cy (SyncE) support and for both constant and varying temperature. For example, G.8273.2 (10/2020) Table 7-9, titled "Performance allowance during loss of PTP input (MTIE) for T-BC/T- TSC classes A and B with constant temperature," indicates that a clock can increase its MTIE by 25.25 ns per second and still pass. Think how badly misaligned the phase would be after several weeks of that level of holdover performance. 11.1.1 Third-Party Circuits An important consideration when building an accurate timing distribution network is the character- istics of the back-haul and front-haul network. Depending on the situation in individual markets, to connect their cell sites to the mobile infrastructure, the mobile service provider (MSP) may either substantially own its own network or rely upon other service providers. These third-party organizations offer services such as Metro Ethernet or Carrier Ethernet circuits to the MSP, SO this book will refer to them as Carrier Ethernet (CE) service providers (SP). They are also referred to by numerous other names, including alternate access vendors (AAV) or third-party circuit providers. Figure 11-4 shows the basic service offered by a CE SP. Service Service demarcation demarcation Carrier Ethernet network MSP leased circuit Mobile Central cell site mobile core Figure 11-4 Carrier Ethernet Service Provider Many of the services offered by the CE SP are defined by the MEF Forum (MEF), which is an in- dustry association of more than several hundred member companies. MEF has released Implement- ation Agreements (IA) that detail the transport services needed to support the back-haul needs of mobile networks. The appropriate documents to consult are known as 22.3 and the amendment to as 22.3.1. For more details on MEF, see "Chapter 4. Standards Development Organizations". The first problem with using CE SP circuits is that any PTP- (and SyncE-) aware nodes in the third- party network can only run a single PTP clock instance belonging to a single clocking domain. Any significant CE SP is