Grandstream Networks HT813 Administration Manual page 49

Preferred Configures vocoders in a preference list (up to 7 preferred vocoders) that will be included with same order
Vocoder in SDP message. Vocoder types are G.711 A-/U-law, G.726-32, G.723, G.729, iLBC and OPUS.
Voice Frames Transmits a specific number of voice frames per packet. Default is 2; increases to 10/20/32/64 for
per TX G711/G726/G723/other codecs, respectively.
Operates at specified encoding rate for G.723 vocoder. Available encoding rates are 6.3kbps or 5.3kbps.
G723 Rate
Default is 6.3kbps.
Specifies iLBC packet frame size (20ms or 30ms).
iLBC Frame
Size
Default is 20ms.
Disable OPUS
Disables OPUS stereo in SDP. Default is No.
Stereo in SDP
iLBC Payload
Determines payload type for iLBC. Valid range is between 96 and 127. Default is 97.
type
OPUS
Determines payload type for OPUS. Valid range is between 96 and 127. Default is 123.
Payload Type
Allows detecting the absence of audio and conserves bandwidth by preventing the transmission of "silent
VAD
packets" over the network. Default is No.
Symmetric Changes the destination to send RTP packets to the source IP address and port of the inbound RTP packet
RTP last received by the device. Default is No.
Specifies the fax mode: T.38 (Auto Detect) FoIP by default, or Pass-Through (must use codec
Fax Mode
PCMU/PCMA)
Re-Invite
after Fax Allows the unit to send out the re-INVITE for T.38 or Fax Pass Through if a fax tone is detected. Default is
Tone Enabled
Detected
Jitter Buffer
Selects jitter buffer type (Fixed or Adaptive) based on network conditions.
Type
High (initial 200ms, min 40ms, max 600ms) Note: not all vocoders can meet the high requirement.
Jitter Buffer
Medium (initial 100ms, min 20ms, max 200ms).
Length
Low (initial 50ms, min 10ms, max 100ms).
Selects SRTP mode to use ("Disabled", "Enabled but not forced", or "Enabled and forced"). Default is
Disabled.
It uses SDP Security Description to exchange key. Please refer to
SRTP Mode
SDES: https://tools.ietf.org/html/rfc4568
SRTP: https://www.ietf.org/rfc/rfc3711.txt
Configures the SRTP Key Length , it can be set to :
1. AES 128&256 bit
SRTP Key
2. AES 128 bit
Length
3. AES 256 bit
Default value is AES 128&256 bit