I decided to go PoE to run a few of the phones. So far only the 7970 is using it, as the switch I got is IEEE 802.3af PoE (because I didn’t feel like selling the motorbike so that I could afford a Cisco switch to run the older Cisco phones). Not one to let mere electronics stand in the way of me running the Cisco pre-standard phones on 802.3af, I set about with crimp-tool and soldering iron to try and make it work.
It’s probably fair to say that success has been limited.
The problem arises because Cisco implemented a PoE mechanism prior to the IEEE 802.3af standard being ratified. This method, not surprisingly, is completely incompatible with IEEE 802.3af. It’s generally referred to as “Cisco pre-standard PoE” — Cisco gear that does PoE usually supports both their pre-standard and IEEE 802.3af.
(When I come across something like this I wonder if a lot of the cost of Cisco gear is soaked up by the added complexity of having to support both the “system they designed and rushed into the market to try and preempt the standards process” as well as the “system the industry agreed on and ratified because the right way is not always the Cisco way”. But I digress.)
Thanks to some information like this, I found out how an 802.3af-compliant switch senses that it should supply power to the wire. It’s quite simple: the switch looks for a “signature resistor” across the cable (follow the linky-trail for more info). This meant it was quite easy to convince my switch that it should apply voltage. (Susan was a little disturbed by how excited I got about making a little green light come on.)
Actually getting power into the phone is a different matter. The page I link to above has about five different versions of fact and fiction when it comes to running the Cisco phones off 802.3af PoE gear, but it misses out on one critical piece of information (or at least it will until I get an ID there and update it).
The IEEE 802.3af standard defines the partners in a PoE transaction as the Power Source Equipment (PSE) and the Powered Device (PD). The PSE can be either a 802.3af-compliant switch, or some kind of intervening device like a PoE midspan or injector. The standard also defines two “modes” of PoE: Type A, where the power is supplied over the same cable pairs as the Ethernet signal; and Type B, where the power is supplied over the spare pairs in the cable.
The voip-info.org page says that with a resistor and a crossover-wired cable, you can run a pre-standard Cisco device off an 802.3af PSE. This is only partly true, because of a very important little piece of info that’s only alluded to on the page. The critical info is this: while an 802.3af PD should be able to work on either Type A or Type B (as you don’t know what kind of PoE source you’re going to be connected to), a PSE can be Type A or Type B. This is particularly important when it comes to the pre-standard Cisco phones, as they can only work as a Type B-style device — the Cisco pre-standard had no way to receive power over the data pairs.
This is why folks have success running Cisco phones off midspans and injectors — because they are 802.3af Type B devices. Type B is used when you are injecting power along the cable run (i.e. without access to the Ethernet PHY). They then crow on about how they got their Cisco phone working with 802.3af — as always, the devil is in the detail. In the case of trying to run Cisco phones off 802.3af Type A devices like switches, you are left with the problem of extracting the 48V out of the data pairs without breaking the Ethernet link to the end device. Not simple.
The page above lists one switch that appears to work with the crossover-cable trick: the Netgear FS726TP. Knowing what I know about 802.3af now, it would seem that Netgear decided to make their switch a Type B PSE instead of a Type A. Is it wrong? Well no, but some folk may be surprised why they don’t get power over some cables when a different switch works fine.
The good thing about the Cisco pre-standard (if there can be a good side to it) is that it should be quite easy to rig up a DIY injector using the original power supply. Since the phone expects power over the spare pairs, there’s no need to use an adaptor to split the cable out again at the phone end. A DIY midspan using a single 48V PSU would reduce the losses in running a number of separate power bricks too.
So if there are budding Cisco PoE hackers out there, be aware of the need to know a little bit more about your 802.3af switch than what the manufacturer says on the glossy brochure. 😉
PS: While researching this, I came across a forum comment from someone who said that one of their pet hates was people referring to “Power over Ethernet” when it should be “Power over Cat5”. Well, one of my pet hates is over-generalisation. There can be no argument that the way that power is delivered in PoE is specific to the Ethernet wiring of Cat5-style cable. Power over Token-Ring, if such a thing existed, could not be the same as PoE because different pairs of wires are used. Likewise Power over ISDN U-Bus, Power over POTS, whatever. My advice to Mr “Power over Cat5”: keep your generalisations to yourself, if you please!
Crossed Wires 