1.2. Key Systems and PBXs
As the telephone became more important in the business world, innovators extended its capabilities and made it more convenient . They did so using enterprise telephony devicesgateways that connect privately owned phones together into a private voice network with self-managed calling features. When the enterprise gained ownership over its own voice network, it set about building telephony applications specific to its business.
One such device is a KTS, or key telephone system. In many small businesses, telephones can share a group of telephone company POTS lines through the use of a KTS. Each phone in a key system has direct access to one or more of the telephone company's lines, just as a simple residential phone has access to a single line. Unlike a single-line phone setup, KTSs allow a group of phones to use more than one telephone line at a time. This allows a single operator to place a call on hold while answering a call on another line, among other things, without using any phone company calling features. Generally, KTSs are not referred to as switches, because they rely upon the circuit-switching abilities of the central office in order to connect calls.
In many larger offices, telephones connect to a private, on-premises switch that interfaces with the telephone company's lines. This switch is called a PBX, or private branch exchange. PBXs are smaller, enterprise-friendly versions of the heavy-duty switches used by the telephone company, and they allow businesses to run their own telephony applications in-house. Unlike with a key system, PBX phones in the office can call other phones in the office without tying up an external telephone line. So several simultaneous conversations between parties in the same office can occur without making use of the PSTN at all. One job of the PBX is to determine how to "route" callsthat is, how to ascertain whether the calling party is trying to reach another person within the same office or trying to reach somebody via the PSTN. Most PBX vendors refer to the call-routing scheme as the dial-plan .
Single-line phones, key systems, and PBX systems all connect to the PSTNbut for various reasons. Single-line phones and key systems connect calls to the PSTN even if they are from one phone to another in the same office, but a PBX connects calls to the PSTN only if they are bound for an outside organization.
Despite their various capabilities, POTS, KTSs, and PBXs are all based on the same circuit-switching, electromagnetic-signaling technologies. Even POTS' higher-capacity digital cousins, ISDN and T1, which are able to squeeze many simultaneous phone calls onto a single copper loop, are members of the same technology family. All the standards that govern these traditional telephony systems stem from the International Telecommunications Union (ITU), and many have been unchanged for decades because they are incredibly reliable. They'd have to be reliable in order to run the global telephone system, wouldn't they?
1.2.1. Lines and Trunks
Connections from the phone company switch, like the ones that connect to residential analog telephones, are called lines . POTS, Centrex, ISDN, and T1 connections are referred to as lines when their purpose it to deliver dial-tone to a phone or group of phones.
But when the connections deliver services to a PBX, they are referred to as trunks . Trunks are phone company lines that run from one switch to another, so a connection from the central office switch to a PBX switch would be called a trunk. Connections between two PBX switches are called trunks, and so are connections from one central office switch to another, coincidentally.