Audio/Video Transport Working Group A. Clark
Internet-Draft Telchemy
Intended status: Standards Track G. Zorn, Ed.
Expires: July 09, 2014 Network Zen
C. Bi
STTRI
Q. Wu, Ed.
Huawei
January 05, 2014

RTCP XR Report Block for Concealment metrics Reporting on Audio Applications
draft-ietf-xrblock-rtcp-xr-loss-conceal-09.txt

Abstract

This document defines two RTCP XR Report Blocks that allows the reporting of concealment metrics for audio applications of RTP.

Status of This Memo

This Internet-Draft is submitted in full conformance with the provisions of BCP 78 and BCP 79.

Internet-Drafts are working documents of the Internet Engineering Task Force (IETF). Note that other groups may also distribute working documents as Internet-Drafts. The list of current Internet-Drafts is at http://datatracker.ietf.org/drafts/current/.

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This Internet-Draft will expire on July 09, 2014.

Copyright Notice

Copyright (c) 2014 IETF Trust and the persons identified as the document authors. All rights reserved.

This document is subject to BCP 78 and the IETF Trust's Legal Provisions Relating to IETF Documents (http://trustee.ietf.org/license-info) in effect on the date of publication of this document. Please review these documents carefully, as they describe your rights and restrictions with respect to this document. Code Components extracted from this document must include Simplified BSD License text as described in Section 4.e of the Trust Legal Provisions and are provided without warranty as described in the Simplified BSD License.

Table of Contents

1. Introduction

1.1. Loss Concealment and Concealment Seconds Metrics Reporting Block

At any instant, the audio output at a receiver may be classified as either 'normal' or 'concealed'. 'Normal' refers to playout of audio payload received from the remote end, and also includes locally generated signals such as announcements, tones and comfort noise. Concealment refers to playout of locally-generated signals used to mask the impact of network impairments or to reduce the audibility of jitter buffer adaptations.

This draft defines two new concealment related block types to augment those defined in [RFC3611] for use in a range of RTP applications. These two block types extend packet loss concealment mechanism defined in section 4.7.6 of RFC3611.

The first block type provides metrics for actions taken by the receiver to mitigate the effect of packet loss and packet discard. Specifically, the first metric (On-Time Playout Duration) reports the duration of normal playout of data which the receiver obtained from the sender's stream. A second metric (Loss Concealment Duration) reports the total time during which the receiver played out media data which was manufactured locally, because the sender's data for these periods was not available due to packet loss or discard. A similar metric (Buffer Adjustment Concealment Duration) reports the duration of playout of locally-manufactured data replacing data which is unavailable due to adaptation of an adaptive de-jitter buffer. Further metrics (Playout Interrupt Count and Mean Playout Interrupt Size) report the number of times normal playout was interrupted, and the mean duration of these interruptions.

Loss Concealment Duration and Buffer Adjustment Concealment Duration are reported separately because buffer adjustment is typically arranged to occur in silence periods so may have very little impact on user experience, whilst loss concealment may occur at any time.

The second block type provides metrics for concealment seconds, which are measured at the receiving end of the RTP stream. Specifically, the first metric (Unimpaired Seconds) reports the number of whole seconds occupied only with normal playout of data which the receiver obtained from the sender's stream. The second metric (Concealed Seconds) reports the number of whole seconds during which the receiver played out any locally-generated media data. A third metric (Severely Concealed Seconds) reports the number of whole seconds during which the receiver played out locally-generated data for more than SCS Threshold.

These metrics belongs to the class of transport-related terminal metrics defined in [RFC6792].

1.2. RTCP and RTCP XR Reports

The use of RTCP for reporting is defined in [RFC3550]. [RFC3611] defined an extensible structure for reporting using an RTCP Extended Report (XR). This draft defines a new Extended Report block that MUST be used as defined in [RFC3550] and [RFC3611].

