18.1.5. Session Description (SDES) Protocol 
The "Advanced Encryption Standard (AES)" encryption system can be
used for payload encryption on the vHG3500 for SIP connections along
with SDES as the encryption attribute. SDES is described in RFC 4568
"Session Description Protocol (SDP) Security Descriptions for Media Streams".
The vHG 3500/STMIX/STMIY supports crypto suites with 80-bit authentication
and 32-bit authentication and SRTP with optional m lines.
IMPORTANT:
SDES is not supported on the STMI2 or STMI4 boards.