Where are the signals and supervisory tones generated, and how are they used?

The following call scenarios are based on scenarios in the ITU specifications. Note that around the world things may vary quite a bit from these call scenarios. For example, the busy tone is shown as being generated from the caller's local switch. In some systems it is generated from the called party's local switch. The signals used for various purposes are often different.

The signaling between the switches and the telephones could be by DTMF or pulse dialling on an analogue pair (FXO/FXS signaling). It could be ISDN signaling, over a BRI connection. It could be a VoIP connection. The actual signals passing between the switches and the telephone will be similar in most cases, and details do not affect the overall discussion of how MFC/R2 works.

The first scenario describes a call where the called party answers and releases the call. There is no transfer of the caller ID.

Caller                  Switch A      [MFC/R2 signaling]      Switch B                  Called party

                              <------------ Idle -------------->
   --- Off-hook --------->
   <======== Dial tone ===
   --- Digits ----------->
   <========== Silence ===
                              --- Seize ----------------------->
                              <------------------- Seize ack ---
                              --- Dialed digit 1 (I-x) -------->
                              <--------------- Get N+1 (A-1) ---
                              --- Dialed digit 2 (I-x) -------->
                                            ......
                              <--------------- Get N+1 (A-1) ---
                              --- Last dialed digit  (I-x) ---->
                              <------ Address complete (A-3) ---
                              --- User w/o priority (II-11) --->
                              <---- User free, charge (B-62) ---
   <========================================== Ringback tone ===    --- Ring signal ------->
                                                                    <---------- Off-hook ---
                              <---------------------- Answer ---
   <====================================== Speech =========================================>
                                                                    <----------- On-hook ---
                              <------------------ Clear Back ---
   <=== Silence or tone ===   --- Clear forward --------------->
   --- On-hook ----------->
                              <------------ Idle -------------->

The next scenario describes a similar call, but this time the ANI (effectively the caller ID) is transferred. When the ring signal is sent to the called party's phone it might carry the ANI with it.

Caller                  Switch A      [MFC/R2 signaling]      Switch B                  Called party

                              <------------ Idle -------------->
   --- Off-hook --------->
   <======== Dial tone ===
   --- Digits ----------->
   <========== Silence ===
                              --- Seize ----------------------->
                              <------------------- Seize ack ---
                              --- Dialed digit 1 (I-x) -------->
                              <-- Get caller category (A-55) ---
                              --- User w/o priority (II-11) --->
                              <--------- Get next ANI (A-55) ---
                              --- ANI digit 1 (I-x) ----------->
                              <--------- Get next ANI (A-55) ---
                              --- ANI digit 2 (I-x) ----------->
                                         ......
                              <--------- Get next ANI (A-55) ---
                              --- Last ANI digit (I-x) -------->
                              <--------- Get next ANI (A-55) ---
                              --- End of ANI (I-156) ---------->
                              <--------------- Get N+1 (A-1) ---
                              --- Dialed digit 2 (I-x) -------->
                                           ......
                              <--------------- Get N+1 (A-1) ---
                              --- Last dialed digit  (I-x) ---->
                              <------ Address complete (A-3) ---
                              --- User w/o priority (II-11) --->
                              <---- User free, charge (B-62) ---
   <========================================== Ringback tone ===    --- Ring signal ------->
                                                                    <---------- Off-hook ---
                              <---------------------- Answer ---
   <====================================== Speech =========================================>
                                                                    <----------- On-hook ---
                              <------------------ Clear Back ---
   <=== Silence or tone ===   --- Clear forward --------------->
   --- On-hook ----------->
                              <------------ Idle -------------->

The following describes the difference in clearing a call where the called party answers and the calling party releases the call.

Caller                  Switch A      [MFC/R2 signaling]      Switch B                  Called party

   <====================================== Speech =========================================>
   --- On-hook ----------->
                              --- Clear forward --------------->
                                                                    === Silence or tone ===>
                                                                    <----------- On-hook ---
                              <------------------ Clear Back ---
                              <------------ Idle -------------->

The following describes the difference when the called party's line is available, but nobody answers the phone.

Caller                  Switch A      [MFC/R2 signaling]      Switch B                  Called party

                              <---- User free, charge (B-62) ---
   <========================================== Ringback tone ===    --- Ring signal ------->
                                           ......
                         Timeout4
   <=== Silence or tone ===   --- Clear forward --------------->
                              <------------- Clear back/idle ---
   --- On-hook ----------->
                              <------------ Idle -------------->

The following describes the difference when the called line is busy. The pattern would be similar if the called line is faulty, or out of service, but the signal set from switch B to switch A would be different.

Caller                  Switch A      [MFC/R2 signaling]      Switch B                  Called party

                              --- User w/o priority (II-11) --->
                              <----------------- Busy (B-33) ---
   <========= Busy tone ===   --- Clear forward --------------->
                              <------------- Clear back/idle ---
   --- On-hook ----------->
                              <------------ Idle -------------->

The following describes the signaling behaviour when meter pulses are sent by pulsing bit A of the CAS ABCD bits, in accordance with Supplement No 6 of the ITU specifications. In this example the called party clears first, causing the additional forced release signal, defined in Supplement No 6, to be sent.

Caller                  Switch A      [MFC/R2 signaling]      Switch B                  Called party

   <========================================== Ringback tone ===    --- Ring signal ------->
                                                                    <---------- Off-hook ---
                              <---------------------- Answer ---
   <====================================== Speech =========================================>
                              <----- Metering pulse on bit A ---
                              <----- Metering pulse on bit A ---
                                           ......
                              <----- Metering pulse on bit A ---
                                                                    <----------- On-hook ---
                              <-------------- Forced release ---
   <=== Silence or tone ===   --- Clear forward --------------->
   --- On-hook ----------->
                              <------------ Idle -------------->