VOCAL’s lawful facsimile/fax interception library provides fax demodulation software for image extraction and decoding from intercepted PSTN audio signals. It allows interception, monitoring, and analysis of both Super Group 3 (V.34) Fax and low speed fax communications from previously recorded input files or real-time sources. The facsimile intercept module has been deployed in surveillance equipment worldwide to assist law enforcement officials and government institutions.The fax image 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.
VOCAL’s Fax Lawful Interception Library may be licensed by software developers as source code or binary. It is available either standalone or as part of an embedded library suite for integration with developer applications. Custom solutions are also available to meet specific developer requirements.
Fax Image Decoder
The facsimile monitoring module for PSTN lines includes a fax image decoder for both standard facsimile 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 fax demodulation using NSF negotiation or where control data may not be readily available. The fax image decoder ensures accurate image recovery through the use of blind data retrieval and consistency checks, thus avoiding possible compatibility issues with non-standard facsimile machines.
Figure 1: Facsimile Lawful Intercept software decodes fax images from intercepted PSTN audio signals
Fax Decoder Input Data Format
For successful fax image decoder processing, it is preferred that the connection is recorded from the beginning of the call. Connection speed is typically negotiated between the sender and receiver at or near the beginning of a FAX call. Unless some form of blind demodulation is used, an external probe must decode this negotiation in order to properly set up the demodulator.
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.
File-Based Processing Interfaces
The file based processing interface for the fax demodulation software in the 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.
Fax Decoder Output Data Format
The fax image decoder 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 from the fax demodulation software 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 Demodulation software 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.
VOCAL’s Lawful Intercept module includes the following functionality:
- Extract and decompress page data to viewable image files
- Partial page output may be available even if recordings are partial or damaged
- Support for real-time processing on a continuous stream and pre-recorded data
- Automatically classify traffic by type, including voice, data, facsimile and non-standard or custom types
- Automatically classify FAX traffic by modulation and non-standard or custom types
- Support for both V.34 (also known as Super Group 3 or Super G3) FAX and low speed FAX
- Supported Protocols :
- Half duplex extraction support
- Ability to extract page data from single-sided recordings
- Phase hit and other line disturbance correction and recovery
- Partial page recovery for damaged recordings
- Automatic and forced detection of non-standard signaling
- Extract fax tones in auto, manual and polling modes
- Extract DTMF/MF tones and pulse dial information
- Support for Windows and Linux operating systems, as well as embedded system platforms
- Available as an executable or library (.dll/.so)
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.
VOCAL’s Lawful Fax Interception software is available for the following platforms:
- Texas Instruments – TMS320 C5000 and C6000 families, DaVinci, OMAP
- Analog Devices – Blackfin, ADSP-21xx, TigerSHARC, SHARC
- AMCC – PowerPC
- MIPS – MIPS32, MIPS64, MIPS4Kc
- ARM – ARM7, ARM9, ARM9E, ARM10E, ARM11, StrongARM
- iPhone / iPad
- CEVA – Teak, TeakLite, Palm, Oak
- Microchip dsPIC
- AMD x86 (MMX) – AMD Geode SC1200
- PowerQUICC – PowerQUICC Pro (83xx), PowerQUICC III (85xx) families
- Ikanos – Fusiv processors
- ARC Processor IP
- NXP Philips Trimedia – TM1000, TM1100, TM1300
- VeriSilicon (LSI Logic) ZSP
Please contact us for other supported platforms.
VOCAL’s Lawful Fax Interception software is available for the following Operating Systems:
- Unix, Linux, uClinux
- Microsoft Windows ACM / RTC / CE / Mobile
- Mac OS
Please contact us for other supported Operating Systems.