Signaling System No. 7 (SS7/C7): Protocol, Architecture, and Services

Signaling System No. 7

By Lee Dryburgh and Jeff Hewett

The History of Signaling

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.


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. Figure 1-1 shows this process; on the switchboard, each terminal jack represents a subscriber.


Figure 1-1 Simple Call Setup Via a Manual Operator with Automatic Equivalent

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. See "Address Signals" in the "Subscriber Signaling" section of this chapter for a discussion of pulse and touch-tone dialing. It is interesting to note that early users did not like the dial pulse handset because they felt they were doing the "telephone company's job."

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.

From 1889 to 1976, signaling had three main characteristics, which resulted because only basic telephone services were available [102]:

  • Signaling was fairly simple. All that was required of the signaling system was the setting-up and releasing of circuits between two subscribers.
  • Signaling was always circuit-related; that is, all signals related directly to the setting-up or clearing of circuits.
  • There was a deterministic relationship, known as Channel Associated Signaling (CAS), between the signaling and the voice traffic it controlled. The "Channel Associated Signaling" section of this chapter discusses CAS.

1976 to Present Day

Another form of signaling was introduced in 1976: Common Channel Signaling (CCS). The "Common Channel Signaling" section of this chapter further explains CSS.

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. The following sections discuss these types of signaling.

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