Hosted PBX or Virtual PBX
A hosted PBX system delivers PBX functionality as a service, available over the Public Service Telephone Network (PSTN) and/or the internet. Hosted PBXs 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.
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 choose 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.
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.
However, recent open source projects (in particular, VoiceLogix, Asterisk and SIPfoundry) combined with cheap modern hardware are sharply reducing the cost of PBX ownership.
For some users, the Private Branch eXchange has gone full circle as a term. Originally having started as an organization's manual switchboard or attendant console operated by the telephone operator or just simply the operator, they have evolved into VoIP centers that are hosted by the operators or even hardware manufacturers. These modern IP Centrex systems offer essentially the same service, but they have moved so far from the original concept of the PBX that the term hardly applies at all.
Even though VoIP gets a great deal of press, the old circuit switched network is alive and well, and the already bought PBX's are very competitive in services with modern IP Centrex's. So currently we have 4 distinct scenarios in use:
PBX (Private and Circuit Switched)
Hosted/Virtual PBX (Hosted and Circuit Switched) or traditional Centrex
IP PBX (Private and Packet Switched)
IP Centrex or Hosted/Virtual IP (Hosted and Packet Switched)
Since in reality people want to call from the IP side to the circuit switched PSTN (SS7/ISUP), the hosted solutions usually have to maneuver in both realms in one way or another. The distinctions are seldom visible to the end user.
Home and Small Business Usage
Historically, the expense of full-fledged PBX systems has put them out of reach of small businesses and individuals. However, since the 1990s there has been a large set of small, consumer-grade and consumer-size PBXs available. These systems are not comparable in size, robustness or flexibility to commercial-grade PBXs, but still provide a surprising set of features.
The first consumer PBX systems were for the analog telephone systems, typically supporting four private analog and one public analog line. They have the size of a small cigar box or smaller and are inexpensive (e.g. US$50).
Particularly in Europe these systems for analog phones were followed by consumer-grade PBXs for ISDN. Using small PBXs for ISDN is a logical step, since already the Basic Rate Interface of ISDN (which is the phone interface individuals and small businesses typically get) provides two logical phone lines (two B channels) which can be used in parallel. Small, entry-level systems are also extremely cheap (e.g. US$100).
With the pickup of VoIP by consumers, of course consumer VoIP PBXs have seen the light, and PBX functions have become simple additional features of consumer-grade routers and switches.
Proponents of recent open source projects (in particular, Asterisk and SIPfoundry) claim that their initiatives have finally brought PBXs within the reach of individuals and small businesses. However, some affordable off-the-shelf solutions have been available since the beginning of the 90s. The mentioned open source projects provide more flexibility and more features (often not needed or understood by average users) on standard hardware platforms, plus means to actually inspect and change the inner working of a PBX. They have also opened business opportunities for newcomers to the market of mid-size PBX, since they have lowered the entry barrier for new manufacturers .
History
Historically a PBX was a company's manual switchboard, and 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 PMBXs are almost unheard of these days, the terms PABX and PBX have become synonymous. When PABXs were built using solid state and digital components the term EPABX came into use but PBX is still more widely recognized. The PBX term was dominant for so long, that it is now being applied very liberally to systems providing complex services regarding telephony, even if they are not Private, Branches or eXchanging anything.
PBXs were 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 the escape code (usually 9, 0 in some systems) followed by the external number; an outgoing trunk line is automatically selected upon which to complete the call. This practice is becoming obsolete as number analysis systems have been changing with modern technology - you can use internal numbers or external numbers without any need for escape codes.
Overview
The Private Branch eXchange has gone full circle as a term. Originally having started as an organization's manual switchboard (operated by a person plugging cables into sockets), they have evolved in to VoIP centers that are hosted by the operators or even hardware manufacturers. These modern IP Centrex systems offer essentially the same service, but they have moved so far from the original concept of the PBX that the term hardly applies at all.
PBXs are differentiated from "Key Systems" in that users of Key systems manually select their own outgoing lines while PBXs select the out going line (trunk) automatically. Hybrid Systems combine features of both.
Initially, it was private since in general it was owned by a company, which wanted to have its own branch to save some money in, for example, internal calls. This was done by having the exchanging or switching of circuits done locally, inside the company. Because fax machines, modems (though the PBX may degrade line quality) and many other communication devices can be connected to a PBX, extension developed to a generic term describing all devices connected to a PBX. The PBX's are connected to the outside world by a number of lines called trunk lines.
After the PBX business took off, PBX's started offering services that were not available in the operator network to small businesses and home users. The status quo was changed after several decades by two significant developments during the 1990's. One was the massive growth of data networks (and the concept of packet switching entering public consciousness) and the other one was the trend of focusing on your core competence. Data networks meant that companies had to have packet switched networks anyway, so putting the telephone calls there was tempting as it was. The Internet - and its low price on global communications - was the final straw, and the VoIP PBX was created. Technically nothing was being eXchanged anymore, but the term PBX was very well recognized, and there apparently were no major pushes for a different acronym.
On the other hand, most companies realized that handling their own telephony was in fact not their core competence. Not to mention, PBX services had always been hard to arrange for smaller companies. These combined to create the concept of Hosted PBX, where the PBX was actually managed by the telephone company, and the customer company just had to sign up for a service rather than buy an expensive piece of hardware and then maintain it. This essentially removed the branch from the private premisis, making it centrally managed.
Even though VoIP gets a great deal of press, the old circuit switched network is alive and well, and the already bought PBX's are very competitive in services with modern IP Centrex's. So currently we have 4 distinct scenarios in use:
PBX (Private and Circuit Switched)
Hosted/Virtual PBX (Hosted and Circuit Switched) or traditional Centrex
IP PBX (Private and Packet Switched)
IP Centrex or Hosted/Virtual IP (Hosted and Packet Switched)
Since in reality people want to call from the IP side to the Circuit Switched (SS7/ISUP), the hosted solutions usually have to maneuver in both realms in one way or another. The distinctions are seldom visible to the end user.
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 calling features and capabilities, with different manufacturers providing different features in an effort to differentiate their products. Common capabilities include (manufacturers may have a different name for each capability):
Auto Attendant
Automatic call distributor
Automated directory services (where callers can be routed to a given employee by keying or speaking the letters of the employee's name)
Automatic ring back
Call accounting
Call forwarding on absence
Call forwarding on busy
Call park
Call pick-up
Call transfer
Call waiting
Camp-on
Conference call
Custom greetings
Customised Abbreviated dialing (Speed Dialing)
Direct Inward Dialing
Direct Inward System Access (DISA) (the ability to access internal features from an outside telephone line)
Do Not Disturb (DND)
Follow-me
Music on hold
Night service
Shared message boxes (where a department can have a shared voicemail box)
Voice mail
Voice paging (PA system)
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 is 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.
RBS - (Robbed bit signaling) - delivers 24 digital circuits over a four-wire (T1) interface.
Internet Protocol - H.323, SIP, MGCP, and Inter-Asterisk eXchange 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 then 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.
Mobile PBX
A mobile PBX is a hosted or virtual PBX service that extends fixed-line PBX functionality to mobile devices such as cellular handsets, smartphones and PDA phones by provisioning them as extensions. Mobile PBX services also can include fixed-line phones. Mobile PBX systems are different from other virtual PBX systems that simply forward data or calls to mobile phones by allowing the mobile phone itself, through the use of buttons, keys and other input devices, to control PBX phone functions and to manage communications without having to call into the system first.
