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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