Traditional Signaling And Control In Telecommunications: “Only in rare instances can a telephone operator know how momentous are the spoken messages for which she established the connections.
Life and death may depend upon the swiftness and sureness with which she handles any call. All Bell System telephone calls are treated as if marked “ urgent. ”
Methods of operation and mechanisms are so planned that every connection may be established with all possible speed.
Bell System service was the fastest in the world, when the average time required to complete a long distance call was seven minutes.
Telephone Almanac , American Telephone & Telegraph Co., 1935
The two basic types of information transfer are user data (content or payload) and signaling and control.
In order for the network to function properly, the various devices, components, or elements of the network must have the capability to signal (i.e., alert and inform) each other, indicating their status and condition.
Typical status indications include available (dial tone), unavailable (busy), and alerting (ringing signal).
The terminal devices also must pass identification information, as well ascertain instructions through the network, perhaps as far as to the receiving terminal.
Such information and instructions might include the originating number or circuit and the target number, based on the dialed digits.
Within the carrier network, such information includes route preference and route availability.
Additionally, the LEC network must determine and honor the end user ’ s IXC designation in order that it can hand off a long distance to the carrier of choice.
Signaling and control systems and networks also handle billing matters, perhaps querying centralized databases in the process.
Billing options might include bill to originating number (e.g., DDD and WATS), bill to terminating number (e.g., INWATS, or toll – free), bill to third number (third party), and bill to calling card with verification of a PIN against a database.
Finally, certain network management information often is passed over signaling and control links.
Such information is used for remote monitoring, diagnostics, fault isolation, and network control.
In this fashion, a centralized Network Operations Center (NOC) can monitor the network, and faults or degradations in performance can be determined and isolated.
Diagnostic routines can then be invoked in order to determine the specific nature of the difficulty.
Finally, a network management system or network manager can instruct the network element in difficulty to resolve the problem, perhaps by resetting or re-initializing itself, or by disabling a failed port and activating a standby port.
