So as you can see - Voice over Internet Packets (VoIP) is a communications suite protocol that not only involves streaming Internet Packets from one IP address to annother; but also "implies" a collection of "software" and "equipment" that together make up a Telecommunications "Soft Switch". Theory and actual practice tends to get foggy at this point. I know of a few companies that have successfully integrated their local LAN into their on-site PABX. They use their Ethernet Workgroup Switches to do the actual switching by modifying their MAC Address scheme on their LAN Network to provide separate addressing for their Computer and Phones. Their PABX takes the incomming calls, routes them into the VoIP System Server through a PABX "Port" that is connected to a "virtual" port on their VoIP Server. Their customers don't know they are connected through a VoIP (or Voice over LAN. Connections for "out going" calls are made by the usual access number (#29 in their case).

Congradulations, you now have a "Basic" understanding of how VoIP works. A Basic Private Area Branch Exchange (PABX) is a very basic "Telephone Central Office" that connects multiple Telephones(Lines) to a limited number of Telephone Lines (Trunks) - do you now get the association to the loose use of the terms "Trunk" and "Lines"?

I hope you now have a reasonable grasps of this concept. There are many additional conditions and signals which must be processed in order to bring it all together and make it work. The Telephone Switch or "Central Office" provides for on-hook and off-hook line sensing. They provide "Dial Tone" which lets the person who picked up the phone know that the line is ready for them to dial the number. Once the person starts dialing, the Dial-tone ceases and talk battery is maintained to allow for voice communications once that call is connected - but the caller is not ready to talk just yet. The number sequence consiste of a set pattern of numbers which tell the "central office" "switch" just what is happening. The numbers zero (0), one (1), and nine (9) are reserved for "special features" and they will automatically route calls to internal "Special Function" ports within the Central Office Switch - 0 = indicates that the call will be routed through an Operator for further assistance, 1 = indicates a long distance "trunk call requests, 9 = a special call and the associated 1 and 1 is anticipated to connect to emergency services. The rest of the numbers are pre-determined based upon other conditions. In a Seven (7-) Digit call exchage the first three numbers (other than 0, 1, or 9) will be the Central Office Exchange Identifier. The next four digits (in the United States - five digits in Germany, Mexico and other countries) will identify the Number Groups and sub-groups, with the last two digits being the line identifiers in the final group. Now the call has been routed to the person's phone that is being called. But first the Central Office Switch will check to see if the line is on-hook and availiable to receive a call - or off hook, meaning it is busy and therefore unable to receive a call at this time - this will result in a "busy tone" being sent back up the chain to the call originator, letting them know the line is busy. If the line is on-hook (or availiable to receive the call - the Central Office switch will intitate a ring signal (usually 20Hz and 90VAC). It is important to understand that since the phone is on-hook it is disconnected from the line. To allow for ring signalling the ringer circuit is bridged accross the line through capacitors. These capacitors allow the 90 Volts Alternating Current (90 VAC) to pass to the ringer and therefore "alert" the person or people that somebody is calling them. When they pick-up their telephone it goes off-hook and draws 48 Volts Direct Current accross the cable pair and causes the Central Office Switch to stop ringing and connect the calling parties so that they can now talk. When the phone is once again placed on-hook the DC path is broken and the call is dropped. Everything now resets to allow for the next event (call).

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