PoE Refresher Course
Power over Ethernet (PoE) technology is used to pass electrical power, along with data, over Ethernet networks. PoE requires Category 5 cable or higher for high power levels, but can operate with Category 3 cable for low power levels. Power can come from a supply within a PoE-enabled networking device such as an Ethernet switch or from a device built for "injecting" power into the Ethernet cabling called a midspan. The high power market is experiencing the highest growth in the PoE market, the total solution is in need of this power and beyond. Future PoE standards can be expected within three years. Ahead of these new standards, some vendors have already introduced higher power midspans to deliver more power to meet the future energy needs of new technology; while keeping power current levels low.
802.3af and 802.3at Standards
The IEEE 802.3af PoE standard ratified in June 2003 provides up to 15.4 W of DC power to each device. Only 12.95 W is assured to be available at the powered device as some of the power is dissipated in the cable. The IEEE 802.3at PoE standard, known also as PoE+ or PoE Plus, ratified September 2009, provides up to 25.5W of power up to 100 meters. Some manufacturers can supply up to 51W of power over a single cable by using all four pair of Cat 5 cable. Current and power are the differences between the two standards.
Technology Match Up
Midspan and endpoint devices are part of the new techno-jargon landscape but what is their practical relevance?
By Jonathan Sohnis
Some clarification is in order here. PoE power supplies come in two basic flavors, midspan and endspan (also known as endpoint). Both will inject managed power onto the Cat 5. In a typical IP surveillance network a camera communicates with a video IP server through a network switch. A midspan is placed between the switch and the camera, passing through its video data and injecting power, whereas an endspan incorporates the switch in the same enclosure.
A common question is: why not just use an endspan device? It is not quite as simple as it may seem and there are benefits to applying each. Both midspan and endpoint devices enable PoE capabilities using IP camera infrastructure, but most video systems incorporate IP cameras that are not PoE-compliant and require separate power sources. The combination of a midspan device and a switch provide optimum flexibility to support PoE cameras using products like Altronix's NetWay midspan solutions, while also managing non-compliant PoE IP cameras directly from the switch. The NetWay midspan solution also includes provisions for PTZ and IR cameras that require more power than conventional IP cameras and provide security professionals with the flexibility to specify different cameras for specific surveillance applications within the same system.
Another advantage of this configuration is that network switches are low-cost commodities that can be replaced or upgraded separately without affecting the midspan device, making the use of a midspan and switch combination more economical. Finally, an endpoint switch's aggregate power may not be sufficient to provide the power required for maximum usage on all ports simultaneously.
This is not an issue using a midspan device like Altronix's NetWay which has been designed to deliver maximum output power on all of its ports.