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Description
Echo cancellation
Silence Suppression
DTMF Through Dial
553-3001-202
Standard 1.00
All telephony voice services now in use reflect some level of echo back to the
user. The term "echo" refers to the return of a signal's reflection to the
originator.
Packet voice networks introduce sufficient latency to cause what a caller
would consider an audible echo. The echo path is round-trip. Any speech
coding, packetization, and buffering delays accumulate in both directions of
transmission, increasing the likelihood of audibility.
The purpose of Silence Suppression is to reduce bandwidth consumption.
With the H.225 protocol, coders can send silence frames before the end of
transmission, during a period of silence. Coders may omit sending audio
signals during periods of silence after sending a single frame of silence, or
send silence background fill frames, if these techniques are specified by the
audio codec in use.
For applications that send no packets during silence, the first packet after a
silence period is distinguished by setting a marker bit in the Real Time
Protocol (RTP) data header. Applications without Silence Suppression set the
bit to zero.
Preservation and transport of tones through the IP network is critical for
Interactive Voice Response (IVR) services. The ITG makes sure that DTMF
tone information is included in the packets that are sent through the IP
network, and that the tones are retransmitted by the far-end gateway. The
duration information for DTMF signals is not transmitted, i.e., long DTMF
bursts are reduced to a short standard duration.
Callers can access traditional Voice Mail or IVR services, including "Press 1
for more information" or "Press 2 to be connected to our customer service
department". Services that depend on long DTMF bursts cannot be accessed.
April 2000
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