Evolution of Signaling

Now that we know what the voice circuit between the switches is, we can talk about how it is established. In the so-called plain old telephone service (POTS), establishing a call is routing, for once the call (for example, an end-to-end virtual circuit) is established, no routing decisions are to be made by the switches. There are three aspects to call establishment: First, a switch must understand the telephone number it receives in order to terminate the call on a line or route the call to the next switch in the chain; second, a switch must choose the appropriate circuit and let the next switch in the chain know what it is; third, the switches must test the circuit, monitor it, and finally release it at the end of the call. We will address the (quite important) concept of understanding the telephone number later. The other two circuit-related steps require that the switches exchange information. In the PSTN, this exchange is called signaling.

Initially, the signaling procedure was much closer to the original meaning of the word—the pieces of electric machinery involved were exchanging electrical signals. The human end user was (and still is) signaled with audio tones of different frequencies and durations.

As far as the switches are concerned, in the past, signaling was not unlike what our telephones do when we push the buttons to dial: switches exchanged audio signals using the very circuit (that is, trunk) over which the parties to the call were to speak. This type of signaling is called in-band signaling, and quite appropriately so, because it uses the voice band. There are quite a few problems with in-band signaling. Not only is it slow and quite annoying to the people who have to listen to meaningless tones, but also telephone users can produce the same tones the switches use and thereby deceive the network provider or disrupt the network.

To prevent fraud and also to improve efficiency, another form of signaling that would not use the voice band was needed. This could be achieved by using for signaling the frequencies that were out of the voice band (thus called out-of-band frequencies). Nevertheless, a channel in the telephone network is limited to the voice band, so there is no physical way to send frequencies beyond the voice band on such a channel. This limitation necessitated out-of-channel rather than out-of-band signaling. It was also obvious that much more information (concerning the characteristics of the circuits to be established, calling and called parties’ numbers, billing information, and so on) was required, and that this information could be stored and passed in the same form that was used for data processing. Hence (1) the information had to be encoded into a set of data structures and (2) these data structures had to be transformed over a separate data communications network. Thus, the concept of common channel signaling was born. Common channel signaling is signaling that is common to all voice channels but carried over none of them. Although it is clearly a misnomer, this type of signaling is often still called out-of-band signaling.

Let’s get back to the question of the switch understanding the telephone number. First of all, there are two types of numbers: those that actually correspond to the telephones that can be called and those that must be translated to the numbers of the first type. An example of the first type is a U.S. number +1-732-555-0137, which translates to a particular line in a particular central office (in New Jersey). An example of the second type is any U.S. number that starts with 1-800. The 800 prefix signals to the switch that the number by itself does not identify a particular switch or line (there is no 800 area code in the United States). Such a number designates a service (called toll-free in the United States or freephone in Europe) that is free to the caller but paid by the organization or person who receives calls.

Handling numbers of the first type is relatively straightforward—they end up in a switch’s routing table, where they are associated with the trunks or lines to be used in the act of establishing a call. The other (toll-free) numbers need translation. Naturally, a switch could translate the toll-free number, too, but such a solution would require tens of thousands of switches to be loaded with this information. The only feasible solution is to let a central database do the translation. The switch then needs to communicate with the database. [Note: The solution was figured out as early as 1979—see Faynberg et al. (1997) for the history.]

Another example where a database lookup is needed is implementation of local number portability (LNP). In the United States, the Telecommunications Act passed by the U.S. Congress in 1996 mandates the right of telephony service subscribers to keep their telephone numbers even when they change service providers. With that, subscribers can keep not only the numbers but also the features (such as call waiting) originally associated with the numbers. In the United States, the solutions are based on switches’ capabilities to query databases so as to locate the terminating switch when they encounter numbers marked as ported. (To be precise, this process requires two database dips—one to determine whether a dialed number is portable and the other to find the terminating switch.)

For both types of communications—out-of-band signaling among the switches and querying the database—the Bell Telephone System has designed a special data network called a common channel interoffice signaling (CCIS) network. When this network was introduced—in 1976—it was used only for out-of band signaling (hence interoffice). Thus the network served as a medium for communicating information about any trunk (channel) without being associated with that particular trunk. In other words, it was a medium common to all trunks, hence the term common channel. In the early 1980s, the network databases were connected to the network; thus signaling ceased to be strictly interoffice, and the I was taken away from the CCIS. Both the network and the concept became known as common channel signaling (CCS).

The architecture of the CCS network is depicted in Figure 1. The endpoints of the system are switches and network databases. The CCS routers are called signaling transfer points (STPs). Since all signaling has been outsourced to it, the CCS network must be as fast and as reliable as the network of the telephone switches. The reliability has been achieved through high redundancy: All STPs within the network are fully interconnected. Furthermore, each STP has a mated STP, with which it is connected through a high-speed link (C-link). Interconnection with other STPs is achieved through a backbone link (B-link). Finally, switches and databases are connected to STPs by A-links.

Figure 1: The common channel signaling (CCS) architecture.

Historically, there are two distinct types of protocols within common channel signaling: (1) interactions between the switches and databases that started as simple query/response messages for number translation and have evolved into service-independent protocols that support multiple services for IN technology; and (2) the protocols by means of which the switches exchange information necessary to establish, maintain, and tear down calls.

The CCS network has evolved through several releases and enhancements in the Bell System, and subsequently other telephone companies, which eventually resulted in multiple CCS networks. To ensure the interoperability of these networks as well as multivendor equipment interoperability in each of them, ITU-T has developed an international standard for common channel signaling. The latest release of this standard is called Signalling System No. 7 (SS No. 7).

Note that the official ITU-T abbreviation of this term is SS No. 7; however, the unofficial (but much easier to write and pronounce) term SS7 is used throughout the industry.We use the official term whenever we refer to the standard or its implementation in the network; we use SS7 when we refer to new classes of products (such as the SS7 gateway).