Title

PBX
Private Branch Exchange

20 years ago, small businesses and corporations with numerous telephones used "key systems" to provide telecom service to their employees. These were cumbersome and very limited, in that they had no intelligence. The caller punched down a key, which acted as a relay to connect his telephone to a particular line. The company would purchase a large number of voice lines and wired them throughout their facility. Today many key systems are still used by small businesses, and they have advanced quite a bit.

However, for large buildings with hundreds of employees - the advent of intelligent, digital PBX systems such as the Meridian, Norstar, Avaya, etc. - has greatly improved the capabilities of end-user phone systems. Users of the PBX share a certain number of outside lines for making telephone calls external to the PBX. Most medium-sized and larger companies use a PBX because it's much less expensive than connecting an external telephone line to every telephone in the organization. In addition, it's easier to call someone within a PBX because the number you need to dial is typically just 3 or 4 digits. The PBX also offers extended services. The most common PBX functions are to supply the employee a phone, phone extension, voice mail and a mailbox.

NOTE: Hotels have the most complex PBX units, with multitudes of features and restrictions

The following diagram shows a typical PBX system. The phones have twisted pair lines that connect them the the PBX, which is usually a large unit in a wiring closet. It is typically about the size of several 19-inch racks placed side-by-side, but can be as small as a PC (for small businesses). The two trunks shown are usually dedicated T1 lines going into the LEC CO. The stations are digital phones for the employees. The attendant console (the "switchboard") is a powerful telephone with additional management features. It is used for a human attendant, who can intervene and supply extension numbers, directory service, and to connect station calls manually.

Intra-building calls can be placed by simply dialing the last 4 digits, or the user's "extension" and the PBX realizes that the call should not be sent out any of the trunks - but be connected directly to the recipient of the call in the building.
Local calls are sent via the CO trunks. The PBX senses local calls whenever a 9+ number is dialed
Long distance calls are either ported through the LEC and handed off to the IXC - or the PBX can be configured with both LEC and IXC trunks. In the latter case, the PBX will sense if a call is long distance (by the 9+1+ dial pattern) and port it out the IXC trunks.
if the company has a VPN confgured (Virtual Private Network), the PBX senses the call, usually by an 8+ dial pattern, and connects it as a long distance call with only 8 digits required instead of the usual 10..

PBX Trunks

The PBX trunks are usually purchased in sets of T1's, but each trunk is actually a DS0. Therefore, if a customer buys a "T1 trunk" - they are actually buying 24 DS0 Trunks. It is important that the customer has an idea of how much traffic they are expecting, so that they can size their trunks properly. Keep in mind that these are shared resources, so you only have to have enough DS0's to handle the peak number of simultaneous calls. In fact, there will be periods where the call volume becomes extreme, and rather than paying a huge amount to handle all of these rare situations - most customers can deal with an occasional "fast-busy" signal. When the LEC cannot handle the call due to call volume a recording of "we're sorry - all circuits are busy" is played. When the PBX is full, the caller usually receives a "fast-busy" signal, which is similar to a standard busy signal, but with a higher frequency.

Trunks can be configured as incoming only, outgoing only, or 2-way. Do not be confuseed about these trunks - all three types support a full-duplex conversation once the call is connected. They refer only to how the call must be originated. 2-way is the default and is slightly more expensive. Incoming only trunks (In-only), means that the call must originate from an outside telephone. Out-only trunks will only support calls made from within the PBX system. If a company wants 2-way calls, they must purchase either 2-way trunks - or both In-Only and Out-only trunks. Some companies, such as telemarketers, will buy mostly out-only trunks. Others, such as information services, will buy mostly In-only trunks.

DID (Direct Inward Dial)

This term is used loosely and confuses many. DID simply means that the outside caller can dial your office phone directly (an inward dialed call - direct to your phone). The only other scenario, "non-DID, would be for an attendant to answer all calls, and connect you to a user's extension.

DID works over 2-way trunks and In-only trunks. It is a rather sophisticated service and must be set up carefully with the phone company. The reason for this complexity, is that a small number of trunks support a large number of stations.

*** see the "DID" section

Starting a Call

When a caller lifts the phone to make a call, they expect a dial tone to be there. Although the caller may not be aware of it, there's a process going on "behind the scenes"; the PBX notifies the Central Office (CO) that a trunk is needed. The start method used and the start options are usually combined in the terminology. For example one might say that ACME corporation is using "Loop/Wink start".

*** see the "Start Signaling" section

Hunting

Every customer wants the best possible phone service at the lowest possible price. They want their customers to be able to get through on the phone lines, but they don't want to pay for more trunks than they need.

One way to make the most efficient use of trunks is by using a feature called hunting. It's the way the network finds available incoming trunks to complete incoming calls. When a trunk is busy with a call, it's not available for other calls. Hunting allows the network to forward an incoming call to the first available trunk. The CO Switch hunts for the first available trunk. If all trunks in the hunt group are in use, the caller gets a busy signal.

*** see the "Hunt Groups" section

Centrex
A new variation on the PBX theme is the centrex, which is a PBX with all switching occurring at a local telephone office instead of at the company's premises. When you think Centrex - think "offsite PBX" managed by someone else.

For more info on PBX systems, see : http://www.pbxinfo.com