[asterisk-users] Why Aastra uses 48V whereas other IP Phones use much less, i.e. 5-12V

[Michael Graves]

On Wed, 22 Nov 2006 19:20:54 +0000, Steve Kennedy wrote:

>On Wed, Nov 22, 2006 at 06:58:13PM +0100, Huib van Wees wrote:

>>    On 11/22/06, Zeeshan Zakaria <[1]zishanov at gmail.com> wrote:
>>      Why Aastra phones use more electricity, i.e. 48VDC whereas other
>>      phones use much less, e.g. Grandstream and Linksys both use only
>>      5VDC. I first thought it was because of PoE, but the ones with 5VDC
>>      also run fine on PoE. What is the difference in power consumption
>>      then?
>>    48V is also a sort of "standard" for telco devices.... if I remember it
>>    correctly...

>Power is nothing to do with voltage (well it is, but not alone), you
>need the current too i.e. V * A.

>Pylon electricity lines run at very high voltage (several hundred
>thousand volts) or the current going down the lines would heat the
>cables and you'd lose a lot of power.

>48V is just a telco standard, and most telco equipment (that runs in
>racks) is 48V. Probably because 110 (or 220/240 here in EU) is enough to
>electrocute an engineer, and 5V/12V would require too many Amps so
>wiring would have to be huge to carry the current.

Yes, 48v dc is a telco standard. It has to do with how they build their facilities and efficiencies in electrcal use.

When your entire plant has to be on a UPS you can save much money and gain reliability by NOT having AC power supplies in every bit of gear. Thus they have standard 48v DC UPS 
infrastructure and everything plugs into it. This is the way to 99.999% uptime from a power perspective.

It's interesting to note that outfits such as Google are now going down a similar route in planning huge new datacenters.

Michael Graves


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