Scheduling Algorithms in Broad-Band Wireless Networks

報告者 : 李宗穎

IEEE PROCEEDINGS OF THE IEEE, VOL. 89, NO. 1, JANUARY 2001

YAXIN CAO AND VICTOR O. K. LI, FELLOW, IEEE

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Outline

Introduction System Model Major Issue in Wireless Scheduling Different Scheduling Methods Compared and Conclusion

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Introduction

The characteristics of wireless communication pose special problems that do not exist in wireline networks high error rate and bursty errors location-dependent and time-varying wireless link capa

city scarce bandwidth user mobility power constraint

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Wireless Network Model

Downlink the base station has full knowledge of the status

of downlink queues Uplink

The base station performs uplink scheduling based on these requests and related information

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Wireless Link Model

good (or error-free) the wireless link is assumed to be error-free

bad (or error) packets transmitted on the link will be corrupte

d with very high probability

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Major Issues

Wireless Link Variability Fairness QoS Data Throughput and Channel Utilization Power Constraint and Simplicity

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Wireless Link Variability

wireless channels are more error-prone and suffer from interference, fading, and shadowing

some mobile hosts may enjoy error-free communication with the base station, while others may not be able to communicate at all

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Fairness

wireline media may be considered error-free ,the wireless link is actually in an error-state

the packet will be corrupted and the transmission will waste transmission resources in error-state

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QoS

at least prioritized scheduling service for aggregated traffic with QoS differentiation

per-flow-based guaranteed QoS performance, such as delay or jitter bound

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Data Throughput and Channel Utilization

minimize unproductive transmissions on error links

maximize the effective service delivered and the utilization of the wireless channels

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Power Constraint and Simplicity

minimal number of scheduling-related control messages

the scheduling algorithm should not be too complex

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Channel state dependent packet scheduling (CSDPS)

Bad stateLSMmark

Waitingtime out

It does not have any mechanism to guarantee bandwidth andthe algorithm does not provide any guarantees on packet delay

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CSDPS + CBQ (class-based queueing)

A class is called unsatisfied if it has persistent backlogs, and the service it recently received is less than its allocated fraction

When class exceeds its allocated bandwidth share and contributes to any other class’ unsatisfied state. Such a queue is called a restricted queue

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Idealize Weight Fair Queue (IWFQ) (1/3)

Queue size leading lagging in sync

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Idealize Weight Fair Queue (IWFQ) (2/3) When a packet of sequence number of flow arrives, it is tagg

ed with virtual service start time Si,n and finish time fi,n

Si,n = max{v(A(t)), fi,n-1}

fi,n = Si,n + Li,n/ri

The scheduler always chooses to serve the packet with the smallest finish time

Li,n : packet size of the arrived packetV(A(t)) : system virtual time defined in WFQri : service rate allocated to flow

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Idealize Weight Fair Queue (IWFQ) (3/3)

Lagging bound all flows that will be compensated is bounded by B bits A flow i with weight ri is allowed to compensate a maxi

mum of Leading bound

for more than li bits, it will only surrender up to li bits of service share to other flows later on

To implement this bound, the scheduler checks each leading flow after transmitting one packet

)/(

Fj jii rrBb

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channel-condition independent packet fair queueing (CIF-Q) (1/4)

Each flow has its own queue, and the real error-prone scheduling system is S associated with an error-free system Sr

Arrived packets are put into queues both in S and Sr (virtual queue)

No link error, packet is chosen in Sr and served in both S and Sr

Link error, the real packet in the queue of S is kept, but the virtual packet in the queue of Sr is still served

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channel-condition independent packet fair queueing (CIF-Q) (2/4)

lagi is flow i serving different between S and Sr

To achieve graceful degradation, a parameter α is used to define the minimal average rate (αri)

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channel-condition independent packet fair queueing (CIF-Q) (3/4) packet in S is transmitted unless one of the followi

ng situations occurs a) Link is an error state b) Leading flow and receive more than αri

Lagging flows have higher priority to receive additional service in a) and b)

the compensation is distributed among the lagging flows

If no lagging flow, the additional service is distributed to nonlagging flows

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channel-condition independent packet fair queueing (CIF-Q) (4/4)

Compared with IWFQ, CIF-Q improves scheduling fairness by associating compensation rate and penalty rate with a flow’s allocated service rate and guaranteeing flows with error-free links with a minimal service rate

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Server-based fair approach (SBFA) (1/3)

SBFA allocated to some compensation server(s), called long-term fairness server

The scheduler maintains two queues, packet queue (PQ) and slot queue (SQ) for each flow

SQ is the virtual queue in this system

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Server-based fair approach (SBFA) (2/3)

Round Robin Policy

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Server-based fair approach (SBFA) (3/3)

Problem LTFS needs prealloc

ated network resources

the algorithm does not work well if the packet size of a flow is variable

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Improved channel state dependent packet scheduling (I-CSDPS) (1/2)

deficit counter (DC) keeps a record of the total credit received less the credit

used compensation counter (CC)

CC keeps track of the amount of lost service for each flow

quantum size (QS) Determines how much credit, in number of bits or bytes,

is given to a flow in each round

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Improved channel state dependent packet scheduling (I-CSDPS) (2/2)

At the beginning of each round αCC amount of credit is added to DC, and CC is decreased by the same amount, where 0 < α < 1

QS1 = 100, QS2 = 50α 1 = 1/3, α2=1/2

bad state

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Comparison

Delay Bound Long-tern Throughput Guarantee

Short-tern fairness

Pre-allocated Resource for

CompensationCSDPS

CSDPS + CBQ ☆IWFQ ☆ ☆CIF-Q ☆ ☆ ☆SBFA ☆ ☆I-CSDPS ☆ ☆

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Future Work

Adaptive Error-Correction Coding and Deferment of Transmissions

Scheduling in CDMA Networks—Multiple Servers and Multiple Link States

Integration of Admission Control, Scheduling, and Congestion Control

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Conclusion

This paper presented a comprehensive and in-depth survey on current research in wireless packet scheduling.

The major issues in wireless scheduling were discussed