Cross-Layer DesignCross-Layer Design

-Kalpana -Kalpana UppalapatiUppalapati

AgendAgenda:a:

IntroductionIntroduction Problem definition (3 issues) Problem definition (3 issues) Background Background Description of the 3 issues Description of the 3 issues Solutions for the issues Solutions for the issues Results of the methods usedResults of the methods used Future ResearchFuture Research ConclusionConclusion

IntroductionIntroduction::

Wireless Networking:Wireless Networking:o Applications: wireless Internet access, ad hoc Applications: wireless Internet access, ad hoc

networks, sensor networksnetworks, sensor networkso Diverse requirements: high-bandwidth video Diverse requirements: high-bandwidth video

and data, low-bandwidth voice and dataand data, low-bandwidth voice and datao Goal: Goal: reliable communication-on-the-movereliable communication-on-the-move in in

highly dynamic environments, highly dynamic environments, QoS QoS provisioningprovisioning

A central problem:A central problem: How to How to efficientlyefficiently transmit transmit heterogeneous traffic over wireless links?heterogeneous traffic over wireless links?

Design Approach:Design Approach:• Layered designLayered design

Each layer can be optimized independentlyEach layer can be optimized independentlyChanges in one layer do not require changes in Changes in one layer do not require changes in

the other layersthe other layers• Cross-layer designCross-layer design

Layered approach is not adequate in Layered approach is not adequate in wireless!!wireless!!• InflexibleInflexible• No AdaptationNo AdaptationWould the advantages of cross-layer Would the advantages of cross-layer

design lead to design lead to new network architecturenew network architecture??

Why Why CLD?CLD?

Looking at the Status of Lower Looking at the Status of Lower Layer May Improve PerformanceLayer May Improve Performance

send receive

Expose lower layer status may improve performance

status

get

Configuring the Lower Layer to Configuring the Lower Layer to Improve PerformanceImprove Performance

send receive

status

get set

Control lower layer behavior may improve performance

Application+Link:perf. gain in e2e SNR by joint channel coding and compression for video over wireless

Examples of Examples of CCross-layer ross-layer DDesignesign

APPLICATIONTRANSPORT

ROUTING/CONTROL PLANEMACLINK

PHYSICAL

MAC+Link: multiuser diversity gain of throughput if scheduling is based on link availability instead of FIFO (2x)

Routing+Link: perf. improvement in percentage of packets not meeting a delay deadline through joint rate allocation and routing (10x)

(MAC+PHY) perf. gain in channel utilization through channel reservation based on physical layer parameters

MAC+Routing: perf. gain in channel capacity via joint MAC/routing protocol design. 3x difference for 802.11

Application+Link+Physical: instead of using “target bER” power adaptation, consider modifying the target bER based on application requirements

Workdone:Workdone: At application-layerAt application-layer, a media server can track , a media server can track

packet losses and adjust media source rate packet losses and adjust media source rate accordingly.accordingly.

At transport-layerAt transport-layer, several cross-layer approaches, , several cross-layer approaches, such as EBSN, Snoop TCP, and freeze TCP, have such as EBSN, Snoop TCP, and freeze TCP, have been proposed as TCP alternatives to distinguish been proposed as TCP alternatives to distinguish congestion loss from non-congestion loss and congestion loss from non-congestion loss and invoke different flow control mechanisms.invoke different flow control mechanisms.

At link and network-layerAt link and network-layer, the persistence level of , the persistence level of the MAC layer ARQ mechanism should adapt to the MAC layer ARQ mechanism should adapt to each application’s latency and reliability each application’s latency and reliability requirementsrequirements..

3 ISSUES:3 ISSUES:Issue 1:Issue 1:Cross-layer Design to achieve Quality of Service andCross-layer Design to achieve Quality of Service andresource efficiency in a multi-user resource efficiency in a multi-user

context(PHY+MAC+App)context(PHY+MAC+App)Issue 2:Issue 2:TCP–over Wireless CDMA TCP–over Wireless CDMA

Links;SnoopTCP(Link+Transport)Links;SnoopTCP(Link+Transport)Issue 3:Issue 3:Cross-Layer Approach for Video over Time-Varying Cross-Layer Approach for Video over Time-Varying

CDMACDMAChannelsChannels

ISSUE 1ISSUE 1ISSUE:ISSUE: A joint cross-layer design for achieving A joint cross-layer design for achieving

