NON-SELECTIVE PARTY-LINE SYSTEMS
A party line is a line that is for the joint use of several stations.
It is, therefore, a line that connects a central office with two or
more subscribers' stations, or where no central office is involved, a
line that connects three or more isolated stations with each other.
The distinguishing feature of a party line, therefore, is that it
serves more than two stations, counting the central office, if there
is one, as a station.
Strictly speaking, the term _party_ line should be used in
contradistinction to the term _private_ line. Companies operating
telephone exchanges, however, frequently lease their wires to
individuals for private use, with no central-office switchboard
connections, and such lines are, by common usage, referred to as
"private lines." Such lines may be used to connect two or more
isolated stations. A _private_ line, in the parlance of telephone
exchange working, may, therefore, be a _party_ line, as inconsistent
as this may seem.
A telephone line that is connected with an exchange is an exchange
line, and it is a party line if it has more than one station on it. It
is an individual line or a single party line if it has but a single
station on it. A line which has no central-office connection is called
an "isolated line," and it is a party line if it has more than two
stations on it.
The problem of mere speech transmission on party lines is comparatively
easy, being scarcely more complex than that involved in private or
single party lines. This is not true, however, of the problem of
signaling the various stations. This is because the line is for the
common use of all its patrons or subscribers, as they are termed, and
the necessity therefore exists that the person sending a signal, whether
operator or subscriber, shall be able in some way to inform a person at
the desired station that the call is intended for that station. There
are two general ways of accomplishing this purpose.
(_1_) The first and simplest of these ways is to make no provision for
ringing any one bell on the line to the exclusion of the others, and
thus allow all bells to ring at once whenever any station on the line
is wanted. Where this is done, in order to prevent all stations from
answering, it is necessary, in some way, to convey to the desired
station the information that the call is intended for that station,
and to all of the other stations the information that the call is not
intended for them. This is done on such lines by what is called "code
ringing," the code consisting of various combinations of long and
short rings.
(_2_) The other and more complex way is to arrange for selective
ringing, so that the person sending the call may ring the bell at the
station desired, allowing the bells at all the other stations to
remain quiet.
[Illustration: Fig. 163. Grounded-Circuit Series Line]
These two general classes of party-line systems may, therefore, be
termed "non-selective" and "selective" systems. Non-selective party
lines are largely used both on lines having connection with a central
office, and through the central office the privilege of connection
with other lines, and on isolated lines having no central-office
connection. The greatest field of usefulness of non-selective lines is
in rural districts and in connection with exchanges in serving rather
sparsely settled districts where the cost of individual lines or even
lines serving but a few subscribers, is prohibitive.
Non-selective telephone party lines most often employ magneto
telephones. The early forms of party lines employed the ordinary
series magneto telephone, the bells being of low resistance and
comparatively low impedance, while the generators were provided with
automatic shunting devices, so that their resistance would normally be
removed from the circuit of the line.
Series Systems. The general arrangement of a series party line
employing a ground return is shown in Fig. 163. In this three ordinary
series instruments are connected together in series, the end stations
being grounded, in order to afford a return path for the ringing and
voice currents.
[Illustration: Fig. 164. Metallic-Circuit Series Line]
In Fig. 164 there is shown a metallic-circuit series line on which
five ordinary series telephones are placed in series. In this no
ground is employed, the return being through a line wire, thus making
the circuit entirely metallic.
[Illustration: Fig. 165. Series Party Line]
The limitations of the ordinary series party line may be best
understood by reference to Fig. 165, in which the circuits of three
series telephones are shown connected with a single line. The receiver
of Station A is represented as being on its hook, while the receivers
of Stations B and C are removed from their hooks, as when the
subscribers at those two stations are carrying on a conversation. The
hook switches of Stations B and C being in raised positions, the
generators and ringers of those stations are cut out of the circuit,
and only the telephone apparatus proper is included, but the hook
switch of Station A being depressed by the weight of its receiver,
includes the ringer of that station in circuit, and through this
ringer, therefore, the voice currents of Stations B and C must pass.
