The 802.3af Power over Ethernet (PoE) standard is helping to give life to VoIP implementations by ensuring that VoIP phones can have a battery backed-up lifeline power source equivalent to that of older digital PBXs. With PoE, equipment designers must add power supply considerations to the long list of new design considerations – ranging from latency and performance expectations to network security - that come with developing VoIP networking equipment.
How PoE is implemented in an enterprise – either end-span or mid-span - will determine how big the issue is for the network equipment designer and whether they will need to adopt an external bulk DC power supply to cope with the load.
Introduction to PoE
802.3af is an IEEE standard that specifies how power is delivered via category-5 cable from power sourcing equipment (PSE) to a powered device (PD) in Ethernet (10BaseT), Fast Ethernet (100BaseTX) and Gigabit Ethernet (1000BaseT) networks.
The job of the PSE is to discover PDs on a network and to provide the required power. The PSE can supply up to 350mA at 48V to the PD. Optionally, the PSE can determine a lower power need and scale back the power levels on a per PD basis. To discover PDs, the PSE issues two small current-limited voltage signals on a port and if a certain resistance level is detected it transmits power on that line.
This process helps to protect non-powered devices that may also be on the network, such as computers and other switches. If the PSE doesn’t detect that resistance, it knows that the device is non powered, and in fact could be damaged by a current. The PSE also monitors the connection continuously and if it detects the power draw dropping below a minimum of 10mA for at least 75 milliseconds out of a 500 ms period, it terminates the power flow to that device and reverts to detection mode.
PoE Deployment Scenarios
PoE can be deployed in one of two scenarios; either an enterprise replaces its LAN switching gear with PoE-enabled “end-span” switches, or they install a “mid-span” PoE-enabled patch panel or power hub that sits between the switch and the powered devices.
In an end-span network (left image), power for all switching and powered devices comes from the Ethernet switch, whereas in a mid-span network (right image) a powered patch panel or power hub powers these devices. Designers need to know the impact that each architecture has on how the design power into their networking equipment.