(DDD) Is a Term Used to Designate Calls Dialed by Customers to Points Outside Their Local Or Extended Service Area

CHAPTER 11

DIRECT DISTANCE DIALING

11.1 INTRODUCTION Direct Distance Dialing (DDD) is a term used to designate calls dialed by customers to points outside their local or extended service area. When such calls are dialed by operators, the phrase "Operator Distance Dialing" is used. Distance Dialing (nationwide dialing) has been accepted as an ultimate objective of the telephone industry, since it will usually provide the fastest, most accurate, and most dependable telephone service and at the same time give over-all operating economies. The direct distance dialing plan might more appropriately be referred to as "continentwide" rather than "Nationwide" since it provides for the handling of long distance traffic both within and between the United States and Canada and to Alaska and Hawaii.

Successful operation of the DDD program depends primarily on three factors: 1. A standard nationwide numbering plan. 2. A fundamental plan for automatic toll switching. 3. A method of charging the customer. The first two requirements will be considered further in this chapter. The third requirement is met by automatic message accounting systems which are covered in Chapter 14. 11.2 THE NATIONAL NUMBERINGPLAN A primary requisite for distance dialing is that each customer be assigned a distinctive telephone number that does not conflict with the number of any other customer in the United States and Canada. It is essential that these numbers be similar in form, convenient to use, and compat ible with local and extended area dialing arrangements. This is accomplished by giving each local central office 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,

11.1 1

:nave

one

avoid

code

•

2000.

are

area

The

maximum

to switch of 3

equip

use,

area

own each

the

which

remaining

year

"destination

designations to

required telephone telephone

office

dial

part

when

its

divided •

the

The

2-letter

the The the the

the

operator the minimum.

PLAN

called

(foreign switching required

each

been

3-digit the

DIALING

having

and

Station N N N N Number

and

become

beyond

except

when

convenient

1947

Z up

office.

The

9 9 Number 9 areas 9

have

through

relations. call

comprise

areas,

in listed

and last

purposes.

code"

number" to to to to NUMBERING

office

calls

make

designation

0 0 0

designation

plan

DISTANCE

effect,

they

must

plan

circuits

Canada

office

central

each

"area

office.

number

Code

that

customer

adopted

Directory from L L N Office from from from 1

11.2

routing

INITIAL

number

except

and

DIRECT

the

each

3-digit

use

with misdialed

call.

for

expected

plan

numbering

the"S'tation

Together,

numbering

required

of subscriber

for are

11-1

Listed

can

offices,

the

savings understandable

letter

numeral States numeral numeral numeral

city improved

station was

(zero)

customer's

particular

the

into

CH.

for

All

0 Any Any

Any Any Any

and

TABLE

Code

Area

between

identity

number

the digits code."

= = =

= =

been

numbering

distinctive

United

readily (2-5)

call 3

located,

reach

constitute 4-digit

X X' N a

the

has

from

designation The

Calls

The

charged as·well

digits

multioffice the

network.

first

"office

Where

digits

Notes:

ing

code"

distinctive reduced result

wherever huge selected,

the 5-numeral misdialed 4 ment, of being

The unique served

directory.

plus code.

assigned geographically CH. 11 - DIRECT DISTANCE DIALING

calls) 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, require dialing only the called customer's listed number, which consists of the office code and the station number. In this geographical division of numbering plan ar~as,border lines between states and between Canadian Provinces have generally been used as area boundaries. Since, as will be shown later, only about 540 central offices can be served in a numbering plan area, 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 to have as much of the toll traffic as possible terminate in the originating area. 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 this way, much intrastate dialing can be kept on a 7-digit basis. As shown in Table 11-1, the numbering plan "Area Code" consists of three digits. 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 XOX codes), digits "2" to "9" in the first position, and all digits "0" to "9" in the third position. It is not practical to use either "1" or "0" for the initial digit of the area code since many customers dial "0" to reach the operator, and an initial "1" is either used for service or toll codes, or the local dial equipment is arranged to ignore it since it may be a preliminary pulse. Only 72 usable "XlX" 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 combinations of which more than 132 have been assigned. Because of the limited supply, assignment of

