[asterisk-users] Polycom 430 , 501 and 550

[Stephen Bosch]

Jeff Davis wrote:
> The IP 501 supports both Cisco and 802.11af with different cables. While
> there are pin assignments differences, there are also electrical
> differences in the discovery protocols. The special cable is an artifact
> of this.
> 
> I don't know of anyone who was able to make the phone work without the
> cable, and the 501 is not designed to work without it. If it were just a
> matter of pin assignments, then people would be selling cables on eBay.
> 
> I notice that now that 802.11af is THE standard, Polycom is supporting
> it on their new phones without any special cables.

The IEEE PoE standard -- for those who care -- is actually IEEE 802.3af,
not 802.11af. The important one is the 802.3af-2003, because that
introduced provisions for preventing ground loops between the PoE
midspan and the switch.

There's a white paper on the topic at Polycom's website which explains
the need for non-standard cable (the newer version of which does in fact
have some additional electronics in it):

http://polycom.com/common/pw_item_show_doc/1,1276,2766,00.pdf

I suspect that the 330, 430 and 550 phones have some sort of built-in
ground loop detection and prevention.

For anybody contemplating running the 501/301 phones with a straight
Cat5 cable: in most cases, it won't work; in some cases, it will work,
but you can bake your PoE injector, unless said injector has a ground
loop prevention circuit built-in; all the newer ones are required to
have this to be truly 802.3af compliant. Remember that a label
indicating 802.3af compliance is no assurance, since the standard has
been through numerous revisions in the last few years.

The moral of the story -- it is probably safer to use the special cable
if you have 301s or 501s.

Thanks for the feedback!

-Stephen-