on baseline. Generally, atomic oscillators are more stable over longer observation intervals and quartz over shorter time intervals, meaning that an atomic clock running alone typically only offers better sta- bility over the longer term. As a rule of thumb, a cesium oscillator has better stability for observa- tion intervals of more than 24 hours, while a rubidium oscillator provides better stability for periods of 5 minutes to 24 hours. A good-quality quartz oscillator will generally perform better than both atomic oscillators for short observation intervals, at least for less than approximately 5 minutes. Clocks based on atomic oscillators generally contain a good quartz oscillator, typically an OCXO, for short-term performance, which is disciplined over the long term by the atomic component. Pair- ing an OCXO with an atomic clock combines good long-term and short-term stability, making them great reference clocks. 3.2.1 Clock Modes During the normal operation of a clock, it goes through different modes of operation. Figure 3-10 illustrates a state diagram with some of the typical transitions (based on ITU-T G.781 and G.810) that a clock can pass through. This figure also shows the transitions between states and an example of a triggering event that might cause such a transition. Warming Up Power on initialization Free-running Source goes unavailable Valid source with invalid available holdover data Source goes Forced free- unavailable running Forced free- running Out of holdover specification Locked Acquiring Achieved Out of locked alignment/locked specification Source goes unavailable Forced holdover Valid source with valid holdover data Holdover available Forced holdover Figure 3-10 Clock Mode States and Transitions Where an event is shown as "forced," it means that the event was triggered by an external event such as operator intervening and executing a control function. Some clock types might not contain every one of these states, as you will see when we discuss specific clock types, such as frequency clo