XI. LAN

PoE Modes

Power over Ethernet is a convenient way to provide power to a device over its wired Ethernet cable rather than having to provide a separate DC power source - particularly useful where no local power is easily reachable, or outdoors.  PoE also provides a convenient way to remotely power cycle a product by turning the PoE power off and back on again, or just to turn it off when not required. 

In earlier RJ45 systems (up to 100BaseT), only 4 of the 8 wires were used for data. In Gigabit Ethernet, all 8 wires wire are used for data but this does not prevent the usage of PoE - it can happily run on Gigabit Ethernet too.

Power is inserted into the Ethernet cable by either an Ethernet switch or a PoE injector. An injector takes the existing Ethernet data cable at its input, adds the power and then its output is the data and power combined. An Ethernet switch does the same thing but combines it inside your Ethernet switch.   The injector or switch is known as the Power Sourcing Equipment (PSE).  The device that you are powering (phone, wireless access point etc.) is known as the Powered Device (PD).

802.3af vs. 802.3at

There have been various PoE standards historically, but the main ones in use today are designated under the specifications 802.3at (Standard PoE) and 802.3at (PoE+ or High-Powered PoE).   802.3af provides up to 15.xW per port.  802.3at provides up to 30.x Watts per port. Both use a nominal voltage of 48 volts DC which can then be stepped down to the required voltage by the powered device. 

Power Budgets

Each Ethernet injector or switch will have a maximum power 'budget' - that is the total power, in watts that it can provide across all of its power. For example if you have a 24 power switch with a 240W total PoE budget, you could have 10 802.3at devices drawing 24 watts each, or 20 devices drawing up to 12 watts each.  High Power requirements tend to be lmited to outdoor IP cameras (which have heaters) or higher power wireless access points. For devices like IP phones, they will typically draw up to 5 watts only.

Non-Standard PoE

The two PoE standards ensure that devices are fully compatible with each other and also include various safety mechanisms to ensure that devices get the right power and there is consistency. For example, an 802.3af compliant switch will only apply (send) power once it has established that there is a compatible device on the other end of the cable. 

There are, however, non-standard injectors and devices which put also provide power over Ethernet but not in any standards-compliant way.  This saves cost and reduces component count but it means that unknown voltages without safety mechanisms will be going down your cables - this can damage devices or switches if you're not careful and is really not recommended especially if more than one person is ever going to be administering on the network or there's any chance of devices being plugged into the wrong port.

Mode A vs. Mode B

As well as the power provision, another variation within the standard is the wiring pattern.   In Mode A, Pins 1,2,3 & 6 are used to carry the PoE power.  In mode B, Pins 4,5,7 & 8 are used.  This isn't generally a problem as any 802.3af or 802.3at compliant PD (powered device) must be able to switch automatically between the two alternative wiring patterns. The PSE (injector or switch) will normally not switch and just be Mode A or B.

A problem might arise if a PD is not 802.3af compliant (merely labelled as 'PoE compatible') can cannot switch modes.

DrayTek Products

Most Draytek products now use Mode A but remember that it normally doesn't matter as a compliant PD can detect and switch automatically.

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