V.21/T.30 Signal Analysis: TRsSim / AI-5620The optional V.21/T.30 Signal Analysis option expands the capability of the TRsSim software and the AI-5620 enabling detection and decoding of the CNG/CED tones present during a fax call. It also decodes all of the V.21 HDLC signaling defined by T.30. This allows investigation into problems present during fax calls over not only the public PSTN, but also at private networks or VOIP calls where channel distortion and latency can cause problems. The T.30 Fax transmission standard identifies five phases as the call progresses to completion.
- Phase A Call Establishment
- Phase B Pre-Message Procedure
- Phase C In-Message Procedure and Transmission
- Phase D Post Message Procedure
- Phase E Call Release
The TRsSim analysis option allows a detailed look at the phases that use the V.21 signaling protocol. Examples of each of the phases are show below:
1. Call Establishment
The Call Establishment phase is characterized by the transmission of the calling tone (CED) followed by the response from the answering fax machine with the Answer Tone (CNG). The tone levels and duration of the CED and CNG are automatically measured by the Waveform Capture Option and annotations are displayed on the waveform.
2. Pre-Message Procedures
The two fax end points now move on to the next phase which is to negotiate the capabilities that each can provide and a common set is chosen to complete the transmission. This phase is started with the T.30 mandatory DIS message sent from the answering device followed by the DCS of the transmitting device. Shown below is a typical example with the measurements of mark and space and the baud rate.
The HDLC data frame that contains the DIS, DCS and other T.30 formatted messages is displayed below the waveform. As the zoom level is expanded more information is displayed. A complete description of the message is provided in the Event listing window. This includes the interpretations on the meaning of each bit as outlined in T.30. The following an example DSC.
3. In-Message Procedure and Transmission
The AI-5620 does not decode the high speed data in the message and can not capture the actual fax data but it is possible to look at the spectrum to determine possible sources of interference or distortion that may exist. With a bandwidth of over 18 kHz even codec clock noise other high frequency interference can be measured.
4. Post Message Procedure
Once the high speed data has been sent the transmission is returned back to V.21 signaling. The confirmation MCF message shown below is an indication that the fax was received correctly. At this point if there was an error in the page transmission it is resent or more pages transmitted if required.
5. Call Release
The final phase of the fax call indicates to the other end point that the call is complete. The disconnect DCS message is transmitted and the line is place back on hook.