QoS (Quality of Service) and Resource EfficiencyQoS (Quality of Service) and Resource EfficiencyProposed Scheme:Proposed Scheme:• QoS-awareness schedulerQoS-awareness scheduler and and power adaptation power adaptation

schemescheme is used at both uplink and downlink MAC layer tois used at both uplink and downlink MAC layer tocoordinate the behavior of the lower layers for coordinate the behavior of the lower layers for

resource efficiency and QoSresource efficiency and QoS• Central to the proposed cross-layer design is the Central to the proposed cross-layer design is the

concept of concept of adaptationadaptation

• The conventional protocol stack is The conventional protocol stack is inflexibleinflexible and and the layers doesn’t adapt to the changing the layers doesn’t adapt to the changing conditions.conditions.

• Adaptation Adaptation represents the ability of the network represents the ability of the network protocols to observe and respond to channel protocols to observe and respond to channel variationvariation

• Central to adaptation is the cross-layer designCentral to adaptation is the cross-layer design• The proposed QoS-awareness scheduler and The proposed QoS-awareness scheduler and

power adaptation scheme deals with power adaptation scheme deals with ApplicationApplication, , MACMAC and and physical layersphysical layers

Purpose of a Purpose of a Scheduler:Scheduler:

MAC scheduler selects appropriate transmission power/format and priorities of the packets for each user depending on its present channel condition and the associated QoS requirements

At MAC layer, different QoS-aware MAC states for both uplink and downlink transmission are used. The IP-base station dynamically schedules users in different MAC states based on resource availability and overall QoS

QoS (combined QoS criteria) = QoS class* QoS stream

QoS class is determined by service pricing QoS stream is determined by characteristics of data

traffic

QoS-aware & Power-adaptive MAC QoS-aware & Power-adaptive MAC StatesStates

a. High-QoS state: Users are actively sending and receiving traffic. In the high-QoS state, users have a dedicated control channel with both control and traffic channel power and timing controlled. Traffic segments are instantaneously assigned to any high-QoS user when there is data to send or receive

b. Media-QoS state: While in media-QoS state, users have contention-free uplink request slots to indicate to an IP-base station that they have data to send. Users also have shared downlink message slots that are timing

c. Low-QoS state: Users only maintain connectivity to an IP-base

station. Low QoS mode users have shared downlink paging slots, where they know to wake up periodically and listen for incoming pages from the IP-base station

Transition among differentTransition among different QoS QoS statesstates

QoS-aware & Power-adaptive QoS-aware & Power-adaptive Transmission:Transmission:

Different applications have different QoS network requirements. QoS vector constitutes Cost network of a stream .The network parameters combined with QoSclass andQoSstream compose a multi criteria decision for a given user and overall cost measurement is defined as:

Cost = QoSclass ∗ QoSstream ∗

Costnetwork

QoS-aware and Power-QoS-aware and Power-adaptive Transmission:adaptive Transmission:

The MAC scheduler selects appropriate transmission power/format and priorities of the packets for each user depending on its present channel condition and the associated QoS requirements.

MAC scheduler dynamically computes Cost real based on the current channel condition.

Simulation Simulation Results:Results:

Four different SNR environments, Location A(Dis=0.5Miles & SNR=20dB) B (Dis=1.4 Miles & SNR=20dB) C (Dis=2.0 Miles & SNR=15dB) D(Dis=1.6 Miles & SNR=10dB), were located within

one cell.Three applications are chosen to run the test:Mobile1: A FTP download of 50 distinct 2MB files (B-C)Mobile 2: A webpage of 205kb was periodically

refreshed (C-D)Mobile 3: A 128 kb/s media stream served from within

the core network (D-A)

Test #1:Test #1:Equal QoS assignment, theyEqual QoS assignment, theysimultaneously request resources.simultaneously request resources.QoS “Gold” user (mobile#1)with FTP downloadMobile #1 has throughput 2.5Mb/s

initiallyMobile#2 has 160Kb/s initiallyWhen Mobile #3 comes,MAC scheduler arbitrates

resource assignments so that all users are supported.

