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exchanges.
Read
on about what a telephone exchange is and the history of
telephone exchanges...
Telephone Exchanges
A telephone exchange connects
telephone calls. It is what makes phone
calls work in the sense of making connections and
relaying the speech information. In the telephone
industry, an exchange refers to a central office.
The term exchange can
also be used to refer to an area served by a particular
switch. Commonly, it can refer to the first three digits of a
local telephone number, sometimes called a
"prefix."
In the past, the first two or
three digits would map to an exchange name, e.g. 524–2368
was formerly JAckson 4–2368.
In December of 1930, New York
City became the first locality in the United States to adopt
the two-letter, five-number format, and it remained alone in this
respect until well after World War II, when other
municipalities across the country began to follow suit (in
some areas, most notably much of California, telephone numbers
in the 1930s through early 1950s consisted of only six digits,
two letters which began the exchange name followed by four
numbers. Prior to the mid-1950s, the number immediately
following the name could never be a "0" or
"1;" indeed, "0" was never pressed into
service at all, except in the immediate Los Angeles area (the
"BEnsonhurst 0" exchange mentioned in an episode of
the popular TV sitcom The Honeymooners was fictitious).
In 1955, the Bell System
attempted to standardize the process of naming exchanges by
issuing a "recommended list" of names to be used for
the various number combinations. In 1961, New York Telephone
introduced "selected-letter" exchanges, in which the
two letters did not mark the start of any particular name
(example: FL 6-9970), and by 1965 all newly-connected phone
numbers nationwide consisted of numerals only (Wichita Falls,
Texas had been the first locality in the United States to
implement the latter, having done so in 1958) Pre-existing
numbers continued to be displayed the old way in many places
well into the 1970s. A Chicago carpet retailer frequently
advertised their number NAtional 2-9000 on WGN until the
1990s; not to mention, the number TYler 8-7100 for a Detroit
construction company.
The first telephone exchange
opened in New Haven, Connecticut in 1878. The switchboard was
built from "carriage bolts, handles from teapot lids and
bustle wire" and could handle two simultaneous
conversations
Later exchanges consisted of
one to several hundred plug boards manned by operators. Each
operator sat in front of from one to three banks of ¼-inch
phone jacks fronted by several rows of phone cords, each of
which was the local termination of a phone subscriber line. A
calling party (known as the 'subscriber'), would lift the
receiver, a light near the plug would light, and the operator
would switch into the circuit to ask "number
please?" Depending upon the answer, the operator might
plug the plug into a local jack and start the ringing cycle,
or plug into a hand-off circuit to start what might be a long
distance call handled by subsequent operators in another bank
of boards or in another building miles away.
On March 10, 1891, Almon
Strowger, an undertaker in Topeka, Kansas, patented the
Strowger switch, a device which led to the automation of the
telephone circuit switching. While there were many extensions
and adaptations of this initial patent, the one best known
consists of 10 layers or banks of 10 contacts arranged in a
semi-circle. When used with a dial telephone, each pair of
numbers caused the shaft of the central contact
"hand" to first step up a layer per digit and then
swing in a contact row per digit.
These step switches were
arranged in banks, beginning with a "line-finder"
which detected that one of up to a hundred subscriber lines
had the receiver lifted "off hook." The line finder
hooked the subscriber to a "dial tone" bank to show
that it was ready. The subscriber's dial pulsed at 10 pulses
per second (depending on standards in particular countries).
Exchanges based on the Strowger
switch were challenged by crossbar technology. These phone
exchanges promised faster switching and would accept pulses
faster than the Strowger's typical 10 pps — typically about
20 pps. The advent of DTMF (Touch-Tone) tone-signaling solid-state switches
cut off the crossbar's takeover before it could really get
going.
A transitional technology (from
pulse to DTMF) had DTMF "link finders" which
converted DTMF to pulse and fed it to conventional Strowger or
crossbar switches. This technology was used as late as the mid
to late 1990s.
Because the switches were
hard-wired together and fairly difficult to re-wire (re-grade),
telephone exchange buildings in many larger cities were
dedicated to circuits that began with the first two or three
numbers of the standard 7-digit phone
numbers. In a holdover from the days of plug-board exchanges,
the exchanges were typically named with a name whose first two
letters translated to the digits of the exchange's prefix on a
common telephone dial. Examples: CApital (22), TRinity (87),
OVerland (68). Certain number combinations were not amenable to
this naming process, such as "57," "95,"
and "97"; it was in part due to this factor that the
name system was eventually abandoned, as more numbers were
needed to prevent a given area
code from running out of available numbers.
