The HDLC data rate adapting encoder accepts a T.30 message stream as input. The V.21 HDLC messages generally arriving byte by byte, in separate T.38 messages. ECM HDLC messages usually arrive in larger chunks, but they are still generally fragmented. The rate adapter dynamically buffers the T.30 data, generates preamble, and adjusts its length if the incoming message data is falling behind. It may insert additional flag octets between frames, as a flow control mechanism. However, the HDLC protocol is synchronous, so the adapter cannot perform any flow control within a frame. It must, therefore, buffer enough octets of a frame to provide reasonable jitter tolerance, before it emits the first octet of that frame. If the arriving messages fall too far behind mid-frame, there will be corruption of the outgoing stream. In the case of ECM image data frames, the frames are fast enough, that buffering whole frames is quite proactical within the T.30 timing constraints. This ensures mid-frame underflow can never occur.