1.3. Performance Metrics Framework

The Performance Metrics Framework [RFC6390] provides guidance on the definition and specification of performance metrics. The RTP Monitoring Architectures [RFC6792] provides guideline for reporting block format using RTCP XR. The Metrics Block described in this document are in accordance with those guidelines.

1.4. Applicability

These metrics are applicable to audio applications of RTP and the audio component of Audio/Video applications in which the packet loss concealment machinery is contained at the receiving end to mitigate the impact of network impairments to user's perception of media quality.

2. Terminology

2.1. Standards Language

The key words "MUST", "MUST NOT", "REQUIRED", "SHALL", "SHALL NOT", "SHOULD", "SHOULD NOT", "RECOMMENDED", "MAY", and "OPTIONAL" in this document are to be interpreted as described in RFC2119 [RFC2119].

2.2. Notations

This report block makes use of binary fractions. The terminology used is

3. Loss Concealment Block

The metrics block described here are intended to be used as described in this section, in conjunction with information from the Measurement Information block [RFC6776]. Instances of this Metrics Block refer by Synchronization source (SSRC) to the separate auxiliary Measurement Information block [RFC6776] which describes measurement periods in use (see [RFC6776] section 4.2). This Metrics Block relies on the measurement period in the Measurement Information block indicating the span of the report and SHOULD be sent in the same compound RTCP packet as the measurement information block. If the measurement period is not received in the same compound RTCP packet as this Metrics Block, this metrics block MUST be discarded.

3.1. Report Block Structure

       0               1               2               3
       0 1 2 3 4 5 6 7 0 1 2 3 4 5 6 7 0 1 2 3 4 5 6 7 0 1 2 3 4 5 6 7
      +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
      |    BT=NLC     | I |plc|  rsv. |       block length=5          |
      +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
      |                         SSRC of Source                        |
      +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
      |                 On-time Playout Duration                      |
      +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
      |                  Loss Concealment Duration                    |
      +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
      |              Buffer Adjustment Concealment Duration           |
      +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
      |    Playout Interrupt Count    |           Reserved.           |
      +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
      |                 Mean Playout Interrupt Size                   |
      +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+

Figure 1: Report Block Structure

Loss Concealment Metrics Block

3.2. Definition of Fields in Loss Concealment Metrics Block

Block type (BT): 8 bits


A Loss Concealment Metrics Report Block is identified by the constant NLC.

[Note to RFC Editor: please replace NLC with the IANA provided RTCP XR block type for this block.]

Interval Metric flag (I): 2 bit


This field is used to indicate whether the Loss Concealment metrics are Sampled, Interval or Cumulative metrics:

In this document, Loss Concealment Metrics can only be measured over definite intervals, and cannot be sampled. Also, the value I=00 is reserved for future use. Senders MUST NOT use the values I=00 or I=01. If a block is received with I=00 or I=01, the receiver MUST discard the block.

Packet Loss Concealment Method (plc): 2 bits


This field is used to identify the packet loss concealment method in use at the receiver, according to the following code:

Note that the enhancement method (plc =3 )for packet loss concealment offers an improved audio quality and or a better robustness against packet losses [G.711] and is equivalent to enhanced in section 4.7.1 of [RFC3611],



Reserved (resv): 4 bits


These bits are reserved. They MUST be set to zero by senders and ignored by receivers (See [RFC6709] section 4.2).

block length: 16 bits


The length of this report block in 32-bit words, minus one. For the Loss Concealment Block, the block length is equal to 5.

SSRC of source: 32 bits


As defined in Section 4.1 of [RFC3611].

On-time Playout Duration: 32 bits


'On-time' playout is the uninterrupted, in-sequence playout of valid decoded audio information originating from the remote endpoint. This includes comfort noise during periods of remote talker silence, if VAD [VAD] is used, and locally generated or regenerated tones and announcements.

An equivalent definition is that on-time playout is playout of any signal other than those used for concealment.

