The timing performance of the transmission system is mainly reflected in two aspects: jitter and drift. （1） Jitter, jitter, and drift are related to the timing characteristics of the system. Jitter refers to the short-term deviation of a digital signal from its ideal time position at a specific moment (such as the optimal sampling time). The so-called short-term deviation refers to the phase change with a change frequency higher than 10Hz. The amplitude of jitter is the time range within which a digital signal shifts relative to its ideal reference time position at a specific moment, in units of UI, 1UI=1/fb. For example, for a signal with a rate of 139.264Mbit/s, the unit of jitter amplitude is 1UI=1/fb=1/139.264Mbit/s=7.18nm. Jitter comes from system wiring and equipment. The jitter introduced by optical cable lines is generally negligible, and equipment is the main source of jitter, including adjustment jitter, mapping/unmapping jitter, and multiplexing/demultiplexing jitter. 1. The impact of jitter on the performance of transmission systems is manifested in the following aspects:

(1) For digitally encoded analog signals, the random phase jitter of the decoded digital stream causes the recovered samples to have irregular phases, resulting in distortion of the output analog signal, forming so-called jitter noise In the regenerator, the irregularity of timing causes effective decisions to deviate from the center of the receiving eye map, thereby reducing the signal-to-noise ratio redundancy of the regenerator until an error occurs (3) In transmission systems, network units equipped with buffers such as synchronous multiplexers, excessive input jitter can cause buffer overflow or emptying, resulting in sliding damage. 2. Common parameters for measuring jitter performance in transmission systems include input jitter tolerance, output jitter tolerance, and jitter transfer function - jitter transfer characteristic. The larger the input jitter tolerance, the better, while the smaller the output jitter, the better. (1) Input jitter tolerance is defined as the sinusoidal peak to peak jitter value that can cause an optical device to generate a 1dB optical power cost. The SDH line intersection input jitter tolerance is divided into two types: PDH input port (branch intersection) and STM-N input port (line intersection). For the PDH input port, the input jitter tolerance refers to the maximum input jitter that the branch input port can withstand without causing errors in the device. To meet the needs of SDH network elements in transmitting PDH services in the transmission network, the branch input port of the SDH network element must be able to accommodate the maximum jitter of the PDH branch signal, that is, the jitter tolerance of the branch intersection can withstand the jitter of the transmitted PDH signal. The input jitter tolerance of the line port (STM-N) is defined as the sinusoidal peak to peak jitter value that can cause the optical device to generate a 1dB optical power cost. This parameter is used to regulate that when SDH network elements are interconnected to receive STM-N signals, the input jitter tolerance of the local network element should be able to accommodate the output jitter generated by the higher-level network element