History
of Signaling
By:
Lysandria G Serrano
Introduction
For those of us in the world of technology, specifically the
telecommunications world, who are NOT engineers but may work in the supporting
roles such as myself, may struggle with learning just what exactly is signaling
and where does it originate? I am
currently completing my MBA in Organizational Development and am building my
career in a great telecommunications company that specializes in a technology called
Diameter. So here is what I have learned
with the help of a great textbook and the fantastic minds of our
Products/Solutions Engineers for Diametriq.
Origination
If you who are looking for a better understanding of
signaling within the world of Diameter, it may help to understand just where
signaling originated. Upon researching, it was discovered that all routes of
information on the history of Signaling came from the same source: Signaling
System No. 7 (SS7): Protocol, Architecture, and Service, authored by Mr.
Lee Dryburgh and Jeff Hewett. Below are
some excerpts from this book to help aide in understanding where Signaling
originated.
To appreciate signaling in today's network and its role in
future networks, let's examine the history of signaling. The history of
signaling has been inextricably linked to the history of telecommunications
and, in particular, switching. As telecommunications advances, so do the
signaling systems that support it.
What is signaling? The International Telecommunications Union
Telecommunication Standardization
Sector (ITU-T) defines signaling as: "The exchange of information
(other than by speech) specifically concerned with the establishment, release
and other control of calls, and network management, in automatic
telecommunications operation." SS7 was defined as an
international standard by ITU-T in its 1980 (Yellow Book)
Q.7XX-series recommendations.
In telecommunications, the network's components must indicate
(that is, signal) certain information to each other to coordinate themselves
for providing services. As such, the signaling network can be considered the
telecommunications network's nervous system. It breathes life into the
infrastructure. Richard Manterfield, author of Telecommunications
Signaling, has stated this poetically:
"Without
signaling, networks would be inert and passive aggregates of components.
Signaling is the bond that provides dynamism and animation, transforming inert
components into a living, cohesive and powerful medium."
1889-1976
The earliest telephone switches were manual; operators used a
switchboard and wire cords to connect and disconnect all calls. The first
manual exchange occurred in 1878 in New Haven, Connecticut. It was introduced
to avoid the imminent problem of running wires from each telephone to every
other telephone (a fully meshed topology). The first manual switch appeared in
Great Britain in 1879. It was also within this same year that subscribers came
to be called by numbers rather than by names. Within a decade of introducing
the manual switch, the United States had 140,000 subscribers and a staggering
8000 exchanges—that is, a switch for every 17.5 subscribers!
A subscriber who was connected to a manual switch would crank
a lever to electronically send an alerting signal that lit up a bulb on the
operator's switchboard. The operator would then connect her telephone to the
calling line, and ask for the called number. Next the operator would connect
her telephone to the called line, where she would place a ringing signal. If
the called party answered the call, the operator would establish the connection
by plugging in a cord between the two terminal jacks on the switchboard.
Signaling, as we know it today, began around 1889 with the
invention of the Strowger exchange (which was patented 1891). The Strowger
exchange was an electromechanical device that provided automatic switching
using the simple idea of two-motion selectors for establishing calls between
two subscribers. It was also known as a step-by-step switch because it followed
pre-wired switching stages from start to finish. Strowgers' dial telephone is
considered the precursor of today's touch-tone phone. It had three buttons: one
for hundreds, one for tens, and one for units. To call the number 322, the
caller had to push the hundreds button three times, the tens button two times,
and the units button two times. In 1896 the Automatic Electric Company developed
a rotary dial to generate the pulses. This method of transmitting the dialed
digits became known as pulse dialing and was
commonplace until the latter half of the twentieth century, when tone dialing became available.
Even in Great Britain in 1930, the majority of all local and
long distance calls were still connected manually through an operator. But
gradually, calls placed between subscribers served by the same local switch
could be dialed without the help of an operator. Therefore, only subscriber signaling
was required because an operator would perform any inter-switch signaling
manually. In the decades that followed, it became possible to dial calls
between subscribers who were served by nearby switches. Thus the requirement
for network signaling was born. Most large U.S. cities had automatic
exchanges by 1940.
Direct Distance Dialing
(DDD) was introduced in the United States in
the 1950s. DDD allowed national long distance calls to be placed without
operator assistance, meaning that any switch in the United States could route
signaling to any other switch in the country. International
Direct Distance Dialing (IDDD) became possible in the 1960s, thus
creating the requirement for signaling between international switches.
1976 to Present
Day
Another form of signaling was introduced in 1976: Common Channel Signaling (CCS). CCS has been used to
implement applications beyond the scope of basic telephone service, including
Intelligent Networks (INs), supplementary services, and signaling in cellular
mobile networks. As you will learn, SS7 is the modern day CCS system that is
used for network signaling. As with any technical subject, signaling can be
split into a number of classifications. The broadest classification is whether
the signaling is subscriber or networked signaling.
Network signaling takes place between nodes in the core
network. This is generally from the local switch, through the core network, and
to the destination local switch—in other words, between the calling and the
called party switch.
In comparison, a modern core network must perform very
complex signaling, such as those to support database driven services like Local
Number Portability (LNP), credit or calling card validation, and cellular
roaming. Therefore, subscriber signaling systems are simple compared to modern
network signaling systems.
Network signaling was previously implemented using Channel Associated Signaling (CAS) techniques and
systems. However, for the past two decades, it has been replaced with Common Channel Signaling (CCS) systems. Signaling System
No. 7 (SS7) is almost the exclusive CSS system; thus, CCS can almost be taken
to refer exclusively to the use of SS7. SS7 is both network architecture and a
series of protocols that provide telecommunications signaling.
SS7 to Diameter
When cellular/mobile networks were being built the SS7
technology was built into its architecture, the 2G mobile core. This architecture
in the 2G mobile core uses SS7 to control signaling that controls communication
from voice to data network elements then to the centralized database.
As 2G grew and evolved into 3G, the SS7 controlled the
signaling communication between the voice networks and the centralized
databases. What about data networks? The
SS7 was utilized in the 3G in conjunction with Diameter to control signaling to
communicate from the data networks to the centralized databases. This is the point to where Diameter is
introduced in the signaling realm.
What is Diameter? In layman terms Diameter is the evolved
product of SS7 and controls the signaling within networks of voice, data and
multi-media services. In today's 4G/LTE world Diameter has now overtaken SS7
and controls the signaling communication from the data network to the
centralized databases. Diameter also
does this same control of signaling from the voice networks to the data
networks then to the centralized databases.
In closing it is truly amazing when you look back at how
signaling has evolved trough time. Beginning with signals telling an operator
to connect one line to another for people to communicate; to the simple light
handed tough of a button or touch screen to relay a text, picture or voice to
another. Behind the scenes, or shall we say "behind the network"
there is an ongoing evolution in the mobile core world of how voice and data is
controlled and sent to centralized databases that allows us to stay connected
in our small networked worlds!
Resources
ITU-T Rec. Q.7 http://www.itu.int/rec/T-REC-Q.700/en
ITU-T Rec. Q.9 (11/88) Vocabulary of Switching and
Signaling Terms.
Manterfield, R. Telecommunications
Signaling. New York, New York; IEEE Publishing, 1999.
Dryburgh, L., Hewett, J. Signaling
System No. 7 (SS7/C7): Protocol, Architecture, and Services
Dan Wonak
