Wireless Resource Management through Packet Scheduling

• Outline for this lecture identify the design challenges for QoS support

over wireless mobile networks an initial solution ongoing research

Environment: Packet Cellular Networks

Base Station

Fixed Host

Wireless Cell

Backbone

Mobile Host

Refresh your memory: Packet Scheduler

Select the next packet for transmission

Endhost

switch

Scheduling: Achieving QoS at the packet level time scaleScheduling: Achieving QoS at the packet level time scale

InputLink

FabricOutputLink

Scheduler

Issues for Wireless Packet Scheduling

• #1: Location-dependent wireless channel error

backbone

MH #1

MH #2

Base Station

Sender

Scheduling policy

Channel state

21

2

1

Issues for Wireless Packet Scheduling

#1 a channel state unware scheduler may schedule wrongly

#2 channel capacity for each user is dynamically changing

• #1: Location-dependent wireless channel error

backbone

21MH #1

MH #2

Base Station

Sender

Scheduling policy

2

1

Channel state

• #2: Bursty wireless channel error

Observation 1: case for accurate channel state estimation Observation 2: case for deferring transmission

Issues for Wireless Packet Scheduling

[Source: D. Eckhardt, P. Steenkiste, “A trace-based evaluation of adaptive error correction for a wireless LAN,” ACM

MONET, 1998]

• #1: Location-dependent wireless channel error

• #2: Bursty wireless channel error

• #3: MHs do not have global channel state for scheduling

• distributed scheduling• #4: MHs are often constrained in terms of processing power

• “dumb terminal, smart base stations”

• #5: Contention in channel access among MHs Close interaction among scheduling and Medium Access Control (MAC)

Issues for Wireless Packet Scheduling

Goals for Wireless Packet Scheduling

• Throughput: Short-term throughput bounds for flows that perceive error free channel Long-term throughput bounds for flows that perceive bounded channel error

• Fairness: Short-term fairness for flows that perceive clean channel Long-term fairness for flows that perceive bounded channel error

Goal: Provide channel-conditionedQoS for multimedia over wireless

A Comprehensive Quality of Service Model for Wireless Packet Scheduling cont’d.

• Channel-conditioned delay bounds for packets

• Support for diverse applications: Both delay-sensitive and loss-sensitive applications Accept flows with different decoupled delay/bandwidth requirements

optimization of the schedulable region

Graceful service degradation and compensation

Goal: Provide channel-conditionedQoS for multimedia over wireless

Conventional Approaches for Wireless Packet Scheduling

• #1: FIFO, WRR, etc.: do NOT address wireless link issues Location dependent channel error Bursty channel error inefficient link utilization users are exposed to all channel errors

• #2: address wireless link issues but NO QoS P. Bhagwat et. al. “Channel State Dependent Packet

Scheduling (CSDPS)”, INFOCOM’96 Not able to support multimedia and provide fair service

Two Design Principles for QoS oriented Wireless Packet Scheduling

• #1: Fair Queueing providing QoS in the error-free case

• #2: Adaptation to location dependent and bursty

channel error via compensation addressing wireless link issues

Introduction to Wireline Fair Queueing

• A popular paradigm to achieve QoS at the packet level throughput guarantees packet delay guarantees fairness various algorithms, WFQ, WF2Q, SCFQ, STFQ, ... ...

• Key idea: flow separation a fluid fair queueing system packetized approximation of the fluid model works regardless of differences in packet size

Review: Wireline Fair Queueing Cont’d

F1

F2

F3

F1: weight = 0.25

F2: weight = 0.5

F3: weight = 0.25

t=1t=0 t=2

Review: Wireline Fair Queueing Cont’d

F1

F2

F3

1/2

1/4

1/4

F1: weight = 0.25

F2: weight = 0.5

F3: weight = 0.25

t=1t=0

Key Idea: Complete flow separation !

Fair share of excess resources

Review: Wireline Fair Queueing Cont’d

F1

F2

F3

t=1t=0

1/2

1/4

1/4 1/3

2/3

F1: weight = 0.25

F2: weight = 0.5

F3: weight = 0.25

t=2

t [0,1]

backbone

Base StationSender1/3

2/3

Equal weights

t=0 1 F1

F2

Why Wireline Fair Queueing Fails in Wireless Networks

F3: CBR

backbone

Base StationSender1/3

2/3

Equal weights

F1

F2

Why Wireline Fair Queueing Fails in Wireless Networks

1/3

1/31/3

t=0 1 2

Instantaneous fairness is NOT equal to long term fairness !

“Memoryless” allocation of WFQ --> no fairness among F1, F2 and F3 !

F3: CBR

How to adapt to wireless channel conditions and provide QoS ?

• Approach: book-keeping the (recent) history of channel allocation and explicitly controlling future allocations

Channel swapping & compensation

t [1,2]

backbone

Base StationSender

2/3

1/31/3

2/3

Equal weights

t=0 1 2F1

F2

F3: CBR

Case for Graceful Compensation

• To prevent flow starvation over a short time scale

backbone

Base StationSender1/31/3

2/3

t=0 1 2 3

Equal weights

F1

F2

F3: CBR

A Comprehensive Wireless QoS Model • Throughput:

Short-term throughput bounds for error-free flows Long-term throughput bounds for error-prone flows

• Fairness: Short-term fairness for error-free flows Long-term fairness for error-prone flows

• Channel-conditioned delay bounds for packets

• Support for both delay sensitive & loss sensitive applications

• Delay and bandwidth decoupling

• Graceful Service Degradation and Compensation: Graceful service degradation for leading flows Graceful service compensation for lagging flows

Unified Framework for Wireless Fair Queueing: Key Components

• Error-Free Service Model: defines an ideal fair service model assuming no channel error

