--- title: "Working principle of a digital telephone exchange" subject: "Telecommunications" chapter: 4 key: "semester_8/telecommunications:digital-telephone-exchange-principle" source: https://www.bctengineer.com/q/semester_8/telecommunications/digital-telephone-exchange-principle site: BCT Engineering --- # Working principle of a digital telephone exchange Telecommunications · Chapter 4: Digital Switching - Asked in: 2070 Asar, 10 marks; 2069 Bhadra (old course), 12 marks; 2065 Magh (old course), 10 marks; 2065 Baisakh (old course), 4+12 marks (asked 4 times) - Video: [T 5.3 Inside a telephone exchange and a call flow](https://www.youtube.com/watch?v=5FxEgdaOw3A) — Ramanikumar animations -Telecom and Wireless - Answer checked by Claude Opus 5.5 (high effort) on 2026-10-14 - Page: https://www.bctengineer.com/q/semester_8/telecommunications/digital-telephone-exchange-principle - Whole chapter: https://www.bctengineer.com/md/semester_8/telecommunications/chapter-4 ## Question With a block diagram explain the working principle of a Digital Telephone Exchange. ## Short answer A digital telephone exchange switches PCM voice channels using a TST network under stored-program control. Subscriber line interfaces perform BORSCHT functions (battery, overvoltage, ringing, supervision, coding, hybrid, testing). Concentrators reduce line numbers; control processors manage call processing, signalling, tones, and synchronization; digital trunks interconnect exchanges. ## Answer A **digital telephone exchange** is a stored-program-controlled (SPC) exchange in which speech is switched in digital (PCM) form, using time and space switching of 64 kbps channels, under the control of computer processors. Examples: Alcatel E10, Siemens EWSD, Ericsson AXE, and the exchanges installed by Nepal Telecom. #### Block diagram ```text analogue +-------------+ +------+ subscribers --| Subscriber |--| | (2-wire) | line units | | conc-| +-----------+ (BORSCHT) | + codec | |entr. |===| | +-------------+ +------+ | Digital | digital trunks (E1)--[trunk interface]====| switching | analogue trunks--[ATI + codec]===========| network | | (TST) | tones/announcements --[tone generator]====| | signalling --[CAS/CCS (SS7) units]=======| | +-----+-----+ | +-------------------------------+---+ | Control subsystem (processors, | | memory, stored programs) | +----+-------------------------+----+ | | [O&M terminal, billing] [network sync clock] ``` #### Main blocks 1. **Subscriber line interface (line card).** Each analogue line needs the **BORSCHT** functions: - **B**attery feed (−48 V DC for the telephone), - **O**vervoltage protection (lightning, power-line contact), - **R**inging (75 V, 25 Hz AC applied to the line), - **S**upervision (detecting off-hook, on-hook, dial pulses), - **C**oding (codec: A-law PCM at 64 kbps, with filters), - **H**ybrid (2-wire to 4-wire conversion), - **T**esting (access for line tests). 2. **Concentrator / remote line unit.** Since only a fraction of subscribers are busy at once, many lines (e.g. 128–1000) are concentrated onto fewer PCM channels (E1 links) to the switching network. Remote units can be placed near subscriber clusters. 3. **Trunk interfaces.** Digital trunks (E1, 2.048 Mbps, HDB3) are connected directly after frame alignment and clock adaptation; analogue trunks need codecs and signalling conversion. 4. **Digital switching network.** Usually a **TST** (time–space–time) or multi-stage network. **Time switches** (time slot interchangers) move a sample from one time slot to another using speech and control memories; **space switches** connect a time slot on one PCM highway to the same slot on another. This connects any 64 kbps channel to any other. The network is duplicated for reliability. 5. **Signalling equipment.** Receivers for dial pulses and DTMF, CAS units (E1 TS16) and **common channel signalling** (SS7) terminals for inter-exchange signalling. 6. **Service circuits.** Tone generators (dial, busy, ring-back), announcement machines, conference bridges. 7. **Control subsystem.** Duplicated processors with stored programs carry out call processing, routing translation, charging and resource management. Control may be centralised or distributed. 8. **Operation and maintenance (O&M) and billing.** Terminals for subscriber data changes, traffic measurement, fault alarms, diagnostics and call detail records. 9. **Synchronisation.** A clock locked to the national network clock keeps all PCM links in step to avoid slips. #### Working principle (a local call) 1. **Off-hook detection:** subscriber A lifts the handset; loop current flows; the line card's supervision scans and reports to the processor. 2. **Dial tone:** the processor checks A's class of service, assigns a free path and a digit receiver, and connects dial tone. 3. **Digit reception:** dialled digits (pulses or DTMF) are received and stored; dial tone is removed after the first digit. 4. **Analysis and routing:** the processor translates the number. For a local number it checks B's line; for a distant number it selects an outgoing trunk and sends signalling (e.g. SS7 IAM). 5. **Path set-up:** a free time slot path through the TST network is found and the speech and control memories are written. 6. **Ringing:** ringing current is sent to B and ring-back tone to A. 7. **Answer:** when B lifts the handset, ringing is tripped, the speech path is through-connected and **charging** starts. 8. **Conversation:** speech from each phone is coded to 64 kbps, switched digitally in its time slots and decoded at the other line card. 9. **Clear-down:** on on-hook, the processor releases the path, stops charging and records the call details. #### Advantages - No noise build-up; uniform digital quality and easy integration with digital transmission (no codec at each trunk). - Small size, low power, fast set-up, high reliability (duplicated control). - Many subscriber services (call waiting, forwarding, conference, abbreviated dialling), easy changes by software, centralised O&M and accurate billing. - Supports ISDN, data and common channel signalling. ## Check yourself The answer's key points as multiple-choice questions, with the correct option and why. 1. In the subscriber line card, what does the 'S' in BORSCHT (Supervision) do? A. Supplies -48 V battery feed B. Converts 2-wire to 4-wire C. Detects off-hook, on-hook and dial pulses D. Codes speech into 64 kbps A-law Answer: C. Supervision detects the subscriber's loop state; battery feed, hybrid and coding are other BORSCHT letters. (Key point: BORSCHT functions) 2. Why is a concentrator used between line units and the switching network? A. Only a fraction of subscribers are busy at once, so many lines share fewer channels B. To amplify speech C. To convert PCM to analogue D. To generate ringing current Answer: A. Concentration uses the low calling rate to connect many lines onto fewer PCM channels, saving switch size. (Key point: Concentration) 3. In the digital switching network (TST), what does a time switch do? A. Connects one highway to another in the same slot B. Moves a sample from one time slot to another using speech and control memories C. Generates the dial tone D. Duplicates the processors Answer: B. A time slot interchanger changes the time slot; the space switch changes highways keeping the slot. (Key point: Time and space switch roles) 4. In setting up a local call, what happens right after the processor detects off-hook? A. Ringing is sent to B B. Charging starts C. It checks class of service, assigns a path and digit receiver, and sends dial tone D. The path is released Answer: C. After off-hook detection comes dial tone, then digit reception, analysis, path set-up, ringing, answer. 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