BS IEC 62843-2013 pdf Standard for N times 64 kilobit per second optical fiber interfaces between teleprotection and multiplexer equipment 

6. Clock timing The optical signal comprises a binary (on-off) modulation with a 2048 kilobit per second bit stream with the following timing requirements. 6.1 Frequency The frequency shall be 2048 kilobits per second +100 ppm. Each multiplexer end unit shall not use the signal from the teleprotection equipment for synchronization. Each teleprotection equipment end unit shall use the signal from the multiplexer unit for synchronization; upon LOS, the unit shall free-run with the above tolerance. Each multiplexer end unit shall be able to detect the overhead “yellow” bit correctly, even when the multiplexer end unit has lost synchronization (due to loss of optical input). For clarification, the above requires that a) The multiplexer unit normally receives data from the teleprotection equipment, at the same frequency as the multiplexer unit’s outgoing data, but during the failure of the outgoing fiber, the multiplexer unit must accept incoming data up to +100 ppm off frequency (detect framing, the overhead bits, and the data bits). It is acknowledged that being asynchronous to the multiplexer will cause bits to be lost, or filled (as in an asynchronous 64 kbit/s G703 interface), but this is considered preferable to squelching the data. b) The teleprotection equipment shall be able to receive signals with up to +100 ppm frequency tolerance on its optical input, and use this for the frequency of its outgoing signal to the multiplexer. If its input is bad, or lost, the teleprotection equipment shall use an internal reference with土100 ppm tolerance. 6.2 Jitter For the multiplexer unit output, the jtter shall be within +50 nanoseconds (+0.1 U). For the teleprotection equipment output, the jtter shall be within +100 nanoseconds (+0.2 UI). (This allows its clock recovery circuit to add +50 nanoseconds to the jitter it receives.)
The multiplexer unit’s receiver shall tolerate a jitter of +100 nanoseconds. For clarification, the above requires that if the data are observed on an oscilloscope triggered on either rising or falling transitions, the next transition should be at a minimum of 438 nanoseconds for the multiplexer unit output and a minimum of 388 to 438 nanoseconds for the teleprotection equipment output (depending on the jitter on its input data).