Audio and video codec requirements and usage for the provision of bidirectional audio services over cable television networks using cable modems (Study Group 9)
This Recommendation specifies the media aspects of the interfaces between IPCablecom client
devices for audio and video communication. Specifically, it identifies the audio and video codecs
necessary to provide the highest quality and the most resource-efficient service delivery to the
customer. This Recommendation also specifies the performance required in client devices to
support future IPCablecom codecs. Additionally, this Recommendation describes a suggested
methodology for optimal network support for codecs.
1.1 Introduction and overview
The quality of audio and video delivered over the IPCablecom architecture will depend on multiple
factors: the end device performance, the network's inherent quality, and the intelligence of the
system resource allocation policy. This Recommendation specifies codecs and capabilities
supporting audio and video applications, with a particular emphasis on the stringent requirements of
IP-based voice communications.
Acceptable voice communications functionality imposes strict delay and packet-loss criteria on IP
implementations and will thus stress system resources, particularly if bandwidth becomes congested
or saturated. Video applications – while more forgiving to dropped packets and delay – require
bandwidth of at least an order of magnitude more than audio applications. The IPCablecom
architecture is designed to support both voice and video applications simultaneously.
Speech and video compression are evolving technologies. New algorithms are being enabled as more
sophisticated and higher performing processors become available at lower cost. Additionally, the
system infrastructure and mechanisms for allocating resources will evolve. Due to this dynamism, the
priority in designing the architecture is to define a robust system to accommodate evolving
technology without creating a legacy burden.
Therefore, the IPCablecom philosophy is to establish cost-effective envelopes for network and device
performance to enable the most appropriate current technology, while allowing upgrades as
technology and market needs evolve. To address near-term market needs, this Recommendation also
specifies codec and performance mandates to deliver the quality-of-service necessary for launching
competitive services.
1.2 Purpose of this Recommendation
The purpose of this Recommendation is to extend the existing IPCablecom v1Codec
Recommendation by introducing two new low-bit codecs, [ITU-T T.38] fax relay for reliable fax
transmission, [IETF RFC 2833] DTMF Relay for reliable DTMF transmission and metrics to
measure voice quality. It is issued to facilitate design and field-testing leading to the
manufacturability and interoperability of conforming hardware and software by multiple vendors.
1.3 Phasing of requirements
The codec requirements contained in this Recommendation cover both audio and video multimedia
terminals (MTAs) and trunking gateways (media gateway). The term MTA-2 is used to define a
terminal supporting video.
In the initial phase of IPCablecom, MTAs are not required to support the MTA-2 requirements as
defined in clause 8. MTAs MUST support the requirements for audio terminals as defined in
clauses 5, 6, 7, and 9.
Support for video terminals will be REQUIRED in later phases of IPCablecom. All MTA-2s MUST
support the requirements defined in clause 8.