As organizations migrate to high-speed, sophisticated network infrastructures to provide services such as voice over IP (VoIP), the fundamental issue of reliability of service remains a concern. Unlike plain old telephone service, where power for telephone service is delivered right over the wiring, VoIP presents its own reliability challenges for network engineers.
Final approval of the power-over-Ethernet (PoE) standard last year removed a significant barrier to widespread adoption of IP telephony. The standard. IEEE 882.3af, specifies how to supply electrical power to compliant devices over Ethernet LAN cabling. Armed with uninterruptible power supplies (UPS), IT staff can now deploy VoIP telephones without worrying about them not functioning during a power outage.
A single-standard Ethernet cable will new be able to provide both power and communication services to IP telephones, guaranteeing power to all connected devices and ensuring reliability of service. Among the advantages of PoE:
* Reduced cabling costs. Because one set of wires provides both power and data to IP telephones connected on the network, installing a local power source at the device is net necessary.
* A worldwide power standard. This means that PoE devices, such as IP handsets, will plug in anywhere around the world.
* Simple network management protocol (SNMP) network management This system can be used to monitor and control connected devices, such as IP telephones, including resetting and shutting them down remotely.
The 802.3af standard defines two new hardware components. The first component, called the power sourcing equipment (PSE), is responsible for supplying up to 15.4 watts of power (at 48 volts) to the second component, called the powered device (PD). There are two types of PSEs-data terminal equipment (DTE) and midspan PSEs (also called midspan controllers).
DTE PSEs are PoE-compliant Ethernet switches and are sometimes called end-span PSEs. They use pins 1/2 and 3/6 of the eight-pin Ethernet cable for both data and power. Midspan controllers are used on LANs that do not have PoE-compliant switches. Sometimes called midspan injectors or midspan hubs, these devices stand between a legacy Ethernet switch and a PD. and supply the PD with power. These devices use pairs 4/5 and 7/8. Midspan controllers cannot be used on 1000Base-T LANs.
Because DTE PSEs and midspan PSEs place power on different pins, the 802.3af standard requires that powered equipment, such as IP hand-sets, be engineered to accept power from either alternative. To power non-compliant equipment, devices called splitters can he installed. Splitters separate the DC voltage and data into different outputs that the non-compliant device can use.
To avoid damaging devices that are not PoE compliant, such as personal or laptop computers, the standard specifies a method of detecting PoE compliance before significant voltage is placed on the cable. The PSE applies a small current-limited voltage to check for the presence of a 25k ohm impedance in the PD. Only if the PSE detects this impedance is the full 48V applied.
When evaluating PoE solutions, pay attention to the total power available to all ports. Many designs cannot deliver the full 15.4 watts per port as specified by 802.3af. For example, a midspan controller that has a 200 watt power supply to serve 24 ports will not be able to provide 15.4 watts on each port. A good method to verify if the product meets the power requirements is to multiply the number of Ethernet ports by 15,4 watts; this number should be less than the total output of the power supply.
