on Sector (ITU-T), the European Telecommunications Standards Institute (ETSI), and the American National Standards Institute (ANSI), started developing new standards specifying more stringent and complex requirements for jitter and wander in synchroniza- tion networks. As a result, operators settled on two basic approaches to achieve frequency synchronization: Using the physical layer, which involves distributing frequency electrically or optically over the network interface. The synchronous digital hierarchy (SDH), synchronous optical networking (SONET), and synchronous Ethernet (SyncE) are examples of technologies using the physical layer from node to node. SDH/SONET and SyncE provide predictable carrier-class frequency distribution but require system/equipment changes in case any nodes in the network are not cap- able of supporting compatible physical interfaces. This is usually not an issue with legacy SDH/SONET network elements, but it becomes a challenge for Ethernet-based systems if they are not capable of supporting SyncE. Frequency can also be distributed using packet-based networks. Over the recent past, engineers have developed several protocols, such as structure-agnostic TDM over packet (SAToP), circuit emulation over packet switched network (CESoPSN), circuit emulation over packet (CEP), Net- work Time Protocol (NTP), and precision time protocol (PTP), to distribute frequency via pack- ets. At first glance, the packet method of distributing frequency looks flexible and simple; how- ever, carrying frequency information over nondeterministic packet-based networks requires careful engineering and can be more difficult than using physical methods. This chapter focuses mainly on frequency distribution using physical interfaces. Frequency distribu- tion over packet is covered in "Chapter 9. PTP Deployment Considerations". 6.2 BITS and SSU For traditional PSTN, frequency synchronization is based on the concept of the building or office clock, where an entire building or office is supplied with frequency to