VL on VoIP               Isseue 78

The VoIP Payphone Idea

By William Flanagan
Remember the public phone booth by the side of the road?  Plain Old Telephone Service (analog technology POTS lines) worked pretty well without environmental controls.  Cell phones have almost eliminated phone booths, but we still have work environments in tough neighborhoods like power substations, pit heads at mines, and remote railway switch points.  Multi-tenant building with no air conditioning in the wiring closets qualify as harsh, too. 

Moving from circuit switching to IP introduces three key concerns.  
  1. Power for active routers/switches where passive copper loops needed none. 
  2. Cooling for that electronic equipment. 
  3. Extending Ethernet LANs to match the reach of analog phone lines.
Sales literature for VoIP equipment typically shows high-end phones on executive desks or in business call centers.  The assumption is that the user and the wiring closet enjoy a controlled "office" environment.  It ain't necessarily so when the phone system extends beyond the office.

Electrical power is seldom a problem.  The utilities will drop a feed if you're willing to pay.  For low-power equipment you can deploy a solar panel and a storage battery to run through the night, or a fuel cell.  Low drain reduces the cost of power infrastructure. 

There are items that boast of low power.  Snom says its model 300 IP handset is the lowest, at 1.7 to 2.7 watts.  Encore Networks has a ruggedized router that draws only 7 W.  That's a total of less than 1 amp at 12 V for voice and data connectivity.  These days, wherever voice is wanted there's almost always a need for data as well, so it's best to budget for both.

With power that low, the need for cooling may disappear.  Encore's router is hardened to operate in ambient temperatures from -40 to 85 C (-40 to 193 F)--and that's not a typo, that's the IEEE 1613 standard.  Phones to cover that range are "less common":  most phone are 4 to 40 C (40 to 104 F) Cisco's 6901 IP phone, relatively low power at 3.11 W, is rated for -5 to 45 C (23 to 113 F).  An interesting exception is a ruggedized adaptation of the Cisco 7961 IP phone that's "dust proof,"  but it needs a heater for the LCD when it's cold.

In a migration from a legacy switch to VoIP, we can assume a copper loop is in place, perhaps even a 4-wire cable capable of T-1 transmission.  Old copper is not much good for native Ethernet, as in 10 Mbit/s LAN, but well-proven technologies (ISDN BRI or PRI, DSU/CSU, or DSL) can push IP and Ethernet over legacy cable for more than a mile.   These digital services from a local exchange carrier (LEC) typically cost more than the existing POTS line, but enterprise versions of the hardware are available if you have the right of way or own the cable.  As an alternative, Encore's router offers an internal cellular radio (for a data service) in addition to a CSU (one can back up the other for higher availability). 

VoIP payphones don't look like a hot item with LECs in the US.  More likely the technology will find use in private networks, for control functions (think SCADA protocol) and voice connections on sprawling infrastructure such as power lines, pipe lines, and railroads.  To find IP versions of real payphones look on the WWWeb--not at the side of your road.

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