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Survey of Telephone Switching
Chapter 2

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Nationwide dialing as commonly understood within the telephone industry, means the maximum completion of dialable calls (both local and long distance) by customers, with operators dialing as much as possible of the remainder traffic which cannot be customer dialed. To designate calls dialed by customers to points outside their local or extended service area, the phrase "Direct Distance Dialing" has been adopted by the Bell System, for use in advertising and all other System material directed to customers. Nationwide dialing has been generally accepted as an ultimate industry-wide objective since this method of operation will usually provide the fastest, most accurate, and most dependable telephone service and at the same time should result in overall operating economies. It is essential that any new switching system or addition to existing system be keyed to this objective. Let's briefly discuss Nationwide Dialing, its minimum requirements and some of the reasons for these requirements.

The National Numbering Plan

An essential element of nationwide dialing is a uniform system wherein each local central office will have a unique designation which is, nevertheless, similar in form to that of all other offices connected to the nationwide network. With such an arrangement operators or customers, wherever located, may use that designation as a "destination code" to reach the required office through the dial switching network. All offices, in effect, become a part of one huge multi-office city with each office having its own distinctive identity for routing purposes. In addition to being readily understandable and convenient to use, these designations should be capable of being used accurately, since a mis-dialed call results in uneconomic circuit and switching usage and, more important, may result in a charge to a customer's account if not reported to the operator.

The numbering plan requires a maximum of 10 digits. The first three are the "area code" and the next three are the "office code". Together they comprise the required unique designation for each central office. The remaining four digits constitute the "station number" of the telephones served from the particular office. The 3-digit office code plus the 4-digit station number make up the 2-letter 5-numeral (2-5) customer's number as listed in the telephone directory. The arrangement is shown in Figure 2-1.

The United States and Canada have been divided geographically into numbering plan areas each of which is assigned a distinctive 3-digit designation called the area code. Calls between numbering plan areas (foreign area calls) will, in general, require dialing the code of the area in which the called station is located as well as the called customer's listed telephone number. Home area calls, which originate and terminate within the same area, will ordinarily require dialing only the called customer's listed number which consists of the office code and the station number. In this geographical division into numbering plan areas, borderlines between states and between Canadian Provinces have generally been used as area boundaries. Since, as will be shown later, only about 500 central offices can be served in a numbering plan area without office code conflicts, it was necessary to divide the more populous states and provinces into two or more areas.

In fixing the intrastate numbering plan boundaries of subdivided states, effort was made to avoid cutting across heavy toll traffic routes in order to have as much of the toll traffic as possible terminate in the area in which it originates. Also, wherever possible, the boundaries have been set to avoid having central offices in one area be tributary to toll offices in an adjacent area. With the numbering plan areas arranged in this manner much intrastate dialing can be kept on a 7-digit basis.

The numbering plan "area code" consists of three digits (Figure 2-1). If the middle digit is either a "1" or a "0" the switching equipment will be able to distinguish the area codes from the central office codes, for the latter will always have a letter (corresponding to a numerical digit from 2 through 9) in the middle position. Accordingly, the area codes consist of three digits with either a "1" or "0" in the middle position; e.g., 516, 201, 607, etc.

There are 80 possible combinations with "0" in the middle (called X0X codes), digits "2" to "9" in the first position, and all digits "0" to "9" in the third position. However, only 72 usable "X1X" combinations are available for area codes since "1" may not be used in the third position because such codes as 211, 411, etc., are used in many places for service codes. There are, then, 152 possible area code combination of which more than 100 have been assigned to date. Because of the limited supply, assignment of area codes must be made on the basis of actual needs. Figure 2-2 covers the present area code assignments. Assignments are made from the X0X and X1X series without regard as to whether the areas are entire states or subdivisions of states, although at one time it was thought that such a distinction might be made.

Any one numbering plan area is limited to 500 or fewer central office units. The 2-5 numbering system will theoretically furnish 640 office code combinations (8 x 8 x 10, for letters appear only on dial positions 2 to 9, inclusive), but it is difficult, and in some cases impossible, to find suitable names with initial letters corresponding to certain code combination such as 55, 57, 95, and 97. Accordingly, these combinations are reserved for radiotelephone use. In addition it has been considered desirable to avoid the use of the digit "0" as the third digit of an office code because of possible confusion with the letter O. These factors, together with some other limitations on the availability of office codes, make the practical upper limit of the 2-5 numbering system about 500 codes.

