If media encryption is configured, the algorithm used during the audio channel setup of the call is maintained for most FAX relay and pass-through modes. The exception is the T.38 standard for FAX over IP, for which encryption is not used.

Encryption is applicable as shown in the following table.

Table 1: Encryption options

Call Type	AEA	AES	SRTP	Transport
Modem Pass-through	Y	Y	Y	RTP (RFC2198)
Modem Relay	Y	N	N	Proprietary
V.150.1 Modem Relay	N	N	N	Simple Packet Relay Transport (SPRT)
FAX Pass-through	Y	Y	Y	RTP (RFC2198)
FAX Relay	Y	N	N	Duplicate Packets
TTY Pass-through	Y	Y	Y	RTP (RFC2198)
TTY Relay	Y	Y	Y	RFC2198
T.38 FAX Standard	N	N	N	T.38 UDPTL Redundancy
Clear Channel	Y	Y	Y	RTP (RFC2198)

If the audio channel is encrypted, the FAX digital channel is also encrypted, except for the limitations described above. AEA-encrypted FAX and modem relay calls that switch back to audio continue to be encrypted using the same key information used at audio call setup.

For the cases of encrypting FAX, modem, and TTY pass-through and TTY relay, the encryption used during audio channel setup is maintained during the call.

The software works in the following way for encryption:

• For FAX, modem, and TTY pass-through and relay, VoIP firmware encrypts calls as administered on the CODEC set screen. These calls begin in voice, so VoIP encrypts the voice channel as administered. If the media stream is converted to FAX, modem, or TTY digital, the VoIP firmware automatically disables encryption as appropriate. When the call switches back to audio, VoIP firmware encrypts the stream again.

• For T.38 FAX, VoIP firmware encrypts the voice channel as administered on the CODEC set screen. When the call is converted to FAX, VoIP firmware automatically turns off encryption. If the call later reverts back to audio, VoIP firmware encrypts the stream again.