FaxBack Port Server RADIUS Support (RFC 2865, 2866) - Manual (Page 8)

FaxBack Proprietary and Confidential Page 8 of 58
Fax ATA Call-Ahead
Call-ahead support in the RADIUS server is optional. If the port server call-ahead feature is
intentionally disabled, or the CalledNumbersDb database is configured to use SQL, or if only real-
time fax clients will be using the port server, then the RADIUS server does not need to implement
support for call-ahead.
Background
The port server fax ATA supports the ability to make a VoIP call directly to a fax number through
a SIP media gateway in order to verify the accuracy of the fax number. Due to the limitations of
the T.38 protocol, the fax ATA never sends a fax directly in this manner; the direct VoIP call from
the ATA is only used to verify that a fax device is answering calls made to the fax number. The
process of calling the fax number directly from the ATA, before the fax is uploaded to the port
server is referred to as “call-ahead”. In order to perform this behavior, the ATA must have call-
ahead support enabled, and be configured with the IP address of the SIP media gateway to use
for the direct VoIP call.
When a fax is sent from a fax machine that is connected to the ATA, the ATA uses the dialed
number to make a direct VoIP call to determine if a fax device answers. If a fax device answers,
then the call with the dialed fax device is terminated, and the connection with the fax machine
continues until the fax is uploaded to the port server. Later, the uploaded fax will be delivered. If
a fax device does not answer the direct VoIP call, then audio from the VoIP call is relayed to the
fax machine, where the fax machine operator can listen to, and possibly interact with the call.
Although the call-ahead to the fax number is performed locally on the fax ATA, the port server
determines whether the call-ahead should be made. When a send session is started, the fax
ATA contacts the port server to ensure that the ATA will be allowed to send a fax to the server. In
response, the port server returns a flag that tells the ATA whether or not it should perform the
call-ahead. The port server can be configured to instruct all ATAs to Never or Always call-ahead,
or it can be configured to query a database of called fax numbers to determine whether the fax
number had previously been used to successfully contact a fax device. If the port server finds the
fax number in the called numbers database, it can return the flag that instructs the ATA to not
perform an unnecessary call-ahead.
Implementation
To provide call-ahead support to the port server, the RADIUS server must do the following:
1. Return the FB-Call-Ahead-Mode attribute when a send session is starting for a Queue
transfer (FB-Transfer-Type of 2). The value of FB-Call-Ahead-Mode should be obtained
from querying a database of previously called fax numbers that will determine if the fax
number has been used previously to successfully contact a fax device. See “Get-Call-
Ahead-Mode / Pre-Send-Session” on page 20 for when the RADIUS server is expected
to return this attribute.
2. Maintain a database of fax numbers that are provided to the RADIUS server in
Accounting Stop requests in the FB-Dial-Number attribute. To simplify this process, the
port server includes an FB-Called-Number-Result attribute only in Accounting Stop
requests where the RADIUS server should take some action to maintain the Called
Numbers database. When present, the value of the FB-Called-Number-Result attribute
identifies the dial number as being verified as a fax number (even if the fax session
began and later failed), or a voice number (the fax failed due to a voice answering the
call).
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For a verified fax number, future queries of the called numbers database should
return 0 (false) to avoid an unnecessary call-ahead.