Reference11r1:Interfaces/FXS/Physical

The physical settings of the analogue interfaces can be made here:

Pulse:	A checked check box enables the recognition of pulse dialling on the relevant interface.
Reverse:	A checked check box inverts the wiring of the relevant interface. This is only necessary in the event of incompatibility of the terminals, since some terminals (for example, in the US) are wired the opposite way.
Ring cadence and CLIP transmission sequence:	Set the interworking of CLIP and Ring cadence to different combinations. Some telephones can explicitly suppress the first ring just to receive and present CLIP before audible ringing (often called 1.st Ring OFF in menues). In this case the 'short first AC ring' option is recommended, but not essential for correct working. There are also telephones that do not accept CLIP information after the second AC ring pulse. Here the 'short first AC ring' option is mandatory to get CLIP working. One known model of that kind is the T-COM Concept P412. In some countries it is common that CLIP is preceded by a line polarity reversal instead of a AC ring pulse prior to CLIP transmission. In this case the 'Line polarity reversed' option must be chosen. This procedure is very common with CLIP using DTMF transmission and can be found in Sweden and other countries.
Receive gain:	Set the loudness a user receives at this interface. Default is -10dB for short lines and -7 dB for long lines.
Transmit gain:	Set the loudness a user transmits at this interface. Default is -3dB for short lines and 0 dB for long lines. Some fax devices may need a lower transmit level to avoid clipping.
Country:	Set the line impedance for different countries. In Europe CTR21 is used.
Internal/External call distinction:	Normally internal and external calls are distinguished by terms of different rythms of the ringing voltage. Normally external calls use 1sec ringing voltage - 4sec pause. Internal calls normally use 375ms ringing voltage - 250ms pause - 375ms ringing voltage - 4sec pause. Select swiss to assign this rythms the opposite way. You can also select one rythm to be used permanently, no matter if the calls are external or internal.
Metering pulse frequency:	possible values : 16kHz or 12kHz. 16kHz is common for most countries. Austria and Swiss use 12kHz.
Loop Current:	As described before this topic changes the off-hook loop current and can be modified if 'Optimization grade' is set to 'low'. 'low' sets 20mA, 'mid' set 23mA, 'high' sets 26mA. Specific gateways may offer additional higher values.
RingTrip Threshold:	This topic modifies the current-threshold which lets the gateway distinguish between a ringing condition and a hook-off condition while the ringing voltage is applied from the gateway to the phone. When this threshold is exceeded by the phone, the gateway assumes a hook-off and therefore stops applying the ringing voltage and enters normal off-hook state. Phones that consume abnormal amounts of current while the AC ringing voltage is applied, cause the FXS to false detect a hook-off, which is quickly followed by a hook-on due to a missing loop current after the AC is switched off. This call is connected and released within fractions of a second and is lost. This typical symptom requires the RingTrip Threshold to be increased. 'low' sets 34mA, 'mid' sets 37,5mA and 'high' sets 40mA.

Receive and transmit sides

FXS Gateway representing central office/PABX	::::::::::::::::::::::::::::::::::::::::::::::::::::::::::::::::::::direction:::::::::::::::::::::::::::::::::::::::::::::::::::::::::::::::::::::::::	phone
PCM DAC	>>>>>>>>>>>>>>>>>>>>>>>>>>>>>----receive direction---->>>>>>>>>>>>>>>>>>>>>>>>>>>>>>>>	phone speakerphone
PCM ADC	<<<<<<<<<<<<<<<<<<<<<<<<<<<<<----transmit direction----<<<<<<<<<<<<<<<<<<<<<<<<<<<<<<<	phone microphone

Some considerations about Flash, R and Dial-pulses

The a/b ports support further signaling through Flash-key (other sources call it R-key, what means exactly the same) followed by DTMF-Key combinations. For example, pressing Flash while already having a peer connection will set the current connection into call-hold state (the peer side hears Music-On-Hold) and establish a new call.

From a technical point of view Flash is a short interruption in offhook loop-current. You can imagine Flash as a very short hook-on/hook-off event. The on-hook duration of a Flash event is normally between 100ms and 300ms. Shorter durations are regarded as dial-pulses (if enabled), longer durations are regarded as seperate hook-on and hook-off events. If you encounter difficulties regarding Flash, be sure you have configured a flash-time between 100ms and 300ms in your phone.

Dial-pulses are also short interruptions of the offhook loop-current and can also be imagined as short hook-on/hook-off events. The only difference to Flash is, Dial-pulses have a much shorter duration of 40ms to 80ms.
Several successive Dial-pulses form a digit of a number. The pulses of one digit are seperated by a pause of about 60-80ms. Different digits are seperated with pauses that are much longer (at least 80ms, but can be much more). The digit '1' consists of one single pulse, '2' is two successive pulses and so on, '0' is ten successive pulses.

Pinout of the a/b connector

Pin	Function
3	TIP
4	RING