Home Office Phone Systems

PBXs make connections among the internal telephones of a private organization - usually a business - and also connects them to the public switched telephone network (PSTN) via trunk lines. Because they incorporate telephones, fax machines, modems, and more, the general term "extension" is used to refer to any end point on the branch.
PBXs are differentiated from "key systems" in that users of key systems manually select their own outgoing lines, while PBXs select the outgoing line automatically. Hybrid systems combine features of both.
Initially, the primary advantage of PBXs was cost savings on internal phone calls: handling the circuit switching locally reduced charges for local phone service. As PBXs gained popularity, they started offering services that were not available in the operator network, such as hunt groups, call forwarding, and extension dialing. In the 1960s a simulated PBX known as Centrex provided similar features from the central telephone exchange.
Two significant developments during the 1990s led to new types of PBX systems . One was the massive growth of data networks and increased public understanding of packet switching. Companies needed packet switched networks for data, so using them for telephone calls was tempting, and the availability of the Internet as a global delivery system made packet switched communications even more attractive. These factors led to the development of the VoIP PBX . (Technically, nothing was being "exchanged" any more, but the abbreviation PBX was so widely understood that it remained in use.)
The other trend was the idea of focusing on core competence. PBX services had always been hard to arrange for smaller companies, and many companies realized that handling their own telephony was not their core competence. These considerations gave rise to the concept of hosted PBX . In a hosted setup, the PBX is located at and managed by the telephone service provider, and features and calls are delivered via the Internet. The customer just signs up for a service, rather than buying and maintaining expensive hardware. This essentially removes the branch from the private premises, moving it to a central location.
A PBX will often include:

• The PBX's internal switching network.
• Microcontroller or microcomputer for arbitrary data processing, control and logic.
• Logic cards, switching and control cards, power cards and related devices that facilitate PBX operation.
• Stations or telephone sets, sometimes called lines.
• Outside Telco trunks that deliver signals to (and carry them from) the PBX .
• Console or switchboard allows the operator to control incoming calls.
• Uninterruptible Power Supply (UPS) consisting of sensors, power switches and batteries.
• Interconnecting wiring.
• Cabinets, closets, vaults and other housings.
• 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 are the size of a small cigar box.
• 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 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.
• Open source projects have been available since the beginning of the 90s. These projects provide flexibility and features (often not needed or understood by average users), plus the 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 PBXs , since they have lowered the entry barrier for new manufacturers.

Residential gateways (often called routers) are frequently used in homes to connect to a broadband service, such as IP over cable or DSL. Such a router may also include an internal DSL modem. Residential gateways and SOHO routers typically provide network address translation and port address translation in addition to routing. Instead of directly presenting the IP addresses of local computers to the remote network, such a residential gateway makes multiple local computers appear to be a single computer. SOHO routers may also support Virtual Private Network tunnel functionality to provide connectivity to an enterprise network..