SIGNALING

1. SIGNALING

2. To establish a telephone call, a series of signaling messages must be exchanged. • There are two basic types of signal exchanges: • (1) between the user and the network and (2) within the network. Both types of signaling must work together to establish the call. In this section we consider the signaling that takes place inside the network. • In general, the signaling messages generate control signals that determine the configuration of switches; that is, the messages direct a switch to a state in which a given input is connected to the desired output. • As shown in Figure 4.38, in traditional networks signaling information would arrive in telephone lines and be routed to the control system. • Initially, hard-wired electromechanical or electronic logic was used to process these signaling messages. The class of stored program control (SPC) switches emerged when computers were introduced to control the switch, as shown in Figure 4.38.

4. Through the intervention of the stored-program control computer, a request for a call would come in, a check would be made to see whether the destination was available, and if so, the appropriate connection would be made. • The use of a program to control the switch provided great flexibility in modifying the control and in introducing new features. • As shown in Figure 4.39, setting up a call also required that the computers controlling the switches communicate with each other to exchange the signaling information. • A modem and separate communication lines were introduced to interconnect these computers. This situation eventually led to the introduction of a separate computer communications network to carry the signaling information.

6. Consider the operation of the signaling network. Its purpose is to implement connectivity between the computers that control the switches in the telephone network by providing for the exchange of messages. Figure 4.40 shows the telephone network as consisting of two parts: a signaling network that carries the information to control connections and a transport network that carries the user information. Communications from the user are split into two stream service switching point (SSP).

8. The signaling information is directed toward the signaling network where it is routed and processed as required. The signaling system then issues commands to the switches to establish the desired connection. • The signaling network functions much like the ``nervous system'' of the telephone network, directing the switches and communication lines in the network to be configured to handle the various connection requests. The second stream in the SSP consists of the user information that is directed to the transport network where it flows from one user to the other. Note that the signaling network does not extend to the user because of security concerns. • Separate user-to-network signaling procedures are in place

9. The function of the signaling network is to provide communications between the computers that control the switches. The computers communicate through the exchange of discrete messages. • The best way of implementing such a network is through a packet-switching network that transfers information in the form of packets between network elements. The introduction of a packet-switching • signaling network in digital telephony is important, since it is at this point that the evolution of the signaling network for telephony converges with the evolution of computer networks.

10. Because uninterrupted availability of telephone service is extremely important, reliability was built into the packet-switching network for signaling. The • packet-switching nodes (signal transfer points or STPs) are interconnected as shown in Figure 4.40. Any given region has two STPs that can reach any given office, so if one STP goes down the other is still available. • The processing of a connection request may involve going to databases and special purpose processors at the service control points (SCPs) in Figure 4.40. • As signaling messages enter the network, they are routed to where a decision can be made or where information that is required can be retrieved. • In the early 1970s telephone companies realized that the signaling network (and its computer control) could be used to enhance the basic telephone service. • Credit-card calls, long-distance calls, 800 calls, and other services could all be implemented by using this more capable signaling network

11. In the case of credit-card calls, a recorded message could request the credit-card number. The digits would be collected, and a message would be sent to a database to check the credit-card number; if authorized, the call would then be set up. • Telephone companies use the term intelligent network to denote the use of an enhanced signaling network that provides a broad array of services. These services include identification of the calling person, screening out of certain callers, callback of previous callers, and voice mail, among others.

12. As shown in Fig4.41, the addition of new devices, ``intelligent peripherals,'' to the intelligent network enables other new services. • For example, one such device can provide voice recognition. When making a call, your voice message might be routed to this intelligent peripheral, which then decodes what you are saying and translates it into a set of actions that the signaling network has to perform in order to carry out your transaction

13. Another service that intelligent networks provide is personal mobility.Personal mobility allows the user who subscribes to the service to have a personal ID. Calls to the user are not directed to a septic location in the network. Instead the network dynamically keeps track of where the user is at any given time and routes calls accordingly

14. Signaling System #7 Architecture • The Signaling System #7 (SS7) network is a packet network that controls the setting up, managing, and releasing of telephone calls. • The network also provides support for intelligent networks, mobile cellular networks, and ISDN. • The SS7 network architecture is shown in Figure 4.42. This architecture uses ``parts'' instead of ``layers.'' The message transfer part(MTP) corresponds to the lower three layers of the OSI reference model.

16. Level 1 of the MTP corresponds to the physical layer of the signaling links in the SS7 networks. Physical links have been defined for the following transmission speeds. E-1 (2.048 Mbps = 32 channels at 64 kbps each), T-1 (1.544 Mbps = 24 channels at 64 kbps each), V-35 (64 kbps), DS-0 (64 kbps), and DS-0A (56 kbps). • MTP level 2 ensures that messages are delivered reliably across a signaling link. This level corresponds to the data link layer in the OSI reference model. • The MTP level 3 ensures that messages are delivered between signaling points across the SS7 network. Level 3 provides routing and congestion control that reroutes traffic away from failed links and signaling points.

17. The ISDN user part (ISUP) protocols perform the basic setup, management, and release of telephone calls. The telephone user part (TUP) is used instead in some countries. • The MTP addresses the signaling points, but is not capable of addressing the various applications that may reside within signaling point. These applications include 800-call processing, calling-card processing, call management services, and other intelligent network services. The signaling connection control part (SCCP) allows these applications to be addressed by building on the MTP to provide connectionless and connection-oriented service.

18. The SCCP can also translate ``global titles'' (e.g., a dialed 800 number or a mobile subscriber number) into an application identifier at a destination signaling point. • This feature of SCCP is similar to the Internet DNS and assists in the routing of messages to the appropriate processing point. • The transaction capabilities part (TCAP) defines the messages and protocols that are used to communicate between applications that use the SS7 network. • The TCAP uses the connectionless service provided by the SCCP to support database queries that are used in intelligent networks.