What is a PBX? - 5/11/10
Posted by Kevin Box on Fri, Nov 05, 2010 @ 07:37 AM
What is a PBX?
A private branch exchange (PBX) is a telephone exchange that is used in an organisation. PBXs are also referred to as:
* PABX - private automatic branch exchange
* Switchboards or switches
PBXs connect internal extensions within an organisation and also connect them to the public switched telephone network (PSTN). An extension can be terminated by a phone, a modem, a fax or other devices. Some vendors used to sell "key systems" where users manually select their own outgoing lines, while PBXs select the outgoing line automatically.
Without a PBX, an organisation needs one telephone line for every telephone. With a PBX system, a small or large business only needs as many lines as necessary for the maximum number of outside calls at one time. This is usually around 25% of the number of extensions.
Circuit Switching Vs Packet Switching
When phones were first invented a conversation was held between two people who had wires which ran from phone to phone. When people wanted to call others they were connected via cables that were “patched” together for the duration of a conversation. Originally this involved a man making these connections by plugging one cable which connected to the calling phone into the socket which connected to the phone being called. This very rapidly led to multiple telephone exchanges and people were replaced by machines making connections. However the principle remained of a single channel being dedicated to a single conversation reserved for the entire communication. This is the so called circuit switched network.
It is obvious that this networking schema is very inefficient – you can only have a limited number of conversations. To improve this situation time division multiplexing (TDM) was introduced into the PSTN in the 1960s. This allowed each channel to be cut into time slices and allowed multiple channels on one set of cables with each conversation being cut into time intervals so small that the callers didn’t hear the cuts.
By contrast in packet based networks (such as the Internet) the message gets broken into small data packets that seek out the most efficient route as circuits become available. Each packet may go a different route: its header address tells it where to go and describes the sequence for reassembly at the destination. This is self-evidently a far more efficient use of resources. Eventually even the entire PSTN will operate along these lines.
Development to Hosted VoIP
As PBXs gained popularity, they started offering services that weren’t available from the PSTN such as hunt groups, call forwarding, and extension dialling
In the 1990s there was a massive growth in the Internet and the development of infrastructure to support it. This in its turn made people look toward the Internet as a global delivery system for voice calls. These factors led to the development of the VoIP PBX
The VoIP PBX started life as a physical box of electronics which allowed calls to be made internally using the same protocols as the Internet. The cables within the building could either be dedicated to the phones or shared with the computer network. The PBX then connected to the PSTN using either the normal analogue circuits or ISDN (an early digital service which is still in use) provided by BT or other telcos. The VoIP PBX was also known as the IPPBX
However, as time progressed it became apparent that the physical box of electronics was an expensive and unnecessary item where the number of concurrent calls was low. It was possible to connect a special kind of phone called an IP phone (or SIP phone) which had its own electronics directly into a broadband connection. The routing of the calls and all the other functions of the PBX could then be handled by a remote server. Because thousands of phones could share the same server (and each seem as if it was connected to small local PBX ) the cost of the PBX was massively reduced to the point where small business VoIP companies like Cloud Net regard it as being essentially a free service for customers. This set up is called hosted PBX.
Conclusion
The PBX is the device which allows internal phones to connect to each other cost effectively. The new generation of VoIP based services based on the Internet is driving down costs massively whilst increasing functionality. The hosted PBX is the way of the future, today.
Written by David Hill, Chairman, Cloud Net