11.3 CH. 11 - DIRECT DISTANCE DIALING

area codes must be made on the basis of actual needs Tables 11-2 and 11-3 show the present and proposed number ing plan areas. Assignments are made from the XOX or XIX series without regard 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 about 540 central office codes. The 2-5 Numbering System will theoretically furnish 640 office code combinations. (Eight dial pulls for the first digit times eight dial pulls for the second digit times ten for the third digit; only eight holes on the dial have letters.) In practice, however, there are only about 60 usable letter combinations for the first two digits instead of 64. This is because of the difficulty in finding names to fit the dial pulls 5-5, 5-7, 9-5, 9-7 (5 corresponds to JKL, 7 to PRS, and 9 to WXY). Also, since there is considerable confusion between the letter "0" and the numeral zero, the latter is usually avoided in the central office designation. This leaves us with 60 usuable letter combinations multiplied by nine numerals for a total of 540 codes. The use of all-numerical central office codes, when preceded by "1" prefix dialing arrangements, increases the number of codes from a theoretical maximum of 640 of the 2L-5N type (about 540 generally usable) to 792.(8 x 10 x 10 minus 8 {Nll service codes) = 792.) The 55, 57, 95 and 97 series, for which no suitable names are available, can be assigned on an all-numeral basis. However, ten of these codes (950-954 and 975-979) are reserved for possible future use for new services. It is planned to change to another series of codes in the future. The 55 office code is reserved for use as the code for a universal distant information number. Also, 844 is reserved for time service, and 936 for weather service. With the above Name-Numeral Plan (ZL, 5N), a shortage of numbers arose. Eighty-six of the 152 area codes were assigned when operator nationwide dialing was started in 1947. The remaining 66 area codes were expected to care for many years of growth. However, there has been a telephone explosion in the United States. In the decade 1950 to 1960, while the United States population grew from about 150 million to 180 million people, telephones grew from about 41 million to over 75 million. The rise in population of 30 million people required the addition of 34 million telephones.

11.4 Island

and

(U.S.)

Islands

Mexico

and

WATS

Edward

TWX

Carolina

Location

TWX

Carolina

TWX

Scotia

York

4-Row

California Inward

Newfoundland

Pennsylvania

New Utah Indiana Unassigned Unassigned Illinois Texas Wisconsin Florida

NUMBER 4-Row Caribbean Florida Vermont South California Texas Unassigned Ontario Bermuda

4-Row Hawaii Pennsylvania

Unassigned

Prince Texas Nova Quebec Northwest Kansas Tennessee Unassigned Georgia Missouri Unassigned

.Iowa California Oklahoma

Texas Unassigned North New Michigan Alaska

715 709 710 712 713

716

717 718 719 714

800 806 801 802 803 804 805

807 813 814 815 CODE 810 812 819

816 817

Code

901 902

Area 906

903 904 910 917 905 912 919 914 908 915 909 918

916

913

*808

*809

*907

DIALING

AREA

)

•

only.

(U.S

(Canada)

11-2

DISTANCE

Columbia

CODES-BY

TWX Dakota

Dakota

TWX

Carolina dialing

Location

11.5

Brunswick

Mexico

York

Hampshire Jersey

York

AND

TABLE

Row

DIRECT

Iowa

Kentucky 4- Louisiana

Ohio Unassigned Missouri

New New Oregon Quebec Arkansas Ohio

4-Row Texas Minnesota New Illinois Washington

British New Ontario New Quebec New Mississippi Arizona Michigan South Ohio

North Kentucky New Nevada Wisconsin Tennessee California Unassigned Unassigned Ontario Unassigned Michigan Minnesota

North Ontario Virginia Massachusetts

•

AREAS

operator

501 502

417 418 419 503 504 505 512 S06 5.07 508 509 510

514 515 516 517 518 513 519

Code 601 607 602 608 603

706 Area 615 701 707 609 702 708 604 610 617 703 605 612 704 606 613 705 614 616 618 - 619

CH.

PLAN

customers

Columbia

NUMBERING

Location

Island

Virginia

Jersey

York

Idaho Connecticut

District New

Illinois Illinois Indiana California Indiana Illinois Iowa California Texas Pennsylvania Delaware

Louisiana Washington Nebraska Maine New Rhode Kansas Manitoba Ohio Alabama Saskatchewan California Colorado Florida

New Nebraska Missouri Georgia Unassigned Maryland Wyoming Alberta Unassigned California Minnesota West Pennsylvania Onta!'io Michigan Oklahoma Montana

reachable

Massachusetts Wisconsin

202

203

206 201

204 205 214 207 215 208 216 209 217 301 212 218 213

306 314 307 315 302 308 219 309 304 318 305 313 Area 316 303 312 406 319 401

~ 317 414 407 415 402 408 403 409 404 412 405 413

416

*Not

506 506

809 809

709 709

306 306

514 514 819

705 705

418 418

519 519 902 902

403 403

807 807 903 903

604 604

715 715

307 307

703 703

304 304

206 206

509 509

416 416

613 613

713 713 414 414

214 214

512 512

802 802

801 801

817 817

608 608

806 806

915 915

.204 .204

615 615

901 901

Code Code

Area Area

Mexico Mexico

Columbia Columbia

Area Area

and and

Scotia Scotia

Brunswick Brunswick

Virginia Virginia

LOCATION LOCATION Numbering Numbering

Plan Plan

Northwest Northwest

New New

Nova Nova

Newfoundland Newfoundland

British British

Saskatchewan Saskatchewan Quebec Quebec

Manitoba Manitoba

Ontario Ontario

Quebec Quebec

Ontario Ontario Alberta Alberta

Ontario. Ontario.