Thousands of central offices do not yet have 2-5 type numbers and it will necessarily be several years before they can all be converted. During this period it may be necessary to employ office codes for operator toll dialing which are unlike those used for local dialing. Nationwide customer dialing is feasible only when the central office code of the called number is an integral part of the listed number. Accordingly, only places using 2-5 numbers will be connected to the nationwide network for customer toll dialing.

Switching Plan

The Telephone Systems in the United States and Canada handle more than seven million toll calls a day. These are routed over a network of more than 100,000 long haul intertoll trunks which interconnect approximately 2,600 toll switching offices and, with a few exceptions, all of the telephones in these two countries.

Large volumes of traffic between toll offices are generally routed economically over direct intertoll trunks. When the volume of traffic is small, however, the use of direct trunks is usually not economical. In these cases the -traffic is then handled by connecting together, by means of switching equipment at intermediate toll offices, two or more intertoll trunks to "build up" the required circuit. "Built-up" connections may involve several intertoll switching points if the originating and terminating points are a great distance apart and the traffic volume is small. Although this multi-switched traffic constitutes only a small portion of the total, it is important that telephone plant be designed to care for it as well as for the greater volume that is handled via the less complex direct and single switch routes.

The conditions under which toll traffic will be automatically switched on a nationwide scale are quite similar to those found in large cities with large volumes of traffic between many separate switching centers. Therefore, experience gained in these places was applied to the nationwide dialing job.

The needs of multi-office exchange areas are met by switching and trunking plans that employ a new principle, "automatic alternate routing", to provide rapid and accurate connections with few occasions for repeated attempts. With this principle, a call which encounters an all trunks busy signal on the first route tested is automatically and rapidly "route advanced" and offered to one or more alternate routes, in sequence.

Office Classification

The general toll switching plan places all switching points into a pattern. Each point is given a number (1 to 5) which indicates its position in this pattern. Figure 2-3 shows this numbering and designation - Class 1 "Regional Center"; Class 2 "Sectional Center"; Class 3 "Primary Center"; Class 4 "Toll Point" or "Toll Center"; Class 5 "End Office".

The general toll switching plan as originally conceived called for one of the regional centers to be designated "National Center" with final trunk groups to and from all regional centers. However, this "National Center" concept has now been abandoned. Instead, final trunk groups will be provided between all regional centers in the United States. (See Figure 2-4). This does not affect the overall flexibility of the plan since it will be possible to route inter-regional traffic via a third regional center on an emergency basis.

Collectively, the Class 1, 2 and 3 offices (Regional Centers, Sectional Centers, and Primary Centers) will constitute the control switching points (CSP's) for nationwide dialing. A control switching point is a key switching location in the nationwide automatic switching network which may have some or all of the features indicated on Figure 2-5.


Through switching toll offices suitable for routing multiswitched traffic to and from the nationwide network will usually have some or all of the following Control Switching Point features:

1. Storing of digits received.
2. Variable spilling - deletion of certain digits when not required for out pulsing.
3. Prefixing of digits when required.
4. Code conversion - a combination of digit deletion and prefixing (also termed substitution).
5. Translation of 3, 4, 5, or 6 digits.
6. Automatic alternate routing.
7. 27 db office balance in offices with 2-wire switching.

It is not necessary that Class 5, 4 or 3 offices home on the next higher ranking office, the complete intermediate final route chain is not necessary. For example, Class 5 offices may be served directly from any of the higher ranking through switching centers. For the general case Figure 2-6 shows the backbone network of "final" intertoll trunk groups, or final route chain, interconnecting the several classes of offices. One final circuit group will always be provided from each office to an office of a higher rank. That one higher ranking switching point to which an office is connected over a final group is called its "home" office; the dependent office is spoken of as "homing" on it. The network of final trunk groups will be engineered on a low delay basis so that, on the average, not more than three calls in each hundred that are offered to such a trunk group in the busy hour will find all circuits busy.

Since the general toll switching plan will make extensive use of alternate routing, the flow of traffic, in many ways, will be different from what it would be 'if present routing methods were continued. The present layout of intertoll trunks may not then be the most advantageous. In a particular cross section of a toll route, the number of intertoll trunks between given termination is likely to be changed appreciably. Furthermore, it is probable that the number of intertoll trunks terminated at a toll center designated as a CSP will be considerably different under the nationwide dialing plan from what it would be if the present trunk estimates were projected assuming continuation of the present method of operation.

