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Cabling Requirements

Typically, APs are connected to the network via some type of Ethernet cabling. This could be Category (Cat) 6 cable, Cat 5 cable, fiber-optic cable, or (in some of the older WLAN products) coax (although not many products support this today). In almost all cases, the same limitations apply to data cable that feeds an AP as would apply to feeding a wired client device. Limitations include distance, number of connections, plenum ratings, and so on.

Network engineers usually have a good understanding of Ethernet cabling, but an AP has one other requirement: power. Because APs are typically placed in the ceilings or other areas where the availability of AC power might not be easily accessible, vendors sought a way to use the same Ethernet cable to provide power to the AP.

Power over Ethernet

Most network devices require some form of power, whether central LAN infrastructure devices such as switches and routers, or peripheral components such as IP telephones and APs. Power over Ethernet (PoE) combines both data and power onto existing LAN infrastructure cabling, which is typically Cat 5 or Cat 6 cable.

PoE had a very high adoption rate for VoIP applications, and its usage in WLANs has recently been increasing (primarily because it can reduce installation cost by removing the need for local AC power at the AP and eliminates the need for extra cabling to feed power to the AP). In many cases, the cost of adding an AC power source (by a licensed electrician) for every AP location exceeds the overall cost of the AP itself.

PoE is used for many different applications. VoIP was one of the initial PoE applications, but now many other network devices benefit from this feature, including the following:

Printers and print servers

Web cameras and security devices

Alarm systems

Security locks/security door systems

Any device connected to the LAN

You can provide PoE in three primary ways:

Placing a standalone device anywhere on the Ethernet cable

Using a multiport PoE device

Using an end-span device

Placing a standalone device anywhere on the Ethernet cable provides a single Ethernet input and a single Ethernet with PoE output for connecting to a single device. Figure 9-11 shows examples of single-port power injectors.

The second method for supplying PoE is through the use of a multiport PoE device. These come in several configurations, supporting as few as 6 or 8 devices (as the one shown in Figure 9-12) or up to 24 or more devices. Basically, these devices provide one Ethernet input and one Ethernet output with PoE for every device supported.

For both the single-port power injector and the multiport power injector, the Ethernet input for the injector comes from the wired network, usually the local Ethernet switch. The device is typically located in the same location as the Ethernet switch, where power is readily available. The output of the injector is connected to the Ethernet cable that feeds the device (such as the AP). Both of these types of devices are known as mid-span devices because power is injected in the middle of the Ethernet run. It is important to remember that this does not increase the length of the Ethernet run, and the limitation of the cabling applies to the total length of the cable between the switch or network connection and the end device. Figure 9-13 shows a typical connection for both a single-port power injector and a multiport power injector.

The third type of PoE-supporting device is the end-span device. This type of device is both the data source and the power injector. An Ethernet switch that provides power over the Ethernet without any external devices is such a product.

802.3af

802.3 Group, the 802.1af specification was developed. Although many companies were involved in the development of this standard, PowerDsine was a major driving force behind completing the standard.

The standard defines the following key elements:

The physical method for power distribution Defines the basic requirements for power delivery.

The voltage, power, and other parameters Defines the maximum current capacity, the minimum and maximum voltage, and the overall maximum power availability.

A standard method for end-device power request (signature recognition) Defines a specification to determine whether the end device requires PoE. This provides protection of non-802.3af devices. By the use of a physical "signature" on the end device, the power source will know whether power should be applied to the cable, protecting the end device from improper voltage on Ethernet cables.

Proprietary PoE Methods

As previously mentioned, it was only recently that 802.3af was ratified, even though PoE has been around for several years. Because many hardware vendors took it upon themselves to develop their own PoE solution, it is important to understand the basic differences between implementations to avoid integration and installation problems.

The Cisco PoE implementation was originally for supplying power to the IP Cisco IP phone products. The power requirements for the early IP phones were very minimal. Therefore, the Cisco PoE solution provided approximately 7 watts per port. When PoE became a requirement for Cisco APs, the same Cisco PoE scheme was used. At that time, the power consumption of the AP fell within the capabilities of most Cisco PoE-supplying switches. However, as new APs were developed, power requirements increased, requiring more power be made available. As a result, newer APs require either in-line power injectors or the use of newer PoE-capable switches.

When designing and installing PoE for APs, you must verify that the intended PoE switch can support the power required by the AP model that will be used (today, and in the future with upgrades). If not, you might have to find an alternative method such as the single-port power injectors.

Cisco Aironet power injectors provide an alternative powering option to local power or using a PoE switch. The single-port Ethernet power injector provides the required 48V DC power and carries it over the same cable as the data signal, by utilizing the unused pairs of the cable.

Cisco took the idea of power injectors one step further. Many networks today use fiber-optic connections, especially for long runs in places such as manufacturing facilities and warehouses. However, providing power over fiber is not possible (at least today). It is possible, however, to provide power from a location near the factory floor or warehouse, but again that is not where the AP is likely to be mounted.

The Cisco Media Converter converts fiber media to Cat 5 media and combines the resulting data signal with power for delivery to the AP or bridge. The power-injector media converter accepts 48V DC power from either the barrel connector of the local power supply or an alternative 48V DC power source. When powered by an alternative 48V DC power source connected using the provided power-supply pigtail, the power injector media converter is UL2043 certified and suitable for installation in plenum areas.

This device enables you to run the fiber to a location in the factory where AC is available, and from that point you can run a Cat 5 cable to the AP location (see Figures 9-14 and 9-15).

There are other possible PoE schemes, and you need to be aware that not all PoE schemes are created equal. Cisco PoE uses 48V DC over unused pairs of the Cat 5 cable, whereas the PoE scheme offered by 3Com provides 24V DC power over the spare pins in standard Cat 5 cable.

Because the 3Com devices use 24V rather than 48V, and the Cisco devices use a different pin-out for the power than an 802.1af system uses, both APs require a converter to receive power from an 802.3af source. PowerDsine and 3Com offer converters for this requirement.

If you are dealing with an existing network for which it would be financially unacceptable to swap or install new LAN switches to add APs, you might find that using a mid-span device is a more feasible and cost-effective option. When purchasing equipment for either mid-span application or for powered switches, ensure that there is an easy and practical upgrade path.