multimedia & comm. lab rate control for abr service in atm networks 98/9/30 multimedia &...

Multimedia & Comm. LabMultimedia & Comm. Lab

Rate control for ABR service in ATM Networks

98/9/30

Multimedia & Comm. Lab

정승훈

2Multimedia & Comm. LabMultimedia & Comm. Lab

Contents

Introduction

ABR Service

Congestion Control Mechanisms

Source-level Rate Adaptation

Examples

Open Issues


Introduction

Classes of Service

Why need Congestion Control

Traffic Management Functions

What is Expected from Congestion Control


Classes of Service

ABR (Available bit rate) Follows feedback instructions. Network gives max throughput with minimum loss.

UBR (Unspecified bit rate) User sends whenever it wants. No feedback mechanism, No guarantee. Cells may be dropped during congestion.

CBR (Constant bit rate) Throughput, delay, and Jitter guaranteed.

VBR (Variable bit rate)


Why need Congestion Control

Will the congestion problem be solved when: Memory becomes cheap (infinite memory)? Links become cheap (very high speed links)? Processors become cheap?

Congestion is a dynamic problem Static solutions are not sufficient

Bandwidth explosion More unbalanced networks.

Buffer shortage is a symptom, not the cause.



Connection Admission Control (CAC) Verify that the requested bandwidth and QoS can be

supported

Traffic Shaping Limit burst length, Space-out cells

Usage Parameter Control (UPC) Monitor and control traffic at the network entrance

Network Resource Management Scheduling, Queueing, Virtual path resource reservation



Priority Control Cell Loss Priority (CLP) = 1 cells may be dropped

Selective Cell Discarding Frame discard

Feedback Controls Network tell the source to increase or decrease its load Explicit forward congestion indication (EFCI) Explicit rate (ER) Backward explicit congestion notification (BECN)


What is expected ?

Objectives Support a set of QoS parameters and classes for all ATM

services Minimize network and end-system complexity while

maximizing network utilization

Selection Criteria Scalability Fairness Robustness Implementability


ABR Service

The Nature of the ABR Service

Some Early Debates

The Role of the Network

The Role of the End Systems


The Nature of the ABR Service

ABR Service ABR connections will share the available bandwidth The share of available bandwidth for each ABR connection

is dynamic and may diminish down to a specified minimum cell rate (MCR)

The dynamic nature of the ABR service can be seen from the feedback model

The ABR service is appropriate only for applications which can adapt their rates to the time-varying available bandwidth and tolerate unpredictable cell delays

a low or zero cell loss rate is guaranteed to users who adapts their rates properly


Some Early Debates

Open-Loop vs. Close-Loop

Credit-based vs. Rate-based

Binary Feedback vs. Explicit Feedback


Open-Loop vs. Close-Loop

Open-Loop not need end-to-end feedback prior-reservation and hop-to-hop flow control

Close-Loop the source adjust its cell rate in responding to the feedback

information from the network. Too slow in high-speed networks But, ABR service is designed to use any bandwidth ATM Forum specified that feedback is necessary for ABR

flow control


Credit-based vs. Rate-based

Credit-based hop-by-hop per-VC window Static : Full round-trip worth of credit per VC Adaptive : Credits depend upon activity

Rate-based End-to-end rate control Binary : Feedback via congestion bit in cells Explicit : Feedback via resource management (RM) cells


Credit vs. Rate Debate : Issues

Per-VC queueing Switch complexity, Nonscalable

Switch vs. end-system complexity

Zero cell loss

Isolation and misbehaving users

Buffer requirements Full round-trip per VC


Binary vs. Explicit Rate

Binary Feedback One-bit Feedback Explicit forward congestion indicator (EFCI) set to 0 at

source Congested switch set EFCI to 1 Every nth cell, destination sends a RM cell to the source

indicating increase amount or decrease factor



Explicit Rate Feedback Every Nrm cells, the sources send a control cell

The switches measure load over a period The destination returns the cell to the source The switches specify explicit rate in cell The source adjusts the transmission rate



ER feedback schemes have several advantages The switches know more information along the flow path Faster to get the source to the optimal operating point Policing is straight forward

Two ways for ER feedback Forward feedback Backward feedback


The Role of The Network

The network might provide information directly to the users No information A binary congestion indication

EFCI Detailed congestion indication

RM cells with queue levels and severity level Explicit bandwidth (or rate) information

RM cell with the current available bandwidth that can be adjusted by nodes along the connection in the forward direction

The destination returns the RM cell to the source with either and absolute rate or a relative rate adjustment


The role of the End systems

How the source and destination end systems work with Feedback information to adapt the source rate Negative Feedback

source increments its rate by default Positive Feedback

source decrements its rate by default Explicit Feedback

source maintains its rate by default


Congestion Control Mechanisms

Fairness

Binary Feedback EFCI PRCA

Explicit Rate Feedback EPRCA ERICA


Fairness

Max-Min available bandwidth = C / N

MCR plus equal share available bandwidth = MCR + (C - MCR) / N

Maximum of MCR or Max-Min share available bandwidth = max{MCR, Max-Min share}

Allocation proportional to MCR The bandwidth allocation for a connection is weighted

proportional to its MCR

Weighted allocation


EFCI

Mechanism The network uses EFCI to convey congestion information

(in the forward direction) Feedback is returned via RM cells from the destination end

system to source. The sources adjust their rates by additive increase and

multiplicative decrease (at periodic update intervals). The feedback is negative, the source increase their rates by

default and decrease only if an RM cell is received


PRCA

Proportional rate control algorithm positive feedback RM cells are generated at a rate proportional to the source

rate End system requires a means to discover when to generate

an RM cell. Every Nrm cells, only one cell with EFCI=0

Source DestATM node ATM node

RM cell

If EFCI=0cellreceived

EFCI=1 EFCI=0

Nrm


EPRCA

Enhanced proportional rate control algorithm Source behavior

The source sends data cells with EFCI set to 0 and sends RM cells every n data cells.

