.4 and 61.5. At certain points Jitter transfer in time the pulse is significantly offset from its correct position. If this offset becomes large, then there will be an error when sampling (Jo/Ji) and decoding the digital signal. Ideal pulse positions Figure 61.6 Three jitter specifications Superimposed pulses with jitter modulation The permitted levels for these parameters are specified in CCITT recommendation G.823 for European hierarchy and G.824 for North American hierarchy. A similar set of North American specifi- cations will be found in ANSI T1.102 and in Bellcore TR-TSY- Eye area 000499. The measuring methods and receiver bandwidths are defined in CCITT recommendation O.171. An example of the jitter Demodulated jitter signal fj = jitter frequency tolerance specification and jitter tolerance mask from G.823 is shown in Figure 61.7 and Table 61.4. Jitter measurements have become important in the new generation of multiplex equipment for SONET and Synchronous Digital Hier- archy (SDH). Synchronisation of payload signals in the SONET/SDH frame is handled by pointer movements. The pointer movements can lead to significant amounts of jitter at the tributary pk- pk jitter amplitude Characteristic of a typical frame aligner 61.5 Effect of jitter modulation on the eye diagram Slope equivalent to 20dB/decade The disturbance or offset of the timing instant is measured in Unit Intervals peak-to-peak (UI), equivalent to one bit period. One of the advantages of recovering a timing clock from the data stream is that it will tend to track any timing jitter present in the data. Provided this tracking occurs, then no errors will result when the recovered clock is used to sample the data. In view of the limited Jitter frequency (log scale) bandwidth of clock recovery circuits, this advantage only exists at low jitter modulation frequencies. At higher jitter frequencies, the tracking diminishes and the equipment's tolerance to input jitter is greatly reduced. For this reason, jitter standards are always specified 61.7 Lower l