Does POE work with 79XX phones that are connected to Cat 5 (all 8 pairs are terminated) or do we need to run power bricks?
Yes it shouldn't be a problem for most phones.
Category 5e cable uses 24 AWG conductors, which can safely carry 360 mA at 50 V according to the latest TIA ruling. The cable has eight conductors (only half of which are used for power) and therefore the absolute maximum power transmitted using direct current is 50 V Ã 0.360 A Ã 2 = 36 W. Considering the voltage drop after 100 m, a PD would be able to receive 31.6 W. The additional heat generated in the wires by PoE at this current level (4.4 watts per 100m cable) limits the total number of cables in a bundle to be 100? at 45 Â°C, according to the TIA.
Drawbacks of IEEE 802.3af are:
* Excessive voltage with peak of 60 V (many components are limited to ~30 V).
* Undefined polarity (requires a diode bridge which causes a voltage drop and require more board space and components).
* Undefined wire pairs (multiple configurations must be handled which requires more board space and components).
A partial solution to the drawbacks of IEEE 802.3af is to assume pin 4 + 5 as positive (+) and pin 7 + 8 as negative (-). This would not be standards compliant but will make PD implementation easier and not damage anything. Any incompatibilities with IEEE 802.3af will only result in an unpowered device.
Another modification is to limit voltage from the PSE to 30 V and thus enable the use of standard components. But this may destroy the PD if it is connected to a PSE that isn't modified to keep the voltage low enough. It also limits the amount of power that can be used.
More info you can find here: http://en.wikipedia.org/wiki/Power_over_Ethernet
We have an older area of our Campus that is strictly CAT5 cabling. In this area there are approx. 500 Cisco IP Phones that have never seen any PoE issues that were related to the Cable type. In other areas (depending on install time frame) we also use CAT5E and CAT6. There is even an old note from Cisco about support for IP Phones on CAT3 :( so you should be good to go!
Hope this helps!