Level 1由H.223附录A定义，它拥有的同步机制可以有效增强易出错信道的传输性能。为了提高MUX-PDU的同步传输性能，在Level 0中MUX-PDU帧使用的8位HDLC同步 ...

If the PDU is the first PPP packet of a superframe, 2-bytes of header space is added. Then a stream editor is used to edit the header by moving the PPP header found in the packet forward by 2-bytes ...