(Continuation of Question 24/16)

Background and justification

ITU has noted a rapid increase in the use of IP for data and a growing commercial interest in deploying voice over IP as an application. In addition, the worldwide usage of wireless communication systems is rapidly increasing, which includes the possibility of deploying certain IMT-2000 networks as IP-oriented systems. IP traffic is evolving to become the dominant global communication system with both wired and wireless access. The transport of IP voice traffic within and between wired and wireless networks is of special interest.

A key issue therefore is the design of new voice coding technology that can best exploit the properties of these networks to provide efficient use of bandwidth and minimize the need for transcoding. As traditional circuit-switched networks will stay with us for a long time, easy transcoding with them is also an important design aspect.

Hence, new work is proposed to examine the applications, technical issues of IP and wireless networks, and investigate the design of a suitable variable bit rate voice coder.

Examples of bearer networks for VoIP carriage which have been identified as benefiting from a variable bit rate voice coder are:

• Ethernet Networks.
• ATM Systems.
• Third Generation Wireless Systems, including CDMA and TDMA.

Additionally, multimedia applications may benefit from a variable bit rate voice coder.

For the activities of this question, the term variable bit rate (VBR) has been defined as follows. A VBR encoder outputs a bit stream which may have a variable number of bits in successive frames. That is, each frame may contain a different number of bits relative to the last frame. Bit rate may vary, for example, in large predefined increments/decrements or it may vary by as little as one bit resolution. The variability in bit rate may be either network controlled or source controlled according to the input audio signal.

Study items

What algorithm should be specified for the coding of voice/audio, with specific attention being paid to the applications, and the consequent constraints imposed jointly by the network and application.

Items for study include:

1) Study and definition of applications and performance for variable bit rate voice coding.

2) Consideration of network requirements and constraints.

3) Definition of performance requirements and system constraints.

4) Definition of the test conditions and evaluation procedures to be applied in selecting between candidate algorithms on the basis of subjective performance, as specified by SG 12, and non-voice performance.

5) Study of encoding techniques and evaluation of the possibility of selecting a single type of VBR technique capable of operating at different transmission rates with an associated improvement in speech quality as the transmission rate is increased.

6) Selection and specification of procedures to be used in verifying the implementation of selected algorithms.

7) Study of the support of text-telephone (TDD) devices in systems using variable rate coding of speed.

While studying these work items, specific efforts will be put on the key issues of reducing cost and complexity, minimizing transcoding, interworking with other systems, and quality of service.

Specific tasks and deadlines

• Definition of performance requirements and objectives: February 2000.
• Approval (Decision) of the Recommendation: November 2001.

Relationships

• Other relevant Questions of SG 16.
• ITU-T SG 2 to identify other potential user applications.
• ITU-T SG 11.
• ITU-T SG 12 for the performance evaluation of specified algorithms with respect to voice quality.
• ITU-T SG 13.
• ITU-T SG 15.
• ITU-R SG 8 to ensure compatibility with mobile transmission system constraints.
• ISO/MPEG.
• ETSI (Tiphon).
• TIA.
• IETF.
• ATM Forum.
• 3GPP, 3GPP-2.