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SECTION 4
NETWORK-CONTROL SIGNALING
INTRODUCTION
The network-control signaling functions are associated with the
initiation, placing, answering, and charging of calls over the switched
network.
Malfunctions can causeincompleted calls, or calls completed to
other than the intended terminaL Processing such calls reduces the
capacity of the network to serve "normal" calls. The effects of these
malfunctions may be felt by all users of the system, not just those
originating and answering imperfect calls.
The present state of the switched telephone network does not
permit easy identification of the source of this kind of malfunction; that
is, to locate it as occuring in the subscriber's station or in the central
office. Carrier-maintenance personnel, tests, and administrative procedures
become involved in the attempt to localize these malfunctions as they come
to light. .
Consequences of Improper NetworkcControl Signaling
The consequences of improper network signaling pervade the entire
network and can be grouped into the following categories:
(a) Wasteful use of central office and
transmission facilities
(b) Annoyance to other users
(c) Incorrect billing
(d) Wasted testing and maintenance effort
(e) Added administrative expense
Following are examples of each category:
1. Wasteful Use of Central Office and Transmission'Facilities.
Wrong numbers caused by a faulty network-control signaling
unit represent a waste of switching equipment and a source
of annoyance to those who are wrongly called.
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Furthermore, a wrong number resulting from faulty signaling
can cause a call to end up in the wrong city. In the near
future, a wrong number may tie up, for a time, a trans-
atlantic cable or satellite trunk connection. There are
other sources of faulty control signaling. If, when a
call is completed, the switch hook contacts fail to open
properly, or some extraneous impedance remains bridged
across the line, it is arranged that the connection will
release after a time-out of thirty seconds. This is thirty
seconds during which the circuits are not available to other
users.
2. Annoyance to Other Users. In the example mentioned above, in which
the call is not released properly, the user himself will be
unable to place calls during this interval and others trying
to reach him will receive busy signals.
3. Incorrect Billing. On a two-party line, the billing equipment
at the central office recognizes which party is making a
call because there is a high-impedance DC connection to
ground on one side 9f the line. If the connection is not
made in the telephone, or if the telephone is installed or
maintained improperly, the wrong party will be charged for
some calls. On lines with more than two parties, more
complex party identification schemes are used, which
depend upon the telephone instrument having particular
identifying characteristics that differ from the
instruments on the same line.
4. Added Testing and Maintenance Effort. When excessive wrong
numbers occur, action must be taken to identify the source.
It might be on the loops, in the line circuit, or in the
central office. On the other hand, it might be in the
network-control signaling unit. The user unable himself
to determine where the problem is located will normally
call the telephone company. Faulty network contact signaling
often shows up as an intermittent trouble. These are the
hardest to trace and to diagnose.
·5. Added Administrative Effort. Improper network-control
signaling can result in customer demands for credit
against his telephone bill due to false charges.
Since the source of the trouble, as previously mentioned,
is difficult to trace and correct, the added administra-
tive effort required can be considerable.
Conclusions
Improper network-control signaling leads to inaccurate billing,
wasteful use of the telephone plant and administrative effort, as well as
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In planning for the use of user-owned network-
a nnoyance tn other users.
the quality of network-control signaling
control signaling devices,
must be preserved.
AT&T Company·Expetiance with NetwotkCC6ntt6lSignaling
The only available reliable source of information to the Panel
on network-control signaling·is experience With this function in the
operation of the sWitched telephone network. In this section, information
and data furnished by AT&T are summarized.
Dial-Pulse Signaling
Network-control signaling failures are largely related to the
familiar rotary dial. Sources of trouble here are:
Finger wheel and stop
(a)
(b) Contact
Mechanism
(c)
(d) Noise
(e) Other
The dial mechanism itself was the most frequent source of
difficulty. The mechanism is required to operate at speeds nominally
between 9.5 and 10.5 pulses per second and with a percentage break
between 58 and 64 percent. Generally, the units used by the Bell System
fail in such a manner as to fall outside the percentage-break tolerances.
This type of failure can lead to dialing wrong numbers.
Data on units supplied by others is sketchy. AT&T and Bell
Laboratories, however, had reported experience with some equipment they
have tested and found deficient. For example, one unit tested had a
low-priced "antique" telephone with these two faults:
(a) Low ringer impedance
(b) Percentage break 67 percent outside
allowable range of 58 - 64 percent
The first fault is attributable to poor design. The second
may indicate either poor design or maladjustment. Bell has also tested
commercial answering machines and repertory dialers. Some answering
machines had the characteristic of failing to disconnect promptly. One
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repertory dialer tested exhibited· improper percentage break as a function
of line voltage, missed digits on low line voltage, and had inadequate
interdigital time. A second repertory dialer exhibited dial speed and
percentage-break characteristics that aged beyond specified limits.
On the other hand, general experience with· telephones made by
reputable manufacturers of telephonic equipment has indicated that the
quality of network-control signaling units is on a par with those supplied
by Bell. No comparative statistics are available.
Based upon the statistics provided AT&T, the mean ·time between
failures for Bell station sets is 8.5 years. The mean time between
failures for rotary dials is 46 years and for ringers 59 years.
The combination of rotary dial. and ringer has a mean time between
failures of 26 years.
