Each asynchronous data frame sent to the TNC is converted back into ``pure'' form and queued for transmission as a separate HDLC frame. Although removing the human interface and the AX.25 protocol from the TNC makes most existing TNC commands unnecessary (i.e. they become host functions), the TNC is still responsible for keying the transmitter's PTT line and deferring to other activity on the radio channel. It is therefore necessary to allow the host to control a few TNC parameters.
To distinguish between command and data frames on the host/TNC link, the first byte of each asynchronous frame between host and TNC is a ``type'' indicator. This type indicator byte is broken into two 4-bit nibbles so that the low-order nibble indicates the command number and the high-order nibble indicates the port number for that particular command. These type indicators are explained in [K3MC \& KA9Q 85]. The type primarily of interest here is type ``0'' or data frame, which indicates that the rest of the frame is data to be sent on the HDLC channel.