Signaling has at two basic functions: control and transmission of information.
Control SignalingThe visual illustrates signals that control something or someone, such as traffic signals at intersecting streets and at railroad crossings. Other examples of signaling used for control purposes are abundant. A referee at a sports event uses a whistle to signal stoppages of play to the athletes. When using a remote control with a television set or VCR, a controlling signal is sent to change the channel or play a tape. Finally, a buzzer going off in a fast food restaurant signals that the fries are done. These, as well as other everyday examples too numerous to mention here, illustrate the use of signaling for control.
In transmitting information, for example, semaphores or signal flags can be used to send alphanumeric characters to another party. Morse code can be used to send characters by radio or light waves. Television signals are broadcast as radio waves through the ether or over coaxial cable to millions of homes each day.
The telephone network depends on signaling for both control and information transfer. To alert the called subscriber to the presence of an incoming call, a signal is sent to the telephone causing the phone to ring. A call appearance might also light up on the phone, but the primary signaling event is ringing. This is an example of signaling used for control in the network.
When a subscriber places a telephone call, the network must be informed of the called party’s address. The user provides it by dialing the telephone number. This number is used by the network to route the call. This is an example of signaling used to pass information, specifically address information, across the network.
Control signaling has two primary functions in the telephone network: call establishment/disestablishment and call progress.
Call establishment/disestablishment includes the procedures necessary to establish and manage telephone calls. These procedures appear in the following list.
- Signaling a request for service (off-hook)
- Signaling the network’s readiness to service a request (dial tone)
- Sending the called party’s address (customer dialing and transmission of address information between switches)
- Signaling the seizure of an idle trunk between switches
- Monitoring the state of a call and sending control signals that indicate a change of state (supervision)
Signaling call progress includes notifying the called and calling parties of disposition of the call. The following list shows these procedures.
- Ringing the telephone
- Providing audible ring to the caller
- Notifying the caller of a blockage in the network (reorder tone or announcement)
- Notifying the caller that the called party’s line is busy (busy tone)
- Providing information to the caller about network inability to complete the call for some other reason (disconnected number, change in area code, etc.)
If a customer dials a call, the call is not completed, and the customer receives no indication of why this occurred, this condition is called “high and dry.” This undesirable condition can occur, but the network engineers have gone to great lengths to avoid it.
Telephone Signaling Relationships
We can identify signaling relationships in telephony by first identifying the two parts or parties that originate and terminate the signals, and then looking at the types of events that occur.
First, there is signaling between a user and the network. Signaling across this interface includes off-hook, dial tone, ringing, and tones and announcements.
Second, to set up a call requires that network nodes (or switches) signal one another. Some of the same kinds of events need to be signaled, as in the user-to-network case, but the signaling must account for the fact that both endpoints are machines. For example, just as a customer signals initiation of a call by going off-hook, one switch must signal call initiation to another by seizing a trunk between the two. Similarly, a switch must signal its readiness to receive digits and route the call. Finally, digits must be transferred between switches, just as between a user and a switch.
The last signaling relationship is between two users. This occurs only after successful call setup and is transparent to the network. It is the responsibility of the network to establish a connection between two user telephones. Once that task is completed, what and how the parties communicate is irrelevant to the network.
Signaling in the Public Network
Customer line signaling has developed rapidly in the past century. The magneto’s hand-powered crank has been replaced by direct current (DC) batteries that power the connection from the central office (CO) to switchhooks on customers’ sets. Addressing has evolved from the spoken word to dial pulses (i.e., the digits 0–9) and subsequently, paired frequency tones (i.e., the digits 0–9 plus special characters # and *). Information gathering, an activity once limited strictly to telephone operators, can be performed electronically, stored in a database, and invoked automatically. Finally, the Integrated Services Digital Network (ISDN) continued the evolution with an elaborate mechanism for message-oriented, user-to-network signaling on the out-of-band, associated D-Channel.
The necessity of managing interoffice facilities for supervisory and addressing functions has existed as long as there have been two telephone sets separated by many miles. Interoffice signaling and network control advances have been introduced more quietly in recent years. Yet these might have a much greater impact on future networks than developments in customer access signaling, switching, or transmission.
In the early 1970s, the majority of interoffice transmission facilities were carrier systems such as the analog L-carrier or digital T-carrier. In all carrier systems, each trunk carried its own signaling information, referred to as per trunk signaling.
Furthermore, analog systems employed in-band signals in the form of frequency-dependent tones that occupied the same passband as the voice traffic. For these systems, a single frequency (SF) tone of 2600 hertz (Hz) was used to indicate busy/idle status of an interoffice trunk. Multifrequency (MF) tones were used for the more involved task of passing addressing information. MF is similar to dual tone multifrequency (DTMF) signaling, which is used to provide touch-tone dialing, in that pairs of selected frequencies are employed to pass distinct digits/characters.
T-carrier systems use a variant of in-band signaling by “robbing” bits from every sixth frame to pass supervisory signals. E-carrier systems implement a separate signaling channel.