It is this kind of performance, or better, that must be
realized where new devices and systems are attached to the telephone
system if present network-control performance levels are not to be
degraded.
Touch-Tone Signaling
Touch-tone signaling uses two tones per digit generated by
pushing buttons on the telephone. One tone is selected from four fre-
quencies between 697 and 941 Hz. The second tone is selected from four
frequencies between 1,209 and 1,633 Hz. Both tones must be received by
the central office for it to be accepted as a valid digit. Frequencies
have a + 1.5 percent tolerance. Output power is made a function of line
current-to regulate the received power at the central office for various
loop lengths. Other tolerances are specified to hold the two sets of
tones at appropriate power levels. The unit must operate within tolerance
over a -30°C to +55°C temperature range and during its service life.
Reliable statistics on types and frequency of failures are not
available on touch-tone dialers. Failure of the multi-frequency dialer
due to improper frequency or power level, for example, will not be
interpreted .by the central office as a wrong number. The more likely
condition is a register time-out due to its failure to recognize all
the transmitted digits. This use of central office facilities is
considered relatively insignificant as a harmful effect when compared
to harmful effects due to malfunctioning rotary dials.
We conclude failure of touch-tone (multifrequency) signaling to
be considerably less harmful to the network than failure of dial-pulse
signaling.
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Haintenance Data
In the switched telephone network, network-control signaling is
exercised by the customer through the telephone instrument and over his
wire loop to the central office. It is pertinent, therefore, to examine
available data on station troubles and the costs associated with mainten-
ance and trouble clearing. The following data were supplied to the Panel
by representatives of AT&T.
In 1967, Bell had 42,586,551 customer-trouble reports - 27,392,760
troubles were found as a result. These troubles broke down as follows:
8,608,962 30.8%
Station set
Other station
equipment 4,302,696 15.4%
Station wiring 4,802,760 17.2%
Outside plant 5,390,924 19.3%
Central office 2,485,913 8.9%
Customer action 6.4%
1,801"505
System study of station troubles made in 1966 showed the
~~ell
folloWing breakdown:
Trouble rate/100 stations/month
0.21
Cord
0.18
Dial
0.14
Ringer
0.12
Key and lamp
Mounting and plastic 0.12
0.08
Circuit
0.03
Receiver
Transmitter 0.03
0.03
Other
Whether these data reflecting carrier experience would be valid
for customer-furnished station equipment, would depend on the performance
of this equipment relative to that furnished by the common carrier. It
would also depend on the extent of use of touch-tone control instead of
rotary dial.
FAULTY NETWORK-CONTROL SIGNALING WITH USER-OWNED EQUIPMENT
It is difficult to evaluate the effect of interconnection on
network-control signaling, since it is not known at present what precrse
instrumentalities users will employ for this function. Network-control
signaling performance is closely related to the very detailed design and
performance of the device used (switch hook, rotary dial, touch-tone pad).
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The best that can be done, therefore, is to cite present experience
of the carriers using their own devices. Starting from this as a reference
point, it may be postulated that devices owned and used by customers will
be either (a) as good as, or (b) poorer than, these carrier-furnished
devices. The consequences of these assumptions are drawn in the following
section.
ECONOMIC PENALTIES OF NET CONTROL SIGNAL-DEVICE FAILURES
Data on Bell System rotary dial and ringer units show a mean
time between failures O1TBF) of 26 years. This is equivalent to
a failure rate of 0.0385 per year.
Except for the special case in which competent maintenance personnel
are continually at hand, trouble visits will be required and costs will be
incurred and must be paid for.
Some vendors and users might be satisfied with a seemingly
reasonable, though lower, MTBF. Reliability, however, has a profound
impact on network operation and cost. Based on a large volume and using
the Bell System experience of $15 per maintenance visit, Table 1 shows
the annual average per phone cost for maintenance alone as a function of
MTBF. The distribution of this cost between the user and the carrier
cannot be determined at this time; however, it represents a substantial
factor to be considered in specifying the performance of network-control
signaling units.
TABLE 1
Annual Maintenance Cost
MTBF Allocated·to Each Phone
26 years .57
$
.75
20 years
15 years 1.00
10 years 1.50
5 years 3.00
1 year 15.00
Another cost (to the carrier) associated with improper network-
control signaling failures is that attributed to wrong numbers, wrong toll
charges, etc. It is difficult to estimate the frequency of such occurrences
as a function of MTBF.
A third cost associated with network-control signal-unit failures
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is that due to false calls for assistance by the user. Where limited
free interconnection has been permitted in the past, it has been the
experience of the carrier that he is frequently called to perform the
maintenance when, in fact, the interconnected equipment is at fault.
This phenomenon can be expected to persist with any form of
interconnection in which a specific ~nterface between vendor equipment
and the telephone company is not clearly defined.
The three types of costs described are a function of the MTBF
of the net-control signal unit. The costs are very significant when
evaluated in terms of a large number of subscribers. These costs will
be borne by both users and the carrier; since some costs cannot be
easily allocated.
CONCLUSIONS
Net-control signaling is a critical element, and a high order of
reliability is necessary to avoid loss of net performance and excessive
costs to both carrier and user~
