SDH equipment clock (SEC) nodes. The section "Synchronization Network Chain" in "Chapter 6. Physical Frequency Synchronization", covers this limit and recommendations by ITU-T in greater detail. To ensure interoperability between devices and to minimize the signal degradation due to jitter and wander accumulation across the network, the ITU-T recommendations (such as G.8261 and G.8262) specify jitter and wander performance limits for networks and clocks. The normal network elements (NE) and synchronous Ethernet equipment clocks (EEC) are usually allocated the most relaxed limits. For example, ITU-T G.8262 specifies the maximum amount of peak-to-peak output jitter (within a defined bandwidth) permitted from an EEC. This is to ensure that the amount of jitter never exceeds the specified input tolerance level for subsequent EECs. "Chapter 8. ITU-T Timing Recommenda- tions", covers the ITU-T recommendations in greater detail. 5.1.1 Frequency Error While jitter and wander are both metrics to measure phase errors, the frequency error (or accuracy) also needs to be measured. The frequency error (also referred to as frequency accuracy) is the degree to which the frequency of a clock can deviate from a nominal (or reference) frequency. The metric to measure this degree is called the fractional frequency deviation (FFD) or sometimes just the frequency offset. This off- set is also referred to as the fractional frequency offset (FFO). The basic definition is given in ITU-T G.810 by the following equation: where: y(t) is the FFD at time t v(t) is the frequency being measured and Unom is the nominal (or reference) frequency FFD is often expressed in parts per million (ppm) or parts per billion (ppb). For example, the free- running frequency accuracy of a synchronous Ethernet (SyncE) clock is < 4.6 ppm, while the carrier frequency accuracy required in cellular mobile radio is < 50 ppb. Taking an example calculation, if the nominal frequency of an oscillator is 20 MHz (which is 20,000,000 Hz and represents Unom as per the previ