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Overlay Carriers
Overlay Carriers build networks which overlay the traditional PSTN. While such networks are unusual in highly developed countries, they are common in developing nations. In some countries of Eastern/Central Europe, for example, private carriers have been franchised to build overlay digital microwave networks to provide service to large government, education and commercial organizations. In such cases, the PSTN simply could not be upgraded quickly to provide satisfactory communications, which are considered vital to economic growth.
Wireless Carriers
While wireless traditionally has been limited to mobile radio, cellular telephony and wireless data networks, wireless carriers will soon challenge the LECs through Personal Communications (PCS) and Wireless Local Loop (WLL) technologies. The ability to bypass the incumbent telephone company on a wireless basis is a significant competitive threat. In a number of developing countries, however, PCS/WLL is likely to be the primary means by which the telephone companies extend their networks in order to provide more universal service. Examples abound in Asia and Central/Eastern Europe.
In order for the network to function properly, the various components or of the network must have the capability to signal each other, indicating their status and condition. Typical status indications might include available (dial tone), unavailable (busy), and available (ringing signal).
Additionally, the devices must pass identification information, as well as certain instructions through the network, perhaps as far as to the receiving terminal. Such information and instructions might include originating number or circuit, and target number, based on dialed digits. Within the carrier network, such information includes route preference and route availability [6-3]. Additionally, it is necessary that the network determine and honor the end users designated IXC carrier in order that a long distance call can be handed-off to the carrier of choice.
Signaling and control systems and networks also are responsible for handling billing matters, perhaps querying centralized databases in the process. Billing options might include bill to originating number (DDD and WATS), bill to terminating number (INWATS or 1-800/1-888), bill to third number (third party) and bill to calling card, with verification of PIN against a database.
Finally, certain network management information is often passed over signaling and control links. Such information is used for remote monitoring, diagnostics, fault isolation, and network control. In this fashion, the network can be monitored from a centralized operations center, 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, the network element in difficulty often can be instructed to resolve the problem, perhaps by resetting or reinitializing itself, or by disabling a failed port and activating a standby port.
In-band signaling and control functions take place over the same physical path as the conversation, and occupy the same frequency band. As the impact of simultaneous conversation and signaling is most unpleasant, it is seldom used in contemporary networks, with the exception of analog local loops.
By way of example, the author frequently calls long distance to talk with his son in East Texas. During those conversations, the young man has a habit of accidentally depressing the buttons on the tonepad with his chin. The resulting multifrequency tones, interpreted by the network as a priority instruction set, interfere with the conversation as they occupy the same range of frequencies at the same moment in time.
Out-of-band signaling and control, in the simplest analog application, takes place over frequencies separate from those that carry the information; therefore, there is no interference between the two functions. In a digital network, signaling and control occupies separate, designated time slots; once again, interference is not an issue. Out-of-band signaling and control is the standard approach in digital networks and, most certainly, in the internal carrier networks.
Common Channel Signaling and Control (CCS) systems are used by the carriers in order to carry large volumes of signaling and control information in support of high-traffic networks. CCS links are digital in nature and involve high-speed links that generally are fiber optic. The CCS network is, in fact, a highly robust subnetwork that supports the operations of the primary communications network. The CCS network often is in the form of a separate network which connects the various network switches to centralized computer systems of significant intelligence and on which reside very substantial databases. Through the use of centralized intelligence supported by carefully synchronized databases, the operations of an entire network can be controlled and its performance monitored, from end to end. While complicated and expensive to design and deploy, CCS networks are more effective and less costly than the alternative of placing lesser levels of intelligence in each of the various network switches, each of which would be required to perform redundant processes in complete harmony.
Signaling System 7 (SS7), the current version, was developed and is deployed based on ITU-T standards. Thereby, all carriers can achieve and manage interconnection on a standard basis (Figure 6.5). SS7 significantly speeds call setup and call completion processes. Additionally, SS7 is responsible for the delivery of many enhanced custom calling features often associated with ISDN. These CLASS (Customer Local Access Signaling Services) services include Caller ID, which recently has been enhanced in some areas as Name ID, including the name of the calling party as listed in the telephone directory. Other services include Selective Ringing (or Priority Ringing), Selective Call Forwarding, Call Block (or Call Screen), Repeat Dialing, Call Trace, and Automatic Call-Back (Call Return). In 1995, most RBOCs planned to have SS7 installed in at least 90% of their backbone networks [6-1].
Figure 6.5 Signaling System 7 (SS7) in support of LEC and IXC networks, interconnected.
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