FIG. 1 shows a conventional network environment consisting of a central office switch 110 of the Public Switched Telephone Network (PSTN) and a PBX switch 130, such as the Merlin Legend.TM. PBX, interconnected by one or more DS1 digital trunks 120-122. The central office switch 110 may be embodied, for example, as the DMS-100 central office equipment, commercially available from Northern Telecom, Inc. of Ontario, Canada. While PBX switches typically process call dialing signals from the central office switch 110, the signals are generally DTMF signals. FIGS. 2a through 2c illustrate the spectrum of DTMF signals from the central office switch 110, corresponding to digits "3," "1" and "5," respectively. The DTMF signal for digit "1," for example, as shown in FIG. 2(b), will be the same regardless of the position of the "1" within a ten (10) digit telephone number.
As shown in FIG. 1, PBX switches, such as the switch 130, typically include a touch-tone receiver (TTR) 140 for receiving and processing the DTMF signals. The Legend.TM. PBX, for example, includes a TTR embodied as a TMS320C17 digital signal processor (DSP), commercially available from Texas Instruments, Inc., of Dallas, Tex.
Typically, the TTR 140 on a PBX switch 130, such as the Legend.TM. PBX, is a closed subsystem and does not provide an access interface to obtain information for diagnostic analysis. Generally, PBX switches, such as the switch 130, do not provide any diagnostic or debugging support for DTMF signals. Thus, the manner in which a PBX switch processes DTMF data is unknown. In addition, such PBX switches do not provide a mechanism for analyzing a call failure problem due to DTMF signals. Currently, it is difficult, if not impossible, to even collect DTMF data on such a PBX switch.
For example, it has been found that inbound calls from a central office switch 110 to a PBX switch 130 are likely to fail on a subset of extension numbers when certain dial plans are implemented. In particular, a high call failure rate has been observed on incoming PBX calls associated with extension numbers having a digit "1" in the dialed telephone number and followed by at least one additional digit, for example, extension "315" or "7150," even though the incoming DTMF signals comply with the DTMF specification. While preliminary investigations suggested a DTMF detection problem in the TTR of the PBX switch, the PBX switch does not have a diagnostic tool to identify the precise source of the call failure problem. The call failure problem cannot be reproduced with valid test scenarios in a laboratory environment. Furthermore, experimental trials on installed PBX systems are not practical, due to frequent service disruptions.
Since the source of the call failure problem could not be identified, the problem could also not be remedied. Thus, in order to avoid such call failures, PBX customers frequently did not assign extension numbers having a digit "1" followed by at least one additional digit, thereby limiting the effective capacity of the PBX switch.
As apparent from the above-described deficiencies with conventional PBX switches, a need exists for a diagnostic tool that analyzes DTMF signals on such PBX switches. A further need exists for a method and apparatus for simulating the processing of DTMF signals by a TTR. Finally, a need exists for a TTR simulator that facilitates the analysis and debugging of DTMF data.