7.2 Historical Perspective

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In the simplest terms, the PBX is a circuit switch that, through control signaling, performs the following three basic functions:

• In response to a call request, establishes end-to-end connectivity among internal users (on-net) and from its own users to other users (off-net) through intermediate nodes, which may consist of other PBXs or central-office (CO) switches on the public telephone network. In both cases, the connected path is dedicated to the users for the duration of the call.

• Supervises the circuit to detect call request, answer, signaling, busy, and disconnect (hang-up).

• “Tears down” the path when a call is terminated (disconnected) so that another user can access the resources available over that circuit.

These functions closely parallel those of the CO switch. In fact, the PBX evolved from operator-controlled switchboards used on the public telephone network. The first of these simple devices was installed in 1878 by Bell Telephone Company to serve 21 subscribers in New Haven, Connecticut. The operator had full responsibility for answering call requests, making the appropriate connections, supervising the line for answers and disconnects, and tearing down the path upon call completion. Interconnectivity among subscribers was accomplished via cable connections at a patch panel.

Today’s PBXs are much more complicated, of course, but they provide the same basic functionality as the first generation of circuit-switching devices. The difference is that the process of receiving call requests, making the appropriate connections, and tearing down the paths upon call completion is now entirely automated and, because the intelligence necessary to do all this resides on the user’s premises, the PBX allows organizations to exercise more control over internal operations and to incorporate communication planning into their long-term business strategies.

In a little more than 125 years, PBXs have evolved from simple patch panels to sophisticated systems capable of integrating voice and data. The generations characterizing the development of PBXs can be summarized as follows:

• First generation PBXs consisted of the operator-controlled patch panels.

• Second generation PBXs evolved from the electromechanical central-office switches: step-by-step (Strowger) and crossbar. Automatic dialing and space division switching are the capabilities that differentiated second from first generation switches.

• Third generation PBXs include the attributes of second generation PBXs. Instead of electromechanical control, however, third generation PBXs use electronic components under stored program control, making possible the addition of many new features, as well as distributed architectures.

• Fourth generation systems are computer-based to permit the integration of capabilities that previously had to be added on with external components, such as automatic call distribution and voice mail. Fourth generation PBXs also use time-division switching, which permits the integration of voice and data over T1 trunks and ISDN services.

• Fifth generation PBXs are more data-oriented, adding LAN and Internet connectivity, plus support for ATM and management via SNMP.

Stored program control, advanced processing power, large-scale integration, and high-capacity memory have not only reduced the cost of PBX ownership, but endowed PBXs with an array of sophisticated features. In fact, among the dozens of products from which to choose, hundreds of different features are offered (see Table 7.1). The interesting thing about all of these features is that they can be implemented right from the telephone keypad.

Table 7.1: Select PBX Features under Direct User Control

Add-on conference	Allows the user to establish another connection while a call is already in progress.
Call forwarding	Allows a station to forward incoming calls to another station. This includes forwarding calls when the station is busy or unattended, or as needed.
Call hold	Allows the user to put the first party on hold so that an incoming call can be answered.
Call waiting	Lets the user know that an incoming call is waiting. While a call is in progress, the user will hear a special tone that indicates another call has come through.
Camp-on	Allows the user to wait for a busy line to become idle, at which time a ring signal notifies both parties that the connection has been made.
Last number redial	Allows users to press one or two buttons on the keypad to activate dialing of the previously dialed number.
Message waiting	Allows the user to signal an unattended station that a call has been placed. Upon return to the station, an indicator tells the person that a message is waiting.
Speed dialing	Allows the user to place calls using an abbreviated number. This feature also allows users to enter a specified number of speed-dial numbers into the main database. These numbers may be private or shared among all users. Entering and storing additional speed-dial numbers is accomplished via the telephone keypad.

Capabilities are also available that promote the efficiency and administration of the PBX (see Table 7.2). Capabilities are distinguished from features in that the former are typically transparent to most users. Whether certain features and capabilities are standard or optional depends, of course, on the vendor. For example, optional or “value-added” capabilities, which require the purchase of adjunct systems and other specialized hardware, as well as software, might include voice mail, a message center, automated attendant station equipment, local area networking, and telecommunication management systems.

Table 7.2: Select PBX Capabilities Transparent to the User

Automated attendant	Allows the system to answer incoming calls and prompts the caller to dial an extension or leave a voice message without going through the operator.
Automatic call distribution (ACD)	Allows sharing of incoming calls among a number of stations so that the calls are served in order of their arrival. This is usually an optional capability, but it may be integral to the PBX, or purchased separately as a standalone system from a non-PBX vendor.
Automatic least-cost routing	Ensures that calls are completed over the most economic route available. This feature may be programmed so that mail room staff always get the cheapest carrier, while executives get to choose whatever carrier they want.
Call detail recording (CDR)	Records information about selected types of calls. This includes the collection of detailed traffic statistics. But additional hardware and software are typically required to arrange this data into meaningful management reports.
Call pickup	Allows incoming calls made to an unattended station to be picked up by any other station in the same trunk group.
Class-of-service restrictions	Controls access to certain services or shared resources. Access to long-distance services, for example, may be restricted by area code or exchange. Access to the modem pool for data transmission over analog lines may be similarly controlled.
Direct inward dialing (DID)	Allows incoming calls to bypass the attendant and ring directly on a specific station.
Direct outward dialing (DOD)	Allows outgoing calls to bypass the attendant for completion anywhere over the public telephone network.
Hunting	Routes calls automatically to an alternative station when the called station is busy.
Music on hold	Serves as a call progress indicator; during peak hours, for example, incoming calls might have to wait in queue until handed off by the ACD to the next available station operator. Music on hold provides assurance to callers that the connection is still active.
Power-fail transfer	Permits the continuance of communication paths to the external network during a power failure. This capability works in conjunction with an uninterruptible power supply, which kicks in within a few milliseconds after detecting a power outage.
System redundancy	Enables sharing of the switching load so that, in the event of failure, another processor can take over all system functions.

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