Error free communications is something every user would like to enjoy. Digital transmission, with its ability to completely avoid cumulative noise-induced degradation, should provide this. One reason for the digital reality not meeting expectations is mis-timing inside transmission equipment when data is regenerated. When mistiming becomes large, errors are produced and the system can become unusable. Even at low values of mis-timing, sensitivity to amplitude and phase variations is increased and performance suffers.
Jitter is always present within devices, systems and networks to a certain degree. In order to ensure interoperability between devices and minimize signal degradation due to jitter accumulation across long distances, it is important that there are limits set on the maximum level of jitter present at an output interface and the minimum level that can be tolerated at an input. Adherence to these limits will ensure interworking between different vendor equipment and networks, as well as providing the basis for demarcation.
Slow variations in signal timing through a system are called wander. Higher speed variations are termed jitter. The division between the two is taken at 10 Hz. Wander is measured using a single pole lowpass filter with its –3 dB point at 10 Hz while jitter uses a high-pass filter with the same –3 dB frequency.
A fundamental operation in every digital transmission system is to receive a degraded signal and regenerate it. All high capacity systems transmit only a suitably coded data signal, and the first task of a regenerator is to produce a local clock signal from that data. There are two contradictory requirements. First, the local clock should be stable for onward transmission and easier aggregation with other data sources. Second, the local clock should track incoming phase
variations of the data signal so that as the optimum sampling point for the input data varies, the clock tracks it. This leads to the danger of phase variations building up as a signal traverses a network and each regenerator in turn attempts to track incoming phase variations.
There are three measurements that define the jitter performance of a transmission system and specifications and standards can be expected to refer to all three:
• Output jitter - a measurement of the jitter present on an output from a system
• Jitter tolerance - a measurement to check the resilience of equipment to input jitter
• Jitter transfer - a measure of how much jitter is transferred between input and output of network equipment
Source: Agilent Technologies