Thinking about inline power?

Considerations before you buy powered switches.

The ability to provide electrical power to IP phones through the CAT5 Ethernet cable is a boon to many IP Telephony deployments. It has a number of advantages that were largely taken for granted in the old world, where the PBX has been supplying power to phones for over a century. However, there are a couple of big gotchas that you must be aware of if you're planning to use inline power.

Before we discuss some of the gotchas, you should realize that inline power is not a requirement, and many organizations are electing simply to plug their phones into local electrical outlets for power. Of course, if you lose power to the wall outlets, you lose the use of your phone, but just because the users are accustomed to the phones still working during a power outage doesn't mean it's a requirement for your project. Each organization needs to determine this individually.

The first gotcha we'll mention is that your cabling may or may not be an issue. Ethernet only requires two pairs of the four provided in the various wiring specifications, so many organizations either never punched down pairs 3 and 4, or are using some legacy cabling like IBM's Type-1, which they kept from when Token Ring was deployed. Power over Ethernet can work with these types of cables... but here's the catch. Inline power delivered directly from a switch uses the same pairs (1 and 2) as Ethernet, so there's no problem. However, if you're using one of the powered patch panels, they deliver power over pairs 3 and 4, and if they're not punched down, inline power isn't an option for you. Consider this before you spend tens of thousands on powered patch panels.

The second gotcha to mention is that many organizations don't have a UPS in their closets. If their network equipment loses power, their phones are dead anyway, whether they're using inline power or not. Also, some companies have UPSs in their wiring closets, but not enough. Newer switches with inline power have far greater requirements than their predecessors. For instance, a popular Cisco switch supporting 240 phones has 2500W power supplies. This can quickly overrun a UPS that was previously sufficient.


Thomas Alexander Lancaster IV is a consultant and author with over ten years experience in the networking industry, focused on Internet infrastructure.


This was first published in July 2003
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