• Lead and Lag Model: how much service a flow should relinquish or get compensated by

• Compensation Model: compensate for lagging flows at the expense of other flows

• Slot Queues and Packet Queues: support for both delay sensitive and loss sensitive flows in a framework

• Channel State Monitoring and Estimation

• MAC design

Error-free service

Channel state estimation

Lead &lag model

Compen.model

MAC

A Flow Chart for

the Architecture:

how the components

interact

A Few Wireless Scheduling Algorithms

• Channel State Dependent Packet Scheduling (CSDPS) and its enhanced version (CBQ-CSDPS)

• Idealized Wireless Fair Queueing (IWFQ) and its variant WPS

• Channel-condition Independent Fair Queueing (CIF-Q)

• Server Based Fairness Approach (SBFA)

• Wireless Fair Service (WFS)

Component #1: Error Free Service Model

Serves as an “ideal” service model that characterizes the

best you want to achieve

In principle, any wireline fair packet scheduling

algorithm is a candidate:• throughput guarantees• packet delay bound• fairness• delay bandwidth decoupling• implementation complexity

Examples: WFQ, WF^2Q, STFQ, SCFQ, ...

Component #2: Lead and Lag model• Keep track of the difference between

service that each flow should receive in the error-free service model

accumulative service that each flow has actually received over the error-prone wireless channel

• Classify a flow as “lead,” “lag,” or “in-sync” accordingly• A flow’s status (i.e., leading, lagging, in-sync) can

dynamically change with time• a small catch in the above definition: for some slots, what

about the case when no flow can transmit (i.e. error prone for all flows)

Lead and Lag Model: An Alternative Definition

• A flow updates its lag if all 3 conditions hold: it is allocated a slot for transmission, it is unable to transmit due to channel error another flow can transmit in current slot and is

willing to give up a slot later

• A flow updates its lead if all 3 conditions hold: another flow gives up its slot due to channel error it uses the slot given up by the error-prone flow it is willing to give up a slot in future to compensate

other flows

Example: Lead and Lag Model

backbone

Base StationSender

F1

F21 2 3

1 2

1 2 3

4

41

t=0

3

Error Free Service: WFQ

r=1/3

r=1/3

r=1/3

Real Service

F1: lag = 0

F3: CBR

Example: Lead and Lag Model

backbone

Base StationSender

F1

F21 2 3

1 2

1 2 3

4

41

t=0

3

Real Service

Error Free Service: WFQ

r=1/3

r=1/3

r=1/3

F1: lag = 0

1

1

1

1

2 2

2 3

2

2

3

4 5

3

3

3

F1: lag = 2F2: lead = 2

F3: CBR

Further Subtle Issues in Lead/Lag Model

• Who should receive the “extra” service that is given up by error-prone flows ? Equal treatment: any flow that perceives a clean

channel Preferential treatment: lagging flows first,

leading flows next, in-sync flows last

Component #3: Compensation Model• Knowing the lead and lag of an individual flow, how

to compensate lagging flows at the expense of leading flows ?

• Control the compensation process: who participate ?

• All flows ?

• Only leading and lagging flows ? when to compensate ?

• Immediate or deferred How fast to compensate ?

• As quick as possible

• in a more controlled manner: graceful service

Component #3: Rate Compensation for Leading Flows in WFS

Slot selection based onminimum service tag

Tra

nsm

it

Com

pens

atio

nAggregate

compensation slots

T

rans

mit

Com

pens

atio

n

• flow i hierarchically decomposes into two flows i: ic and it

• compensation flow ic with rate ri E(i)/Emax(i)

• transmission flow it with rate ri(1-E(i)/Emax(i))

Leading flows

Exponential service degradation during compensation !

Transmit

time

rate

Component #3: Rate Compensation for Lagging Flows in WFS

Slot selection based onminimum service tag

Tra

nsm

it

Com

pens

atio

nAggregate

compensation slots

T

rans

mit

Com

pens

atio

n

Tra

nsit

Tra

nsit

Tra

nsit

WRR for lagging flows

• service comes from normal rate compensation

• maintain a compensation WRR among lagging flows

• traverse WRR when a compensation slot is available

• fair compensation among lagging flows

Leading flows Lagging flows Insync flows

Example: Graceful Service Degradation in WFS

Example: Non-graceful Service Degradation in IWFQ

CSDPS

• Error-free service: WRR is a choice

• Lead & Lag model: no

• compensation model: no

• comments: implications for no compensation: no long-term

fairness, in-sync flows got disturbed, lagging flows have to play luck, etc.

if high-level enforcement is available, may still work

IWFQ

• Error-Free Service: WFQ

• Lead and lag model: yes

• compensation model: maintaining the tagging history -> maintain the

precedence for channel access serve the packet with minimum tag -> earliest lag

first

• comments: if lag is large, may starve other flows

CIF-Q

• Error-free service: STFQ

• Lead & Lag model: yes

• Compensation: leading flow receives a fixed fraction lagging flows receives compensation according

to their rate weights

• Comments: linear service degradation for leading flows

SBFA

• Error-free service: WFQ is a choice

• lead & lag model: no notion of leading flows

• Compensation model: reserve a fraction of bandwidth for compensation

-> a virtual compensation flow any lag is charged to this compensation flow.

• Comments: fundamentally different from others compensation capture effect, HOL blocking, ...

Summary

How to perform packet scheduling over wireless

• necessary components for wireless fair queueing

• interaction with MAC layer

Wireless Fair Packet Scheduling = Fair Queueing+ Adaptation to wireless channel characteristics

Scheduling in Multihop Wireless Networks

• Key issue: distributed packet scheduling

• Solution approaches: Backoff based design Table-driven approach

• Illustration through an example