1、 TIA STANDARD Discontinuous Transmission (DTX) of Speech in cdma2000 Systems TIA-1095 April 2006 TELECOMMUNICATIONS INDUSTRY ASSOCIATION Representing the telecommunications industry in association with the Electronic Industries Alliance NOTICE TIA Engineering Standards and Publications are designed
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22、NDAMENTAL ELEMENT OF THE USE OF THE CONTENTS HEREOF, AND THESE CONTENTS WOULD NOT BE PUBLISHED BY TIA WITHOUT SUCH LIMITATIONS. TIA-1095 1 INTRODUCTION 1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 In circuit-switched calls over CDMA, acoustic
23、background noise is encoded in eighth rate packets and transmitted periodically, for example every 20 msec. Although it is necessary for circuit-switched calls to transmit a packet periodically, the same is not true for packet-switched calls. It is possible to achieve capacity savings and extend bat
24、tery life for VoIP calls by using a discontinuous transmission scheme (DTX). This specification provides requirements for source controlled variable rate vocoders to implement DTX. It is recognized that there exists a great deal of flexibility in the way DTX can be implemented. The purpose of this s
25、pecification is not to specify a particular solution, but to provide a set of requirements to ensure interoperability between different implementations. This specification uses the following verbal forms: “Shall” and “shall not” identify requirements to be followed strictly to conform to the standar
26、d and from which no deviation is permitted. “Should” and “should not” indicate that one of several possibilities is recommended as particularly suitable, without mentioning or excluding others; that a certain course of action is preferred but not necessarily required; or that (in the negative form)
27、a certain possibility or course of action is discouraged but not prohibited. “May” and “need not” indicate a course of action permissible within the limits of the standard. “Can” and “cannot” are used for statements of possibility and capability, whether material, physical, or causal. 2 DEFINITIONS
28、Comfort Noise (CN): Output of the decoder to represent acoustic background noise. Discontinuous Transmission (DTX): Gaps inserted in the media stream during periods of non-active speech. DTX Stream: A bit stream resulting from DTX processing, i.e., a bit stream in which silence has been suppressed.
29、DTX Update Interval: The number of frames from one SID frame to the next. For example, an update interval of 10 frames means that a SID frame is sent once every 10 frames. This interval may be variable between updates. DTX min: The minimum acceptable DTX interval. DTX max: The maximum acceptable DTX
30、 interval. Silence Insertion Description (SID): Packets used to convey acoustic background information during DTX periods. Eighth-Rate Hangover: A number of eighth-rate frames transmitted at the end of active speech but before DTX. DSP: Digital Signal Processor PDSN: Packet Data Serving Node RTP: Re
31、al Time Protocol 3 APPLICABILITY This section describes the applicability of this specification. Page 2 TIA-1095 3.1 Non-DTX-Capable Legacy Vocoders 1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 Modifying an existing vocoder that is not DTX capable to comply with this specification is
32、optional. 3.2 DTX-Capable Legacy Vocoders This specification does not supercede native DTX schemes defined in legacy vocoders, like VMR-WB 2. 3.3 Future Vocoders Future vocoders have the option of adopting this specification for DTX operation. 3.4 Other DTX/CN Schemes This specification does not pro
33、hibit the use of other DTX schemes, such as 1. In addition, other DTX schemes may be negotiated simultaneously with this DTX scheme if more than one vocoder is sharing the same session. The mechanism by which the endpoints negotiate this DTX scheme and its parameters is beyond the scope of this spec
34、ification. 3.5 Intermediaries DTX streams defined by this specification can be converted to continuous streams, and vice versa, with a minimum of computational complexity. For example, a continuous stream can be converted to a DTX stream by discarding eighth-rate frames between SID frames. Likewise,
35、 at a minimum, a DTX stream can be converted to a continuous stream by replicating eighth-rate SID frames. Since the conversion can be achieved without requiring the processing horsepower of a DSP, the conversion can be performed in “wrapper” code outside the DSP, in de-jitter buffers, or in network
