REAL TIME MONITORING AND PROCESSING OF 2 MBPS PCM DATA

Abstract

The present work of "Real Time Monitoring and Processing of 2 Mbps PCM data" deals with the conceptualization, design and development of Facsimile Monitoring System (FMS) operating on the PCM data of a CEPT trunk. Sometimes a fax message may become unintelligible due to problems related to a fax machine or due to PSTN related factors. This facsimile monitoring system is proposed as a solution to analyze the failure mechanism of long distance fax calls. The Group 3 type fax machines are widely used for fax transmission over the PSTN. The transmitting and receiving fax machines (Group 3) use T.30 ( ITU) protocol for signalling between each other. The T.30 protocol specifies the half-duplex frequency shift keying scheme conforming to ITU V.21 channel No. 2 characteristics for signalling between the fax machines. The T.30 protocol uses the HDLC frames used for message transmission and all messages are preceded by a preamble which consists of a series of flags ("01111110" pattern) for one second duration. An analysis of these messages will provide complete information on the performance of the fax call such as the transmission speed, the fall back in transmission speed, the class of modem used and a normal or abnormal reception of the fax message by the receiving machine.

The FMS is capable of simultaneously monitoring the data on ten CEPT trunks. One of the CEPT trunks connected to the FMS carry the CCITT Signal System No. 7 signalling data at 64 kbps and this information is used by the FMS for associating the PSTN signalling information of the fax calls under monitoring. Typically the timeslots on a CEPT trunk can carry voice, data or a fax call. The FMS scans for the occurrence of Group 3 fax call on the timeslots of the digital trunk which is identified by the preamble. If a T.30 preamble is detected, then the FMS captures all the signalling messages both from the transmitting and receiving fax machines and creates a call record which contains all the signalling messages of the fax call.

The FMS is implemented on a IBM-PC host. The FMS consists of ten ISA add-on cards, each capable of supporting one digital trunk. The add-on card is implemented by loosely coupled multi-processor architecture with an efficient inter-processor communication. Each add-on card consists of the modules namely Digital Trunk Interface Module (DTIM), Digital Signal Processing Module (DSPM), T.30 Preprocessing Module( TPM), Signalling System No. 7 Module (SSM) and the ISA bus interface. The TPM communicates with the ISA bus through a Dual Port RAM( DPRAM). At the beginning of a fax monitoring session the host downloads the system configuration details to the add-on cards through the DPRAM.

The DTIM interfaces with the CEPT trunk and captures the PCM timeslots from the digital trunk. The captured PCM timeslots are forwarded to the DSPM. The DSPM consists of eight TMS320Pl 7 digital signal processors (DSP), each capable of processing four PCM timeslots. The firmware for V.21 channel No. 2 demodulation is implemented in the DSP. The demodulated binary data is forwarded to the TPM which removes the stuffed bits of the HDLC and recovers the T.30 protocol messages. The TPM uploads the T.30 messages to the host through the DPRAM. The host forms a call record which is uploaded into a database. The database can be accessed by application programs to generate statistics and failure analysis of fax calls. The functionality the FMS was verified at Singapore Telecom's international CEPT trunks.