PDF of this tutorialWhile most people naturally think of telephony in terms of the voice content that forms the heart of their person-to-person conversations over the phone, in reality, the telephone system could not exist without the underlying signaling mechanisms that provide network-wide coherence, interoperability, and nonvoice communications.
Call Setup and Supervision
In the setup and establishment of any voice connection, signaling provides everything from the caller’s initial dial tone, through the addressing functions involved in dialing, to the ringing of the recipient’s telephone. In the process, the underlying signaling system also has to gracefully handle a variety of conditions and provide appropriate responses, such as busy signals, error-tones, distinctive ringing, call-waiting indicators, and/or caller I.D. functions. Once the connection is established, the signaling system also continues to stay involved for the duration of the call to provide critical billing and system management functions such as definitive answer supervision, call-waiting notification, and disconnection indicators.
Digital Set Control
In addition to supporting POTS functions, today’s signaling schemes also must provide for controlling feature-rich digital telephony equipment at the customer’s premises. These requirements can include functions such as turning line indicators on and off, providing alphanumeric display messages, setting special features, automatic forwarding to voice mail, managing ISDN functionality, etc.
Node-to-Node Communication
Going beyond individual telephones, the signaling system is responsible for node-to-node communications such as enabling switches (including private branch exchanges [PBXs]) to “talk” to each other. These functions can include the following:
- Defining and assigning trunk paths
- Handling loop-back testing functions, providing routing instructions for number portability
- Dynamically adapting the overall network to the current state of each system, such as in-service or out-of-service status of line cards.
Dealing with network anomalies is a critical signaling capability that is only appreciated when things go wrong. Routing calls around failures is an obvious requirement, but the network also is expected to deal gracefully with externally induced events. A simple example would be a radio contest that results in a huge flood of simultaneous calls.
Network Management
At the network level, modern message-based signaling methods can provide the flexibility to dynamically manage high-level network functions, such as provisioning new services, remotely updating software versions, and maintaining network security systems.
