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Wireless networks are wireless only up to a point: The access point, power cord, or electrical outlet.
Although data can flow without wires, power typically does not—at least not for anything but very short distances. As a result, the free-to-roam promise sometimes cannot fulfilled in areas where plugging the access point into the wall is easier said than done.
The tyranny of power cords was loosened considerably in 2003 when the IEEE adopted its Power-over-Ethernet IEEE 802.3af-2003 standard. PoE was created to allow users to treat Ethernet devices in the same manner they have treated plain old telephones for over 100 years: Just plug it and it works, even when there is a power failure.
PoE applications included IP phones, WLAN access points and network cameras that required 12.95W or less, as this is the maximum power 802.3af-2003 allows a powered device to consume.
Being able to provide power to a WLAN access point over the same cable that carries the data onto the wired network can drastically reduce make WLAN installation costs and provide service over much greater distances.
PoE includes the following basic mechanisms: Detection (to prevent powering non-powerable devices); disconnection (to ensure power is removed from the line when the device is disconnected); protection (against over current and voltage); and, a classification option (to allow the PSE to allocate power a-priory to devices that require different power levels).
It works very well up to a point. And that point is about to arrive.
Because the soon-to-be realized 802.11n standard consumes considerably more power than the 12.95 W maximum of 802.3af-2003 we need a new solution for so-called "n" networks.
PoE and WLAN
Using Power over Ethernet, Wireless access points can be deployed in the location which makes most sense from the RF connectivity point of view, without the constraint of locating the access point near an AC outlet.
This is because category 3/5/6/7 cables can be up to 300ft (100m). In addition to this, there is no need for a specialized electrician to install Ethernet cabling infrastructure, as the PoE voltages never exceed 57V, and are therefore considered to be SELV (separated extra low voltage) by Underwriters Laboratories.
So for Wireless LAN access points PoE provides ease, low cost and flexibility in installation. And it also allows centralized power backup of the devices, as well as centralized and remote management.
For example, if an IT manager wants to reset a misbehaving access points, all that needs to be done is a PoE power cycle. And with sophisticated PoE power sourcing equipment it is possible to schedule times of the day in which the access points connected to the PoE power source will be turned off automatically.
Power sourcing equipment
802.3af-2003 specifies two types of power sourcing equipment (PSE): Endpoint PSE's (PoE switches) and Midspan PSE's (or just Midspans). While PoE switches include both the data switching and the power injection capabilities, Midspans are have only power injection capabilities.
This makes PoE switches more appropriate for greenfield installations, while Midspans are more useful to retrofit existing networks to support PoE with minimal disruption. Midspans have three added advantages, specifically for WLAN networks:
- Midspans come in various port counts, which make it easy to buy only the number of PoE ports needed to power the WLAN access points required. Since WLAN APs are typically not deployed very densely, Midspans with 1 to 12 ports are mostly used for WLAN applications.
- Unlike switches, Midspans are not replaced every three to four years but can last for over ten years, since they include only the power functionality, which does not necessarily change
- In thin access point client deployments, in which the WLAN switch performs most of the functionality, there is normally no direct link between the WLAN switch and the WLAN APs, and there is no need for special data switches, meaning that adding Midspans is the only change needed in the network, minimizing installation costs
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