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ISSUES IN WIRELESS MAC PROTOCOLS Mohit Virendra Peng Lin Vidhya Seran

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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?. - PowerPoint PPT Presentation

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Page 1: ISSUES IN WIRELESS MAC PROTOCOLS

ISSUES IN WIRELESS MAC PROTOCOLS

Mohit VirendraPeng Lin

Vidhya Seran

Page 2: ISSUES IN WIRELESS MAC PROTOCOLS

OUTLINE MAC Fairness in Wireless Ad-

Hoc Networks MAC Fairness in Wireless Cellular

Networks Power Controlled Multiple Access

Protocol for Wireless Ad-Hoc Networks

Page 3: ISSUES IN WIRELESS MAC PROTOCOLS

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

Page 4: ISSUES IN WIRELESS MAC PROTOCOLS

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)

Page 5: ISSUES IN WIRELESS MAC PROTOCOLS

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.

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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

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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

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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)

Page 9: ISSUES IN WIRELESS MAC PROTOCOLS

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

Page 10: ISSUES IN WIRELESS MAC PROTOCOLS

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)

Page 11: ISSUES IN WIRELESS MAC PROTOCOLS

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.

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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…..

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The Fair Share Estimation Algorithm

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HowFair….(contd):Adjustment of Contention Window:

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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.

Page 16: ISSUES IN WIRELESS MAC PROTOCOLS

Simulation and Results(NetWk Configs)

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Results (contd..)(a) Station throughput (b)fairness index versus station’s offered load for the 4-station scenario.

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Results (contd..) Station throughput (a)original algorithm (b) modified algorithm

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Results (contd..) (c) fairness index versus station offered load for the 5 station scenario

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Results (contd..) (a) Link throughput algorithm (b) link throughput (modified algorithm,Ø=0.5 for all)

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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

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Results (contd..) (a) Station throughput, (b) fairness index versus station’s offered load for the 6-station scenario.

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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.

Page 24: ISSUES IN WIRELESS MAC PROTOCOLS

OUTLINE MAC Fairness in Wireless Ad-Hoc

Networks MAC Fairness in Wireless

Cellular Networks Power Controlled Multiple Access

Protocol for Wireless Ad-Hoc Networks

Page 25: ISSUES IN WIRELESS MAC PROTOCOLS

Wireless Fairness Scheduling

Why we need wireless scheduling?

1. Provide short-term fairness 2. Provide short-term throughput bounds 3. Provide delay bounds for packets4. Decouple delay/bandwidth requirements

Page 26: ISSUES IN WIRELESS MAC PROTOCOLS

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

Page 27: ISSUES IN WIRELESS MAC PROTOCOLS

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

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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

Page 29: ISSUES IN WIRELESS MAC PROTOCOLS

SBFA

Server-based fairness approach, proposed by P. Ramanthan and P. Agrawal

Statistically reserve a fraction of the bandwidth, no compensation

Page 30: ISSUES IN WIRELESS MAC PROTOCOLS

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

Page 31: ISSUES IN WIRELESS MAC PROTOCOLS

Wireless Channel Characteristics

Channel capacity is dynamically time-varying, due to fading/contention

Channel errors are in nature location-dependent and bursty

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Wireless Fair Service

Scheduling Targets:1. Short-term fairness among backlogged flows

with clean channels.2. Long-term fairness among backlogged flows

with bounded channel error3. Short-term throughput bounds for flows with

clean channels4. Long-term throughput bounds for flows with

bounded channel error

Page 33: ISSUES IN WIRELESS MAC PROTOCOLS

Wireless Fair Service (Cont)

Definitions1. Error free service2. Lead & Lag Model3. Compensation Model4. Slot queues & packet queues5. Channel monitoring & prediction

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WFS Service model

Page 35: ISSUES IN WIRELESS MAC PROTOCOLS

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

Page 36: ISSUES IN WIRELESS MAC PROTOCOLS

Lead and lag model

Three types of flows:1. Leading flows: the flows which

receive excess service 2. Lagging flows: the flows which

relinquish slots due to expected channel errors

3. In-sync flows: the flows which follow the idealized service model

Page 37: ISSUES IN WIRELESS MAC PROTOCOLS

Compensation model

Swapping slots between leading & lagging flows

In-sync flows unaffected Gradually swapping to avoid the

grabbing of the channel

Page 38: ISSUES IN WIRELESS MAC PROTOCOLS

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

Page 39: ISSUES IN WIRELESS MAC PROTOCOLS

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

Page 40: ISSUES IN WIRELESS MAC PROTOCOLS

Comparison of Wireless Scheduling Algorithms

Scenario: Flow 1 is in error till t = 100 sec, Flow 2 & 3 are always error-free

Page 41: ISSUES IN WIRELESS MAC PROTOCOLS

Comparison of Wireless Scheduling

Algorithms

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Comparison of Wireless Scheduling Algorithms(cont)

Page 43: ISSUES IN WIRELESS MAC PROTOCOLS

Comparison of Wireless Scheduling Algorithms(cont)

Page 44: ISSUES IN WIRELESS MAC PROTOCOLS

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

Page 45: ISSUES IN WIRELESS MAC PROTOCOLS

OUTLINE MAC Fairness in Wireless Ad-Hoc

Networks MAC Fairness in Wireless Cellular

Networks Power Controlled Multiple

Access Protocol for Wireless Ad-Hoc Networks

Page 46: ISSUES IN WIRELESS MAC PROTOCOLS

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.

