Forms and types of signaling

Signaling in a telecommunication network is the transfer of control information that establishes, supervises, charges and releases connections, and manages the network. It is the "language" between telephones and exchanges and between exchanges.

Forms (functions) of signals

1. Supervisory signals – show the state of a line or trunk: idle/busy, seize, off-hook, answer, clear-forward, clear-back. Example: loop closed by lifting the handset.
2. Address signals – carry the called number and other routing information: rotary dial pulses, DTMF tones, MF digits, or the called-party number in an SS7 IAM.
3. Call-progress (information) signals – inform the user: dial tone, ringing current, ring-back tone, busy tone, congestion tone, recorded announcements.
4. Network management / maintenance signals – blocking/unblocking of circuits, alarm, test, re-routing information.

Types of signaling by location

• Subscriber (access) signaling: between telephone and local exchange — DC loop (off-hook/on-hook), dial pulses at 10 pps or DTMF (697–941 Hz row, 1209–1633 Hz column tones), 75 V/20–25 Hz ringing. In ISDN, DSS1 on the D-channel.
• Inter-exchange (trunk) signaling: between exchanges over trunks — line signals and register signals.

Types by frequency relation

 0   300                 3400 3825      Hz
 |----|=====voice band=====|----|-----
   DC   in-band (e.g.2600)   out-band

• DC signaling: loop or polarity changes; used on short metallic lines.
• In-band signaling: tones within 300–3400 Hz, e.g. 2600 Hz SF, MF R1/R2. Passes through any voice channel but can be imitated by speech; signals cannot be sent during speech.
• Out-of-band signaling: a tone outside the speech band but inside the 4 kHz channel, e.g. 3825 Hz. Can signal during conversation; needs filters.

Types by relation to speech channel

• Channel Associated Signaling (CAS): signals tied to each speech channel, e.g. ABCD bits in TS16 of E1 for each of 30 channels, MF R2.
• Common Channel Signaling (CCS): one separate data link carries labelled messages for many channels, e.g. CCITT No. 6 and SS7 (MTP, SCCP, ISUP, TCAP).

Other classifications

• Line vs register signaling: line signals supervise the trunk (seize, answer, clear) throughout the call; register signals carry digits during set-up only.
• Compelled vs non-compelled: in compelled signaling (R2) each signal stays until acknowledged; non-compelled signals have fixed duration.
• Link-by-link vs end-to-end: link-by-link signals are regenerated at each exchange; end-to-end signals pass from origin to destination exchange directly.

Comparison of main approaches

Modern networks use DTMF/DSS1 on the access side and SS7 (or SIP in IP networks) between exchanges.