36、 elements that handle the RTP packets but have limited processing power, such as the PDSN. This specification permits the use of a network element to act as an intermediary between two endpoints to convert DTX streams into continuous streams, and vice versa. 3.6 Network This DTX specification applie
37、s to the cdma20001access network as well as the core network. 1cdma2000is the trademark for the technical nomenclature for certain specifications and standards of the Organizational Partners (OPs) of 3GPP2. When applied to goods and services, the cdma2000mark certifies their compliance with cdma2000
38、standards. Geographically (and as of the date of publication), cdma2000 is a registered trademark of the Telecommunications Industry Association (TIA-USA) in the United States. Page 3 TIA-1095 4 ENCODER REQUIREMENTS 1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31
39、32 33 34 35 4.1 SID Frame Format The vocoders existing eighth rate packet format shall be used as the SID frame format. SID frames shall not consist of full, half, or quarter rate packets, nor shall a separate packet format be defined for the purpose of providing SID updates. 4.2 Eighth-Rate Hangove
40、r The encoder shall implement eighth-rate hangover. The length of the hangover is negotiated in signaling, which is beyond the scope of this document. 4.3 DTX Parameters Signaling may be used to negotiate the use of DTX, following an offer/answer model. An endpoint capable of DTX shall communicate t
41、he following parameters: Willingness to use DTX (i.e. on/off) Default value: on for RFC 35583, off for all other RTP formats. Minimum DTX Update Interval Default value: 12 frames. Maximum DTX Update Interval Default value: 32 frames. The DTX max shall be greater than or equal to the DTX min. Eighth-
42、Rate Hangover Default value: 1 frame. 4.3.1 The values for DTX max, DTX min, and eighth-rate hangover shall be represented as unsigned 8-bit integers, i.e. limited to the range 0-255. 4.3.2 For vocoders using the RTP format defined in RFC 3558 3, the default for willingness to use DTX support define
43、d in this specification shall be “on”, and the default values for DTX min, DTX max, and eighth-rate hangover shall be used, unless negotiated otherwise. That is to say, for those vocoders that use the RTP format in RFC 3558 3, DTX support defined in this specification is always assumed enabled unles
44、s the negotiation specifically disables it. For all other vocoders, if all four of the DTX parameters defined above are absent from either the offer or the answer, it is assumed that DTX support defined in this specification is not available in the endpoint(s). 4.3.3 If the offer as well as the answ
45、er contains DTX parameters, the value of DTX min in the answer shall be greater than or equal to the value of DTX Min in the offer, and the value of DTX max in the answer shall be less than or equal to the value of DTX max in the offer. Furthermore, the eighth-rate hangover in the answer shall be gr
46、eater than or equal to the eighth-rate hangover in the offer. 4.3.4 If both offer and answer contain DTX parameters, then both sides shall use the values for DTX parameters that are proposed in the answer. 4.3.5 If none or only one of offer or answer contains DTX parameters: The default values for t
47、he DTX parameters shall initially be used by both sides. Page 4 TIA-1095 For vocoders using the RTP format defined in RFC 3558 3, if at any time during the flow, a vocoder detects the remote vocoder as DTX-incapable, it should stop sending SID frames. That is to say that the vocoder shall still cont
48、inue to send non-eighth rate frames as well as eighth rate hangover frames, but should not send eighth rate frames as SID frames. Silence should be suppressed from the end of the eighth rate hangover until the beginning of the next talk spurt. 1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22
49、 23 24 25 26 27 28 29 30 31 32 33 34 It may be determined that a vocoder is DTX incapable if the number of eighth rate frames with contiguous RTP timestamps exceeds the default value of the eighth rate hangover. For vocoders using the RTP format defined in RFC 3558 3, if a vocoder is suppressing SID frames because it detected that the remote vocoder is not DTX capable and subsequently detects that the remote vocoder is DTX capable, the default values for the DTX parameters shall be used. It may be determined that a vocod
