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Scheduling Algorithms in Broad-Band
Wireless Networks
YAXIN CAO AND VICTOR O. K. LI, FELLOW, IEEE
IEEE PROCEEDINGS OF THE IEEE, VOL. 89, NO. 1, JANUARY 2001
報告者 : 李宗穎
Outline
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Introduction
System Model
Major Issue in Wireless Scheduling
Different Scheduling Methods
Compared and Conclusion
2
Introduction
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The characteristics of wireless communication
pose special problems that do not exist in wireline
networks
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high error rate and bursty errors
location-dependent and time-varying wireless link
capacity
scarce bandwidth
user mobility
power constraint
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Wireless Network Model
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Downlink
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the base station has full knowledge of the status
of downlink queues
Uplink
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The base station performs uplink scheduling
based on these requests and related information
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Wireless Link Model
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good (or error-free)
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the wireless link is assumed to be error-free
bad (or error)
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packets transmitted on the link will be
corrupted with very high probability
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Major Issues
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Wireless Link Variability
Fairness
QoS
Data Throughput and Channel Utilization
Power Constraint and Simplicity
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Wireless Link Variability
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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
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wireline media may be considered errorfree ,the wireless link is actually in an errorstate
the packet will be corrupted and the
transmission will waste transmission
resources in error-state
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QoS
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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
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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
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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 state
LSM
mark
Waiting
time out
It does not have any mechanism to guarantee bandwidth and
the algorithm does not provide any guarantees on packet delay
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CSDPS +
CBQ (class-based queueing)
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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)
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Queue size
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leading
lagging
in sync
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Idealize Weight Fair Queue
(IWFQ) (2/3)
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When a packet of sequence number of flow arrives, it is
tagged 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
Li,n : packet size of the arrived packet
V(A(t)) : system virtual time defined in WFQ
ri : service rate allocated to flow
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The scheduler always chooses to serve the packet with the
smallest finish time
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Idealize Weight Fair Queue
(IWFQ) (3/3)
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Lagging bound
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all flows that will be compensated is bounded by B bits
A flow i with weight ri is allowed to compensate a
maximum of bi  B  (ri /  jF rj )
Leading bound
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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
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channel-condition independent
packet fair queueing (CIF-Q) (1/4)
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Each flow has its own queue, and the real errorprone scheduling system is S associated with an
error-free system Sr
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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)
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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)
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packet in S is transmitted unless one of the
following situations occurs
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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)
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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
preallocated 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)
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deficit counter (DC)
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compensation counter (CC)
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keeps a record of the total credit received less the credit
used
CC keeps track of the amount of lost service for each
flow
quantum size (QS)
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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)
bad state

QS1 = 100, QS2 = 50
α 1 = 1/3, α2=1/2
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
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Comparison
Delay Bound
Long-tern
Throughput
Guarantee
Short-tern
fairness
Pre-allocated
Resource for
Compensation
CSDPS
☆
CSDPS + CBQ
IWFQ
☆
☆
CIF-Q
☆
☆
☆
SBFA
I-CSDPS
☆
☆
☆
☆
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Future Work
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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
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This paper presented a comprehensive and
in-depth survey on current research in
wireless packet scheduling.
The major issues in wireless scheduling
were discussed
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