ver with Con- stant Temperature-Based on Figure 7-1 of ITU-TG.8273.2 In the second case of holdover in a T-BC/T-TSC, both the PTP packet timing signal and the physical layer frequency inputs are lost simultaneously, and the clock can rely only on the quality of its own internal oscillator. In that case, the holdover performance in a clock with an oven-controlled crystal oscillator (OCXO) is going to be much worse than one with a reference frequency input. See the section "Network Holdover" later in the chapter for more details. 9.4.1 T-TC The function of a T-TC is to measure the residence time of transit PTP event messages (Sync and Delay_Req), SO that the next slave in the chain can compensate for the PDV suffered by those mes- sages. The T-TC is expected to measure the residence time and use it to update the CF of the PTP message. The performance of a T-TC is based on how accurately it can measure and reflect the delay experienced by the PTP event messages; in other words, how accurately it updates the CF. ITU-T G.8273.3 specifies the limits for an end-to-end T-TC with a frequency input reference provided by the physical layer. It defines the maximum limit of error (or noise) that a T-TC adds to the CF. For a T-TC, this error in the CF is categorized as noise generation. The frequency reference is needed to ensure that the oscillator is correctly calculating the residence interval. If the T-TC has no reference, then the accuracy of the measurement will be wrong if the oscillator is running too fast or slow. The performance of a T-TC without a frequency reference provided by the physical layer is for further study. Refer to the section on G.8273.3 in Chapter 8 for an overview of this ITU-T recommendation. The types of inaccuracy (or noise) in the CF can be either of the following: Fixed error (cTE): When the time written into the CF is offset from the real residence time by a fixed value. The slave clock will incorporate this (erroneous) offset into its calculations to re- cover phase/time. However, this offset 