So, Mobile#1-dropped to 1.4Mb/sWhen Mobile #3 completes ,Mac

Scheduler reallocates bandwidth back to Mobile#1

Test #2:Test #2:When different QoS areWhen different QoS areassigned to mobiles,assigned to mobiles,Mobile #1 was assigned low Mobile #1 was assigned low

QoSQoSLow QoS=> never receive Low QoS=> never receive

more than 150Kb/smore than 150Kb/sFigure shows Figure shows throughput dropthroughput drop

The available throughput to theThe available throughput to theuser was reduced from 2.6Mb/suser was reduced from 2.6Mb/sto 150Kb/sto 150Kb/s

Issue-2:TCP Over CDMA Wireless Issue-2:TCP Over CDMA Wireless LinksLinks

TCP has been optimized for wired networks. Any TCP has been optimized for wired networks. Any packet loss is considered as a congestion and packet loss is considered as a congestion and hence the window size is reduced dramatically as hence the window size is reduced dramatically as a precaution, however wireless links are known to a precaution, however wireless links are known to experience sporadic and usually temporary losses experience sporadic and usually temporary losses due to fading, shadowing, handoff etc. which due to fading, shadowing, handoff etc. which cannot be considered as congestion. cannot be considered as congestion.

ProblemProblem: wireless corruption mistaken for : wireless corruption mistaken for congestioncongestion

SolutionSolution: Snoop Protocol: Snoop Protocol

Channel ErrorsChannel ErrorsInternet

Router

Loss Congestion2321

Loss ==> Congestion

210

Burst losses lead to coarse-grained timeouts

Result: Low throughput

Performance DegradationPerformance Degradation

0.0E+00

5.0E+05

1.0E+06

1.5E+06

2.0E+06

0 10 20 30 40 50 60

Time (s)

Sequ

ence

num

ber (

byte

s)

TCP Reno(280 Kbps)

Best possible TCP with no errors(1.30 Mbps)

2 MB wide-area TCP transfer over 2 Mbps Lucent WaveLAN

Cross-layer protocol design & Cross-layer protocol design & optimizationsoptimizations

At the transport-layer, several cross-layer At the transport-layer, several cross-layer approaches such as freeze TCP, approaches such as freeze TCP, snoop TCPsnoop TCP have have been proposed as TCP alternatives to distinguish been proposed as TCP alternatives to distinguish congestion-loss from non-congestion loss.congestion-loss from non-congestion loss.

In snoop TCPIn snoop TCP, TCP layer knowledge is used by , TCP layer knowledge is used by link layer schemeslink layer schemes

Transport

Network

Link

Physical

Link-aware transport (Explicit Loss Notification)

Transport-aware link(Snoop agent at BS)

Solution: Snoop ProtocolSolution: Snoop Protocol Shield TCP sender from wireless vagariesShield TCP sender from wireless vagaries

Eliminate adverse interactions between Eliminate adverse interactions between protocol layersprotocol layers

Congestion control only when congestion Congestion control only when congestion occursoccurs

Fixed to mobile: transport-aware link protocolMobile to fixed: link-aware transport protocol

Snoop Protocol: FH to MHSnoop Protocol: FH to MH

FH Sender

Mobile Host

Base Station5

1

12346

Snoop agent: Snoop agent: active interposition agentactive interposition agent Snoops on TCP segments and ACKsSnoops on TCP segments and ACKs Detects losses by duplicate ACKs and timersDetects losses by duplicate ACKs and timers Suppresses duplicate ACKs from FH senderSuppresses duplicate ACKs from FH sender

Cross-layer protocol design: Cross-layer protocol design: snoop agent snoop agent state is state is softsoft

