offered load
Desirable properties • Station i’s offered load to channel is less than capacity: W(i)=L(i) • Station’s offered load> Capacity: each station should be able to get its fair share of the channel,i.e. prop. to Ø • Thus ideally for i and j: W(i)/ Ø(i)=W(j)/ Ø(j)
Description • We define Fairness Index (FI): FI=max{ұ i,j :max[W(i)/Ø(i) ,W(j)/Ø(j)] / min [W(i)/Ø(i) ,W(j)/Ø(j)] } • Actual case: Abs(W(i)/ Ø(i)-W(j)/ Ø(j)) should be bounded by smallest value. • Our Goal: Design a dist MAC protocol that minimizes FI and achieves fairness
Description (contd.) • Choice of Ø(i):(open research problem) • Assumption here (no admission control) • Ø(i) = 0.5 (regardless of neighbors) • Ø(o)= 1- Ø(i)=0.5 (per station fairness) • E.g. Station with two active links: Ø(i)/Ø(o) = Ø(i)/(1-Ø(i)) =2/1 Thus Ø(i) ~ 0.67 (per stream fairness)
Description (contd.) Back off Scheme Notations: • W(ei):The estimated share of estimating station itself. • W(eo):The estimated share of other stations. • T(type):Time to transmit a packet of type type.
How Fair-Share Estimation algorithm works: • Station i sees itself competing with a group of stations for channel access. • Stations dynamically estimate what throughput “they” get and what throughput “others” get and adjust their contention window according to the FI. • Station i estimates “others” bandwidth by looking at the packets in its vicinity • FI(e)=(W(ei)/ Ø(i)) / (W(eo)/ Ø(o)) contd…..
How Fair…(contd.):ContentionWindow adjustment • In Algorithm2, C is a constant to adjust adaptivity of the algorithm. • Smaller C:more aggressively contention window adjusted. • C=2, possibility of collision high in high load and large no of competing stations • C close to 1 (1.01), stations busy adjusting their contention windows all the time and algorithm becomes unstable.
Results (contd..)(a) Station throughput (b)fairness index versus station’s offered load for the 4-station scenario.
Results (contd..) Station throughput (a)original algorithm (b) modified algorithm
Results (contd..) (c)fairness index versus station offered load for the 5 station scenario
Results (contd..) (a) Link throughput algorithm (b) link throughput (modified algorithm,Ø=0.5 for all)
Results (contd..) (c)link throughput (modified algorithm,Ø=0.67 f0r station 2,3 and 4) (d) FI versus station offered load for the 5-station scenario
Results (contd..) (a) Station throughput, (b) fairness index versus station’s offered load for the 6-station scenario.
Summary • A different scheme for IEEE 802.11 DFWMAC • Contention window adjustment according to the estimated share . • Achieves far better fairness than others though some throughput sacrificed • Does not assume any knowledge of network topology,thus does not require broadcast packets to disseminate info to other stations:very simple to overlay on existing DFWMAC.
OUTLINE • MAC Fairness in Wireless Ad-Hoc Networks • MAC Fairness in Wireless Cellular Networks • Power Controlled Multiple Access Protocol for Wireless Ad-Hoc Networks
Wireless Fairness Scheduling Why we need wireless scheduling? Provide short-term fairness Provide short-term throughput bounds Provide delay bounds for packets Decouple delay/bandwidth requirements
CSDPS • Channel state dependent packet scheduling algorithm, proposed by P. Bhagwat • One step channel prediction and no compensation • Lagging flows can only make up in long run
IWFQ • Idealized wireless fair queueing algorithm, proposed by S. Lu, V. Bhaghavan and R. Srikant • Lagging flows will capture the channel whenever they perceive clean channels
CIF-Queueing • Channel independent fair queueing Algorithm, proposed by T.S. Ng, I. Stoica and H. Zhang • Leading flows relinquish their leads linearly and distribute to lagging flows proportional to their weights
SBFA • Server-based fairness approach, proposed by P. Ramanthan and P. Agrawal • Statistically reserve a fraction of the bandwidth, no compensation
CBQ-CSDPS • Class-based queueing with channel state dependent packet scheduling • Maintain lead and lag based on the actual number of bytes transmitted during a time window • Lagging flows are given explicit precedence, and hence capture the channel
Wireless Channel Characteristics • Channel capacity is dynamically time-varying, due to fading/contention • Channel errors are in nature location-dependent and bursty
Wireless Fair Service • Scheduling Targets: • Short-term fairness among backlogged flows with clean channels. • Long-term fairness among backlogged flows with bounded channel error • Short-term throughput bounds for flows with clean channels • Long-term throughput bounds for flows with bounded channel error
Wireless Fair Service (Cont) • Definitions • Error free service • Lead & Lag Model • Compensation Model • Slot queues & packet queues • Channel monitoring & prediction
Error-free service model • A reference for how much service a flow may receive in an ideal error-free channel environment • WFQ is adopted as the error-free service model
Lead and lag model • Three types of flows: • Leading flows: the flows which receive excess service • Lagging flows: the flows which relinquish slots due to expected channel errors • In-sync flows: the flows which follow the idealized service model
Compensation model • Swapping slots between leading & lagging flows • In-sync flows unaffected • Gradually swapping to avoid the grabbing of the channel
Slot queues and packet queues • Separate the logic packet flow queue and the MAC slot queue • Packet flow queue may adopt any packet dropping policy • Slot queue follows the swapping policy
Channel monitoring & prediction • Channel errors are highly correlated • One-step prediction: The channel state for the current time slot is predicted to be the same as the monitored channel state for the previous slot
Comparison of Wireless Scheduling Algorithms • Scenario: Flow 1 is in error till t = 100 sec, Flow 2 & 3 are always error-free
Summary • Several wireless fair scheduling algorithms have been proposed to address the fairness issues in wireless networks with time-varying capacity • The performance of such wireless scheduling algorithms depends on the precision of channel monitoring/prediction methods
OUTLINE • MAC Fairness in Wireless Ad-Hoc Networks • MAC Fairness in Wireless Cellular Networks • Power Controlled Multiple Access Protocol for Wireless Ad-Hoc Networks
MOTIVATION • One of the major issue in wireless networks is developing efficient multiple access protocols that optimizes spectral reuse and hence maximize aggregate channel utilization. • Theoretical studies have shown that ideal medium access protocols using optimal power can improve aggregate channel utilization. • This motivates the study for power controlled wireless medium access protocols.
PAST WORK ON POWER CONTROL • Past work on power control has primarily dealt with cellular networks and the base station provides centralized control. • Distributed power control algorithms have also been presented but still require fundamental cellular configuration. • Other work focused on MAC protocols that control transmission power level to conserve power consumption.
PCMA • PCMA differs from the related work in two significant ways: • A) Focus on wireless multiple access networks where all nodes share a channel and there is no centralized control. • B) Focus on power control mechanism for increasing channel efficiency rather that as a mechanism for increasing battery life
PCMA-contd • Dominant wireless MAC is IEEE802.11 standard follows the CSMA/CA paradigm. • There exists no power control MAC that fits within the collision avoidance framework. • Goal is to propose a power controlled MAC that follow the same collision avoidance framework.
PROBLEMS AND APPROACH TO THE SOLUTION • MAC have made the case that a sender receiver pair should first acquire the floor before initiating a data packet transfer. • Acquiring the floor allows sender-reciver pair to avoid collision due to hidden and exposed stations in the shared channel.