re connected to the public telephone network and the DSPs perform the IP-to-circuit-switched conversions to make IP telephone system calls compatible with the PSTN. This conversion is not required for calls that bypass the public network and are transmitted over data networks such as the Internet. Central Power Distribution and Backup via Power over Ethernet Every IP telephone needs its own source of electrical power, unless it's connected to a PC's USB port. To avoid the expense and labor of providing local electricity to each phone at installation, organizations can power them via the Layer 2 switch to which the they are connected. This ensures that all phones are connected to a UPS (Uninterrupted Power Supply) to survive power surges and interruptions. Common power sources and backup also avoid the problem of ensuring that each phone is near an electrical outlet, is plugged in, and has an electrical cord. To bring power to groups of endpoints, organizations use 802.3af, the IEEE standard known as Power over Ethernet (PoE) that defines how power can be carried from the Layer 2 switch to the IP telephone by using the same cabling that transmits voice and data. Battery backups and generators are deployed in wiring closets where switches support mission-critical telephones or attendant consoles. PoE is also used to power corporate www.TechnicalPDF.com wireless Wi-Fi antennas and base stations. (See Chapter 7 for more information about corporate wireless networks.) Session Initiation Protocol-Compatible Trunks Session Initiation Protocol (SIP) signaling is a key protocol for connecting IP networks together. It is also used to connect enterprise locations with data networks for voice calling. Signaling systems used in office phone systems need to be compatible with signals used in a carrier's network if voice calls are to be sent over data links. This is SO that the equipment can interpret addressing signals, ringing signals, and busy signals sent by office IP PBXs, and vice versa. Without this compatibility, organi