The RM cells contain desired explicit rate(ER), current cell rate (CCR) and congestion indication(CI).

The source initializes CCR to the allowed cell rate(ACR) and CI to 0.

Source DestATM node ATM node

RM cell

CI=0 if nocongestionCI=1otherwise

User cellEFCI=0

RM cellCI=1

Nrm


EPRCA (cont’d) Switch behavior

computes a mean allowed cell rate(MACR) for all VCs using: MACR = (1 - ) * MACR + *CCR

and the fair share as a fraction of this average The ER field in the returning RM cells are reduced to fair share. May set the CI bit in the cells passing.

Destination behavior monitors the EFCI bits and mark the CI bit in the RM cell if the l

ast seen data cell had EFCI bit set. Problems

congestion detection is based on the queue length. This method is shown to result in unfairness.

Sources that start up late may get lower throughput than those start early


ERICA

Explicit Rate Indication for Congestion Avoidance Switch behavior

Set target rate at 95% of link bandwidth Monitor input rate and number of active VCs k

Overload = Input rate / Target rate

VC’s share = VC’s current cell rate / Overload Fairshare = Target rate / k ER = Max(Fairshare, This VC’s share) ER in Cell = Min(ER in Cell, ER)

Features Measured overload/load at switch. Small queue lengths during steady state. Fast response.


Source level rate adaptation

Architecture

Encoder-level rate shaping

Rate shaping for precoded video


Rate control architecture

QuantizerRate

shaper

Ratecontrol

ATMNetworks

Output buffer

Quantizationlevel index

MPEG CodecMPEG Codec Rate ShaperRate Shaper

Feedback


Rate shaping for precoded video

Frame discarding

Selective Block dropping

Eliminate some DCT coefficients

Block dropping with Error concealment

Feature-based block dropping


Examples of Rate control

Explicit Backward Congestion Notification

Composite Rate Control Scheme

Weighted Max-Min Fairness

Shaped VBR


EBCN

Using queue occupancy of the VP buffer of an ATM switchRM cells : increase / decrease quantizer step size

qnew = max[qold + qdiff, qmax]

Tc Tn

VideoSources

Server

Buffer occupancymax 0

Explicit Backward Congestion NotificationExplicit Backward Congestion NotificationM. Ghanbari - Essex Univ.GLOBECOM ‘96


CRCS

Composite Rate Control Scheme From

S. Karademir - Garleton Univ in Canada GLOBECOM ‘96

Congestion Notification Explicit Feedback mechanism Feedback cell

Traffic prediction Prediction parameter

feedback info. Average transmission rate during the last two cycles.

Prediction Model TES : nonlinear auto-regressive model


WMMF

Weighted Max-Min Fairness from

T.V. Lakshman - Bell Labs. INFOCOM ‘97

Design Goals simple admission control high statistical multiplexing gain frequent bandwidth negotiation adaptation of source rates to match available bandwidth maintain low end-to-end delays

Key Idea RCBR associate a weight with each flow


WMMF (cont’d)

RCBR Renegotiated CBR

hybrid of the CBR and VBR the simplicity of admission control for CBR the greater statistical multiplexing gains of VBR.

Key Points short-term fluctuations are absorbed in local source buffers long-term changes make the source renegotiate the bandwidth

Weighted fair share Key

Using difference of flow activity


WMMF (cont’d)

Source Adaptation Mechanism Demand Prediction

Discrete Auto-regressive model Xn+k = + k(Xn-) correlation efficient : mean number of cels per frame

Gamma-Beta Auto-regressive Model Heymann ‘96

Encoder Rate Adaptation rate adaptation function

avg = Travg - [ * (Bp - SETPOINT) / Thorizon ]

Travg : Transmission Rate

Bp : Predicted Buffer


SVBR

Shaped VBR From

M. Hamdi - ENST IEEE JSAC Aug. ‘97

Key Idea CBR 의 장점과 VBR 의 장점을 혼합한 형태 비디오 전체에 대해 VBR 로 인코딩

CBR 의 단점인 buffer delay 를 제거 Bursty 한 부분의 영역에 대해서는 CBR 을 적용

VBR 의 burstiness 감소 Shaped Variable Bit Rate


SVBR (cont’d)

Rate Shaping Principle

하나의 GOP 에 할당되는 비트수의 최대값을 설정 (leak rate)

GoP 단위로 비트수를 계산하여 leak rate 를 넘으면 Quantization parameter Q 를 증가

GoP scale rate prediction GoP 단위로 rate prediction 을 적용 다음 GoP 의 크기를 예측한 후 , 해당 GoP 를 위한 Q 를 재조정

Qk+1 = QkRk/Rk+1

Qk : k 번째 GOP 의 quantization parameter

Rk: k 번째 GOP 의 bit 수


Open Issues

Policing dynamic UPC control time lag estimation

Point-to-Multipoint Connections Branchpoint behavior

Priority service for RM cells

Virtual Source / destination


Conclusion

Rate control for ABR Services

Congestion Control algorithm

Source-level rate adaptation

multimedia & comm. lab rate control for abr service in atm networks 98/9/30 multimedia &...

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