Quebec Quebec

Caribbean Caribbean

Bermuda Bermuda

Mexico Mexico

Canada Canada

Utah Utah Texas Texas

Virginia· Virginia· Wisconsin Wisconsin

Vermont Vermont Wisconsin Wisconsin

Wisconsin Wisconsin

Texas Texas

Tennesse.e Tennesse.e Texas Texas

Wyoming Wyoming

Texas Texas

Tennessee Tennessee

Texas Texas

West West

Washington Washington

· ·

DIALING DIALING

717 717

803 803

814 814 215 215

216 216 513 513 401 401

405 405

503 503

412 412

605 605

701 701

704 704

419 419 614 614

716 716

918 918

212 212

7()2 7()2 315 315

308 308

201 201 505 505

919 919

402 402

516 516

607 607

518 518

603 603

914 914 406 406

218 218

417 417

609 609

507 507

816 816

314 314 616 616

601 601

517 517 313. 313.

906 906

61Z 61Z

617 617

Code Code

Area Area

GEOGRAPHICAL GEOGRAPHICAL

· ·

11-3 11-3

DISTANCE DISTANCE

11.6 11.6

Area Area

Island Island

Carolina Carolina

Dakota Dakota

Carolina Carolina

Dakota Dakota

Carolina Carolina

CODES-BY CODES-BY

Jersey Jersey

Hampshire Hampshire York York

York York

Mexico Mexico

York York

York York Jersey Jersey

York York

TABLE TABLE

Plan Plan

Numbering Numbering

DIRECT DIRECT

AND AND

---

Pennsylvania Pennsylvania

Rhode Rhode

Pennsylvania Pennsylvania

South South Pennsylvania Pennsylvania

South South

Pennsylvania Pennsylvania

Oklahoma Oklahoma

Ohio Ohio

Oregon Oregon Ohio Ohio

North North

Ohio Ohio North North

New New

North North

New New New New

New New New New Nebraska Nebraska

New New

New New New New

New New Nevada Nevada

Nebraska Nebraska

Missouri Missouri Michigan Michigan

Missouri Missouri

Minnesota Minnesota

Montana Montana Minnesota Minnesota

Missouri Missouri

Michigan Michigan

Michigan Michigan Mississippi Mississippi

Michigan Michigan

Massachusetts Massachusetts

Minnesota Minnesota

11 11

AREAS AREAS

CH. CH.

207 207

301 301

219 219

712 712

504 504

413 413

318 318

515 515

319 319

502 502

800 800

316 316 317 317

812 812

208 208

203 203

815 815 808 808

312 312

217 217 913 913

309 309

209 209

202 202

305 305

813 813 404 404 714 714

805 805 912 912 707 707 302 302 618 618

SOl SOl

213 213

904 904 303 303

415 415

408 408

916 916

602 602

205 205

907 907

.606 .606

Code Code

Area Area

PLAN PLAN

NUMBERING NUMBERING

Columbia Columbia

Area Area

of of

WATS WATS

Plan Plan

Numbering Numbering

Louisiana Louisiana Iowa Iowa

Iowa Iowa

Inward Inward

Indiana Indiana

Louisiana Louisiana

Illinois Illinois Iowa Iowa

Illinois Illinois

Indiana Indiana

Kentucky Kentucky

Illinois Illinois

Maine Maine Kansas Kansas

Illinois Illinois

Idaho Idaho

Massachusetts Massachusetts

Maryland Maryland Kansas Kansas

Illinois Illinois

Florida Florida

District District

Georgia Georgia

Delaware Delaware

Connecticut Connecticut

Colorado Colorado

Hawaii Hawaii

California California

California California Arkansas Arkansas

Alabama Alabama

Arizona Arizona Alaska Alaska

the the

will will

in in

have have

that that

Gateway Gateway

because because

2000, 2000,

data data

in in

to to

numbers numbers

PBX PBX

number number

that that

expected expected

million million

beJtan beJtan

service service

Each Each

per per

year year

twebre twebre

Kingdom Kingdom

military, military,

was was

Plains Plains

needed needed

600 600

equal equal

Federal Federal

additional additional

services services

the the one one

industry industry

which which

an an

appears appears

telephone telephone

so so

extension extension

with with

plan plan

the the

new new

by by

United United

White White

DIALING DIALING

it it

than than

signaling signaling

service, service,

network. network.

