1. 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

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

3. 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 capacity
scarce bandwidth
user mobility
power constraint

4. 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

5. Wireless Link Model good (or error-free) bad (or error)
the wireless link is assumed to be error-free
bad (or error)
packets transmitted on the link will be corrupted with very high probability

6. Major Issues Wireless Link Variability Fairness QoS
Data Throughput and Channel Utilization
Power Constraint and Simplicity

7. 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

8. 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

9. QoS
at least prioritized scheduling service for aggregated traffic with QoS differentiation
per-flow-based guaranteed QoS performance, such as delay or jitter bound

10. Data Throughput and Channel Utilization
minimize unproductive transmissions on error links
maximize the effective service delivered and the utilization of the wireless channels

11. Power Constraint and Simplicity
minimal number of scheduling-related control messages
the scheduling algorithm should not be too complex

12. 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

13. 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

14. Idealize Weight Fair Queue (IWFQ) (1/3)
Queue size
leading
lagging
in sync

15. Idealize Weight Fair Queue (IWFQ) (2/3)
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
The scheduler always chooses to serve the packet with the smallest finish time
Li,n : packet size of the arrived packet
V(A(t)) : system virtual time defined in WFQ
ri : service rate allocated to flow

16. 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 maximum 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

17. 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

18. 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)

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

20. 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

21. 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

22. Server-based fair approach (SBFA) (2/3)
Round Robin Policy

23. 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

24. 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

25. Improved channel state dependent packet scheduling (I-CSDPS) (2/2)
bad state
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 = 50
α 1 = 1/3, α2=1/2

26. Comparison Delay Bound Long-tern Throughput Guarantee
Short-tern fairness
Pre-allocated Resource for Compensation
CSDPS
CSDPS + CBQ ☆
IWFQ
CIF-Q
SBFA
I-CSDPS

27. 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

28. 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