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VOCAL's facsimile interception (lawful fax intercept) library uses fax demodulator software to extract fax images from intercepted PSTN audio signals. It allows effective security monitoring and analysis of fax communications from previously recorded input sample files or real-time sources. The fax intercept module has been deployed worldwide to assist law enforcement officials and government institutions in their surveillance equipment.
The fax decoder can operate at multiple sample rates, including 16 KHz, in order to ensure the highest input signal quality. VOCAL has substantial experience with recording technologies and can help advise in the proper configuration of your recording system. For important recordings with insufficient signal quality or damage, VOCAL can also provide analysis and manual inspection services which may be able to recover some or all of the relevant data.
The facsimile monitoring module for PSTN lines includes a fax decoder for both standard faxes as well as machines that make use of manufacturer specific T.30 'Non-Standard Facilities' (NSF) features. For recorded files using low-speed demodulation, an auto-detect mode that does not rely on FSK frames is also available. This allows for demodulation of faxes using NSF negotiation or where control data may not be available. Accurate image recovery is ensured through blind data retrieval and consistency checks, avoiding possible compatibility issues with non-standard fax machines.
VOCAL's Lawful Intercept module includes the following functionality:
To assist with embedded development, all combinations of the above modulations, protocols, and features are supported. This allows for optimal use of resources in embedded systems where processing power, memory, or even connect rate are limited.
FAX connection speed is typically negotiated between the sender and receiver at or near the beginning of a FAX call. Unless using some form of blind demodulation, an external probe must decode this negotiation in order to properly set up the demodulator. Therefore, it is preferred that the connection is recorded from the beginning of the call.
The recorded analog signal should have 15 or more bits of resolution and be sampled at an 8 KHz or higher rate. For the V.34 FAX recordings, a sample rate higher than 8 KHz is recommended in order to ensure the highest available input signal quality. Captures from digital (T1/E1) streams are also acceptable.
Either a 2- or 4-wire tap may be used to collect PCM samples. All supported FAX modes use either exclusively half-duplex transmission, or in the case of V.34 fax use split-band signaling.
For recording devices, all automatic gain adjustments, VAD, or any other options that might introduce non-linearities into the signal should be disabled. The data recording should be as close to that seen by the receiving machine as possible.
If the recording device is not negotiable and has undesirable properties, VOCAL may be able to create a special purpose front-end to correct for specific types of distortion.
The file based processing interface for VOCAL's Facsimile Lawful Interception Library has been designed for simplicity of integration and ease of use. Callback support during file-based operation is also available.
For non-real time processing, the standard supported file formats are Microsoft WAVE (mono/stereo, ulaw/alaw/16 bit linear, 8khz/16khz) and 16 bit linear PCM. Not all .wav formats are supported. Stereo recordings are assumed to be from a four wire tap; mono recordings are assumed to be from a two wire tap. Additional interfaces, rates, and data formats can be provided as required for your application.
The output file format commonly will be either TIFF or the standard Microsoft bitmap format (bmp), and can be chosen for a specific implementation. Images are written in these formats and should be readable by nearly any image manipulation utility. Support for alternate page naming mechanisms is also available on request. For page image data, typically one output file is created per page, named in increasing numerical order and extracted from the recordings. A thorough logfile of each FAX session is also available to assist in debug and analysis.
To summarize the extracted images, a short report file is produced that contains the following information: input file, caller ID, and for each page: width, length, resolution, ECM mode, modulation and compression.
In addition, function callbacks are available for the direct extraction of page and T.30 data to allow the VOCAL FAX Demodulator components to work as libraries or in specialty embedded applications. These interfaces are customized on a per-application basis; please contact us with your requirements for this type of interface.