Page 1 박 수 현 컴퓨터 및 정보통신공학부 Scalable Video Coding and Transport over...

박 수 현

컴퓨터 및 정보통신공학부

Scalable Video Coding and Transport over Broadband Wireless Networks

Dapeng Wu, Yiwei Thomas Hou, And Ya-Qin Zhang, Proceedings of the IEEE, Vol. 89, No. 1, January 2001

목 차 Introduction

Scalable Video Coding

Network-Aware End Systems

Adaptive Services

Summary

Problems for real-time video transmission over wireless networks

Unreliability– Compared with wired links, wireless channels are typically much more noisy and

have both small-scale(multipath) and large-scale (shadowing) fades, making the BER very high.

Bandwidth fluctuations– When a mobile terminal moves between different networks [e.g., from a wireless

local area network (LAN) to a wireless wide area network (WAN)].– When a handoff happens, a base station may not have enough unused radio re-

source to meet the demand of a newly joined mobile host.– The throughput of a wireless channel may be reduced due to multipath fading,

co-channel interference, and noise disturbances. Last but not least, the capacity of a wireless channel may fluctuate with the changing distance between the base station and the mobile host.

Heterogeneity– Unicast vs Multicast

Unicast video distribution using multiple point-to-point connec-tions.

Multicast video distribution using point-to-multipoint transmission.

3 basic components of Adaptive QoS support from networks

Scalable video coding– Scalable video is more suitable than nonscalable video under a time-varying wire-

less environment.– Scalable video representation is a good solution to the heterogeneity problem in

the multicast case.– Scalable video representations naturally fit unequal error protection.

Network-aware adaptation of end systems– To solve these problems

• Unreliability• Bandwidth fluctuations

– Network awareness– Network adaptation

Adaptive services– Service contract– Call admission and resource reservation– Mobile multicast mechanism– Substream scaling– Substream scheduling– Link-layer error control


SNR scalability– SNR-scalable coding quantizes the DCT coefficients to different levels of accuracy

by using different quantization parameters

– The SNR-scalable encoder operates in the same manner as both the nonscalable video encoder and decoder one.

SNR-scalable encoder with 2 levels

SNR-scalable decoder with 2 levels

1) The raw video is DCT transformed and quantized at the base level.

2) The base-level DCT coefficients are reconstructed by inverse quantization.

3) Subtract the base-level DCT coefficients from the original DCT coefficients.

4) The residual is quantized by a quantization parameter, which is smaller than that

of the base level.

5) The quantized bits are coded by VLC.

1) Decoded by VLD and inversely quantized.

2) The base-level DCT coefficient values are added to the enhancement-level

DCT coefficient refinements.

3 ) The summed DCT coefficients are inversely DCT transformed, resulting in

enhancement-level decoded Video.

DCT : Diserete Cosine TransformQ : QuantizationVLC : Variable Length Coding

VLD : Variable Length DecodingIQ : Inverse QuantiazationIDCT : Inverse DCT


Spatial scalability– For the base layer, the raw video is

• Spatially down-sampled.• DCT transformed• Quantized• VLC coded.

– For the enhancement layer, the raw video is • The raw video is spatially down-sampled, DCT transformed,and quantized at the base layer.• The base-layer image is reconstructed by inverse quantization and inverse DCT.• The base-layer image is spatially up-sampled.• Subtract the up-sampled base-layer image from the original image.• The residual is DCT transformed, and quantized by a quantization parameter, • The quantized bits are coded by VLC.

Spatially/temporally scalable encoder with 2 levels Spatially/temporally scalable decoder with 2 levels


Temporal scalability– Temporally scalable video is encoded by making use of temporally up-sampled

pictures from a lower layer as a prediction in a higher layer.

– The block diagram of temporally scalable codec is the same as that of spatially scalable codec.

– Temporal down-sampling uses frame skipping.

PredictionGOP border GOP border

Key Picture Key Picture

T0T0T1T2

T2T3T3

T3T3

Tx : Temporal Layer Identifier


Network monitoring

– On-demand monitoring• When applications ask the monitor to collect status information about a certain resource in an on-

line fashion.

– Continuous monitoring• The monitor notifies the application when the status of a previously requested resource changes

in a certain way.

– Centralized case• status information from the entire network is maintained at a central host and shared by all other

hosts.

– Distributed case• Monitors collect only local network status information and obtain nonlocal status information on

demand from other network monitors.

Criteria Type of monitoring

Method of monitoring Active Passive

Monitoring frequency On demand Continuous

Replication of information Centralized Distributed


Adaptation 1

Architecture for transporting scalable video from a mobile terminal to a wired terminal

At the sender side,

the compressed video bit stream is first filtered by the scaler,

the operation of which is to select certain video layers to

transmit.

