Carrier frequency accuracy

For some services, particularly data circuits, it is desirable that the 'virtual' carrier difference over a network is maintained to within 2Hz. (CCITT, 1985.) To achieve this the carrier generation master oscillators receive regular maintenance adjustment using a refer­ence frequency comparison pilot (see Section 20.6.2.2) or in later developed systems are phased locked to the frequency comparison
pilot.

The recommended stability of the carrier frequencies are given in Table 20.2. With this carrier accuracy the end to end difference in virtual carriers will exceed 3Hz for 1 % of the time and 4Hz for 0.1% of the time over a hypothetical international link of 2500km.