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Linksys ATA
Administrator Guide
Document Version 3.1
Chapter 1
Introducing Linksys Analog Telephone Adapters
Feature Descriptions
starts streaming audio to the calling party provided the FXS port is off-hook. If the FXS port is on-hook
when the incoming call arrives, the Linksys ATA replies with a SIP 503 response code to indicate
“Service Not Available.”
If an incoming call is auto-answered, but later the FXS port becomes on-hook, the Linksys ATA does
not terminate the call but continues to stream silence packets to the caller. If an incoming call arrives
when the SAS line has reached full capacity, the Linksys ATA replies with a SIP 486 response code to
indicate “Busy Here”. The SAS line can be set up to refresh each streaming audio session periodically
(via SIP re-INVITE) to detect whether the connection to the caller is down. If the caller does not respond
to the refresh message, the SAS line terminates the call so that the streaming resource can be used for
other callers.
On a connected call, the Linksys ATA may place the remote party on call. The only way to do this on
the SPA2102 is to perform a hook-flash to initiate a three-way call, or to swap two calls during
call-waiting. If the remote party indicates that they can still receive audio while the call is holding, the
SPA2102 can be set up to contact an auto-answering SAS as described in and have it stream audio to the
holding party. When used this way, the SAS is referred to as a MOH Server.
Silence Suppression and Comfort Noise Generation
Voice Activity Detection (VAD) with Silence Suppression is a means of increasing the number of calls
supported by the network by reducing the required bidirectional bandwidth for a single call. VAD uses
a very sophisticated algorithm to distinguish between speech and non-speech signals. Based on the
current and past statistics, the VAD algorithm decides whether or not speech is present. If the VAD
algorithm decides speech is not present, the silence suppression and comfort noise generation is
activated. This is accomplished by removing and not transmitting the natural silence that occurs in
normal two-way connection. The IP bandwidth is used only when someone is speaking. During the silent
periods of a telephone call, additional bandwidth is available for other voice calls or data traffic because
the silence packets are not being transmitted across the network.
Comfort Noise Generation provides artificially-generated background white noise (sounds), designed to
reassure callers that their calls are still connected during silent periods. If Comfort Noise Generation is
not used, the caller may think the call has been disconnected because of the “dead silence” periods
created by the VAD and Silence Suppression feature.
Modem and Fax Pass-Through
Modem Pass-through Mode can be triggered only by predialing the <Modem Line Toggle Code>. FAX
Pass-through Mode is triggered by CED/CNG tone or NSE events. Echo canceller is automatically
disabled for Modem Pass-through Mode only. Echo canceller is automatically disabled only if <FAX
Disable ECAN> (Line 1/2) is set to “yes” for that line (in that case FAX pass-through is the same as
Modem pass-through). Call waiting and silence suppression is automatically disabled for both FAX and
Modem pass-through as before. In addition, out-of-band DTMF Tx is disabled during modem or fax
pass-through.
Adaptive Jitter Buffer
The Linksys ATA can buffer incoming voice packets to minimize out-of-order packet arrival. This
process is known as jitter buffering. The jitter buffer size proactively adjusts or adapts in size, depending
on changing network conditions.