User's Manual 174 Document #: LTRT-42060
Mediant 2600 E-SBC
14.4 Configuring Various Codec Attributes
The following codec attribute settings can be configured in the General Media Settings
page:
AMR coder:
• 'Payload Format': Defines the AMR payload format type.
SILK coder (Skype's default audio codec):
• 'Silk Tx Inband FEC': Enables forward error correction (FEC) for the SILK coder.
• 'Silk Max Average Bit Rate': Defines the maximum average bit rate for the SILK
coder.
For a detailed description of these parameters and for additional codec parameters, see
''Coder Parameters'' on page 609.
To configure codec attributes:
1. Open the General Media Settings page (Configuration tab > VoIP menu > Media >
General Media Settings).
Figure 14-6: Codec Settings in General Media Settings Page
2. Configure the parameters as required, and then click click Submit.
3. To save the changes to flash memory, see ''Saving Configuration'' on page 424.
14.5 Configuring Media (SRTP) Security
The device supports Secured RTP (SRTP) according to RFC 3711. SRTP is used to
encrypt RTP and RTCP transport for protecting VoIP traffic. SRTP requires a key
exchange mechanism that is performed according to RFC 4568 – “Session Description
Protocol (SDP) Security Descriptions for Media Streams”. The key exchange is done by
adding a 'crypto' attribute to the SDP. This attribute is used (by both sides) to declare the
various supported cipher suites and to attach the encryption key. If negotiation of the
encryption data is successful, the call is established.
SRTP supports the following cipher suites (all other suites are ignored):
AES_CM_128_HMAC_SHA1_32
AES_CM_128_HMAC_SHA1_80
When the device is the offering side, it generates an MKI of a size configured by the
'Master Key Identifier (MKI) Size' parameter. The length of the MKI is limited to four bytes.
If the remote side sends a longer MKI, the key is ignored. The key lifetime field is not
supported. However, if it is included in the key it is ignored and the call does not fail.
The device supports the following session parameters (as defined in RFC 4568, SDP
Security Descriptions for Media Streams):
UNENCRYPTED_SRTP
UNENCRYPTED_SRTCP
UNAUTHENTICATED_SRTP
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