The Magic of Cross-Country Communications
September 1959 Popular Electronics
area code system was developed by AT&T and Bell Laboratories
in the 1940's, and went into effect in 1947. It was called the North
American Numbering Plan (NANP) and included the United States and
Canada. States and provinces that had a single area code were assigned
three digit codes with 0 as the middle number. There were 86 area
codes at that time. States and provinces that had more than one
area code distributed to them were given three digit codes with
1 as the middle number. The first and third digits were allotted
according to population density in the city or region, with the
most populated areas getting the lowest numbers.
September 1959 Popular Electronics
Table of Contents
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The Magic of Cross-Country Communications
Here's how telephone calls and TV programs are sent from
coast to coast
By Art Zuckerman
you've been reading your mail lately, you may recall a letter from
your local telephone company. This letter was about the new system
of direct long-distance dialing which makes it possible for you
to call Aunt Minnie in Minneapolis' just "by dialing 612, the area
code number, plus her phone number.
Ingenious new switching
circuits make such long-distance dialing possible. But these fancy
new techniques wouldn't be worth a nickel without a way of getting
messages across the country quickly and economically. At the present
time, we have two efficient carriers of the cross-country electronic
mailbag. These are the coaxial cable and the radio relay systems.
Some 16,000 miles of American real estate are covered by
the radio relay towers of the American Telephone & Telegraph
Co., while another 10,000 miles hold A T & T's buried coaxial
Through Earth and Air. Although
one sends its messages through underground tubes and the other
goes through the air, radio relay and coaxial cable use the same
basic techniques to handle thousands of telephone calls at the same
time. Network television programs are routed around the country
on these same carriers, right along with your phone call.
a coaxial cable and a relay antenna carry a broad band of frequencies.
Some of the latest equipment covers eight times the frequency span
of the entire broadcast radio band. An individual telephone conversation
occupies only a narrow channel of the band, much as a single radio
program represents only a part of the total signal picked up by
your receiving antenna.
In the same way that your radio
receives and separates many stations, so coaxial cables and radio
relay can carry an enormous composite signal and separate the many
individual voice channels at the receiving stations and cable terminals.
Why are there two systems? There are several reasons. First
of all, the type of terrain to be covered can determine whether
coaxial cable or radio relay should be used. In a mountainous area,
radio relay is almost always called for, while coaxial cable is
favored in open country.
Another factor is local needs.
Other things being equal, the system which best suits local traffic
requirements at a given time will receive the nod. At one point
in the game, radio relay may offer the greatest message-handling
capacity; at another time, cable equipment may forge ahead.
The costs of the two systems tend to average out. While radio
relay is cheaper to install, cable is cheaper to maintain and operate.
The two carriers of the cross-country electronic mailbag
are the coaxial cable and radio relay systems. Above is an inside
view of a coax cable. At right is an antenna tower at a radio relay
The Cable System. Coaxial cable is a marvel of design. About as
big around as a man's wrist, it's really a collection of eight copper
tubes, together with a number of standard wire conductors for maintenance
and short-haul telephone purposes. These are all encased in a gas-filled
Radio relay antennas, shaped like tapering horns, focus microwaves
into tight beams and aim them at the next station down the radio
cable is made up of eight coaxial tubes plus a number of standard
wire conductors. Spacers made of polyethylene position the center
wires inside the hollow copper outer tubes.
Television control terminals monitor programs coming in either
on radio relay or coaxial cable. One TV program ties up the
equivalent of 600 telephone circuits.
"Mickey Mouse" plow train can cut through most types of terrain,
planting coaxial cable and filling its trench as it goes along.
Each copper tube is roughly the diameter of
a fountain pen. A copper wire runs down the center of each tube,
held in place by polyethylene insulating discs spaced about an inch
apart. Because the tube and the wire have the same axis, the tubes
are called "coaxial." This design offers unusually good shielding
properties. It prevents interference with other conductors. in the
same cable and protects against outside electrical disturbances.
For telephone purposes, the tubes are used in pairs - each
transmitting in one direction. When equipped with the latest design
of related apparatus, some 1860 simultaneous conversations can be
handled by each pair of coaxials.
Since signals carried
by the cable weaken rapidly, a chain of booster amplifiers spaced
four to eight miles apart is necessary to refresh them. Depending
on the particular system, these repeater stations are mostly small,
unattended units that draw their power from the cable itself. A
series of main stations supplies this power and also provides overall
The cable has an amazing ability to look out for itself. Not so
long ago, for instance, a farm hand in the South was digging post
holes when he hit something. The man was startled when the mysterious
object started hissing at him. He was almost as surprised when a
Bell System maintenance man showed up in short order.
object our farm hand struck was, of course, a "coax." The hissing
came from gas - either nitrogen or. dehumidified air escaping
through a break in the casing. The gas serves two purposes. First,
by escaping under pressure, it keeps moisture from entering the
break. Second, it sets off an alarm at a main repeater station and
allows maintenance personnel to locate the damage from gauges that
show the point of lowest gas pressure.