1) Scale down the received video representation, that is, drop the enhancement layer(s)

2) Transmit what is received, i.e., do not scale the received video representation.

Scaler

1) To notify the sender about the available bandwidth of the wireless channel through a signaling channel

The network monitor

1) The rate control module at the sender conveys the bandwidth parameter to the scaler.

2) The scaler regulates the output rate of the video stream so that the transmission rate is less than or equal to the available bandwidth.

The rate control


Adaptation 2

Architecture for transporting scalable video from a mobile terminal to a wired terminal

At the sender side,

the compressed video bit stream is first filtered by the scaler,

the operation of which is to select certain video layers to

transmit.

1) The network monitor notifies the sender about the channel qual-ity (i.e., BER)

The network monitor

The rate control module at the sender commands the scaler to per-form the following operations.

1) if the BER is above a threshold, discard the enhancement layer so that the bandwidth allocated for the enhancement layer can be utilized by FEC to protect the base layer.

2) otherwise, transmit both layers.

The rate control

Scaler’s operations

Adaptive Services

Functions of adaptive services– Reserve a minimum bandwidth to meet the demand of the base layer. As a re-

sult, the perceptual quality can always be achieved at an acceptable level.– Adapt the enhancement layers based on the available bandwidth and the fair-

ness policy.

Using scaling inside the network has the following advantages– Improved video quality– Low latency and low complexity– Lower call blocking and handoff dropping probability

The required components of the end-to-end adaptive services– Service contract– Call admission control and resource reservation– Mobile multicast mechanism– Substream scaling– Substream scheduling– Link-layer error control

Adaptive Services

Service contract– The service contract between the application and the network could consist of

multiple subcontracts.– A substream is assigned a priority according to its significance.

• The base layer is assigned the highest priority. • The priority can be used by routing, scheduling, scaling, and error control components of the

adaptive network.

Call admission control(CAC) and resource reservation

CAC

To provide a QoS guarantee for individual connections while

efficiently utilizing network resources

CAC algorithm has to check whether admitting the connec-

tion would reduce the service quality of existing connec-

tions, and whether the incoming connection’s QoS require-

ments can be met

Resource reservation

In order to maintain the specified QoS in the long time scale,

the network must reserve some resource along the current

path of a mobile connection.

In order to seamlessly achieve the QoS on the short time

scale, bandwidth must be reserved on the paths from the cur-

rent base stations to the neighboring base stations so that in

the event of a handoff, a termination of the connection can

be avoided

Adaptive Services

Mobile multicast mechanism– The mobile routing protocol needs to be proactive and anticipatory in order to

match the delay, loss, and jitter constraints of a substream.

– As a mobile station hands off from a base station to another, new paths are set up and old paths are torn down

Substream scaling– Scaling is employed during bandwidth fluctuations and/or under poor channel

conditions.

– The scaling decision is made by a bandwidth manager, which obtains the avail-able bandwidth from a network monitor.

– Fariness problem’s solutions• A max–min fairness

• A utility-based fairness– Which represents the relationship

between observed quality (i.e., utility) and bandwidth

Adaptive Services

Substream scheduling

Architecture for substream scheduling at a base station

1) Delay and throughput guarantees for error-free sessions

2) Long-term fairness for error sessions

3) Short-term fairness for error-free sessions

4) Graceful degradation for sessions that have received excess service time

CIF (Channel-condition Independent Fair)’s properties

Hierarchical packet-scheduling architecture where a priority link scheduler is shared among a CIF-Q scheduler for base-layer substreams.

An FIFO scheduler for enhancement-layer substreams.

Service priority is first given to the CIF-Qscheduler and then to the FIFO scheduler.

Adaptive Services

Link-layer error control

FEC(Forward Error Correction)

1) The throughput can be kept constant.

2) Delay can be bounded.

1) The redundancy ratio should be made large enough to

guarantee recovery of corrupted bits under the worst

channel conditions.

2) FEC is not adaptive to varying wireless channel

conditions and it works best only when the BER is stable.

3) FEC is useless when the short-term BER exceeds the

recovery capability of the FEC code.

ARQ(Automatic Repeat reQuest)

1) ARQ is adaptive to varying wireless channel condi-tions.

1) Adaptiveness and efficiency of ARQ come with the cost

of unbounded delay.

VS

Advantages

Disadvantages

Combination ofbounded delay and adaptiveness

H-ARQ (Hybrid-ARQ)

Tc : Current time

Ds : Slack term

Td(N) : Packet N’s scheduled time

Summary

We examined the challenges in QoS provisioning for wireless video trans-port.

To address the challenges, 3 techniques have been studied in great depth individually.

Adaptive QoS support from network

(Adaptive services)

Network-aware adaptation of end sys-

temsScalable video coding

Combinations of 3 techniques

An adaptive framework for scalable video transport over wireless networks

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