Newsletter: FAX Analysis over IP, TDM, & PSTN Networks

Welcome to another February 2014 issue of GL's Newsletter providing information and insight into our enhanced software for FAX Analysis over IP, TDM, and PSTN networks referred to as FaxScan™ application.


Call-center quality engineers, brokerages, government agencies, and other entities have a need to monitor fax transactions. Monitoring is done by recording the analog or IP traffic using suitable call-capture applications. These files need to be decoded to investigate issues with protocol messages and Fax image quality.

FaxScan™ application is used to process 2-wire and 4-wire voice band capture files as well as Win PCAP captures to provide analysis of the T.38 packets, T.30 frames, decode a Fax TIF image, and general call-flow indicators for detail analysis. It is a valuable T.30 and T.38 debug and test tool, aiding significantly in system development. Fax sessions can contain standard G3 or V.34-based sessions.

FaxScan™ has the capability to

  • process up to V.34 T.30 recordings in 2-wire, 4-wire. Different encoding formats like µ-law, A-law, 16 bit, and 13 bit PCM captures are supported (requires VBA038 License)
  • process V.34 T.38 and SIP/RTP PCAP captures (requires PKV104 License) Win PCAP captures can be processed from T.38 packets alone or as part of a capture file with multiple sip calls.

Working principle

FaxScan™ can be operated as a stand-alone from a Batch file or Windows® Command Line, or as a integrated module within the GL Voice Band Analyzer application to produce decoded fax image TIFF files and call logs.

FaxScan™ supports 3 modes of operation depending on the type of input file: PCM, SIP, and T.38. The files captured by the call capture applications can be a single file or a pair of files as explained below -

  • Single file PCM recording containing both the transmitting and receiving fax-machine signals
  • Dual PCM recordings, one containing the transmitting fax machine signal and a second with the receiving fax machine signals
  • Single PCAP file containing only T.38 packets; IPv6 are also supported.  PCAP New Generation (PCAPNG) is currently not supported.

Analyzing Fax over IP

The main Fax Decode and Analysis figure above depicts a typical call is established between two Fax machines via 2 analog telephone adapters or gateways (ATA's) and transmitted over IP. Fax-Over-IP transmission happens in 2 modes.

  • T.38 Decoding and Analysis mode - In T.38 mode, the call is detected as a T.38
  • Transparent mode (pass through) mode - In this mode, the gateway does not distinguish fax calls from voice calls. The fax traffic is carried between the two gateways in RTP packets using an uncompressed format resembling the G.711 codec. For Internet telephony gateways with support for PCM/G.711 audio and no support for T.38 real-time fax, it is recommended to switch the session to fax pass-through mode.

GL's FaxScan™ can process V.34 captures (requires PKV104 License) in T.38 mode as well as transparent mode. Win PCAP captures can be processed from T.38 packets alone or as part of a capture file with multiple SIP calls.

Outputs of FaxScan™

FaxScan™ for PCM outputs are: Fax Call Flow (contains T.30 Analysis) and Fax Image in TIFF format.

FaxScan™ for PCAP outputs are: Fax Call Flow (contains T.30 or T.38 Analysis), SIP Call Flow, and Fax Image in TIFF format. The T.30 / T.38 analysis takes the form of a sequence of events detected in the session. T.30 analysis is generated in all modes.

FaxScan™ generates fax image output in Class F TIFF (Tagged Image File Format) files, the common file format for the exchange of image files where Class F TIFF files are specific for the exchange of fax images.

SIP analysis is in the form of ladder diagram and is only available in SIP mode.

Optionally, users can also generate verbose log file for monitoring the step-by-step decoding of FaxScan™. Verbose output is useful in examining the T.38 protocol. When log file output is enabled, each packet is decoded and its type is printed. Caution to be exercised when requesting log file output, as log files are much larger than the normal ladder analysis output.

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