Private branch exchange

From Free net encyclopedia

The PBX abbreviation is also used for polymer-bonded explosives.

A Private Branch eXchange (also called PBX or Private Business eXchange) is a telephone exchange that is owned by a private business, as opposed to one owned by a common carrier or by a telephone company.

1 Overview
2 Current trends
3 PBX Functions
4 Interface standards
5 Hosted PBX
6 See also
7 Notes
8 External links

[edit]

Overview

Originally an organization's manual switchboard (operated by a person plugging cables into sockets) was known as a PMBX (Private Manual Branch eXchange). These were gradually replaced by automated electromechanical and then electronic switching systems, called PABXs (Private Automatic Branch eXchange). As PMBXes are almost unheard of, the terms PABX and PBX have become synonymous.

Using a PBX saves connecting all of a business's telephone sets separately to the public telephone network (PSTN or ISDN). Such a set-up would require every set to have its own line (usually with a monthly recurring line charge), and "internal" calls would have to be routed out of the building to a central switch, only to come back in again.

In addition to telephone sets, fax machines, modems and many other communication devices can be connected to a PBX as well (although the PBX may degrade line quality for modems). For this reason, all such devices are generally referred to as extensions.

The PBX equipment is typically installed at a business's premises, and connects calls between the telephones installed there. In addition, a limited number of outside lines (called trunk lines) are usually available for making and receiving calls external to the site (i.e. to the public telephone network). Companies with multiple sites can connect their PBXs together with trunk lines. PBX-like services can also be provided by equipment located off site at a central provider, delivering services over the public telephone network. This is known as a hosted PBX. For example, most local phone companies offer a Centrex service in which each extension has a trunk line connected to the telephone company's Central Office. Other companies offer similar services.

PBXs are distinguished from smaller "key systems" by the fact that external lines are not normally indicated or selectable at an individual extension. From a user's point of view calls on a key system are made by selecting a specific outgoing line and dialing the external number; calls on a PBX are made by dialing 9 (or 0 in some systems) followed by the external number; an outgoing trunk line is automatically selected upon which to complete the call.

[edit]

Current trends

One of the latest trends in PBX development is the VoIP PBX, also known as an IPBX, which uses the Internet Protocol to carry calls. Most modern PBXs support VoIP.

ISDN PBX systems also replaced some traditional PBXs in the 1990s, as ISDN offers features such as conference calling, call forwarding, programmable caller ID, etc.

Historically, the expense of PBX systems has put them out of reach of small businesses and individuals. However, recent open source projects combined with cheap modern hardware are sharply reducing the cost of PBX ownership.

[edit]

PBX Functions

Functionally, the PBX performs three main duties:

Establishing connections (circuits) between the telephone sets of two users. (e.g. mapping a dialed number to a physical phone, ensuring the phone isn't already busy)
Maintaining such connections as long as the users require them. (i.e. channeling voice signals between the users)
Providing information for accounting purposes (e.g. metering calls)

In addition to these basic functions, PBXs offer many other capabilities, with different manufacturers providing different features in an effort to differentiate their products. Here is a short list of common capabilities (note that each manufacturer may have a different name for each capability):

Call transfer
Direct Inward Dialing
Customised Abbreviated dialing (Speed Dialing)
Voice mail
Follow-me
Call forwarding on absence
Call forwarding on busy
Music on hold
Automatic ring back
Night service
Call distribution (ACD, fixed sequences, ...)
Call waiting
Call pick-up
Call park
Call conferencing
Custom greetings
Shared message boxes (where a department can have a shared voicemail box)
Automated directory services (where callers can be routed to a given employee by keying or speaking the letters of the employee's name)
Call accounting

[edit]

Interface standards

Interfaces for connecting extensions to a PBX include:

POTS (Plain Old Telephone System) - the common two-wire interface used in most homes. This is cheap and effective, and allows almost any standard phone to be used as an extension.
proprietary - the manufacturer has defined a protocol. One can only connect the manufacturer's sets to their PBX, but the benefit is more visible information displayed and/or specific function buttons.
DECT - a standard for connecting cordless phones.
Internet Protocol - For example, H.323 and SIP.

Interfaces for connecting PBXs to each other include:

proprietary protocols - if equipment from several manufacturers are on site, the use of a standard protocol is required.
QSIG - for connecting PBXs to each other, usually runs over T1 (T-carrier) or E1 (E-carrier) physical circuits.
DPNSS - for connecting PBXs to trunk lines. Standardised by British Telecom, this usually runs over E1 (E-carrier) physical circuits.
Internet Protocol - H.323, SIP and IAX protocols are IP based solutions which can handle voice and multimedia (e.g. video) calls.

Interfaces for connecting PBXs to trunk lines include:

standard POTS (Plain Old Telephone System) lines - the common two-wire interface used in most domestic homes. This is adequate only for smaller systems, and can suffer from not being able to detect incoming calls when trying to make an outbound call.
ISDN - the most common digital standard for fixed telephony devices. This can be supplied in either Basic (2 circuit capacity) or Primary (24 or 30 circuit capacity) versions. Most medium to large companies would use Primary ISDN circuits carried on T1 or E1 physical connections.
Internet Protocol - H.323, SIP, MGCP, and IAX protocols operate over IP and are supported by some network providers.

Interfaces for collecting data from the PBX:

Serial interface - historically used to print every call record to a serial printer. Now an application connects via serial cable to this port.
Network Port (Listen mode) - where an external application connects to the TCP or UDP port. The PBX the starts streaming information down to the application.
Network Port (Server mode) - The PBX connects to another application or buffer.
File - The PBX generates a file containing the call records from the PBX.

The call records from the PBX are called SMDR, CDR, or CIL.

[edit]

Hosted PBX

A hosted PBX system delivers PBX functionality as a service, available over the Public Service Telephone Network (PSTN) and/or the internet. Hosted PBXes are typically provided by the telephone company, using equipment located in the premises of the telephone company's exchange. This means the customer organisation doesn't need to buy or install PBX equipment (generally the service is provided by a lease agreement) and the telephone company can (in some configurations) use the same switching equipment to service multiple PBX hosting accounts.

The first hosted PBX was introduced to the market in late 1997 by Virtual PBX. Instead of buying PBX equipment, users contract for PBX services from a hosted PBX service provider, a particular type of Application Service Provider (ASP). The first hosted PBX service was very feature-rich compared to most premise-based systems of the time. In fact, some PBX functions, such as follow-me calling, appeared in a hosted service before they became available in hardware PBX equipment. Since that introduction, updates and new offerings from several companies have moved feature sets in both directions. Today, it is possible to get hosted PBX service that includes far more features than were available from the first systems of this class, or to contract with companies that provide less functionality for more simple needs.

In addition to the features available from premises-based PBX systems, hosted-PBX:

Allows a single number to be presented for the entire company, despite its being geographically distributed. A company could even chose to have no premises, with workers connected from home using their domestic telephones but receiving the same features as any PBX user.
Allows multimodal access, where employees access the network via a variety of telecomunications systems, including POTS, ISDN, cellular phones, and VOIP. This allows one extension to ring in multiple locations (either concurrently or sequentially).
Supports integration with custom toll plans (that allow intra company calls, even from private premises, to be dialed at a cheaper rate) and integrated billing and accounting (where calls made on a private line but on the company's behalf are billed centrally to the company).
Eliminates the need for companies to manage or pay for on-site hardware maintenance.

[edit]