The Type Frame field of F frames along with the subclass field determines the function of the package that has been sent or received.They are the control signaling messages.
The Type Frame field is a set of 8 bits (1 byte) and the main values that can take are shown in the following table:
Table 1. Main values of the "Type Frame" field of F frames or Full Frame
"type frame" value
To send DTMF digits
Subclass field show the audio codec used. See table 2.
Subclass field show the video codec used.
Provide session control. They refer to control of the devices connected to the IAX endpoint. Subclass field show the specific control message. See table 3.
Provide IAX protocol specific endpoint management. They are used to manage IAX protocol interactions that are generally independent of the type of endpoints. Subclass field show the specific IAX control message. See table 4.
Table 2. Meaning of the subclass values for Frame Type = "0x02" (voice data)
Subclass Value (Type Frame =0x02)
Description (codec used in the convesation)
G.711 u (u-law)
G.711 a (a-law)
Table 3. Meaning of the subclass values for Frame Type = "0x04" (control)
Subclass Value (Type Frame =0x04)
Table 4. Meaning of the subclass values for Frame Type = "0x06" (IAX control)