Telephony Signaling

In a telephony system, a signaling mechanism is required for establishing and disconnecting telephone communications.

Telephony Signaling Types

The following forms of signaling are used when a telephone call is placed via a PBX:

■ Between the telephone and PBX

■ Between the PBX and PSTN switch

■ Between the PSTN switches

■ Between two PBXs

At a high level, there are two signaling realms, as shown in Figure 8-7:

■ Local-loop signaling: Between a PSTN or PBX switch and a subscriber (telephone)

■ Trunk signaling: Between PSTN switches, between a PSTN switch and a PBX, or between PBX switches

Figure 8-7 Telephony Signaling Includes Local-Loop and Trunk Signaling

Local-loop Signaling Trunk Signaling

Corporate Location/

Figure 8-7 Telephony Signaling Includes Local-Loop and Trunk Signaling

Local-loop Signaling

Simple signaling examples include the ringing of the telephone, a dial tone, and a ring-back tone. Following are the three basic categories of signals commonly used in telephone networks:

■ Supervision signaling: Typically characterized as on-hook, off-hook, and ringing, supervision signaling alerts the CO switch to the state of the telephone on each local loop. Supervision signaling is used, for example, to initiate a telephone call request on a line or trunk and to hold or release an established connection.

■ Address signaling: Used to pass dialed digits (pulse or DTMF) to a PBX or PSTN switch. These dialed digits provide the switch with a connection path to another telephone or customer premises equipment.

■ Informational signaling: Includes dial tone, busy tone, reorder tone, and tones indicating that a receiver is off-hook or that no such number exists, such as those used with call progress indicators.

For a telephone call to take place, all three types of signaling occur.

Analog Telephony Signaling

The most common methods of analog local-loop signaling are loop start and ground start. The most common analog trunk signaling method is E&M (derived from a combination of recEive and transMit, and sometimes known as Ear and Mouth). These methods are described as follows:

■ Loop start: Loop start is the simplest and least intelligent signaling protocol, and the most common form of local-loop signaling. It provides a way to indicate on-hook and off-hook conditions in a voice network. The creation of the electrical loop initiates a call (off-hook), and the opening of the loop terminates the call (on-hook). This type of signaling is not common for PBX signaling because it has a significant drawback in which glare—what the telephone industry calls collisions—can occur. Glare occurs when two endpoints try to seize the line at the same time, resulting in the two callers connecting unexpectedly. Because business callers use telephones regularly and the possibility of glare is high, loop-start signaling is acceptable only for residential use.

■ Ground start: Also called reverse battery, ground start is a modification of loop start that provides positive recognition of connects and disconnects (off-hook and on-hook). It uses current-detection mechanisms at each end of the trunk, thereby enabling PBXs to agree which end will seize the trunk before actually doing so, minimizing the effect of glare. Ground start is preferred when there is a high volume of calls; therefore, PBXs typically use this type of signaling.

■ E&M: E&M is a common trunk signaling technique used between PBXs. In E&M, voice is transmitted over either two- or four-wire circuits, with five types of E&M signaling (Types I, II, III, IV, and V). E&M uses separate paths (or leads) for voice and signaling. The M (Mouth) lead sends the signal, and the E (Ear) lead receives the signal.

Digital Telephony Signaling

On PSTN switches, analog signaling is usually provided through current flow in closed electrical circuits, and digital signaling is provided through channel associated signaling (CAS) or common channel signaling (CCS).

Many varieties of CAS exist, and they operate over various analog and digital facilities.


CAS uses defined bits within T1 or El channels for signaling; this is in-band signaling. Therefore, the signal for call setup and so forth is in the same channel as the voice call.

Examples of CAS signaling include the following:

■ R1 signaling (on T1 facilities): Used in North America.

■ R2 signaling (on E1 facilities): Used in Europe, Latin America, Australia, and Asia.

■ DTMF signals: DTMF signals are the "pulses" used within the call path.

Modern telecommunication networks require more efficient means of signaling, so they are moving toward CCS systems. CCS can have faster connect times than CAS, and it offers the possibility of a number of additional services.


CCS uses a common link to carry signaling information for several trunks. It differs from CAS signaling because it uses a separate channel for call setup; this is out-of-band signaling.

Examples of CCS signaling include the following:

■ Integrated Services Digital Network (ISDN)

The following sections further describe these types of CCS signaling. DPNSS

DPNSS is an industry-standard interface defined between a PBX and an access network. DPNSS expands the facilities normally available only between extensions on a single PBX to all extensions on PBXs connected in a private network.


ISDN provides digital telephony and data transport services. ISDN involves the digitalization of the telephone network, permitting voice, data, text, graphics, music, video, and other source material to be transmitted on the same facility. For example, ISDN enables PBXs to connect over the PSTN and to create voice VPNs by delivering PBX signaling over the network to distant PBXs.

Following are the two ISDN access methods, as illustrated in Figure 8-8:

■ ISDN Basic Rate Interface (BRI): Offers two bearer (B) channels and one delta (D) channel (2B+D). The BRI B channel operates at 64 kbps and carries user data and voice. The BRI D channel operates at 16 kbps and carries both control and signaling information. BRI is typically used for residential and small office/home office applications.

■ ISDN Primary Rate Interface (PRI): Designed to use T1 or E1 circuits, PRI offers 23 B channels and one D channel (23B+D) in North America and 30 B channels and one D channel (30B+D) in Europe. The PRI B channels operate at 64 kbps and carry user data and voice. The PRI D channel also operates at 64 kbps and carries both control and signaling information. PRI is typically used for enterprise business and voice applications.

Figure 8-8 ISDN Digital Signaling



Used For


64 kbps

Circuit-Switched Data

16 kbps for


BRI; 64 kbps

Signaling Information

for PRI

QSIG Digital Signaling

Figure 8-9 illustrates QSIG. QSIG is a peer-to-peer signaling system used in corporate voice networking to provide standardized inter-PBX communications. It is a standards-based mechanism that provides transparent transportation of PBX features across a network.

Figure 8-9 QSIG

Layers 4-7

End-to-End Protocol Network Transparent


QSIG Procedures for Supplementary Services

QSIG Generic Functional Procedures

QSIG Basic Call

Link Layer

Interface-Dependent Protocols



QSIG features include the following:

■ Standards-based protocol that enables interconnection of multivendor equipment

■ Enables inter-PBX basic services, generic feature transparency between PBXs, and supplementary services

■ Interoperability with public and private ISDN

■ Operable in any network configuration and compatible with many PBX-type interfaces

■ No restrictions on private numbering plans

SS7 Digital Signaling

SS7 is an international signaling standard within the PSTN. SS7 defines the architecture, network elements, interfaces, protocols, and management procedures for a network that transports control information between PSTN switches. SS7 works between PSTN switches and replaces per-trunk in-band signaling.

As shown in Figure 8-10, a separate data network within the PSTN implements SS7. SS7 provides call setup and teardown, network management, fault resolution, and traffic management services. The SS7 network is solely for network control. Out-of-band signaling via SS7 provides numerous benefits for internetworking design, including reduced call setup time, bearer capability, and other progress indicators.

Figure 8-10 SS7 Signaling Is Used Between PSTN Switches

SS7 Signaling


SS7 Signaling

SS7 Network


Transmission Network


Transmission Network


When using SS7, all trunk channels are for voice and data, and the SS7 network carries the associated signaling separately.

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