Linksys SPA 900 manual Field Description

Chapter 5 Linksys IP Phone Field Reference

Ext Tab

Field	Description
FAX CNG Detect Enable	Defaults to yes.
FAX Passthru Codec	Codec to use for FAX passthru (G711u or G711a).
	Defaults to G711u.
FAX Codec Symmetric	Force unit to use symmetric codec druing FAX passthru.
	Defaults to yes.
FAX Passthru Method	Choose None, NSE, or ReINVITE.
	Defaults to NSE.
FAX Process NSE	Defaults to yes.
FAX Disable ECAN	Set to yes to automatically disable Echo Canceller when FAX tone is detected.
	Defaults to no.
DTMF Tx Method	Select the method to transmit DTMF signals to the far end: InBand, AVT, INFO, Auto,
	InBand+INFO, or AVT+INFO. InBand sends DTMF using the audio path. AVT sends
	DTMF as AVT events. INFO uses the SIP INFO method. Auto uses InBand or AVT based
	on the outcome of codec negotiation.
	Defaults to Auto.
Hook Flash Tx Method	Select the method to signal Hook Flash events:
	• None: do not signal hook flash events
	• AVT: use RFC2833 AVT (event=16)
	• INFO: use SIP INFO method with the signal line “signal = hf” in the message body. The
	MIME type for this message body is taken from the <Hook Flash MIME Type> parame-
	ter.
	Defaults to None.
Symmetric RTP	Enable symmetric RTP operation. If enabled, the SPA sends RTP packets to the source
	address of the last received valid inbound RTP packet. If disabled, the SPA sends RTP to
	the destination as indicated in the inbound SDP.
	Defaults to yes.
Release Unused Codec	This feature allows the release of codecs not used after codec negotiation on the first call,
	so that other codecs can be used for the second line. To use this feature, select yes.
	Otherwise, select no.
	Defaults to yes.

A codec resource is considered allocated if it has been included in the SDP codec list of an active call, even though it eventually might not be chosen for the connection. If the G.729a codec is enabled and included in the codec list, that resource is tied up until the end of the call whether or not the call actually uses G.729a. If the G729a resource is already allocated (and since only one G.729a resource is allowed per SPA), no other low-bit-rate codec can be allocated for subsequent calls. The only choices are G711a and G711u.

Since two G.723.1/G.726 resources are available per SPA, you should disable the use of G.729a to guarantee support for two simultaneous G.723/G.726 codecs.

	Linksys IP Phone Administrator Guide
Firmware Version 5.1		5-45