Aastra Telecom 55I, 57I CT, 53I manual Silence Suppression

Configuring Network and Session Initiation Protocol (SIP) Features

Global SIP Settings

If an SRTP enabled IP phone initiates a call, and the receiving phone is also SRTP enabled, the IP Phone UI displays a “lock” icon, indicating that the call is secure. If the receiving phone does not support SRTP, the IP phone will send unsecured RTP messages instead of SRTP encrypted messages. However in this case, the IP Phone UI does not display the lock icon - indicating a non-secure call.

Note: If you enable SRTP, then you should also enable Transport Layer Security (TLS). This prevents capture of the key used for SRTP encryption. To enable TLC, set the Transport Protocol parameter (located on the Global SIP Settings menu) to TLS.

You can configure SRTP on a global or per-line basis using the configuration files or the Aastra Web UI.

Silence Suppression

In IP telephony, silence on a line (lack of voice) uses up bandwidth when sending voice over a packet-switched system. Silence suppression is encoding that starts and stops the times of silence in order to eliminate that wasted bandwidth.

Silence suppression is enabled by default on the IP phones. The phone negotiates whether or not to use silence suppression. Disabling this feature forces the phone to ignore any negotiated value.

You can configure silence suppression on a global-basis using the configuration files or the Aastra Web UI.

Configuring the IP Phones

41-001160-00, Release 2.1, Rev 04 4-69