Volume Ii Part 17 (1/2)

[Ill.u.s.tration: Fig. 436. Toll Distributing Frame and Harmonic Converters]

In manual practice it is necessary to place the distributing frames and power apparatus in a separate room from that containing the switchboard, but in an automatic exchange no such necessity exists; in fact, so far as the distributing-frame equipment is concerned, it is considered desirable to have it located in the same room as the automatic switches.

The battery room in an automatic exchange should be entirely separate from the operating room, since the fumes from the battery would be fatal to the proper working of the automatic switches.

_Typical Automatic Office._ The floor-plan and views of a medium-sized automatic office at Lansing, Michigan, have been chosen as representing typical practice. The floor plan is shown in Fig. 429. The apparatus indicated in full lines represents the present equipment, and that in dotted lines the s.p.a.ce that will be required by the expected future equipment.

In Fig. 430 is shown a group of five line-switch units, representing a total of five hundred lines. The length of such a unit is practically fourteen feet and the breadth over all about twenty-two inches.

Fig. 431 shows a general view of this Lansing office, taken from a point of view indicated at _A_ on the floor plan of Fig. 429. Fig. 432 shows the main distributing frame, which is of ordinary type; Fig. 433 shows a closer view of some of the primary line switches; Fig. 434 is a view of the secondary line switches and first selectors, the latter being on the right; Fig. 435 is a view of the frequency selectors and second selectors, the former being used in connection with party-line work; and Fig. 436 is a view of the toll distributing frame and harmonic converters for party-line ringing.

A general view of the main switching room in the Grant Avenue office of the Home Telephone Company of San Francisco is given in Fig. 437, this being taken before the work of installation had been fully completed.

The present capacity of the equipment is 6,000 and the ultimate 12,000 lines. This office is one of a number of similar ones recently installed for the Home Telephone Company in San Francisco, the combination of which forms by far the largest automatic exchange yet installed. The scope of the plans is such as to enable 125,000 subscribers to be served without any change in the fundamental design, and by means merely of addition in equipment and lines as demanded by the future subscriptions for telephone service.

[Ill.u.s.tration: Fig. 437. Grant Avenue Office--San Francisco]

CHAPTER x.x.xIV

PRIVATE BRANCH EXCHANGES

=Definitions.= A telephone exchange devoted to the purely local uses of a private establishment such as a store, factory, or business office, is a private exchange. If, in addition to being used for such local communication, it serves also for communication with the subscribers of a city exchange, it becomes in effect a branch of the city exchange and, therefore, a private branch exchange. The term ”P. B. X.” has become a part of the telephone man's vocabulary as an abbreviation for private branch exchange.

Private exchanges for purely local use require no separate treatment as any of the types of switching equipments for interconnecting the lines for communication, that have been or that will be described herein, may be used. The problem becomes a special one, however, when communication must also be had with the subscribers of a public exchange, since then trunking is involved in which the conditions differ materially from those encountered in trunking between the several offices in a multi-office exchange.

For such communication one or more trunk lines are led from the private branch office usually to the nearest central office of the public exchange and such trunks are called private branch-exchange trunks. They are the paths for communication between the private exchange and the public exchange. For establis.h.i.+ng the connections either between the local lines themselves or between the local lines and the trunks, and for performing other duties that will be referred to, one or more private branch-exchange operators are employed at the switchboard of the private establishment.

The private branch exchange may operate in conjunction with a manual or an automatic public exchange, but whether manual or automatic, the private exchange is usually manually operated, although it is quite possible to make a private branch exchange that is wholly automatic and will, therefore, involve no operator at all.

=Functions of the Private Branch-Exchange Operator.= It is possible, as just stated, entirely to dispense with the private branch-exchange operator so far as the mere connection and disconnection of the lines is concerned. But the real function of the private branch-exchange operator is a broader one than this and it is for this reason that even in connection with automatic public exchanges, operators are desirable at the private branches. The private branch-exchange operator is, as it were, the doorkeeper of the telephone entrance to the private establishment. She is the person first met by the public in entering this telephone door. There is the same reason, therefore, why she should be intelligent, courteous, and obliging as that the ordinary doorkeeper should possess these characteristics.

As to incoming traffic to a private branch exchange, an intelligent operator may do much toward directing the calls to the proper department or person, even though the person calling may have little idea as to whom he desires to reach. This saves the time of the person who makes the call as well as that of the people at the private branch stations, since it prevents their being unnecessarily called.

The functions of the private branch-exchange operator are no less important with respect to outgoing calls. It is the duty of the operator to obtain connections through the city exchange for the private branch subscriber, who merely asks for a certain connection and hangs up his receiver to await her call when she shall have obtained it. This saving of time of busy people by having the branch-exchange operator make their calls for them has one attending disadvantage, which is that the person in the city exchange who is called does not, when he answers his telephone, find the real party with whom he is to converse, but has to wait until that party responds to the private branch operator's call.

This is akin to asking a person to call at one's office and then being out when he gets there. This drawback is greatly accentuated where both the parties that are to be involved in the connection are people high in authority in certain establishments at private branch exchanges. Some business houses have made the rule that the private branch operator shall not connect with their lines until she has actually heard the voice of the proper party at the other end. When two subscribers in two different private branch exchanges where this rule is enforced, attempt to get into communication with each other, the possibilities of trouble are obvious.

All that may be said on this matter is that the person who calls another by telephone should extend that person the same courtesies that he would had he called him in person to his office; and that a person who is called by telephone by another should meet him with the same consideration as if he had received a personal call at his office or home. The arbitrary ruling made by some corporations and persons, which results always in the ”other fellow's” doing the waiting, is not ethically correct nor is it good policy.

=Private Branch Switchboards.= Private branch switchboards may be of common-battery or magneto types regardless of whether they work in conjunction with main office equipments having common-battery or magneto equipments. Usually a magneto private branch exchange works in conjunction with a magneto main office, but this is not always true.

There are cases where the private branch equipment of modern common-battery type works in conjunction with main office equipment of the magneto type; and in some of these cases the private branch exchange has a much larger number of subscribers than the main office. This is likely to be true in large summer resort hotels located in small and otherwise unimportant rural districts. In one such case within our knowledge the private branch exchange has a larger number of stations than the total census population of the town, resulting in an apparent telephone development considerably greater than one hundred per cent.

_Magneto Type._ Where both the private branch and the main office equipments are of the magneto type, the private branch requirements are met by a simple magneto switchboard of the requisite size, and the trunking conditions are met by ring-down trunks extending to the main office. In this case the supervision is that of the ordinary clearing-out drop type, the operators working together as best they may.

_Common-Battery Type._ The cases where the private branch board is of common-battery type and the main office of magneto type are comparatively so few that they need not be treated here. Where they do occur they demand special treatment because the main portion of the traffic over the trunk lines to the city or town central office is likely to be toll traffic through that office over long-distance lines.

The princ.i.p.al reason why the equipment of the town offices under such conditions is magneto rather than common battery is that the traffic conditions are those of short season and heavy toll, and common-battery switching equipment at the main office has no especial advantages for toll work.

[Ill.u.s.tration: Fig. 438. Desk Type, Private Branch Board]