The generator of Station A is not in the circuit of voice currents,
however, because of the automatic shunt with which the generator is
provided, as described in Chapter VIII.
A slight consideration of the series system as shown in this figure,
indicates that the voice currents of any two stations that are in use,
must pass (as indicated by the heavy lines) through the ringers of all
the stations that are not in use; and when a great number of stations
are placed upon a single line, as has been frequently the case, the
impedance offered by these ringers becomes a serious barrier to the
passage of the voice currents. This defect in the series party line is
fundamental, as it is obvious that the ringers must be left in the
circuit of the stations which are not in use, in order that those
stations may always be in such condition as to be able to receive a
call.
This defect may in some measure be reduced by making the ringers of
low impedance. This is the general practice with series telephones,
the ringers ordinarily having short cores and a comparatively small
number of turns, the resistance being as a rule about 80 ohms.
Bridging Systems. Very much better than the series plan of
party-line connections, is the arrangement by which the instruments
are placed in bridges across the line, such lines being commonly known
as bridged or bridging lines. This was first strongly advocated and
put into wide practical use by J.J. Carty, now the Chief Engineer of
the American Telephone and Telegraph Company.
A simple illustration of a bridging telephone line is shown in Fig.
166, where the three telephones shown are each connected in a bridge
path from the line wire to ground, a type known as a "grounded
bridging line." Its use is very common in rural districts.
A better arrangement is shown in Fig. 167, which represents a
metallic-circuit bridging line, three telephone instruments being
shown in parallel or bridge paths across the two line wires.
The actual circuit arrangements of a bridging party line are better
shown in Fig. 168. There are three stations and it will be seen that
at each station there are three possible bridges, or bridge paths,
across the two limbs of the line. The first of these bridges is
controlled by the hook switch and is normally open. When the hook is
raised, however, this path is closed through the receiver and
secondary of the induction coil, the primary circuit being also closed
so as to include the battery and transmitter. This constitutes an
ordinary local-battery talking set.
[Illustration: Fig. 166. Grounded Bridging Line]
[Illustration: Fig. 167. Metallic Bridging Line]
[Illustration: Fig. 168. Metallic Bridging Line]
A second bridge at each station is led through the ringer or
call-bell, and this, in most bridging telephones, is permanently
closed, the continuity of this path between the two limbs of the line
not being affected either by the hook switch or by the automatic
switch in connection with the generator.
A third bridge path at each station is led through the generator.
This, as indicated, is normally open, but the automatic cut-in switch
of the generator serves, when the generator is operated, to close its
path across the line, so that it may send its currents to the line and
ring the bells of all the stations.
When any generator is operated, its current divides and passes over
the line wires and through all of the ringers in multiple. It is seen,
therefore, that the requirements for a bridging generator are that it
shall be capable of generating a large current, sufficient when
divided up amongst all the bells to ring each of them; and that it
shall be capable of producing a sufficient voltage to send the
required current not only to the near-by stations, but to the stations
at the distant end of the line.
It might seem at first that the bridging system avoided one difficulty
only to encounter another. It clearly avoids the difficulty of the
series system in that the voice currents, in order to reach distant
stations, do not have to pass through all of the bells of the idle
stations in series. There is, however, presented at each station a
leakage path through the bell bridged across the line, through which
it would appear the voice currents might leak uselessly from one side
of the line to the other and not pass on in sufficient volume to the
distant station. This difficulty is, however, more apparent than real.
It is found that, by making the ringers of high impedance, the leakage
of voice currents through them from one side of the line to the other
is practically negligible.
It is obvious that in a heavily loaded bridged line, the bell at the
home station, that is at the station from which the call is being sent,
will take slightly more than its share of the current, and it is also
obvious that the ringing of the home bell performs no useful function.
The plan is frequently adopted, therefore, of having the operation of
the generator serve to cut its own bell out of the circuit. The
arrangement by which this is done is clearly shown in Fig. 169. The
circuit of the bell is normally complete across the line, while the
circuit of the generator is normally open. When, however, the generator
crank is turned these conditions are reversed, the bell circuit being
broken and the generator circuit closed, so as to allow its current all
to pass the line. This feature of having the local bell remain silent
upon the operation of its own generator is also of advantage because
other parties at the same station are not disturbed by the ringing of
the bell when a call is being made by that station.
