--- title: "Forms and types of signaling" subject: "Telecommunications" chapter: 5 key: "semester_8/telecommunications:signaling-forms-types" source: https://www.bctengineer.com/q/semester_8/telecommunications/signaling-forms-types site: BCT Engineering --- # Forms and types of signaling Telecommunications · Chapter 5: Signaling Systems - Asked in: 2071 Chaitra, 10 marks - Answer checked by Claude Opus 5.5 (high effort) on 2026-10-14 - Page: https://www.bctengineer.com/q/semester_8/telecommunications/signaling-forms-types - Whole chapter: https://www.bctengineer.com/md/semester_8/telecommunications/chapter-5 ## Question What is signaling in communication system? Explain its forms and types in case of telecom network. ## Short answer Signaling transfers control information to establish, supervise, charge, and release connections. Its forms are supervisory, address, call-progress, and network-management signals. Types include subscriber versus trunk signaling, DC/in-band/out-of-band signaling, and CAS versus CCS (SS7), with line/register and compelled/non-compelled variants. ## Answer **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 ```text 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 | Feature | In-band CAS | Out-band CAS | CCS (SS7) | |---|---|---|---| | Signal path | Speech band | Above speech band | Separate link | | Speed | Slow | Slow | Fast | | During speech | No | Yes | Yes | | Fraud risk | High | Medium | Low | | Services | Basic | Basic | ISDN, IN, mobile | Modern networks use DTMF/DSS1 on the access side and SS7 (or SIP in IP networks) between exchanges. ## Check yourself The answer's key points as multiple-choice questions, with the correct option and why. 1. Which is an example of a call-progress signal? A. Ring-back tone B. Dialled digits C. Seizure D. Clear-forward Answer: A. Call-progress signals inform the user (dial, ring-back, busy tones); seizure and clear are supervisory, digits are address signals. (Key point: Forms of signals) 2. What distinguishes out-of-band signaling from in-band? A. It uses tones inside 300-3400 Hz B. It uses a tone outside the speech band but inside the 4 kHz channel (e.g. 3825 Hz), allowing signaling during speech C. It uses DC loop D. It uses a separate data link Answer: B. Out-band tones can be sent during conversation, but need filters; in-band tones cannot be sent during speech. (Key point: In-band vs out-of-band) 3. What is the difference between line and register signaling? A. Line signals carry digits only B. Register signals supervise the trunk throughout the call C. Line signals supervise the trunk throughout the call, register signals carry digits during set-up only D. They are identical Answer: C. Line signals (seize, answer, clear) last the whole call; register signals carry address digits in set-up. (Key point: Line vs register) 4. Which belongs to subscriber (access) signaling? A. SS7 ISUP IAM B. MF R2 between exchanges C. Changeover between STPs D. DC loop, dial pulses or DTMF, and ringing current Answer: D. Subscriber signaling runs between telephone and local exchange; ISDN uses DSS1 on the D-channel. 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