Voice Attributes

Introduction

You can configure the following voice attributes with the Resource Attribute Configure (0x00E3) message:

• RTP Payload Type

• RTP Payload Size

• Silence Suppression (sometimes referred to as Voice Activity Detection, VAD)

• Echo Cancellation

• Redundancy

• RTCP Port Offset

• Connection Mode

• Jitter Buffer/Adaptation Rate

RTP Payload Type

The RTP payload type indicates the algorithm used for compressing the data or payload portion of the packet. For an active connection, the only configuration change you can make is between G.723 5.3 Kbps and G.723 6.3 Kbps. See the TLV for options.

Default: G.711µ-Law

Use TLV: 0x0100 RTP Payload Type (VDAC-ONE)

RTP Payload Size

RTP payload sizes can be changed in multiples of their individual packetization rates. The default multiplication factor is 1x. The maximum multiplication factor supported is 8x. The higher the multiplication factor, the larger the packets, and the smaller the network throughput.

Example: For G.711, you can compute the payload size by multiplying the packetization rate by the number of bytes of PCM data sampled per second.

160 Bytes = 8000 samples/sec * 20 milliseconds

The RTP payload size factor produces the number of 20 millisecond payloads (30 milliseconds for G.723) to be used for generating one RTP packet. Multiples lower than 20 milliseconds (that is, 5 milliseconds, 10 milliseconds, 15 milliseconds) or 30 milliseconds for G.723 are not supported.

Default: 1x

Use TLV: 0x0101 RTP Payload Size (VDAC-ONE)

Payload Type and Payload Size

The following table describes the relationship between the Payload Type and Payload Size on a VDAC-ONE card.

Table 2-5 VDAC-ONE Voice Coder Packet Rate Information

Silence Suppression

During a normal voice conversation, much of the time is wasted on silence from one or both ends. Ethernet bandwidth can be conserved if, during these periods of silence, RTP packets are sent with silence-encoded, compressed payloads.

You can enable or disable Silence Suppression on an established connection.

Default: Disabled

TLV: 0x0102 RTP Silence Suppression
(Do not send to an active call that is in fax/modem mode.)

The VDAC-ONE card supports silence suppression as follows:

A voice activity detection (VAD) algorithm determines which portions of the input signal contain active speech. At the beginning of silence periods (the end of active periods), a special silence insertion descriptor (SID) packet is generated that describes the background noise. The SID packets are generated at the onset of the silence period, and whenever the characteristics of the background noise change. The speech decoder that receives the SID packet uses a comfort noise generation (CNG) algorithm to reproduce the background noise from the information in the packet and possibly information contained in past active voice packets. The average bit rate required for voice transmission is lowered considerably when using silence suppression.

Echo Cancellation

In compliance with ITU G.168, this feature eliminates echo introduced by impedance mismatched hybrids. You can enable and disable Echo Cancellation on an established connection.

Default: Enabled

TLV: 0x0103 RTP Echo Cancellation

RTP Payload Redundancy

You can set the redundancy level for RTP using the RTP Payload Redundancy TLV. See RFC 2198.

Default: Disabled

TLV: 0x01D1 RTP Payload Redundancy

Connection Mode

During call setup, the Connection Mode is automatically determined according to the presence of destination and source addresses in the Route Control or Outseize Control message.

Source Address Only - Receive Only

Source and Destination Address - Transmit and Receive

To change the Connection Mode after a call is established using Resource Attribute Configure 0x00E3, send the Connection Mode TLV with the appropriate mode set.

In hold mode, no voice is transmitted or received.

Example: After a two-way call has been established to a Call Center, the caller is put on hold and connected to music. The Connection Mode of the Call Center line is changed to Transmit Only. If callers enable the mute feature on their phone, the Connection Mode on their line is changed to Receive Only.

Default

There is no default Connection mode. If you query the VDAC Module Connection Mode, it is reported as 0xFF. This is a generic value and does not indicate a specific mode. You cannot set the Connection Mode to this value.

TLV: 0x01DB Connection Mode

Important! Source Address (IP/RTP Port) is always required in Route Control or Outseize Control messages.

RFC 2833 Enable

This TLV allows DTMF signals to be relayed within the VoIP media stream, using a special RTP payload format (see RFC 2833). Low bit-rate audio codecs (such as G.729 or G.723.1) can compromise the signal integrity of DTMF digits (and other telephony tones and signals), causing inaccurate detection and recognition of the DTMF digits on the recipient side. When this feature is enabled, the detected incoming DTMF digits (PSTN side) are removed from the audio stream by the VDAC-ONE card. The information for the detected and removed DTMF digits is embedded within the RTP stream, using a special RTP payload format. The VDAC-ONE card on the receiving side decodes this special RTP payload carrying DTMF information, and regenerates the DTMF digits toward the receiving PSTN side. This TLV cannot be set for an active call. It can be configured only before or during the setup of the VDAC-ONE card call.

Important! The CSP does NOT support RFC 2833 via H.323 signaling.

Default: Disabled

TLV: 0x01E2 RFC 2833 Enable

See also the follow section in the Call Agent chapter.

• RFC 2833 Multi-Unicast

RTP Suppression for H.245 Signaling Tones

The EXS API has been updated to allow you to configure how DTMF digits are transmitted to the IP network.

Important! This feature is for Clear Channel calls only.

This feature extends the RFC2833 Enable (0x01E2) TLV as follows:

• The new value 0x02 removes the digit from the voice stream and drops the digit.

• If you are using the VDAC-ONE card, this TLV can be configured only before or during the call establishment. It cannot be set during an active call. If you are using the IP Networking Series 2 card, this TLV can be configure on the fly while the call is active.

Source RTCP Port

This TLV is used to specify the local RTCP port number. If this port is not specified by the host during the VDAC-ONE card call setup, the source RTCP port is set one higher than the source RTP port, per RFC 1889. This TLV can be set during an active call.

Default: No default

TLV: 0x01E8 Source RTCP Port

Destination RTCP Port

This TLV is used to specify the remote RTCP port number. If this port is not specified by the host during the VDAC-ONE card call setup, the destination RTCP port is set to one higher than the destination RTP port, per RFC 1889. This TLV can be set during an active call.

Default: No default

TLV: 0x01E9 Destination RTCP Port

RFC 2198 (RTP Redundancy) Dynamic Payload Negotiation

The VDAC-ONE card and the IP Network Interface Series 2 card allow RFC 2198 dynamic payload negotiation for clear channel calls only.

You first must enable RFC 2198 using the RTP Payload Redundancy TLV (0x01D1) in the Resource Attribute Configuration message.

You can configure the default value at configuration time, or change this value per call, as follows:

• To configure the default RFC 2198 Codec Type, use the RFC 2198 Dynamic Payload Type TLV (0x01F2) in the Resource Attribute Configuration message (0x00E3).

• To change the default RFC 2198 per call, use the RFC 2198 Dynamic Payload Type TLV (0x01F2) within the Generic PPL ICB (0x1E) in the Route Control (0x00E8) or Outseize Control (0x002C) messages.

Default: 104

TLV: 0x01F2