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The following parameters in VXbuilder need to be altered to enable Secure Voice. Note: this assumes a configured and established network and secure phones are attached to VX via any of the supported physical interfaces (ISDN, SS7, CAS R2 or E&M, Analog).

Note this document only describes the basic, required settings; there are other settings in the system configuration that can affect secure voice. See the settings under Secure Relay in the General Telephony Settings settings, Secure Relay STU-III De-Scrambler option.

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Edit Media Entry


This setting will be found in VXbuilder under the global system settings, titled "Modem/Fax Relay Mode" and needs to be set to T.38 Fax or Secure Relay.
If a Media Class is not defined to include the secure relay codec, then you must define a setting of ANY as the media class to ensure secure voice operation.
Note: Changing the Modem/Fax Relay mode requires a node restart.

Edit Media Entry

Secure Voice


Edit Media Crypto Class


A media crypto class specifies one or more media-crypto attributes. Using the media crypto classes function controls the secure calls passthrough. The Secure Voice settings take effect at the start of the next call.
To set the STU-III Message A-B Spoofing parameters, consider:
The initiator device typically sends message A that contains the desired modes of operation. The options are:

  • Minimum delay (RTT) in milliseconds as measured by the LQM subsystem that this feature will become active. Note: a value of 0 RTT will deactivate this feature.
  • 1st preference: The 1st preference mode to be selected if present in Message A.
  • 2nd preference: The 2nd preference mode if the 1st preference is not in Message A.
  • 3rd preference: The 3rd preference mode if the 2nd preference is not in Message A.
  • 4th preference: The 4th preference mode if the 3rd preference is not in Message A.The options that can be specified in the preference dropdowns are:
  • 2.4k Interoperable
  • 4.8k Standard (standard training length - typically on AT&T/STE/"white" phones)
  • 4.8k Modified (extended training length - typically on Motorola devices)
  • 9.6k Standard (standard training length - not a common mode)
  • 9.6k Modified (extended training length - most common 9.6k mode)
  • NoneNote: this feature is not used with, and does not apply to, FNBDT devices

General Telephony Settings

Secure Relay

The STU-III Scramble/Descrambler feature is enabled by default. You may have to disable the scrambler when troubleshooting older STU-III devices or STU-III like devices. Disabling the scrambler could result in large one way audio delays when in secure mode. It is not recommended that the user change this setting unless directed to by service or engineering.
Note: This setting takes effect at the start of the next secure call.

The DC Filter setting is disabled by default. The DC signal filter is placed on the sample input of the samples from the TDM side. The DC Filter is placed after the samples are converted from a law to linear and before the samples are process by Secure Voice modems.
This can be useful if there is a signal offset on the Received recorded audio higher than the zero point for silence.
Note: This setting takes effect at the start of the next secure call.

The Clock Rate Compensator compensates for end to end clock drift of synchronous secure calls.

Trunk Group


The codecs used on all trunk groups involved with the Secure Voice call route need to have a valid non-conflicting audio codec and the STU codec, for example, G.723 AND STU) or (ANY).

Call Routing


The codecs used on the secure call route need to have a valid non-conflicting audio codec and the STU codec, for example, G.723 AND STU) or (ANY).

Operational and Performance Characteristics

Bandwidth usage for Secure Voice calls will vary greatly with the following factors:

  • Base bandwidth of the secure data rate (9.6k, 4.8k, 2.4k etc.
  • IP Packet overhead of each packet (IP + UDP + VTP)
  • BESTflow Frame Packing Technology
  • ECC (Error Correction used for Secure)
  • Packet Dispatch Interval (VTPTimeout)/increased latency.

If you need help in determining the impact of these factors, contact your Sonus support representative.