Figure 2-7 makes a comparison of intertoll trunk networks theoretically required (a) with the limited switching under ringdown operation and, (b) with operator and customer toll dialing utilizing common control equipment and full automatic alternate routing at the principle switching centers. It is apparent that there are possibilities for substantially reducing the number of intertoll trunk groups. Alternate routing tends to reduce the size of, or eliminate, direct groups and to concentrate the intertoll trunks in larger and more efficient groups which generally can be provided at lower cost per trunk mile.

Interoffice Signals

Nationwide dialing by customer introduces a basic problem of insuring proper audible signals to customers when equipment is blocked by "trunk busy" and "line busy" conditions. The original design of the toll switching system assumed that calls would always require an originating operators Accordingly, the signals that originate in toll offices consist of flashes without tone. Plans are now being formed to eliminate the toll trains and flashes from the terminating office. All calls will terminate over the local train which provides tone only. This will change the operating practices and will be introduced when the amount of customer dialing justifies it. Until this plan is placed in effect, the toll trains will continue to be used. This makes it necessary to add tone to the toll train signal or use tone appliers. A factor in favor of using a tone applier is that the passage of signals over toll lines involves predominately "in band" single frequency signaling equipments. There is a difference in transmission time between tone and flash signals so that the tone may get out of step with the flash when the toll connection consists of one or more toll lines equipped for SF signaling. The cumulative effect is a suppression or mutilation of some of the tone signals returning to the calling customer.

Switching Equipment

Nationwide dialing places no restriction on the type of dial switching system (SxS, XB, etc.) provided at Class 4 or Class 5 offices. Common control equipment such as registers, senders, directors, etc., is not essential at these offices although it may be used in many instances to effect economies in switching traffic and to provide uniform dialing procedures. Class 1, 2 and most Class 3 offices will employ common control switching facilities and have control switching point (CSP) features.

Outward direct distance dialing requires that the Class 5 office be able to send the complete 7 or 10-digit called number to the toll switching center on which it homes. Common control switching equipment can be arranged to do this. If direct control equipment such as-step-by-step without senders or directors is used at the Class 5 office, it will be necessary to prefix a toll access or transfer code to direct the call to the toll office. An alternate arrangement might provide for the simultaneous registration of digits at both the local and toll offices. At the toll office, common control equipment or its equivalent will store the digits and transmit them forward.

The minimum number of digits that Classes 3, 4C, 4P and 5 offices should be capable of receiving over incoming intertoll or toll connecting trunks is shown in Figure 2-8. In general, the 2-5 number will be sent as far as the toll office upon which the Class 5 office "homes" so that machine switched traffic may be combined with operator dialed traffic received over direct trunk groups from distant toll switchboard positions (operators normally dial the full 2-5 listed numbers).

Adequate intercepting facilities should be provided for unassigned and non-working customer numbers as well as for unassigned central office codes (vacant levels in SxS offices). Where presently provided, "no-such number" tone may be connected to vacant selector levels as an interim measure until standard arrangements can be provided. The use of busy or audible ring back tones for such purposes or the return of no tone at all is considered unsatisfactory.

To avoid false charging on nationwide dialed traffic, intercepting equipment should be arranged so that it will: (a) not return answer supervision, (b) not differentiate between local and toll calls, and (c) not recall originating operator (flashing key should not be provided).

Automatic Recording of Message Billing Data for Customer Dialed Calls

Customer dialing of station-to-station sent-paid messages requires that certain data be obtained automatically in order that chargeable calls may be billed. The industry has developed a number of systems for automatically recording the required message billing data. These may be broadly divided into "local" and "centralized" systems. Local automatic message billing data recording systems have the recording equipment located at the Class 5 office where the extra charge calls are originated. Centralized automatic message billing data recording systems are those in which the recording equipment is installed at a centralized location so that extra charge calls from a number of Class 5 offices may be concentrated and recorded there. Centralized recording may prove economical when local recording equipment in individual Class 5 offices can not be justified.

Present plans contemplate the use of centralized recording equipment at Class 4 or higher ranking toll offices. Extra charge traffic from Class 5 offices not equipped with local recording facilities, and extra charge traffic that can not be served by local recording equipment, when provided, may be trunked to the centralized location where the required billing data can be recorded.

In addition to the central office equipment mentioned above, suitable accounting equipment should be provided as required, to process the message billing data into the form needed for billing the customer.

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