Supplier eBooks

| 31 | In September 2003, the IEEE published the IEEE 802.3AF standard that defines the specifications to deliver power over standard Ethernet cables. The Power over Ethernet (PoE) standard has facilitated the global development of technology that allows IP telephones, wireless LAN access points, webcams, credit card scanners, and many other appliances to receive power as well as data over existing LAN cabling without needing to modify the existing Ethernet infrastructure. Such devices in remote locations may be powered and managed with PoE, eliminating the expense of supplying them with higher-cost 110/220 AC power. The PoE Architecture The IEEE 803.3AF Ethernet specification standard defines the voltage and current requirements of powered Ethernet equipment delivering up to 48 volts of DC power to PoE- compliant devices over eight-wire Category 5 and 6 cabling. There are two types of architecture. One is called mid-span which involves running power over unused wire pairs in a LAN cable. Mid-span products are built into patch panel-like devices that can add PoE to existing LAN infrastructures. In these installations the midspan hub is the power sourcing equipment, while the IP phones, webcams and Wi-Fi access points are the powered devices. The other increasingly popular version of 802.3AF is called end-span. It runs DC power signals over the same wire pairs used for data transmission. Industry experts say end-span devices are becoming popular because they are usually built into new switches with PoE, which users buy for IP telephony or WLAN rollouts. Electrical Requirements The addition of power into a LAN results in the requirement for a power management circuit between the power source and the output of the powered Ethernet devices. Typically, designers choose to back up the power management circuit with a solid- state Bourns ® Multifuse ® PPTC resettable fuse. Bourns ® Multifuse ® PPTC Thermistors for Power over Ethernet (PoE) Protection Figure 1: PoE Mid-span Switch Installation

• Cover