FRAME SYNCHRONISM PROTECTION SYSTEM

PURPOSE:To improve the reliability of data by providing a variable range in the number of steps for the forward and backward protection of the frame synchronism of a digital transmission device and deciding the variable numbers of the frame synchronism protection steps by counting the frequency of generating out-of synchronism in a using state. CONSTITUTION:The number of coincident frame patterns is counted by an OK counter 2, and a synchronism establishment state starts when confirming the prescribed steps of the coincidence number continuously. An out-of-frame frequency monitoring part 8 monitors the step number of the OK counter 2 until the establishing of synchronism and the step number of a NG counter 3 until an out-of-synchronism to decide the protection step number corresponding to the state in protection step number control parts 5 and 6 and to control the count step number of the OK counter 2 and the NG counter 3.