Post on 21-Dec-2015
An Overview ofScheduling Algorithms in
Wireless Multimedia Networks
Hossam Fattah, Cyril Leung(The University of British Columbia)
presented by Metin Tekkalmaz
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Outline Introduction Challenges Scheduler Components Scheduler Properties Classification Scheduling in TDMA Networks Scheduling in CDMA Networks Scheduling in Multihop Networks
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Introduction (1/3)
Function of scheduler is to select the session whose head-of-line (HOL) packet is to be transmitted next
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Introduction (2/3)
Important component for guaranteed QoS parameters delay delay jitter packet loss rate throughput
Scheduling is more difficult for wireless networks
Currently available wireline algorithms cannot be applied directly
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Introduction (3/3)
Different QoS service parameters for different applications as opposed to traditional best effort services
Service classes: nrt-VBR: non-real-time variable bit rate ABR: available bit rate UBR: unsepecified bit rate CBR: constant bit rate rt-VBR: real-time variable bit rate
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Challenges in Wireless Network Scheduler (1/2)
Time- and location- dependent signal attenuation fading interference noise
Result in bursty error time-varying channel capacity
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Challenges in Wireless Network Scheduler (2/2)
For scheduling decisions Number of sessions Reserved rates Statuses of session queues
are necessary Battery consumption Handoffs In CDMA, SIR requirements should be met Rapidly changing topology Communication range
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Components of a Scheduler Error-free service model Lead/Lag counter Compensation model A means of monitoring and
predicting the channel state
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Properties of a Scheduler (1/2)
Efficient link utilization Delay bound Fairness Throughput Implementation complexity
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Properties of a Scheduler (2/2)
Graceful service degradation Isolation Energy consumption Delay/bandwidth decoupling Scalability
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Classification of Schedulers work-conserving vs.
non-work-conserving timestamped round-robin sorted priority frame-based
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Generalized Processor Sharing-Based Scheduling (1/2)
GPS is efficient, flexible and fair Simulated by some timestamp-
based algorithms Work-conserving Provides end-to-end delay bound Provides equal normalized service Fairness index is zero -> optimal
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Generalized Processor Sharing-Based Scheduling (2/2)
GPS is simulted in a TDMA packet network using a virtual time function: v(t) [total work performed in GPS]
•B: Set of backlogged sessions
•S: Start time
•F: Finish time
•L: Length
•R: Reserved Transmit rate
•a: arrival time
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Scheduling in Wireless TDMA Networks – NW Model
One BS, multiple MSs Scheduling is at BS side BS can communicate with all MSs Direct MS-MS comm. is impossible Channel errors may be experienced Channel state and packet queue info
for each session is available at BS
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Scheduling in Wireless TDMA Networks – CSDPS
Channel State Dependent Packet Scheduling
RR, LQF, ETF can be used Errors are avoided at link level rather than
recovering at transport or application level
Channel state of each link is monitored No lead/lag concept
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Scheduling in Wireless TDMA Networks – IWFQ
Idalized Wireless Fair Queuing Realization of PGPS for error prone
sessions Each service is assigned a service
tag: virtual finish time of its HOL packet
Sessions with good channel are services according to their tags
Bounds are set for lead/lag counters
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Scheduling in Wireless TDMA Networks – CIF-Q
Channel-Condition-Independent Fair Queuing
Start Time Fair Queuing is used as error-free service model
Each session has lead and lag counter
an α value (between 0 & 1) is used for compensation between leading and lagging sessions
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Scheduling in Wireless TDMA Networks – SBFA
Server-Based Fairness Approach Any wireline scheduler can be used
as error-free service model A portion of outgoing bandwidth is
reserved for a hypothetical session Packets that cannot be sent are
scheduled for this session
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Scheduling in Wireless TDMA Networks – WFS
Wireless Fair Service Delay and bandwidth are decoupled
Packet with lowest finish time with v(t) + x, which determines schedulable interval, is sent
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Scheduling in Wireless CDMA Networks (1/2)
CDMA provides higher (soft) system capacity
Accurate power control is required A new session can be established as
long as SIRs for all transmitting sessions can be mainteined above their target levels a certain percent of time
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Scheduling in Wireless CDMA Networks (2/2)
Simultaneous transmission is possible as long as following inequality is satisfied:
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Scheduling in Wireless CDMA Networks – packet-by-packet GPS
PGPS model where multiple services can be served simultaneously
Each packet is timestamped according to equation 2
Packet with lowest timestamp is chosen to sent
Opposed to TDMA version, multiple packets can be sent at a time
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Scheduling in Wireless CDMA Networks – Scheduled CDMA
Hybrid CDMA/TDMA scheduler Data is exchanged between BS and MS in capsules
CTR (capsule transmission request) is used
BS sorts requests according to the priorities or delay tolerances
BS returns with permission capsules containing transmission slot and power
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Scheduling in Wireless CDMA Networks – Dynamic Resource Scheduling
Modified version of SCDMA w/o TDMA aspect
BS classifies requests according to traffic characteristics
Two seperate queues are used: Guaranteed Best effort
Predefined rate is provided but there is no delay guarantee
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Scheduling in Wireless CDMA Networks – WISPER
Wireless Multimedia Access Control Protocol with BER Scheduling
Packets arrive in batches All packets in a given batch have the
same expiry time Priority is directly proportional to the
remaining number of packets in the batch and inversly proportional to remaining time before expiration and max MS transmission rate
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Scheduling in Multihop Networks Ad-hoc networks with little
infrastructure support No Base Stations In singlehop networks direct MS-MS
communication is possible Relays are needed as the number of
MSs increase Topology changes rapidly
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Scheduling in Multihop Networks – NW Model
Time is divided into slots MSs have omnidirectional antennas Channel is noise-free Half-duplex communication is used Conflicts occurs:
Primary conflict Secondary conflict