Phone Systems

I’ve been on the technical side of telephony for more than 15 years now – beginning in tactical military communications and phone systems.  As you can imagine, I’ve seen incredible changes occur with IP telephony, business phone systems, and VOIP in general.  This blog post will give our readers a high-level introduction to VOIP technology and how it has evolved to where it is today.  I figure the best way to do this is to give you a glimpse of my own journey down the business telephone systems and VOIP technology road.  Note that this might not be an accurate depiction of the history of the telephone system; this is simply my own experience.

Business Telephone Systems in the Past:

Business telephone systems often consisted of multiple analog lines from the Local Exchange Carrier (LEC) to each basic, analog business telephone.  Often times, the LEC provided PBX-type functionality (transfer, hold, etc).  As you can imagine, this was very expensive because businesses had to pay for a phone line for EACH telephone, even if it wasn’t being used.  Additionally, every internal phone-to-phone call had to go through the all-mighty LEC (Bell).

The PBX (Private Branch Exchange) and Key System:

A PBX, or key system, is a piece of equipment that resides at the business location and centrally connects business telephones to each other and to the telephone company (Public Switched Telephone Network or PSTN).  It usually consists of multiple connections (“ports”) that are hard wired to telephone cabling. The cabling connects to proprietary digital business telephones and ports that are connected to analog lines from the telephone company. This type of business telephone system equipment allowed businesses to have free phone-to-phone calling within a business, and connectivity to the outside world.  A one-to-one ratio between the number of outside PSTN lines and telephones was not required either.  This is because a business could estimate the number of simultaneous inbound and outbound calls at any given time and “right-fit” their number of lines to accommodate that.  So a business that previously had 100 phones–and subsequently 100 phone lines–could  now pair the number of phone lines down to 24 (for example).  As you can imagine, with phone lines at $30 each, the cost savings were huge.  Additionally, PBX systems could offer businesses features like direct extension dialing, hunt groups, and voicemail.  Traditionally PBX or business phone system manufacturers were AT&T (which became Lucent and is now Avaya), Nortel, Tadiran, Executone, etc.

The T1 and PRI:

Using Digital Time Division Multiplex (TDM) technology, phone service providers could now place up to 23 or 24 phone lines on one wire.  Businesses that were paying $30+ per analog line and had over 12 lines could now replace those analog lines with a PRI and save money.   The PBX would need to be equipped with a T1 or PRI card.  A T1 traditionally accommodates 24 lines or channels; a PRI (Primary Rate Interface) accommodates up to 23 lines or channels with one channel being used for information transfer like caller ID.

This post is part 1 of a 2-part series. Click to read Part 2 of our General Introduction to SIP, VOIP, an Internet Phone Systems.

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