On-time playout duration is expressed in units of RTP timestamp and MUST include both speech and silence intervals, whether VAD is used or not.
If the measured value exceeds 0xFFFFFFFD, the value 0xFFFFFFFE MUST be reported to indicate an over-range measurement. If the measurement is unavailable, the value 0xFFFFFFFF MUST be reported.

Loss Concealment Duration: 32 bits


The duration, expressed in units of RTP timestamp, of audio playout corresponding to Loss-type concealment.

Loss-type concealment is reactive insertion or deletion of samples in the audio playout stream due to effective frame loss at the audio decoder. "Effective frame loss" is the event in which a frame of coded audio is simply not present at the audio decoder when required. In this case, substitute audio samples are generally formed, at the decoder or elsewhere, to reduce audible impairment.

If the measured value exceeds 0xFFFFFFFD, the value 0xFFFFFFFE MUST be reported to indicate an over-range measurement. If the measurement is unavailable, the value 0xFFFFFFFF MUST be reported.

Buffer Adjustment Concealment Duration: 32 bits


The duration, expressed in units of RTP timestamp, of audio playout corresponding to Buffer Adjustment-type concealment, if known.

If the measured value exceeds 0xFFFFFFFD, the value 0xFFFFFFFE MUST be reported to indicate an over-range measurement. If the measurement is unavailable, the value 0xFFFFFFFF MUST be reported.

Buffer Adjustment-type concealment is proactive or controlled insertion or deletion of samples in the audio playout stream due to jitter buffer adaptation, re-sizing or re-centering decisions within the endpoint.

Because this insertion is controlled, rather than occurring randomly in response to losses, it is typically less audible than loss-type concealment. For example, jitter buffer adaptation events may be constrained to occur during periods of talker silence, in which case only silence duration is affected, or sophisticated time-stretching methods for insertion/deletion during favorable periods in active speech may be employed.

Concealment events which cannot be classified as Buffer Adjustment-type MUST be classified as Loss-type.

Playout Interrupt Count: 16 bits


The number of interruptions to normal playout which occurred during the reporting period.

If the measured value exceeds 0xFFFD, the value 0xFFFE MUST be reported to indicate an over-range measurement. If the measurement is unavailable, the value 0xFFFF MUST be reported.

Reserved (resv): 16 bits


These bits are reserved. They MUST be set to zero by senders and ignored by receivers (See [RFC6709] section 4.2).

Mean Playout Interrupt Size: 32 bits


The mean duration, expressed in units of RTP timestamp, of interruptions to normal playout which occurred during the reporting period.

If the measured value exceeds 0xFFFD, the value 0xFFFE MUST be reported to indicate an over-range measurement. If the measurement is unavailable, the value 0xFFFF MUST be reported.

4. Concealment Seconds Block

This sub-block described here is intended to be used as described in this section, in conjunction with information from the Measurement Information block [RFC6776] and provides a description of potentially audible impairments due to lost and discarded packets at the endpoint, expressed on a time basis analogous to a traditional PSTN T1/E1 errored seconds metric. Instances of this Metrics Block refer by Synchronization source (SSRC) to the separate auxiliary Measurement Information block [RFC6776] which describes measurement periods in use (see [RFC6776] section 4.2). This Metrics Block relies on the measurement period in the Measurement Information block indicating the span of the report and SHOULD be sent in the same compound RTCP packet as the measurement information block. If the measurement period is not received in the same compound RTCP packet as this Metrics Block, this metrics block MUST be discarded.

The following metrics are based on successive one second intervals as declared by a RTP clock. This RTP clock does not need to be synchronized to any external time reference. The starting time of this clock is unspecified. Note that this implies that the same loss pattern could result in slightly different count values, depending on where the losses occur relative to the particular one-second demarcation points. For example, two loss events occurring 50ms apart could result in either one concealed second or two, depending on the particular one second boundaries used.

The seconds in this sub-block are not necessarily calendar seconds. At the tail end of a session, periods of time of less than one second shall be incorporated into these counts if they exceed 500ms and shall be disregarded if they are less than 500ms.