A difficulty encountered on non-selective bridging party lines, which
at first seems amusing rather than serious, but which nevertheless is
often a vexatious trouble, is that due to the propensity of some
people to "listen in" on the line on hearing calls intended for other
than their own stations. People whose ethical standards would not
permit them to listen at, or peep through, a keyhole, often engage in
this telephonic eavesdropping.
Frequently, not only one but many subscribers will respond to a call
intended for others and will listen to the ensuing conversation. This
is disadvantageous in several respects: It destroys the privacy of
conversation between any two parties; it subjects the local batteries
to an unnecessary and useless drain; and it greatly impairs the
ringing efficiency of the line. The reason for this interference with
ringing is that the presence of the low-resistance receivers across
the line allows the current sent out by any of the generators to pass
in large measure through the receivers, thus depriving the ringers,
which are of comparatively high resistance and impedance, of the
energy necessary to operate them. As a result of this it is frequently
impossible for one party to repeat the call for another because,
during the interval between the first and second call, a number of
parties remove their receivers from their hooks in order to listen.
Ring-off or clearing-out signals are likewise interfered with.
[Illustration: Fig. 169. Circuits of Bridging Station]
A partial remedy for this interference with ringing, due to
eavesdropping, is to introduce a low-capacity condenser into the
receiver circuit at each station, as shown in Fig. 169. This does not
seriously interfere with the speech transmission since the condensers
will readily transmit the high-frequency voice currents. Such
condensers, however, have not sufficient capacity to enable them
readily to transmit the low-frequency ringing currents and hence
these are forced, in large measure, to pass through the bells for
which they are intended rather than leaking through the low-resistance
receiver paths.
The best condenser for this use is of about 1/2-microfarad capacity,
which is ample for voice-transmitting purposes, while it serves to
effectively bar the major portion of the generator currents. A higher
capacity condenser would carry the generator currents much more
readily and thus defeat the purpose for which it was intended.
In order that the requisite impedance may be given to the ringers
employed for bridging party lines, it is customary to make the cores
rather long and of somewhat larger diameter than in series ringers and
at the same time to wind the coils with rather fine wire so as to
secure the requisite number of turns. Bridging bells are ordinarily
wound to a resistance of 1,000 or 1,600 ohms, these two figures having
become standard practice. It is not, however, the high resistance so
much as the high impedance that is striven for in bridging bells; it
is the number of turns that is of principal importance.
As has already been stated, the generators used for bridging lines are
made capable of giving a greater current output than is necessary in
series instruments, and for this purpose they are usually provided with
at least four, and usually five, bar magnets. The armature is made
correspondingly long and is wound, as a rule, with about No. 33 wire.
Sometimes where a bridged party line terminates in a central-office
switchboard it is desired to so operate the line that the subscribers
shall not be able to call up each other, but shall, instead, be able
to signal only the central-office operator, who, in turn, will be
enabled to call the party desired, designating his station by a
suitable code ring. One common way to do this is to use biased bells
instead of the ordinary polarized bells. In order that the bells may
not be rung by the subscribers' generators, these generators are made
of the direct-current type and these are so associated with the line
that the currents which they send out will be in the wrong direction
to actuate the bells. On the other hand, the central-office generator
is of direct-current type and is associated with the line in the right
direction to energize the bells. Thus any subscriber on the line may
call the central office by merely turning his generator crank, which
action will not ring the bells of the subscribers on the line. The
operator will then be able to receive the call and in turn send out
currents of the proper direction to ring all the bells and, by code,
call the desired party to the telephone.
[Illustration: ONE WING OF OPERATING ROOM, BERLIN, GERMANY Ultimate
Capacity 24,000 Subscribers' Lines and 2,100 Trunk Lines.
