Earlier in this section you learned that telephone networks use two basic types of signaling – analog and digital. While this is true of the way in which voice traffic is transported across the network between users, a variety of additional signaling methods are used in the process of establishing and disconnecting a phone call. Examples of signaling include elements you are likely very familiar with, such as dial tone, a phone ringing, and so on. Two major types of signaling exist phone networks, namely subscriber signaling and trunk signaling, as outlined below:
Subscriber signaling. This type of signaling is used between a PSTN switch and a subscriber telephone.
Trunk signaling. This type of signaling is used on trunk links, such as between PSTN switches, between a PBX and PSTN switch, or between PBXs.
These two major signaling types provide signals in four different categories. Supervisor signaling is used to carry out tasks such as initiating a phone call or placing a call on hold. Address signaling is used to forward the dialed number to the PSTN switch or PBX, using methods you are likely already familiar with, such as touch tone or pulse. Call processing signals are also easy to identify, such as the ringing or busy tones encountered as part of trying to establish a connection. Finally, network management signals are used to control how circuits and switches respond when fully loaded, such as routing traffic over another switch connection or circuit.
Depending on whether a PSTN switch is connected to a digital or analog network, different methods are used to transfer signaling information across trunk links. On an analog link, signaling information is sent across the circuit itself. On a digital trunk link (such as a T1 line), signaling information is typically sent using one of two methods, namely channel associated signaling (CAS) or common channel signaling (CCS). In CAS, signaling information (such as for initiating or terminating a call) is sent over the same channel as the voice call will ultimately use. In CCS, signaling information is sent over a dedicated and separate channel from the voice call. In general, CCS is the more popular option because it is faster and supports a wider variety of services, such as call display. A good analogy in this case would be comparing CCS to services like ISDN, where data or voice travels over one channel (a “B” channel) and signaling and call control information travels over another (a “D” channel).
Some of the common signaling systems used on voice networks include ISDN, QSIG, and SS7. Each of these is listed in more detail below.
ISDN. Originally developed as an all-digital phone network capable of allowing home users to transfer voice, data, and video over a single link, ISDN BRI and PRI are still commonly used on voice networks. For example, in situations where a company has PBXs in distant locations, ISDN links are often used to create a type of virtual private voice network between locations across the PSTN.
QSIG. QSIG is a signaling system that was originally developed to allow PBXs from different vendors to interoperate across a network.
SS7. Short for Signaling System 7, this is an international signaling standard that follows the CCS method outlined earlier. In other words, signaling information is sent over a separate channel rather than the channel used for the voice call. Examples of information passed over this control channel include call control (call setup/teardown), network management functions, as well as features like call display, call forwarding, and so forth.