Cisco ATA 180 Series Analog Telephone Adaptors

Field Notice: Fax Passthrough Limitations on ATA 186

Updated September 8, 2004

January 30, 2002

Products Affected




Cisco ATA 186 2-Port Analog Telephone Adaptor, 600 Ohm Config


Cisco ATA 186 2-Port Analog Telephone Adaptor, Complex Imp.

Problem Description

The Cisco ATA186-L adapters do not support fax transmission reliably at speeds above 9600 bits per second (bps). This is due to a design limitation of the platform.

Note:?Although most tests are successful at 9600bps, there may be a few fax machines that will not work with the ATA 186-L.


The Cisco ATA 186 was originally designed as a low-cost voice platform, and manufacturing tolerances were chosen to support voice services only.

Cisco customers have experimented with fax transmission using the ATA 186 and many have been successful.

The Cisco ATA team verified customer testing in Cisco labs and agreed that fax passthrough is generally successful. Cisco announced software support for fax implementations in June 2001.

As more customers began to use the fax feature, field failures were reported. There are two causes for field failures. Some are due to software bugs, but others were found to be a design limitation of the platform.

Fax passthrough using ATA 186-L cannot be guaranteed above 9600bps. Even at 9600bps or below, some fax machines may be unsuccessful with longer documents.

Many fax machines have a default transmission speed of 14400 bps. While they may be successful at initial training, the fax transmission may fail later in the call.

Customers should ensure their fax machines are configured to operate at 9600 bps or slower. Reducing the number of pages in a transmission also improves the success rate.

Problem Symptoms

When transmitting faxes through a Cisco ATA 186-L in fax passthrough or fax mode, faxing may fail either during initial training, or during the transmission of longer documents.

Generally, the fax machines will detect inadequate quality at 14400 bps and will attempt slower transmission rates.

When the initial training is successful, but the end-to-end quality degrades, pages are partially transmitted and the transmission is aborted. The fax at the transmitter side shows a transmission error.

This fax passthrough issue affects all VoIP protocols, including H.323, Session Information Protocol (SIP), Media Gateway Control Protocol (MGCP), and Simple Conference Control Protocol (SCCP), when the fax transmission rate is 14400 bps or higher. Although most tests indicate that 9600 bps fax does not exhibit problems, there may be a few fax machines that will not work with the Cisco ATA 186.

Some fax machines do not attempt slower speeds, and may require extra configuration to allow fallback or fixed lower transmission speeds.


Cisco ATA186-L models do not support fax transmission with all fax machines at high speeds.

The workaround for this limitation is to configure the fax machine to a maximum speed of 9600 bps.

Not all fax machines may be configured to operate at lower speeds, so check the fax machine manual carefully for configuration instructions.

Some fax machines will not work with the Cisco ATA 186-L at any configured speed.

Note:?Software fixes will be provided for fax software defects on the Cisco ATA186-L platforms. Please check the Software Center (registered customers only) for updated software packages.

As stated above, there are two root causes for fax failures. Some are due to software defects, so it is important to test your fax machine with the latest software from Registered users should check the Software Center for updates. Non-registered users should contact TAC for assistance.

If you are using the latest ATA 186-L software, and you still have fax transmission failures, then you may have encountered the hardware limitation of the ATA 186-L platform.

Two new models in the ATA family, the ATA186-I1 and ATA186-I2 have been designed to support fax transmission at speeds up to 14400bps. Customers wishing to use ATA products for fax transmission should purchase these models, and not the ATA 186-L models.


To follow the bug ID link below and see detailed bug information, you must be a registered user and you must be logged in.



CSCdv90018 (registered customers only)

FAX transmission failures @ 14.4K but OK @ 9.6K

For More Information

If you require further assistance, or if you have any further questions regarding this field notice, please contact the Cisco Systems Technical Assistance Center (TAC) by one of the following methods:

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