Because the pulses in a
Strowger switch exchange took time, having a phone number with
lots of 8s or 9s or 0s meant it took longer to dial. The phone
companies typically assigned such "high" numbers to
pay phones because they were rarely called.
To test the basic functioning
of all of the switches in a chain, a special "test"
number was reserved that consisted of all 5s (555–5555) —
half-way up and in on each bank. The "555" (or
KLondike) exchange was never assigned any real numbers, which
is why today's TV and movie shows use 555-xxxx numbers for
their phone numbers (previously, such productions often used
numbers that ended in certain four-number combinations that
were typically set aside for similar uses — "0079"
on the West Coast and "9970" in many other places;
examples include the TV series Perry
Mason and the 1948
film Sorry,
Wrong Number). That way there was no possibility that
a fake number from a show would actually reach someone, thus
avoiding the scenario which arose in 1982
with Tommy
Tutone's hit single 867-5309/Jenny,
which led to many customers who actually had that number
receiving a plethora of unwanted calls. In fact, many US phone
companies either no longer assign this number, or have
relegated to internal testing purposes.
However, today only numbers
beginning with 555–01 are reserved for fiction, and other
555-numbers can be allocated to "information
providers." A side effect of the fictional-number pool
being reduced to 100 numbers is that the same ones now often
recur in different movies or TV shows. The "958" and
"959" exchanges have also been reserved for similar
purposes in most localities, and as a result very few
individuals or businesses have telephone numbers beginning
with those sets of digits either (although this fact is not as
well known, so such numbers have not been used in a fictional
context).
The number in the Glenn Miller
Orchestra's hit 'PEnnsylvania 6-5000' was and is the number of
the Hotel Pennsylvania in New York. If you call the number,
now written as (212) 736-5000, you still get the hotel.
Manual Telephone Exchanges
With all-manual calling, the
customer calls the operator
and asks the operator for the number, and provided that the
number is in the same central office, the operator connects
the call by plugging into the jack on the switchboard
corresponding to that customer's line. If the call is to
another central office, the operator plugs into the trunk for
the other office and ask the operator answering (known as the
"inward" operator) to connect the call.
Most manual telephone exchanges
in cities were common-battery, meaning that the central office
provided power for the telephone circuits, as is the case
today. A customer lifting their receiver would change their
line status to "tip," thereby lighting a lamp on
the operator's switchboard. In smaller towns, early telephones
were often magneto, or crank, phones, where the subscriber
turned a crank to generate current to activate the
"tip" condition, notifying the operator of the call
with a hinged piece of metal dropping. Batteries at the subscriber's home provided the current to
allow conversation. Magneto systems were in use in some small
towns in the U.S. as late at the 1980s.
In large cities, such as New
York City, with hundreds of central offices, it took many
years to convert the whole city to dial service. To help
automate service to manual offices during the transition to
dial service, a special type of switchboard, which would
display the number dialed by the customer, was used. For
instance, if a customer in the MUrray Hill exchange picked up
the phone and dialed a number in the CIty Island exchange, the
customer would never need to know the destination number was
in a manual exchange. Dialing that number would connect to the
CIty Island exchange inward operator, who would see the number
displayed on the switchboard, and plug into the line.
Automatic Telephone
Exchanges
These came into existence in
the early 1900s. They were designed to replace the need for telephone
operators. Before the exchanges became automated,
operators had to complete the connections required for a telephone
call. Almost everywhere, operators have been replaced by
computerized exchanges.
The local exchange
automatically senses an off hook (tip) condition, provides dial
tone to that phone, receives the pulses or touch tounes
tones generated by the phone, and then completes a connection
to the called phone within the same exchange or to another
distant exchange.
The exchange then maintains the
connection until a party hangs up, and the connection is
disconnected. Additional features, such as billing equipment,
may also be incorporated into the exchange.
Early exchanges used motors,
shaft drives, rotating switches and relays.
Some types of automatic exchanges were Step-By-Step, All Relay, X-Y,
Panel
and Crossbar.
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