4.1. Report Block Structure

       0                   1                   2                   3
       0 1 2 3 4 5 6 7 8 9 0 1 2 3 4 5 6 7 8 9 0 1 2 3 4 5 6 7 8 9 0 1 
      +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
      |    BT=NCS     | I |plc|Rserved|       block length=4          |
      +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
      |                         SSRC of Source                        |
      +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
      |                    Unimpaired Seconds                         |
      +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
      |                    Concealed Seconds                          |
      +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
      | Severely Concealed Seconds    | RESERVED      | SCS Threshold |
      +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+

Figure 2: Report Block Structure

Concealed Seconds Metrics Block

4.2. Definition of Fields in Concealed Seconds Metrics Block

Block type (BT): 8 bits


A Concealed Seconds Metrics Report Block is identified by the constant NCS.

[Note to RFC Editor: please replace NCS with the IANA provided RTCP XR block type for this block.]

Interval Metric flag (I): 2 bit


This field is used to indicate whether the Concealment Seconds Metrics are Sampled, Interval or Cumulative metrics:

In this document, Concealment Seconds Metrics can only be measured over definite intervals, and cannot be sampled. Also, the value I=00 is reserved for future use. Senders MUST NOT use the values I=00 or I=01. If a block is received with I=00 or I=01, the receiver MUST discard the block.

Packet Loss Concealment Method (plc): 2 bits


This field is used to identify the packet loss concealment method in use at the receiver, according to the following code:

Note that the enhancement method (plc =3) for packet loss concealment offers an improved audio quality and or a better robustness against packet losses [G.711] and is equivalent to enhanced in section 4.7.1 of [RFC3611],

Reserved (resv): 4 bits


These bits are reserved. They MUST be set to zero by senders and ignored by receivers (See [RFC6709] section 4.2).

Block Length: 16 bits


The length of this report block in 32-bit words, minus one. For the Concealment Seconds Block, the block length is equal to 4.

SSRC of source: 32 bits


As defined in Section 4.1 of [RFC3611].

Unimpaired Seconds: 32 bits


A count of the number of unimpaired Seconds that have occurred.

An unimpaired Second is defined as a continuous period of one second during which no frame loss or discard due to late arrival has occurred. Every second in a session must be classified as either OK or Concealed.

Normal playout of comfort noise or other silence concealment signal during periods of talker silence, if VAD is used, shall be counted as unimpaired seconds.
If the measured value exceeds 0xFFFFFFFD, the value 0xFFFFFFFE MUST be reported to indicate an over-range measurement. If the measurement is unavailable, the value 0xFFFFFFFF MUST be reported.

Concealed Seconds: 32 bits


A count of the number of Concealed Seconds that have occurred.

A Concealed Second is defined as a continuous period of one second during which any frame loss or discard due to late arrival has occurred.

Equivalently, a concealed second is one in which some Loss-type concealment has occurred. Buffer adjustment-type concealment SHALL not cause Concealed Seconds to be incremented, with the following exception. An implementation MAY cause Concealed Seconds to be incremented for 'emergency' buffer adjustments made during talkspurts.

Loss-type concealment is reactive insertion or deletion of samples in the audio playout stream due to effective frame loss at the audio decoder. "Effective frame loss" is the event in which a frame of coded audio is simply not present at the audio decoder when required. In this case, substitute audio samples are generally formed, at the decoder or elsewhere, to reduce audible impairment.

Buffer Adjustment-type concealment is proactive or controlled insertion or deletion of samples in the audio playout stream due to jitter buffer adaptation, re-sizing or re-centering decisions within the endpoint.

Because this insertion is controlled, rather than occurring randomly in response to losses, it is typically less audible than loss-type concealment. For example, jitter buffer adaptation events may be constrained to occur during periods of talker silence, in which case only silence duration is affected, or sophisticated time-stretching methods for insertion/deletion during favorable periods in active speech may be employed. For these reasons, buffer adjustment-type concealment MAY be exempted from inclusion in calculations of Concealed Seconds and Severely Concealed Seconds.

