Signaling is a very important part of a telephone
call. Without signaling, a call cannot be sent through the telephone
network from one end to the other. Signaling methods were developed
to properly send information on who the caller wants to call (the
called party) and who the caller is to the network (identification for
billing). The following are examples of various technologies, both
past and present that were or are still used within the North American
Signaling System Technologies
A brief overview of the history and operation of
telephone signaling technologies.
Pulses (interrupting current on toll trunk lines), especially on
inter-office trunk lines, were common in the days of step by step and
crossbar systems. These were used to pulse out dialed digits, either
directly or via pulse senders from a central office switch, to the
destination switch. Though not in common use today, these may still do
exist in rare instances outside of North America.
DC Voltage and DC Polarity
Voltage changes and polarity changes (as well as pulses) were used in
some inter-office signaling routines. These were sometimes found in
Panel systems when communicating to Panel "tandem" systems. Never was
in common use and are not used anymore.
MF (Multi-Frequency) signaling was developed in the 1930's for
signaling between central office and toll tandem systems. Special
dual frequency tones (not the same as Touch Tones) are used to send
information of the called telephone number throughout the network.
These tones have to be within the band of normal voice communications
in the telephone system (300Hz to 3000Hz). The information is sent to
either tandems and then to local central offices, or from central
office to other central offices directly. The format of a typical MF
sequence on most calls is:
where KP = Key Pulse and ST = Start. Most MF toll
trunks in the US and Canada use 2600 Hz as supervision and for idle
line. Some places used 3750 Hz (out of band) for supervision/idle line
An abbreviated listing of MF tones in the North
American network (sometimes referred to as R1 signaling) are shown in
the chart below. KP2 is not normally used in domestic traffic.
MF signaling is still used in limited amounts in
the US and Canada, though it is not used as much in most toll systems
as it had been in the past. An international version of MF signaling
(sometimes referred to as R2 for bi-directional signaling) is still in
widespread use throughout the world.
Common Channel Inter-Office
Signaling / Signaling System 7
CCIS (Common Channel Inter-Office Signaling)
started in the late 1970s as a way to send more information between
tandems and switches in the network. Sending a call via MF takes up to
15 seconds on a domestic phone call. Also, MF lead itself to toll
fraud since making MF tones is fairly easy to do.
CCIS was invented to send information "out of band" on a data circuit
parallel to the voice circuit. This way the call can be set up and
completed in a shorter amount of time, be able to send more
information, and avoid toll fraud - all at the same time.
The early commonly deployed version of CCIS was version 6. The modern
version of CCIS is version 7 - commonly called SS7 (Signaling System
7) where there are 7 "layers" of the signaling network. SS7 is in
widespread use in the USA, Canada and many industrialized countries in
the world including the UK, Australia and others.
Network Systems Pages
Switching Systems - Main Page
Central Office Switching Systems
Central Office Switching Systems
Network Signaling Technology
Network Transmission Technology
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Page last modified December 20, 2008
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