Calling Line Identifier

FX602

© 1998 Consumer Microcircuits Limited

18

D/602/7

1. INITIAL state

This is the normal state when the CPE is off hook. The FX602 is in the Tone Alert Detect mode, the timer is

reset and the FX602's DET output line is being monitored.

If the FX602 DET output goes high start the timer, and go to the WAIT_T1 state.

2. WAIT_T1 state

During this state the DET output high time is measured so that pulses lasting less than time T1 may be ignored.

If the DET output goes low before time T1 then reset the timer and return to the INITIAL state.

If the DET output is still high when the timer reaches time T1 then initiate muting of the local speech

circuit and go to the MEAS_DET_HI state. Muting should be complete within time T2.

3. MEAS_DET_HIGH state

This state continues to measure the length of the FX602 DET output high pulse, but now the local speech is

muted to eliminate any false CAS signals it may be generate.

If the DET output goes low before time T3 then the signal was most probably due to near end speech,

so cancel the speech muting, reset the timer and return to the INITIAL state.

If the DET output is still high when timer reaches 65ms the signal is too long to be a valid CAS and

must have been caused by far end speech, so cancel the speech muting and go to the

WAIT_DET_LOW state.

If the DET output goes low when the timer is between T3 and 65ms then it could be a valid CAS, in

which case it will be followed by a 'quiet' period (local speech being muted) so set the FX602 to FSK

Receive mode, reset the timer and go to the CONFIRM_QUIET state. These actions should take place

within 5ms of the DET output going low.

4. CONFIRM_QUIET state

During this state the energy present at the FX602's input after the end of the dual-tone detection is monitored

for up to 50ms using the FX602's FSK Level Detector circuits. If the received signal had been a valid CAS then

it would have been followed by a quiet period (the local speech being muted by this algorithm) whereas a false

signal caused by far end speech will contain additional energy in the following period.

If the DET output goes high before the timer has reached 50ms then remove local speech muting, set

the FX602 to Tone Alert Detect mode and return to the INITIAL state.

If the timer reaches 50ms then a valid CAS has been detected so go to the COMPLETE_CIDCW state.

5. COMPLETE_CIDCW state

This state handles the remaining CIDCW transaction functions e.g. determine that no near end extensions are

off hook, send a dtmf ACK signal to the Central Office, receive FSK Caller ID data stream, etc.