Snoop agent

Snoop Protocol: FH to MHSnoop Protocol: FH to MH

Mobile Host

1Base Station

Snoop Agent

FH Sender

Snoop Protocol: FH to MHSnoop Protocol: FH to MH

Mobile Host

1234Base Station

5

FH Sender

Snoop Protocol: FH to MHSnoop Protocol: FH to MH

Mobile Host

Base Station5

1

12346

FH Sender

Snoop Protocol: FH to MHSnoop Protocol: FH to MH

Mobile Host

51234

Base Station

32

6

21

Sender

Snoop Protocol: FH to MHSnoop Protocol: FH to MH

Mobile Host

61234

Base Station

43

1

5

2ack 0

Sender

Duplicate ACK

Snoop Protocol: FH to MHSnoop Protocol: FH to MH

Mobile Host

1234

Base Station

1

1

56

4 3 2

Sender

Retransmit from cacheat higher priority

ack 0

ack 0

ack 0

65

Snoop Protocol: FH to MHSnoop Protocol: FH to MH

Mobile Host

1234

Base Station

1

1

SuppressDuplicate Acks

56

4 3 2

Sender 5ack 0

ack 4

Snoop Protocol: FH to MHSnoop Protocol: FH to MH

Base Station

6

56

1 4 3 25

Senderack 4

ack 5

Clean cache on new ACK

Snoop Protocol: FH to MHSnoop Protocol: FH to MH

Mobile Host

Base Station

6 5 4 3 21

Senderack 4

6

ack 6

ack 5

Snoop Protocol: FH to MHSnoop Protocol: FH to MH

Mobile Host

Base Station

5 4 3 21

Active soft state agent at base stationTransport-aware reliable link protocolPreserves end-to-end semantics

6

Senderack 5 ack 6

789

0.0E+00

5.0E+05

1.0E+06

1.5E+06

2.0E+06

0 10 20 30 40 50 60

Bestpossible TCP (1.30 Mbps)

Snoop Performance Snoop Performance ImprovementImprovement

Time (s)Time (s)

Sequ

ence

num

ber (

byte

s)

Snoop (1.11 Mbps)

TCP Reno(280 Kbps)

2 MB wide-area TCP transfer over 2 Mbps Lucent WaveLAN

ISSUE 3:Cross-Layer Approach for ISSUE 3:Cross-Layer Approach for Video over Time-Varying CDMA Video over Time-Varying CDMA

ChannelsChannelsProblemProblem::Applying multi-user diversity to real-time traffic is Applying multi-user diversity to real-time traffic is

very challenging due to the delay requirement of very challenging due to the delay requirement of such trafficsuch traffic

SolutionSolution::A Cross-Layer approach,A Cross-Layer approach,DWGPSDWGPS(Dynamic Weight (Dynamic Weight

Generalized Processor Sharing) is proposed.Generalized Processor Sharing) is proposed.The proposed cross-layer approach can benefit fromThe proposed cross-layer approach can benefit frominformation in both the application and physical information in both the application and physical

layerslayers

Existing Technology:GPSExisting Technology:GPS

• GPSGPS is an ideal fair scheduling discipline is an ideal fair scheduling discipline originally proposed for wire line networksoriginally proposed for wire line networks

• Principle of GPSPrinciple of GPS:: A fixed-weight is assigned to each session and A fixed-weight is assigned to each session and

bandwidth is allocated to all sessions according to bandwidth is allocated to all sessions according to the weights and traffic loads.the weights and traffic loads.

• GPS can provide each session with a minimum GPS can provide each session with a minimum service rate.service rate.

• The minimum service rate and tight delay bound The minimum service rate and tight delay bound guaranteed in GPS may seem attractive to real-time guaranteed in GPS may seem attractive to real-time video transmissionvideo transmission

• A large weight should be assigned to a video A large weight should be assigned to a video session in order to guarantee the peak ratesession in order to guarantee the peak rate

• This means a video session will get a large portion This means a video session will get a large portion of theof the

total capacity whenever it has traffic to transmit, total capacity whenever it has traffic to transmit, thus leading to service degradation of other thus leading to service degradation of other sessionssessions

• In order to apply GPS discipline to video In order to apply GPS discipline to video transmission and extend it to wireless networks, transmission and extend it to wireless networks, dynamic weights in GPS are used(DWGPSdynamic weights in GPS are used(DWGPS))

The link layer resource allocation benefits from the video compression information such as the batch class and batch arrival size from the application layer; and in return, the link layer provides the video compression with the desired service differentiation.

This DWGPS scheduling procedure in the link layer can achieve low computation complexity and small signaling overhead.

LINK Layer

Application Layer

Provides video compression for the desired service

Video compression information

ConclusioConclusion:n:

Though Cross-layer design has many advantages theThough Cross-layer design has many advantages theAchilles heel of cross-layer design is its potential to destroyAchilles heel of cross-layer design is its potential to destroymodularity, and make the overall system fragilemodularity, and make the overall system fragile..Tradeoff between efficiency and modularityTradeoff between efficiency and modularity Future workFuture work on CLD implementation can be aiming at on CLD implementation can be aiming atreplacing the traditional layered structures completely.replacing the traditional layered structures completely.However this might not even be possible because of theHowever this might not even be possible because of thedemand for compatibility with every other network usingdemand for compatibility with every other network usingthe IPthe IP..

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