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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Introduction

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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

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Scheduling in Multihop Networks Scheduler Properties

Topology transparency Low connectivity information

requirement Basic Principles

Node Activation Link Activation