Siemens-Halske Equipment. Note Horizontal Disposal of Multiple]
Signal Code. The code by which stations are designated on
non-selective party lines usually consists in combinations of long and
short rings similar to the dots and dashes in the Morse code. Thus,
one short ring may indicate Station No. 1; two short rings Station No.
2; and so on up to, say, five short rings, indicating Station No. 5.
It is not good practice to employ more than five successive short
rings because of the confusion which often arises in people's minds as
to the number of rings that they hear. When, therefore, the number of
stations to be rung by code exceeds five, it is better to employ
combinations of long and short rings, and a good way is to adopt a
partial decimal system, omitting the numbers higher than five in each
ten, and employing long rings to indicate the tens digits and short
rings to indicate the units digit, Table X.
TABLE X
Signal Code
+--------------+---------------+--------------+---------------+
|STATION NUMBER|RING |STATION NUMBER|RING |
|1 |1 short |12 |1 long, 2 short|
|2 |2 short |13 |1 long, 3 short|
|3 |3 short |14 |1 long, 4 short|
|4 |4 short |15 |1 long, 5 short|
|5 |5 short |21 |2 long, 1 short|
|11 |1 long, 1 short|22 |2 long, 2 short|
+--------------+---------------+--------------+---------------+
Other arrangements are often employed and by almost any of them a
great variety of readily distinguishable signals may be secured. The
patrons of such lines learn to distinguish, with comparatively few
errors, between the calls intended for them and those intended for
others, but frequently they do not observe the distinction, as has
already been pointed out.
Limitations. With good telephones the limit as to the number of
stations that it is possible to operate upon a single line is usually
due more to limitations in ringing than in talking. As the number of
stations is increased indefinitely a condition will be reached at
which the generators will not be able to generate sufficient current
to ring all of the bells, and this condition is likely to occur before
the talking efficiency is seriously impaired by the number of bridges
across the line.
Neither of these considerations, however, should determine the maximum
number of stations to be placed on a line. The proper limit as to the
number of stations is not the number that can be rung by a single
generator, or the number with which it is possible to transmit speech
properly, but rather the number of stations that may be employed
without causing undue interference between the various parties who may
desire to use the line. Overloaded party lines cause much annoyance,
not only for the reason that the subscribers are often not able to use
the line when they want it, but also, in non-selective lines, because
of the incessant ringing of the bells, and the liability of confusion
in the interpretation of the signaling code, which of course becomes
more complex as the number of stations increases.
The amount of business that is done over a telephone line is usually
referred to as the "traffic." It will be understood, however, in
considering party-line working that the number of calls per day or per
hour, or per shorter unit, is not the true measure of the traffic and,
therefore, not the true measure of the amount of possible interference
between the various subscribers on the line.
An almost equally great factor is the average length of the
conversation. In city lines, that is, in lines in city exchanges, the
conversation is usually short and averages perhaps two minutes in
duration. In country lines, however, serving people in rural
districts, who have poor facilities for seeing each other,
particularly during the winter time, the conversations will average
very much longer. In rural communities the people often do much of
their visiting by telephone, and conversations of half an hour in
length are not unusual. It is obvious that under such conditions a
party line having a great many stations will be subject to very grave
interference between the parties, people desiring to use the line for
business purposes often being compelled to wait an undue time before
they may secure the use of the line.
It is obvious, therefore, that the amount of traffic on the line,
whether due to many short conversations or to a comparatively few
long ones, is the main factor that should determine the number of
stations that, economically, may be placed on a line. The facilities
also for building lines enter as a factor in this respect, since it is
obvious that in comparatively poor communities the money may not be
forthcoming to build as many lines as are needed to properly take care
of the traffic. A compromise is, therefore, often necessary, and the
only rule that may be safely laid down is to place as few parties on a
given line as conditions will admit.
No definite limit may be set to apply to all conditions but it may be
safely stated that under ordinary circumstances no more than ten
stations should be placed on a non-selective line. Twenty stations
are, however, common, and sometimes forty and even fifty have been
connected to a single line. In such cases the confusion which results,
even if the talking and the ringing efficiency are tolerable, makes
the service over such overloaded lines unsatisfactory to all
concerned.