However, an implementation SHOULD include buffer-type concealment in counts of Concealed Seconds and Severely Concealed Seconds if the event occurs at an 'inopportune' moment, with an emergency or large, immediate adaptation during active speech, or for unsophisticated adaptation during speech without regard for the underlying signal, in which cases the assumption of low-audibility cannot hold. In other words, jitter buffer adaptation events which may be presumed to be audible SHOULD be included in Concealed Seconds and Severely Concealed Seconds counts.
Concealment events which cannot be classified as Buffer Adjustment- type MUST be classified as Loss-type.
For clarification, the count of Concealed Seconds MUST include the count of Severely Concealed Seconds.
If the measured value exceeds 0xFFFFFFFD, the value 0xFFFFFFFE MUST be reported to indicate an over-range measurement. If the measurement is unavailable, the value 0xFFFFFFFF MUST be reported.
Severely Concealed Seconds: 16 bits


A count of the number of Severely Concealed Seconds.

A Severely Concealed Second is defined as a non-overlapping period of one second during which the cumulative amount of time that has been subject to frame loss or discard due to late arrival, exceeds the SCS Threshold.

If the measured value exceeds 0xFFFD, the value 0xFFFE MUST be reported to indicate an over-range measurement. If the measurement is unavailable, the value 0xFFFF MUST be reported.

Reserved: 8 bits


These bits are reserved. They MUST be set to zero by senders and ignored by receivers (See [RFC6709] section 4.2).

SCS Threshold: 8 bits


The SCS Threshold is defined as the percentage of packets corresponding to lost or discarded frames that must occur within a one second period in order for the second to be classified as a Severely Concealed Second. This is expressed in numeric format 0:8 and hence can represent a range of 0.1 to 25.5 percent loss or discard.

A default threshold of 5% effective frame loss (50ms effective frame loss ) per second is suggested.

5. SDP Signaling

[RFC3611] defines the use of SDP (Session Description Protocol) [RFC4566] for signaling the use of XR blocks. XR blocks MAY be used without prior signaling.

5.1. SDP rtcp-xr-attrib Attribute Extension

This section augments the SDP attribute "rtcp-xr" [RFC3611] by providing two additional values of "xr-format" to signal the use of two report blocks defined in this document.

xr-format =/ xr-conceal-block
           / xr-conc-sec-block

xr-conceal-block = "loss-conceal"
xr-conc-sec-block = "conc-sec" ["=" thresh]

thresh      = 1*DIGIT          ; threshold for SCS (ms)
DIGIT          = %x30-39

5.2. Offer/Answer Usage

When SDP is used in offer-answer context, the SDP Offer/Answer usage defined in [RFC3611] applies. Note that “thresh” is declared by the offer.

6. IANA Considerations

New block types for RTCP XR are subject to IANA registration. For general guidelines on IANA considerations for RTCP XR, refer to [RFC3611].

6.1. New RTCP XR Block Type values

Name:       LCB
Long Name:  Loss Concealment Block
Value       <LCB>
Reference:  Section 3.1

Name:       CSB
Long Name:  Concealment Seconds Block
Value       <CSB>
Reference:  Section 4.1

This document assigns two block type values in the IANA "RTP Control Protocol Extended Reports (RTCP XR) Block Type Registry ":

[Note to RFC Editor: please replace <NLC> and <NCS> with the RTCP XR block type assigned by IANA for this block.]

6.2. New RTCP XR SDP Parameters

This document also registers two new parameters in the "RTP Control Protocol Extended Reports (RTCP XR) Session Description Protocol (SDP) Parameters Registry":

6.3. Contact information for registrations

The contact information for the registrations is:
      
Qin Wu (sunseawq@huawei.com)
101 Software Avenue, Yuhua District
Nanjing, Jiangsu  210012
China



7. Security Considerations

It is believed that this proposed RTCP XR report block introduces no new security considerations beyond those described in [RFC3611]. This block does not provide per-packet statistics so the risk to confidentiality documented in Section 7, paragraph 3 of [RFC3611] does not apply.

