Introduction

Signal To Noise Ratio (SNR)

Training Check Frame (TCF)

Using SNR and TCF Together

Introduction

Modem metrics provide useful information about the performance of the receive modems during a fax session. This information allows NaturalFax users to determine whether the current line conditions during a fax session are of good or poor quality.

NaturalFax modem metrics provide an estimate of the Signal To Noise Ratio (SNR) and the results of the Training Check Frame (TCF). These metrics are valid when NaturalFax is acting as the receive fax machine. Modem metrics are available to the NFX API through the NFX_FAX_STATUS structure in the snr and rx_training_zeros fields.

typedef struct 
{
 DWORD size;
 DWORD rate; 
 DWORD ecm;    /* error correction mode: NFX_YES or NFX_NO            */
 DWORD resolution;
 DWORD encoding;
 DWORD pagewidth;
 DWORD mode;    /* NFX_MODE_IDLE, NFX_MODE_NEGOTIATING,            */
     /* NFX_MODE_TRANSMITING, NFX_MODE_RECEIVING,            */
     /* NFX_MODE_DISCONNECTING, NFX_MODE_FINISHED            */
 DWORD error;    /* Last error code generated            */
 DWORD docnumber;
     /* Current document in progress            */
 DWORD pagenumber;
     /* Current page of the document            */
 DWORD badlines;    /* Number of bad lines (or bad frames in            */
     /* ECM mode) during current session             */
 char filename[NFX_FILENAME_MAX]; 
     /* Name of current file processed            */
 char remoteSID[NFX_MAX_SID];
     /* Received SID from remote station            */
 BYTE remoteNSF[NFX_MAX_NSF];
     /* Received NSF from remote station            */
 DWORD snr;    /* signal to noise ratio in dB            */
 DWORD rx_training_zeros;
     /* training zeros in the TCF            */
     /* in tens of milliseconds            */
} NFX_FAX_STATUS;

Note: This feature set is currently supported on only the AG 2000 board, AG 4000 board, and QX board. For an AG board, the values returned in these fields will be zero.

Signal To Noise Ratio (SNR)

An estimate of SNR is calculated during two phases of a fax call, when NaturalFax is acting as the receive fax machine. These phases are the initial training check frame (TCF), and the subsequent page or image portion of the facsimile. At the end of each of these phases, a value of SNR is provided to give indication of the line quality during each phase.

The following table illustrates the valid ranges that NaturalFax reports for SNR, as well as what should be considered a good, marginal, or poor value for receive performance during a fax session:

Line Quality	SNR in dB
Poor	15 - 21
Marginal	22 - 24
Good	25 - 32

The values in the preceding table should be used as a guide when determining whether receive performance problems are being caused by line quality problems or compatibility issues. For example, if the fax session is training down to lower rates, failing, or aborting fax calls while receiving a fax, and the reported SNR is below 22 dB, the probable cause is poor line quality. However, if the same symptoms occur with a SNR above 25 dB, it is unlikely that line quality is the reason for failure.

Training Check Frame (TCF)

The Training Check Frame is the phase of a fax call when the sending fax transmits a sequence of zeros to the receiving fax machine at the highest common data rate negotiated during prior phases. This check determines whether the line quality is adequate to pass information at the desired rate. If the line quality is good, the receiving fax machine will receive this sequence of zeros without error. If line quality is not good, bit errors will occur during reception, and the total amount of zeros will not be received. The duration of the zero sequence sent by the transmitting fax is 1.5 seconds. The T.30 protocol states that to successfully receive this training data, 1.5 seconds +/- 10% of these zero symbols should be received. If successful, the receiving fax sends back a confirm signal (CFR) to the transmitting fax. If the standard is not met, the receiving fax sends back a fail to train (FTT) signal, indicating to the transmitting fax that a lower data rate should be tried.

NaturalFax reports the results of training information, which can be used to determine the quality of the training event, and therefore the quality of the line. This metric can be used to determine whether a receive problem is the result of poor line quality, insufficient training, or a compatibility issue. The rx_training_zeros metric reports the length of zeros received in tens of milliseconds. A good value for the TCF is greater than 1.35 seconds. If values less than that are being reported, the receiver is not able to demodulate the incoming signal appropriately, which results in bit errors.

Using SNR and TCF Together

Using both SNR and TCF modem metrics, quick diagnostics of poor or failing receiver performance can be made. The following table shows how these two parameters can be useful:

Indication	SNR	Training Zeros	Possible NFX Receiver Performance
Good Line Quality	> 25 dB	> 1.35 Seconds	No problems encountered
Good Line Quality Non-compliant TCF	>25 dB	> 1.0 Seconds < 1.35 Seconds	Possible FTT due to non-compliant sending fax Possible line errors in non-ECM mode, and retransmits in ECM mode
Marginal Line Quality Compliant TCF	>22 dB < 25 dB	> 1.0 Seconds < 1.35 Seconds	Possible FTT due to line conditions Possible line errors in non-ECM mode, and retransmits in ECM mode
Poor Line Quality Compliant TCF	< 22 dB	< 1.0 Seconds	Probable FTT due to poor line conditions Possible line errors in non-ECM mode, and retransmits in ECM mode

The preceding table shows that the modem metrics clearly indicate when receiver performance may be compromised due to poor line quality. The metrics should be used as an additional tool to help diagnose problems encountered in the field.