PBX PBX willbe willbe

require require

by by

and and

now now

predict predict

Dialing, Dialing,

may may

telephone telephone

requires requires

numbers, numbers,

operation operation

codes codes

rather rather

every every

about about

but but

numbering numbering

1990 1990

The The

in in

Gateway Gateway

two two Kingdom Kingdom

BELLBOYR BELLBOYR

telephone telephone

that that telephone telephone

millton; millton;

11.7 11.7

area area

numbers. numbers.

to to

people people

or or

2000, 2000,

now now

Operator Operator

the the

280 280

brought brought

and and

requires requires

new new

1975. 1975.

is is

United United

one one

extension extension

--DIRECT-DISTANCE --DIRECT-DISTANCE

1985 1985

Japan Japan

Oakland Oakland

Australia Australia

is is

mobile mobile

nationwide nationwide

year year

service service

over over

numbers: numbers:

11 11

million million

the the

o~hers. o~hers.

many many

for for

per per

Gateway Gateway

codes codes

which which

the the

about about

new new

340 340

CH. CH.

phones phones

require require

by by

Germany, Germany,

with with

man~ man~ problem problem

initial initial

service, service,

added added

follo.ws: follo.ws:

number number

and and

Gateway Gateway

of of

office office

beyond beyond

service. service.

switching switching

telephone telephone

will will and and

The The

as as

1963 1963

Overseas Overseas

Centrex, Centrex,

h~s h~s

Another Another

Forecasts Forecasts

This This

Zealand Zealand

York York

last last

outgrown outgrown

this this

data data

people people

7-digit 7-digit

to to

be be Italy Italy

Sweden Sweden

Switzerland Switzerland

Netherlands Netherlands France France

Germany Germany

New New Denmark Denmark

Belgium Belgium

New New Republic Republic

is is

trunk, trunk, of of

points points

numbers, numbers,

station station

March, March,

require require systems systems

a a

approximately approximately

of of

air-ground air-ground phones. phones. CH. 11 - DIRECT DISTANCEDIALING.

United States and Canada has, therefore·, adopted a new plan with several times the code capacity of the present one which should last well into the next century.

11.3 THE NEWNUMBERING PLAN

Under the initial plan, procedures varied with different types of offices, however, the new plan, shown in Table 11-4 provides uniform dialing procedures for all types of central offices. The following are also provided: 1. A prefix for customer dialed person to person collect, credit card, and other calls requiring operator assistance. Theassigned prefix is "0" (zero).

2. A simplified toll office access code for DDD (Direct Distance Dialed) calls in Step-by-Step central offices. With the present plan, a Step-by Step central office customer making a DDD call uses a 3-digit access code such as "112" to reach the toll office. The access code may be followed by as many as ten digits giving a maximum of thirteen digits. With the new plan, the recom mended DDD access code is the single digit "1" and the maximum number of digits is therefore eleven. 3. A prefix to prevent local intended calls from reaching toll points in error in cities using common control type switching systems which store and process the dialed digits like No.1 and No. 5 Crossbar. The new plan uses a prefix, "0" or "1" on all toll calls. Hence, a wrong office code digit on a 7-digit local call (no prefix being dialed) cannot convert a local intended call into a toll call.

11.8 CH. 11 - DIRECT DISTANCE DIALING

TABLE 11-4 NEWNUMBERING PLAN

INITIAL STAGE ULTIMATESTAGE

HOMEAREA DDD (1) + NNX + XXXX 1 + NXX + XXXX HOMEAREA SPECIAL TOLL 0 + NNX + XXXX 0 + NXX + XXXX FOREIGN AREA DDD (1) + NO/IX + NNX + XXXX1 + NXX + NXX + XXXX FOREIGN AREA SPECIAL TOLL 0 + NO/lX + NNX + XXXX 0 + NXX + NXX + XXXX

TABLE 11-5 CAPACITYOF NEWNUMBERING PLAN

INITIAL: AREA CODES N 0/lX ...... •...... 152 OFFICE CODES NNX (NAMESAND NUMBERS)•••.••••••.•• 540 NNX (ALL NUMBERCALLING) •••••••••••• 640 ULTIMATE:

AREA CODES NXX •.•.•••••..•.•••••.•••••••.•••.•• 800 OFFICE CODES NXX (Less Service Codes) •••••••••••. 792 Notes:

N = ANY NUMERAL2 to 9 X = ANY NUMERAL0 to 9 (1) INDICATES THAT THE PREFIX MAYBE USED IN COMMON CONTROLAREAS BUT MUST BE USED IN SXS AREAS.

A. Requirements The new plan was designed to meet the following requirements: 1. Capacity. Additional capacity is a basic require ment. It could be extended for a few years beyond the expected exhaust date with modest changes in the present plan but underestimations have led to

11.9

1 1

J J

to to

"0" "0"

for for

than than

as as

One One

local local

inter

th•~se th•~se

to to

well well

been been

as as

pro

simul

pro

which which

the the

The The

Apart Apart

signal signal

also also

plan plan

of of

present present

a a

without without

such such

the the

for for

would would

to to

has has

both both

a a

users users

specially specially

easy easy

last last

to to

modified modified

suggested, suggested,

such such

cost. cost.

central central

sensible sensible

the the

any any

codes codes

adequate adequate

ultimate ultimate

by by

in in

of of

digits digits

be be

is is

keysets keysets would would

existing existing

of of

open open

sent sent

and and

that that

education education

of of

plan. plan.

will will

been been

require require

the the

handled handled

more more

prefix prefix

on on

use use

area area

be be

step-by-step step-by-step

digits digits

interruptions. interruptions.

the the

cutover, cutover,

"0" "0"

systems, systems,

must must

plan plan

customers customers

new new

Equipment. Equipment.

door door

is is

be be

felt felt

equ1pment, equ1pment,

a a

that that

DIALING DIALING

prefix prefix

a a

of of

the the and and

also also

impractical. impractical.

