There are several factors that can impact audio quality during fax transmissions. When Centralis is contacted via phone or email to troubleshoot issues such as failed or partially received faxes, handshaking problems, or transmission delays, our carrier technicians conduct “line audio quality testing.” The results are typically consistent, revealing poor line audio quality. In such cases, we generally recommend that the sending organization reach out to their phone service provider, long-distance service provider, and local telephone company to test the line and identify any additional issues.
After being assured by the sending carrier that the trouble was not with their lines our participant reaches back to Centralis support, often feeling frustrated and seeking a resolution. Additionally, they might have connected an analog phone to the line and found that test calls sounded clear, without any noise. This leads to skepticism about Centralis Health’s carrier diagnosis, with the question arising: “Why do you believe there is a line audio quality issue? How did you reach that conclusion?”
We can answer these questions, warning… this may get a bit technical.
Fax is a method of data transmission. Traditional faxing uses phone lines to send this data, requiring it to be modulated into audio by the sender and then demodulated back into data by the receiver. For this process to work effectively, the transmission medium must be “lossless,” meaning it should not alter the audio in a way that changes the data during the demodulation process.
Are you bored yet?
The fundamental way to assess whether line audio quality is sufficient is to compare the original data with the data received on the other end. Fax protocols provide a valuable tool for this comparison through ECM (Error Correction Mode).
In ECM, the fax image data is divided into 64-kilobyte sections called “blocks,” which are further segmented into 256-byte units known as “frames.” Each frame includes a 16-bit (two-byte) checksum value. A checksum is a number calculated mathematically from the source data to ensure data integrity. The sender transmits these blocks of frames along with their checksums to the receiver and informs the receiver of the total number of frames sent. The receiver then uses the checksums to verify the integrity of the received frames and communicates back which frames were not received correctly. The sender will retransmit the incorrect frames, and the receiver will recheck their integrity and respond accordingly. This cycle continues until all data is correctly received or until the fax protocol drivers (fax software or modem firmware) determine that reliable communication cannot be achieved and abort the fax session.
Centralis Support carrier uses this fax protocol interaction to determine line audio quality.
In an ideal lossless situation, the data received would exactly match the data sent, eliminating the need for retransmissions. This level of reliability is achievable, typically in cases where the customer is directly connected to the PSTN (public switched telephone network) through a service provider that uses only PSTN circuits, such as AT&T or Verizon, rather than VoIP circuits provided by companies like Comcast or Time Warner.
Unfortunately, not all conditions are ideal. Factors such as local phone systems (PBXes), minor electrical interference, imperfect telephone switches, and other elements can affect audio quality. At Centralis, we classify audio quality based on frame retransmission rates: under 2% is considered “good,” under 4% is “fair,” and above 4% is “poor.” While a 95% success rate might seem excellent in academic terms, it’s not as favorable for fax transmissions. Centralis Health has found that customers with frame retransmission rates over 4% often experience high failure rates for their faxes, including issues with partially received faxes and difficulties sending multi-page documents. For context, VoIP services commonly exhibit frame transmission failure rates around 10%, meaning that 90% success is actually problematic for faxing.
When you consider it in broader terms, the situation becomes clearer. Who would want to regularly retransmit one out of every twenty faxes sent? Clearly, that level of performance is not acceptable.
Fax customers expect higher reliability, and such reliability is indeed achievable. Centralis Support is equipped and trained to help customers achieve that level of dependable fax performance, with line audio quality testing being a key component of our service.