Page 47: ISSUES IN WIRELESS MAC 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.

Page 48: ISSUES IN WIRELESS MAC PROTOCOLS

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

Page 49: ISSUES IN WIRELESS MAC PROTOCOLS

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.

Page 50: ISSUES IN WIRELESS MAC PROTOCOLS

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.

Page 51: ISSUES IN WIRELESS MAC PROTOCOLS

OPERATION

Page 52: ISSUES IN WIRELESS MAC PROTOCOLS

PROBLEMS AND APPROACH TO THE SOLUTION While acquring the floor to enable collision

avoidance from hidden and exposed stations ,this method preculdes multiple concurrent transmissions over the region of the acquired floor.

To optimize spatial channel reuse in a shared wireless channel network, a pair of communicating nodes must only acquire the minimum area of the floor that is needed for it to successfully complete a data transmission.Figure(2)

Unfortunately, it turns out that for collision avoidance mechanisms to work correctly, the control and data packets must be transmitted with a fixed power

Page 53: ISSUES IN WIRELESS MAC PROTOCOLS

Motivation for power controlled in collission avoidance-based medium access

Page 54: ISSUES IN WIRELESS MAC PROTOCOLS

PCMA Goal is to change the on/off fixed power

transmission model to a more flexible bounded and variable power controlled transmission model.

The fundamental change:unlike current protocols that use the reception of control packets as an on/off trigger for transmission/deferral by hidden and exposed stations,this approach uses the signal strength of a received control message to bound the transmission power of these stations.

Page 55: ISSUES IN WIRELESS MAC PROTOCOLS

PRINCIPLES Tow key principles:

Power conserving principle: each station must transmit at the minimum power level that is required to be successfully heard by its intended receiver.

Cooperation Principle : No station that commences a new transmission must transmit loud enough to disrupt ongoing transmissions.

Page 56: ISSUES IN WIRELESS MAC PROTOCOLS

NETWORK AND CHANNEL MODEL Channel Propagation Model

The amount of spatial reuse and transmission power required for a node to send a valid signal to its destination will depend on the gain between each source and destination which models the attenuation of the transmitter power over distance.

Gij actual gain is measured based on sender power and the receiver power.Then overcompensate to account for the distortions introduced from fading.

Page 57: ISSUES IN WIRELESS MAC PROTOCOLS

Channel propagation model Assumptions:

1.The data and busy tone channels observe similar gains.

2.Channel reciprocity hold so that the gain between two nodes is approximately the same in both in both directions.

3.The channel gain is stationary for the duration of the control packet and data packet transmissions.

Page 58: ISSUES IN WIRELESS MAC PROTOCOLS

NOTATIONS Pt_Max and Pt_Min-maximum and minimum

transmission power for the transmitter for data channel respectively.

Rx_Thresh and CS_Thresh-minimum received power for receiving a valid packet and for sensing a carrier respectively.

SIR_Thresh –Capture threshold ,minmum signal to interference ratio for which the reciver can successfully recive a packet.

Pnj is the total noise that node j observes on the data channel.

Page 59: ISSUES IN WIRELESS MAC PROTOCOLS

POWER CONSTRAINTS

iik

kki

rkk

nj

tjijj

ijij

i

boundPtG

EPtAlso

ThreshSIR

PE

P

PGSIR

ThreshRXPG

MaxPtPtMinPt

_min,

_.4

.3

_.2

__.1

Page 60: ISSUES IN WIRELESS MAC PROTOCOLS

Power Constraints Critical Issues:

A)handshaking between a transmitter –receiver pair to determine the minimum transmission power that satisfies constraints 2 and 3(power conserving principle)

B) For every receiver to advertise its noise tolerance so that no potential transmitter will disrupt its ongoing reception applying constraint 4 (cooperative principle)

Page 61: ISSUES IN WIRELESS MAC PROTOCOLS

PCMA PROTOCOL The on/off model proposed is a bounded power

model. Two Main mechanisms to achieve this model:

A request power to send(RPTS)/acceptable power to send(APTS) handshake between the data sender and receiver-used to determine minimum transmission power.

The noise tolerance advertisement is used by each active receiver to advertise the maximum additional noise power it can tolerate ,given its current received signal and noise power levels.

The packet handshake sequence on the data channel is RPTS-APTS-DATA-ACK.

Page 62: ISSUES IN WIRELESS MAC PROTOCOLS

PROTOCOL STEPSi j

l

APTS

Step2RPTSStep

1

DATAStep3 Send Busy tone

Step 4

Step 5

ACK Step 6Step 7

Page 63: ISSUES IN WIRELESS MAC PROTOCOLS

PROTOCOL STEPS Step1:Node I in its IDLE state monitors the busy

tone to determine its power bound Pt_bound by measuring the maximum power received on the busy tone channel over a threshold time window.