800 800

into into

not not

or or

accepted accepted

It It

plan. plan.

would would

which which

the the

from from

the the

considered considered

customer customer

is is

adoption adoption

thousands thousands

was was

premises, premises,

extension extension

implemented implemented

such such

prefix prefix

which which

plan plan

The The

"1" "1"

a a

Such Such

of of

have have

service service

grow grow

will will

systems systems

It It

modified. modified.

dials dials

that that

in in

are are

wide wide

an an

of of

be be

dt)ubled dt)ubled

switching switching

to to

DISTANCE DISTANCE

past. past.

readily readily

be be

on on

Customers' Customers'

and and leaves leaves

mod1fy1ng mod1fy1ng

switch switch

exception exception from from

conceivably conceivably

as as

can can

dials, dials,

completely completely

plan plan

work work

11.10 11.10

local local

equipment equipment

to to

prefix prefix

plan. plan. to to

dialing dialing

of of

It It

the the

world world

prefix prefix

changes changes

discussed discussed

prevented prevented costs costs

capacity capacity

A A

plan plan

customers' customers'

more more

of of

century. century.

a a

not not

be be

of of

such such

in in

new new

DIRECT DIRECT

the the

following: following:

new new

The The

need need requirement requirement

be be

the the

on on

buttons buttons

telephone telephone

have have

been been

possible possible

a a

essent1ally essent1ally

Acceptance. Acceptance.

large large

replace replace

adopted adopted

Other Other

large large the the

next next

the the

control control

desirable desirable

could could

nat1onwide nat1onwide

11 11

would would

a a

• •

to to

as as

will will

has has

difficulties difficulties

object1on object1on

America. America.

by by

office. office.

do do

the the

instead instead

the the

would would

was was

allow allow

translating translating

costs costs

plan plan

up up

new new

CH. CH.

"1". "1".

more more

Modification Modification

to to

radically radically

common common

existing existing

the the

toll toll

received. received.

To To

a a

that that

standpoint. standpoint.

telephones telephones

ideas ideas

Customer Customer

designated designated

or or be be with with

plan plan

the the

from from taneous taneous

No No

ch1ef ch1ef

stages stages into into posed posed

but but

offices offices

push push

The The

co-ordination co-ordination

arrangements arrangements

North North desirability, desirability,

which which major major

Programmable. Programmable.

difficulties difficulties set set

vided vided

will will

them them

1. 1.

4. 4.

All All

3. 3.

2. 2.

equipment, equipment,

. .

toll, toll,

Modifications Modifications

toll toll

enable enable

and and B. B.

two two

a a

is is

When When

are are

four four

and and

as as

a a

or or

loca

In In

known known

The The

the the

office office

digits digits

the the

or or

however, however,

service service

800 800

is is

to to

that that

trunk trunk

to to

a a if if

and and

digit digit

special special

of of

operator. operator.

together, together,

st~rvice st~rvice

to to

code code

without without

transmission transmission

offices, offices,

toll toll

points points

person person

"0" "0"

trunks. trunks.

distinguish distinguish

to to

an an

whether whether

· ·

several several

three three

up up

to to

llX llX

small, small,

level level

two two

to to

dialed. dialed. the the

to to

area area

basis basis

generally generally

auxiliary auxiliary

office office

second second

number number

only only

see see

referred referred

is is

require require

for for

important important

terminating terminating

be be

route route

repeated repeated

an an

"1" "1"

However, However,

connection. connection.

economical. economical.

direct direct

any any

received received

the the to to

If If

when when

is is

are are

adequate adequate

the the

DIALING DIALING

is is

involve involve

connecting connecting

to to

toll toll

person person

is is

waiting waiting

be be

and and

The The

on on routed routed

will will

not not

If If to to

are are

follows. follows. codes. codes.

"0" "0"

It It

intermediate intermediate

over over

as as

arranged arranged

by by and and

by· by·

made made

may may

offices offices

are are

the the

absorbed, absorbed,

be be

there. there.

capacity capacity code code

will will

at at

between between

be be

required required

to to

provide provide

process process

digit digit

a a

be be

two two are are

such such

system system

DISTANCE DISTANCE

which which

calls calls

eliminated eliminated

office office

will will

to to

usually usually

levels. levels.

initial initial

the the

the the apart. apart.