8. Contributors

Geoff Hunt wrote the initial draft of this document.

9. Acknowledgements

The authors gratefully acknowledge reviews and feedback provided by Bruce Adams, Philip Arden, Amit Arora, Bob Biskner, Kevin Connor, Claus Dahm, Randy Ethier, Roni Even, Jim Frauenthal, Albert Higashi, Tom Hock, Shane Holthaus, Paul Jones, Rajesh Kumar, Keith Lantz, Mohamed Mostafa, Amy Pendleton, Colin Perkins, Mike Ramalho, Ravi Raviraj, Albrecht Schwarz, Tom Taylor, and Hideaki Yamada.

10. References

10.1. Normative References

[RFC2119] Bradner, S., "Key words for use in RFCs to Indicate Requirement Levels", March 1997.
[RFC3611] Friedman, T., Caceres, R. and A. Clark, "RTP Control Protocol Extended Reports (RTCP XR)", November 2003.
[RFC4566] Handley, M., Jacobson, V. and C. Perkins, "SDP: Session Description Protocol", July 2006.
[RFC3550] Schulzrinne, H., "RTP: A Transport Protocol for Real-Time Applications", RFC 3550, July 2003.
[RFC6776] Wu, Q., "Measurement Identity and information Reporting using SDES item and XR Block", RFC 6776, August 2012.

10.2. Informative References

, "
[RFC6792] Hunt, G., "Monitoring Architectures for RTP", RFC 6792, November 2012.
[RFC6390] Clark, A. and B. Claise, "Framework for Performance Metric Development", RFC 6390, October 2011.
[RFC6709] Carpenter, B., Aboba, B. and S. Cheshire, "Design Considerations for Protocol Extensions", RFC 6709, September 2012.
[VAD]http://en.wikipedia.org/wiki/Voice_activity_detection", .

Appendix A. Metrics represented using RFC6390 Template

RFC EDITOR NOTE: please change XXXX in [RFCXXXX] by the new RFC number, when assigned.

  1. On-time Playout Duration Metric

  2. Loss Concealment Duration Metric

  3. Buffer Adjustment Concealment Duration Metric

  4. Playout Interrupt Count Metric

  5. Mean Playout Interrupt Size Metric

  6. Unimpaired Seconds Metric

  7. Concealed Seconds Metric

  8. Severely Concealed Seconds Metric

  9. SCS Threshold Metric

Appendix B. Change Log

Note to the RFC-Editor: please remove this section prior to publication as an RFC.

B.1. draft-ietf-xrblock-rtcp-xr-loss-conceal-09

The following are the major changes to previous version :

B.2. draft-ietf-xrblock-rtcp-xr-loss-conceal-07

The following are the major changes to previous version :

B.3. draft-ietf-xrblock-rtcp-xr-loss-conceal-06

The following are the major changes to previous version :

B.4. draft-ietf-xrblock-rtcp-xr-loss-conceal-05

The following are the major changes to previous version :

B.5. draft-ietf-xrblock-rtcp-xr-loss-conceal-04

The following are the major changes to previous version :

Authors' Addresses

Alan Clark Telchemy Incorporated 2905 Premiere Parkway, Suite 280 Duluth, GA 30097 USA EMail: alan.d.clark@telchemy.com
Glen Zorn (editor) Network Zen 77/440 Soi Phoomjit, Rama IV Road Phra Khanong, Khlong Toie Bangkok, 10110 Thailand Phone: +66 (0) 87 502 4274 EMail: gwz@net-zen.net
Claire Bi Shanghai Research Institure of China Telecom Corporation Limited No.1835,South Pudong Road Shanghai, 200122 China EMail: bijy@sttri.com.cn
Qin Wu (editor) Huawei 101 Software Avenue, Yuhua District Nanjing, Jiangsu 210012 China EMail: sunseawq@huawei.com