itional hardware in the switch and an extra cable run from the telephone to the switch. In either case, voice, video, and data share the fiber-optic cabling and ports on Layer 3 switches that transmit traffic between floors and build- ings. This is the enterprise backbone. 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 IP Private Branch Exchange Architecture 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 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 stan- dard 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 wireless Wi-Fi antennas and base stations. (See Chapter 7 for more information about corporate wireless networks.) IP Telephones Connected to USB Ports on PCs In addition to connecting IP devices to RJ45 ports, another option is to connect them to USB interfaces on PCs. There are now IP phones on the market that connect directly to USB ports. One of the benefits of this is that the IP phone uses the PC's electric power. Thus, if the computer has backup power, in the event of a power outage the VoIP device is also backed up. Separate power in the form of either a local outlet or PoE is not required. Connecting to Branch Offices in a Centralized Services Environment The ability to connect IP telephone