Chapter 1. The Evolution of Signaling

This chapter is intended to provide a sound introduction to the world of telecommunications signaling. It is particularly written for those readers who have little or no signaling knowledge. It provides a solid foundation to help you grasp signaling ideas, concepts, terminology, and methods. A strong foundation will provide the novice reader with a better understanding of the book's main topic: Signaling System No. 7. Today, Signaling System No. 7 is the most advanced and widely used signaling system for both cellular and fixed-line telecommunications networks.

This chapter covers the following topics:

• What signaling is and why it is relevant

• Overview of subscriber and network signaling

• The history of signaling and the development of the Public Switched Telephone Network (PSTN)

• Overview of the Channel Associated Signaling (CAS) method of signaling and its common implementations

• Overview of the Common Channel Signaling (CCS) method of signaling and its operational modes

• The limitations of CAS and CCS

Signaling System No. 7, known more commonly in North America as SS7 and elsewhere as C7, is both a network architecture and a series of protocols that provide telecommunications signaling. In order to begin studying SS7, you must first learn what telecommunications signaling is by studying its origins and purpose.

The ITU-T defines signaling as, [47] "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."

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 [103]:

"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."

For example, if a subscriber wishes to place a call, the call must be signaled to the subscriber's local switch. The initial signal in this process is the off-hook condition the subscriber causes by lifting the handset. The action of lifting the handset signals to the network that the subscriber wishes to engage telephony services. The local switch should then acknowledge the request for telephony services by sending back a dial tone, which informs the subscriber that he can proceed to dial the called party number. The subscriber has a certain amount of time to respond to the dial tone by using the telephone keypad to signal the digits that comprise the called party number. The network signals that it is receiving the dialed digits with silence (as opposed to a dial tone).

Up to this point, the signaling is known as subscriber signaling and takes place between the subscriber and the local switch. Subscriber signaling is also known as access signaling. The "Subscriber Signaling" section of this chapter further describes subscriber signaling.

When a complete called party number is received or enough digits are collected to allow the routing process to proceed, the calling party's local switch begins signaling to the other nodes that form part of the core network.

The signaling that takes place between core network nodes (and switches and, over the past two decades, databases) is known as network signaling.

The purpose of network signaling is to set up a circuit between the calling and called parties so that user traffic (voice, fax, and analog dial-up modem, for example) can be transported bi-directionally. When a circuit is reserved between both parties, the destination local switch places a ringing signal to alert the called party about the incoming call. This signal is classified as subscriber signaling because it travels between a switch (the called party's local switch) and a subscriber (the called party). A ringing indication tone is sent to the calling party telephone to signal that the telephone is ringing. If the called party wishes to engage the call, the subscriber lifts the handset into the off-hook condition. This moves the call from the set-up phase to the call phase.

At some point in the call phase, one of the parties will wish to terminate the call, thereby ending the call phase. The calling party typically initiates this final phase, which is known as the clear-down or release phase. The subscriber signals the network of the wish to terminate a call by placing the telephone back in the on-hook condition; hence, subscriber signaling. The local switch proceeds with network signaling to clear the call down. This places an expensive resource (the circuit) back to an idle condition, where it can be reserved for another call.

The previous high-level example relates to a basic telephone service call; that is, simple call setup and clear down. As you will discover, the signaling network can do far more than carry the digits you dial, release calls, notify the network that you went on or off-hook, and so forth. The signaling network can also translate toll-free numbers into "routable" numbers, validate credit and calling cards, provide billing information, remove faulty trunks from service, provide the support for supplementary services (such as caller ID), allow you to roam with your cellular telephone, and makes local number portability (LNP) possible. This list is by no means exhaustive; see Chapters 3, "The Role of SS7," and 11, "Intelligent Networks (IN)," for more example services.

The main function of signaling is still that of circuit supervision: setting up and clearing down circuits (that is, trunks). Traditionally, once a circuit was set up, no other signaling was performed apart from releasing the call; therefore, all calls were simple, basic telephone service calls. However, modern telephone networks can perform signaling while a call is in progress, especially for supplementary services—for example, to introduce another called party into the call, or to signal the arrival of another incoming call (call waiting) to one of the parties. In fact, since the 1980s, signaling can take place even when there is not a call in place. This is known as non-circuit related signaling and is simply used to transfer data between networks nodes. It is primarily used for query and response with telecommunications databases to support cellular networks, intelligent networks, and supplementary services. For example, in Public Land Mobile Networks (PLMNs), the visitor location register (VLR) that is in charge of the area into which the subscriber has roamed updates the home location register (HLR) of the subscriber's location. PLMNs make much use of non-circuit-related signaling, particularly to keep track of roaming subscribers. Chapter 13, "GSM and ANSI-41 Mobile Application Part (MAP)," covers this topic in more detail.

Network signaling is further described in the "Network Signaling" section of this chapter.