VoIP Voice and Fax Signal Processing

8.6 CALL WAIT CALLER ID

Call wait caller ID (CWCID) or calling identity delivery on call waiting (CIDCW) includes all features of Type-1 caller ID plus the capability of call wait caller ID display in the off-hook state. CIDCW uses only MDMF, and this feature is also called Type-2 caller ID or off-hook Caller ID. CIDCW can happen when a call is already in progress. Ring is not used with CIDCW as alert. The channel seizure signal is not used with CIDCW. With CIDCW, the identification of the calling party can be seen without keeping the current call on hold or any additional operations. In this section, the FSK-based Type-2 caller ID method is described.

8.6.1 Call Wait ID Flow in PSTN

In this section, call wait caller ID functioning in PSTN is explained. A and B are the end telephones connected to the PSTN central office as indicated in Fig. 8.7(a). With the help of the central office, A and B will establish the call and continue the conversation. In the example, A receives another call from C through the central office. The basic sequence from the established call between A and B are described as follows:

• The voice call continues between A and B.
• C is dialing A, and the central office senses that A is busy.
• If subscriber A does not have the call wait service, the central office sends a busy tone to C and User-A cannot speak to User-C.

Figure 8.7. Call wait ID. (a) Functional representation ...