2 2

digits digits

handled handled

will will

office. office.

11.11 11.11

released released

stored stored digits digits

forthcoming. forthcoming.

be be

will will

(according (according originating originating is is

third third

trunk trunk

79 79

code code

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selectively selectively

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will

seven-digit

crossbar

required

forward

example,

DIGITS

routed

call

District

Lubbock,

the for

received

the line

station

11-4,

area

area for

customer

Madison

area

the

OF routed

primary

CH.

turn

area

first

the

not assume

Oklahoma

the

and

step-by-step

405 Lubbock one

the

the receives

pulses·

405.

routing

are

the to

Pittsburgh,

another

the

equipment

establishing

915 in

center

Figure

equipment

that

could completion.

and to

and

the

code

As step-by-step In

from

Prefixing

The

Bend

number.

area

necessary call

nearby

PREFIXING for

Amarillo calls

being

conditions,

call

area

example,

·then

the

either

received is in

·a

prefix

call

step-by-step

202

center

South

crossbar NA-4-1234 comprising to Pittsburgh

office

South shown deleted all digit

of 11.8 a is from to

area is through 915 switching it

for For equipment

area. Assuming a Amarillo

The sectional and

when 2-

CITY

1234

LOCAL

MADISON

CUSTOMER

OFFICE

CODE

405

TOLL

OFFICE

MADISON

OKLAHOMA

AREA

,.,.to

...

Digits

...

DIALING

-::-~;;::;.,__...,_,.__,

ro_,()

CUSTOMER

Prefixing

STEP

CENTER

DISTANCE

2-1234

11.20

OFFICE

TANDEM

11-6

Prefixing

AMARILLO

CROSSBAR

•

STEP-BY-

PRIMARY

:'(

DIRECT

OUTPULSED

I I

11-5

eus

_.,

c.o.

Figure

MA2

~c."{

LOCAL

04-MA2-1234

,II

~~ou"

LOCAL

NUMBER

o'~

OFFICE

NUMBER

RECEIVED

MA2-1234

Figure

CH.

CALL

/"._''

/--7

LOCAL

---

OFFICE

TOLL

OUTGOING

TOLL

OFEICE

LUBBOCK CH. 11 - DIRECT DISTANCE DIALING

switches at the primary center "use up" digits for its switching of the call to the toll center. The common control equipment at the. sectional center must therefore prefix digits to accomplish this switching in order to provide the full seven digit number to the toll center for completing the call. · 11.9 AUTOMATICALTERNATE ROUTING The nationwide trunking network is so designed that direct trunks, called "high usage" groups, are provided between individual switching offices of all classes where such trunks are warranted by the traffic load. These high usage groups are not engineered to handle all the traffic offered to them during the busy hour, since it is not prac tical or economical to provide facilities for the busiest five or ten minutes. Traffic offered to a high usage group, which finds all trunks busy, is automatically rerouted to alternate routes consisting of other high usage groups, and finally to a final trunk group for which no alternate route is provided. In intertoll operation there are calls for which no direct trunk groups exist. Such calls are handled over a preferred trunk group, but are automatically rerouted to other trunk groups if the preferred trunks are busy. The characteristic of the common control equipment at a control switching point to select one of several alternate routes automatically, when all choices in the first route are busy, contributes to the economy of the plant and provides addi tional protection against complete interruption of service when all circuits on a particular route are out of service. Figure 11-7 and the following discussion illustrate a particular routing pattern that might be involved in completing a call that appears at an end office served from toll center TC1 destined for an end office served from toll center TCz. In this example, TC1 has trunks to PC! only, hence the call is routed to that primary center. At PC! the call would be offered first to the high usage group to PC2• At PCz the switching equipment would select an idle trunk in the final group to TC2 and the call would be routed to the called customer in EOz. If, however, all the trunks in the fir~thigh usage group (between PC! and PC2) had been busy, the call would next be offered to the high usage group between PC1 and sc2 (if PCl-SCz-PCz is the most economical alternate route). At SC2 the call would have a choice of two routings: 1. Via direct high usage trunks to TC2, or if they were all busy, 2. Over the two final trunk groups sc 2-PCz and PCz-TCz.

11.21 of

the

and

11-8

high

final

and

usage

Traffic

assumed

with

SCI

extent

accord

centers

that

control the auto

inter

the

following

PC1

other

Regional

centers

Figure

which

lastly

the high

selected.

shows

terminal

accomplished

of to

set

the

and then

numbering

primary

the

dialing, reach

codes

routed.

and

different

common

However,

for

be.en

other RCI.

one

economies

lead

between

switching

must

plant

The

yet

equipment

systems.

This

offered

realized.