Signal problems and
power failure are other occasional sources of trouble in the cables.
Here, too, the coax has built-in safeguards. For example, only six
of the eight tubes are usually in operation. Two are kept in reserve
for emergencies, one set to handle signals in one direction and
the other ready to transmit in the opposite direction. If the commercial
power should fail, batteries are automatically cut in. Should these
go out, emergency generators take over.
require somewhat different safeguards. In areas where the soil offers
a high resistance to electricity, a polyethylene insulating layer
wards off lightning damage. Where gophers are found, a wrapping
of steel tape deprives the critters of a tasty lead-sheath snack.
Radio Relay. Natural and man-made hazards
are less of a problem with radio relay. Since it operates in the
microwave frequency range, it isn't affected by static caused by
lightning, nearby ignition systems, or even sunspots.
radio relay chain is made up of a number of stations about 30 miles
apart. Each one must be within the line of sight of the station
in front and behind, because the microwaves they handle are highly
directional and are focused into extremely sharp beams. The relay
stations are placed in a zigzag path rather than in a straight line,
so that there is no danger of a station's overshooting its mark
and being picked up by the next station down the line.
it uses such tightly focused beams, microwave relay employs power
very sparingly. It takes less than a watt - about enough to light
up a pocket flashlight bulb - to span the gap between stations.
The tight focusing arrangement also conserves frequencies at a station
that feeds two or more relay chains. The station can send out the
same frequency in different directions without danger of interference.
In regular practice, however, a relay station receives a
transmission on one frequency band and then converts it to another
frequency for relaying to the next site. Here, the transmission
is again sent out on the original frequency, and the alternation
of frequencies continues right on down the line. This practice eliminates
the possibility of part of a station's transmission feeding back
into its own receiving antenna.
The amplifiers that give
the relay stations their punch really pack a wallop. They can take
a weakened signal and send it on with a million fold gain in strength.
As with coaxial cable, the radio relay system contains a
number of built-in safeguards. It, too, has spare emergency channels
for use in case of trouble. Storage batteries and motor-driven generators
are ready to supply emergency power. An alarm system spells out
any trouble on a coded diagram at the nearest control office.
Multi-Purpose "Highway." Television and
telephone traffic are carried together on both radio relay and coaxial
cable. The average TV circuit ties up the equivalent of 600 telephone
circuits. But, unlike telephone, it only goes one way, and thus
uses only half the two-way circuits involved.
enough, the television picture and the audio portion of the show
are normally carried on separate channels. This means that technicians
in the control centers must coordinate the two sections of the program
at the end of the line.
the audio is carried on lower-priced lines, network television doesn't
offer too much for the hi-fi enthusiast. Bell Telephone can supply
circuits with a top audio frequency response of either 5000, 8000,
or 15,000 cps. The higher the response, the higher the price of
the service. For this reason, though some TV circuits are occasionally
rigged for the higher frequencies, the networks generally content
themselves with a 5000-cycle audio cutoff. Regular telephone frequency
response, incidentally, ranges from about 300 to 3000 cycles.
Ground Work. A lot of planning goes into
the construction of either system. For radio relay, topographical
maps are used to locate clear paths between sites and to avoid reflective
surfaces, such as flat land or lakes, that might harm the signal.
Portable transmitters, receivers, and antennas are spotted for field
tests of prospective sites. Borings and soil samples tell what kinds
of foundations are in order.
Cables are laid in open country
by tractor-drawn plow trains. A 27-ton job, tagged the "Mickey Mouse,"
is the most powerful type in use. This monster's hydraulically controlled
plowshare can cut through almost any kind of terrain, feeding cable
into the ground and filling in the trench as it moves ahead.
In cities, on the other hand, or under highways and railroads, the
cable is generally placed in a conduit or pipe. Sometimes it must
be run over bridges. Or, at river crossings where there are no bridges,
the cable may be laid by barges and dredges using high-pressure
water jets to trench it in the stream bed.
Aside from consideration
of local problems, the telephone companies think it is just plain
good sense to use both radio relay and coaxial cable, instead of
putting all their eggs in one basket, since a disaster that affects
one system isn't likely to harm the other. This way, there is a
good chance that an alternate route will always be available should
something go wrong with the prime channel.
And if this belt-and-suspenders
approach is important under everyday conditions, from the viewpoint
of national defense it could be a matter of life and death.