Step 2:Channel Gain is computed,Gij and the receiver then requires the data to be sent at

Step 3: Source receives APTS packet and transmits the DATA at Pti_des on the data channel if the bound is satisfied

Step 4: Receiver starts sending busy tone pulses on busy tone channel

ijij G

DjPnDesSIR

G

desRXdesPti

_._,

_max_

Page 64: ISSUES IN WIRELESS MAC PROTOCOLS

Protocol Steps contd Step 5: When A node l receives the busy

tone , it calculates its transmission power bound imposed by j

Step 6: When the destination receives the entire data packet without errors, it sends an ACK.

Step 7: if the source receives a valid ACK it resets the max back off and returns to the IDLE state ,otherwise, it increase the max back off and starts over.

Page 65: ISSUES IN WIRELESS MAC PROTOCOLS

PERFORMANCE OF PCMA Parameter settings

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Performance of PCMA to 802.11 and IPC

100 nodes in a 1000x 1000 meter network with 100 flows each sending 2KB packets and a connectivity range of 250 meters.

X axis-Flow rate,Yaxis-Utilization

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Performance of PCMA to 802.11 and IPC

Throughput for a 100x100 meter network with 100 flows each sending 2Kb packets and a connectivity range of 250 meters.

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Performance of PCMA to 802.11 and IPC

Throughput for 100x100meter network with nodes separated into clusters regions

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Performance of PCMA to 802.11 and IPC

Destination range distribution for PCMA with Pt_max=Pt+4dB

X axis-Range, Y axis-Fraction of Packets sent to range

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Performance of PCMA to 802.11 and IPC

Destination range distribution for PCMA with Pt_max=Pt+8dB

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Performance of PCMA to 802.11 and IPC

Throughput for different amounts of busy tone distortion with varying compensations in a 1000x1000 meter network.

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CONCLUSION The performance results show that PCMA can

achieve more than 2 times improvement in aggregate bandwidth compared to 802.11 for highly dense networks.

When users communicate locally , the protocol provides improvements in throughput and increases scalability.

PCMA is a protocol design in progress.Future work may include fairness properties of PCMA, performance under mobility and evaluating in a multihop wireless networks.

Page 73: ISSUES IN WIRELESS MAC PROTOCOLS

Reference Wireless Fair Scheduling: P. Bhagwat, P. Bhattacharya, A. Krishma and S. Tripathi, “Enhancing

throughput over wireless LANs using channel state dependent packet scheduling”, IEEE INFOCOM’96

T.S. Ng, I. Stoica and H. Zhang, “Packet fair queueing algorithms for wireless networks with location-dependent errors”, IEEE INFOCOM’98

M. Srivastava, C. Fragouli and V. Sivaranan, “ Controlled multimedia wireless link sharing via enhanced classbased queueing with channel-state-dependent packet scheduling”, IEEE INFOCOM’98

P. Ramanathan and P. Agrawal, “Adapting packet fair queueing algorithms to wireless networks”, ACM MOBICOM’98

S. Lu, T. Nandagopal and V. Bharghavan, “Fair scheduling in wirless packet networks”, ACM MOBICOM’98

Page 74: ISSUES IN WIRELESS MAC PROTOCOLS

Reference PCMA:

Jeffrey P. Monks, Vaduvur Bharghavan, and Wen-mei Hwu, "A Power Controlled Multiple Access Protocol for Wireless Packet Networks," IEEE INFOCOM 2001, Anchorage, Alaska, April, 2001

Jeffrey P. Monks, Vaduvur Bharghavan, and Wen-mei Hwu, "Transmission Power Controlled for Multiple Access Wireless Packet Networks," Proceedings of The 25th Annual IEEE Conference on Local Computer Networks (LCN 2000), Tampa, FL, Nov., 2000

Jeffrey P. Monks, Transmission Power Control for Enhancing The Performance of Wireless Packet Data Networks, PhD Thesis, Department of Electrical and Computer Engineering, University of Illinois, Urbana, IL, March, 2001

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Reference

Ad-Hoc Mac Fairness: DFWMAC:

http://www.ietf.org/html.characters/manet-characters.html MACAW: A Media Access Protocol forWireless LAN's (1994):  Vaduvur

Bharghavan, Alan Demers, Scott Shenker, Lixia Zhang , 1994 SIGCOMM Conference

Fair Medium Access in 802.11 based Wireless Ad-Hoc Networks:Brahim Bensaou,Yu Wang, Chi Chung Ko,Mobihoc 2000

Achieving MAC Layer Fairness in Wireless Packet Networks:Thyagrajan Nandgopal,Tae-Eun Kim,Xin Gao,Vaduvur Bhargavan,Mobicom 2000

A New Model for Packet Scheduling in Multihop Wireless Networks: Haiyun Luo, Songwu Lu,Vaduvur Bhargavan, Mobicom 2000