PC!

for offer

DIALI.NG

group, group, directly

group,

routing

national

calls.

distance customers practice

needed

group

advantages

not

SC1

obtain

feature.

point

intended

not

this

routings

ranking

must

the

required

step-by-step

plan.

switched

would

the

usage

U$age

usage

using

are

are next from

have

this

codes

offered

actual

above

centers.

direct

DISTANCE in

that four the

11.22

some

used

lower

service

step-by-step

under used

be high high

high

step-by-step

digits

available

layout

originated

switching

group

These

which

the

should

the

feature

numbering

items

time,

not

the

routing codes

trunks

DIRECT

are

other

vary

primary

customer

offices

restrict

described path

plant

call

final RC1. choice

SC1-PC2

scl-SC2 the SCI-RCz

the

assigned

the

are

control

to though

possibilities

operator

arbitrary

could There

should

and

application

the

traffic

the the the and

the

through

other

national

present

route

all

that

the

CH.

groups. those

arrangement

the

conversion

there

even

PCI

have

and

routing

CONVERSION

with

route

RC2

the

Via the Via

equipment Via Via

provide

SCI.

busy,

other

reach

step-by-step

final event

via

from

code The

office

This

rank

trunk

desirable

CODE

economics

would

with

are

introduction

the

essential

In It

in lower

group alternate groups call

certain that

routed

SC2 sequence:

the

conditions is

Centers

switching equipment usage centers 11.10

called illustrates acceptable

ance

differ

substitutes

matically

by plan. mediate

. Group

DIALING

Pattern

Group

4C Usage

High

Final

Class

-Class

DISTANCE

Routing

EO-

TP-

PC-

TC-

LEGEND SC RC-

11.23

•

e D D. 0

11-7

DIRECT

CSP'S

I Figure

CH.

If If

on on

the the

call. call.

more more

a a

this this

code code

arrives arrives

the the

step-by

code, code,

from from

by by

"C". "C".

the the

crossbar crossbar

code code

substitute substitute

number. number.

the the

area area

when when

or or

center center

reaching reaching

I I

calls calls I I

GAS-1234 GAS-1234

area area

call call

the the

central central

office office

California California

and and

Area. Area.

translation translation

through through

the the

complete complete

via via

"B" "B"

used used

the the

toll toll

office office

for for

call call

toll toll

218 218

this this

To To

indirectly indirectly

local local

Ohio Ohio

code code

A A

completing completing

call call

GAB-1234 GAB-1234

digit digit

the the

in in

a a

CENTER CENTER

the the

or or

keys keys

route, route,

DIALING DIALING

218 218

the the

6 6

completing completing

code code

mainly mainly

AREA AREA

Oakland, Oakland,

deletes deletes

for for

the the

to to

LOCAL LOCAL

of of

Conversion Conversion

in in

is is

national national

area. area.

office office

STEP-BY-STEP STEP-BY-STEP

CUSTOMER CUSTOMER

PRIMARY PRIMARY

the the

subscriber subscriber

and and

since since for for

necessary necessary

area area

3 3

in in

"C" "C"

customer customer

be be

Code Code

directly directly

operator operator

direct direct

number number

center center

DISTANCE DISTANCE

the the local local

alternate alternate

a a

11.24 11.24

218-GA8-1234. 218-GA8-1234.

details details

equipment equipment

nA" nA"

to to and and

either either

both both

center, center,

an an

adjacent adjacent

will will

-1234 -1234

the the

toll toll

for for

arranged arranged

11-8 11-8

I I

the the

' '

+ +

digit digit

translation translation

appropriate appropriate

"B" "B"

to to

it it

062 062

an an

DIRECT DIRECT

-GAB -GAB

ROUTE ROUTE

reached reached

Oakland Oakland TRANSLATION TRANSLATION

delete delete

ROyTE ROyTE

DIRECT DIRECT

NUMBER NUMBER

reach reach

shows shows

tandem tandem

ALTERNATE ALTERNATE

OUTPULSED OUTPULSED

in in

number, number,

062 062

the the

to to be be

to to

the the

seven seven

11 11

primary primary

office office

Figure Figure

digit digit

8, 8,

forward forward

subscriber subscriber

11-9 11-9

by by

digits digits

DIGIT DIGIT

can can

6 6

GAS-1234 GAS-1234

center, center,

receiving receiving

6 6 the the

switched switched

CH. CH.

