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Originally, all Customer Premise Equipment (CPE) was owned by the telephone company and was rented to the end user. The CPE included every device that was connected to the network, including single-line telephones, KTS and PBX systems, and all telephone sets connected behind them. As answering machines and other peripheral devices appeared, they too were rented to the user by the telephone company. In concert, the regulators and telephone companies maintained that any privately-owned equipment could conceivably cause damage to the network switches and even to telephone company personnel by introducing uncontrolled levels of electrical current. Further, such devices might well be incompatible with the highly standardized network and, therefore, might cause uncontrollable disruption. Interestingly, even acoustically-coupled devices were banned, unless provided by Telco. After a series of lengthy legal battles (Chapter 16), the Federal Communications Commission (FCC) reversed that policy through the 1968 Carterphone decision. That decision allowed user ownership through a special interconnecting device, or coupler, which allowed interconnection but protected the network from damage. Currently, virtually all such equipment is customer-owned; in fact and with rare exception, the U.S. telephone companies are barred from owning such equipment. The rest of the world more recently has followed this lead, although there always are exceptions.

Key Telephone Systems (KTSs)

Key Telephone Systems (KTSs) are business communications systems intended for small businesses. Key telephone dates to the beginnings of telegraphy and telephony when mechanical keys were employed to open and close a circuit. The buttons on a key telephone set, used to mechanically open and close the line circuit, were also referred to as keys. While contemporary KTSs provide the same features as small PBXs, and while they also act as contention devices for network access, KTSs are not switches. They do not possess the intelligence to accept a call request from a user station, switch it through common equipment, and determine over which pooled circuit to connect it to the outside world. Rather, the end user must make the determination and select the appropriate facility (e.g., local circuit, WATS circuit or FX circuit) from a group of pooled facilities.

1A1 and 1A2 KTSs

As was the case with early PBXs, KTSs originally were electromechanical in nature. The common control unit, known as a Key Service Unit (KSU), and the key sets were hardwired. Early KTS feature content was limited to hold, intercom (highly limited), speaker phones (separate unit), and auto dialers (mechanical card dialers).

The first standard key system, known as the 1A, was a hardwired system developed by the Bell System and first marketed in 1938. The 1A1 KTS systems, first introduced in 1953, were truly hardwired as well, with components wired together to form a complete system. 1A1 systems added a few features, including line status lamps which lit steadily to indicate a line in use and flashed to indicate a line on hold. 1A2 systems, introduced in 1963, were modular to some extent, as hardwired circuit packs plugged into a pre-built chassis that included cable connectors for attaching station equipment [4-2]. Additionally, a limited number of enhanced features could be added through the addition of common control cards in the form of circuit packs. In either case, wiring between the KSU and the sets required labor-intensive home runs of expensive 25-pair or 50-pair cables between each device and the KSU.

Electronic KTS

Electronic KTSs entered the market in the 1970s, offering many of the same advantages of Electronic Common Control (ECC) as did contemporary PBXs, although they remained intended for a small business environment. Currently, few such systems are manufactured, having been replaced by Hybrid KTSs.

Hybrid Key Systems

Hybrid Key Systems have the unique ability to function as a Key System (direct circuit selection) or as a PBX (switched access to pooled facilities). Additionally, they can function as both, simultaneously, for separate work groups. Just as is the case with contemporary PBXs, Hybrids are digital systems comprising microprocessors, printed circuit boards, memory, software, and so on (see Figure 4.1). They can be upgraded and expanded, within finite limits. Cabling is reduced to that required of PBXs, i.e., 1-4 pair. Hybrid port capacities generally are limited to 256 ports, most of which can be configured as either station ports or trunk ports.


Figure 4.1  Hybrid key system configuration.

Electronic KTSs and Hybrids offer a relatively significant set of features—virtually everything a small business might require. Features include nonblocking switching matrix, call transfer, voice-over paging, authorization codes, ACD, SMDR, voice mail, call park, T/E-Carrier Interface, wireless sets, and display sets. As these features traditionally have been restricted to PBX systems, these will be discussed in some detail later in this chapter. Leading manufacturers include Lucent Technologies (nee AT&T), Nortel, Toshiba, Executone, Comdial and Inter-Tel [4-3].


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