Evolutionary Stored Program Control and Distributed Processing

11.3.1 Introduction. The presently installed switching plant represents an extremely large investment yet to be fully amortized. The small amount of plant that remains consists of crossbar equipment that is quickly being replaced by digital switches. It was estimated that in 1973, SPC served less than 10% of Bell System subscribers (North America). SPC was just one small step behind a full digital switch. To accelerate SPC implementation, it was common practice to adapt existing plant by modifying crossbar hard-wired control to a computer-controlled installation (SPC) with distributed processing. ITT's Pentaconta 2000 was a step in this direction. (Also see Refs. 8 and 13.)

11.3.2 Modification of Existing Plant. As we mentioned above, the remaining analog plant was crossbar with its inherent distributed control, typically markers, registers, translators, directors, and finders. It was costly to install a large computer in these switches, but cost-effective to replace the markers, registers, and so on, with small microprocessors. This allowed most of the amenities of digital switching without actually being digital. We break these amenities down into two categories: subscriber-related and administration-related. Table 3.2 lists new service features and facilities available to the subscriber, and Table 3.3 lists new service features and facilities administration (telephone company) related.

There were also disadvantages to the implementation of computer control. Electromechanical exchanges were rugged and worked well over wide ranges of temperature, humidity, and vibration. They could withstand high levels of EMI (electromagnetic interference). Not so with solid-state devices. None of the above is valid. Microprocessors, computers, and other solid-state control elements are much more finicky, with heat being a particular problem. In fact it is advisable to keep these devices under air conditioning, vibration, EMI, and humidity control. EMI may turn out to be the worst of them all. Many depend on chopper power supplies which emit high levels of RF noise, often related to the chopping rate.

TABLE 3.2 Service Features and Facilities, Subscriber-Related

Line Classes

Subscriber Facilities

One-party lines

Abbreviated numbering

With dial or push-button-type subscriber sets

One or two digits

With or without special charging categories

For individual lines, or groups

Two-party lines

of lines, or for all lines

With or without privacy, separate ringing, separate

Transfer of terminating calls

charging, revertive call facility

Conversation hold and transfer

Multiparty lines (up to 10 main stations)

Calling party's ring-back

Without privacy

Toll-call offering

With selective or semiselective ringing

Hot line


Automatic wake-up

Unlimited number of trunks

Doctor-on-duty service

Uni- or bidirectional trunks

Do-not-disturb service

With or without in-dialing

Absentee service

Coin-box lines

Immediate time and charge

Local traffic


Toll traffic

Conference calls

Special applications

Centrex facilities (optional)

Restricted lines

Voice mail

To own exchange


To urban, regional, national, or toll areas


To some specified routes

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