"A", "A",

full full

TANDEM TANDEM

crossbar crossbar

step-by-step step-by-step

701 701

AREA AREA

CROSSBAR CROSSBAR

route. route. subscribers subscribers

is is

and and

at at

tandem tandem

national national

area area

1..------,,------J 1..------,,------J

GArden GArden

a a

pulses pulses

AND AND

Figure Figure

3 3

the the

Upon Upon

equipment equipment

3 3

the the

between between

followed followed

and and

one one

the the

primary primary

-1234 -1234

numericals, numericals,

call call

arbitrary arbitrary

and and

-+ -+

NUMBER NUMBER

216, 216,

RECEIVED RECEIVED

and and

foreign foreign

than than

218 218 the the

tandem tandem

calls calls

the the

subscriber subscriber

Area Area

through through

11.11 11.11

switches switches call, call,

step step

quires quires

crossbar crossbar

office, office,

crossbar crossbar

with with £1,!-GAS £1,!-GAS

2 2

as as

with with

"B". "B".

spill spill

the the

must must one one

the the

toll toll

to to

toll toll

the the

and and by by

MAin MAin

trunk trunk

must must

operator operator

spills spills

and and

a a

Ohio, Ohio,

that that

the the

determines determines

Area Area

manner manner

office office

the the (one (one

Chicago Chicago

which which

216, 216,

toll toll

to to

digits digits

622-1234, 622-1234,

variable variable

code code

216 216

Chicago Chicago

center center

Oakland Oakland

the the

Chicago Chicago

receives receives

622-1234, 622-1234,

The The

same same

directly directly

subscriber subscriber

(622) (622)

select select

Area Area

office office

since since

in in

six six

the the

variable variable

the the

2 2

the the

Code; Code;

The The

to to

area area

group group

toll toll

to to

the the

an an

216 216

Cleveland, Cleveland,

Oakland Oakland

to to

determine determine MA MA

office office

and and

forward forward

way way

in in

served served

in in because because

"A" "A"

the the

Area Area

in in

first first

2 2

receives receives

the the

DIALING DIALING

the the

trunk trunk

is is

arrives arrives

of of

code code

digits digits

must must

"C" "C"

this this

Chicag9 Chicag9

to to 2-1234. 2-1234.

The The

"B" "B"

the the

Cleveland, Cleveland, groups groups

MAin MAin the the

spills spills

illustrates illustrates

office, office,

In In

216-622-1234. 216-622-1234.

and and

Canton, Canton,

call call

center center

to to

The The

(623) (623)

2 2

three three

office, office,

The The

office office

11.25 11.25

DISTANCE DISTANCE

3 3

terminated terminated to to

subscriber subscriber

trunk trunk

216-MA 216-MA

groups groups

the the

This This

toll toll

center center

select select

digits. digits.

MAin MAin

group group

toll toll

subscriber subscriber

combination combination

the the

forward forward

call. call.

first first

subscriber subscriber

then then

translates translates

MAin MAin

to to

Canton), Canton),

.keys .keys

trunk trunk

When When

trunk trunk six six

DIRECT DIRECT

to to

-1234. -1234.

the the

toll toll

of of

to to

is is

a a

Code. Code.

the the

Cleveland. Cleveland.

to to

national national

the the

connects connects

trunk trunk

this this

code code

to to

code code

two two

again again

Canton Canton

Center. Center.

on on

one one

spills spills

11 11

call call

Cleveland Cleveland

The The

first first

one one

on on

call call

and and

Area Area the the

digits digits

has has

operator operator

the the

Chicago Chicago

and and

CH. CH.

selects selects

the the

trunk trunk

The The

is is

Toll Toll that that

the the

and and

the the

office office

The The

the the

used used

·office ·office

the the on on

a a

-1234 -1234 control control

On On

translates translates

The The

"homes" "homes"

translates translates

On On

be be

time time

216-623-1234. 216-623-1234.

this this

office office

Oakland Oakland

selected. selected.

Chicago Chicago

this this

above. above.

common common

Canton. Canton. national national

keys keys

"homes" "homes"

forward forward

digits digits

feature. feature.

selects selects skipping skipping

Cleveland Cleveland center center

that that

indicates indicates translate translate

NAtional NAtional

office office

to to shall shall

toll toll

be be Cleveland Cleveland

the the office office R

22}

23)

I8ER

t--

1234

LOCAL 1234

OFFICE r--

LOCAL

r--

.__

CALL

OFFICE

CALLED

SUBSCR

MAIN

MAIN3(8

OHIO

216

OHIO

AREA

OFFICE

DIGIT

CLEVELAND, 3

CLEVELAND

C\1

TOLL

...

TOLL

CANTON,

TRANSPORTATION

N!!:

..,. •"

!~.!]

I I I i I I I

I I I I I I I I I I I I I I

SPILLING

DIALING

623-1234

622-1234

,_JARIABLE

Translation

N ..J

..,

ILL.

312

(ftiL ;;•

....

121~

OFFICE

6-DIGIT

j~ _,N NN Q:N ..,

••

.!.J

AREA

.... Digit

DISTANCE

TOLL

J.!.2

t2161 r

IQ.

TRANSPORTATION

_,.

N-

OON ..,. ·- Ntn

CHICAGO,

11-26

1234 3

1234

216-623

DIRECT

I I 1216-622

I I I I

I l I I I I I I I I I I

11-9

CH.

Figure

CALIF

415

AREA

OAKLAND,