--- title: "Switching techniques in data communication" subject: "Telecommunications" chapter: 8 key: "semester_8/telecommunications:switching-techniques-data-comm" source: https://www.bctengineer.com/q/semester_8/telecommunications/switching-techniques-data-comm site: BCT Engineering --- # Switching techniques in data communication Telecommunications · Chapter 8: Data Communication - Asked in: 2080 Chaitra, 8 marks; 2069 Bhadra (old course), 10 marks; 2065 Baisakh (old course), 2+14 marks (asked 3 times) - Video: [Switching Techniques in Computer Networks](https://www.youtube.com/watch?v=-HlJ4psu5aU) — Neso Academy - Answer checked by Claude Opus 5.5 (high effort) on 2026-10-14 - Page: https://www.bctengineer.com/q/semester_8/telecommunications/switching-techniques-data-comm - Whole chapter: https://www.bctengineer.com/md/semester_8/telecommunications/chapter-8 ## Question What are the switching techniques (strategies) used in data communication system? Explain. ## Short answer Data communication uses circuit switching (dedicated path with setup, transfer, disconnect), message switching (store-and-forward entire messages), and packet switching (datagram or virtual circuit). Packet switching divides messages into packets for pipelined transmission, reducing delay and improving link efficiency. ## Answer **Switching** is the method of connecting a sender to a receiver through intermediate nodes of a network, so that every pair of users does not need a dedicated link. Data communication uses three main switching techniques: **circuit switching**, **message switching** and **packet switching** (datagram and virtual circuit). ```text Switching +-------------+--------------+ Circuit Message Packet +------+------+ Datagram Virtual circuit ``` #### 1. Circuit switching A **dedicated physical path** is set up between the two users before data transfer and kept for the whole call. The telephone network (PSTN) uses it. **Three phases** 1. **Circuit establishment:** signalling sets up a path through the switches. 2. **Data transfer:** data flows continuously at a fixed rate over the reserved path. 3. **Circuit disconnect:** the path is released. ```text A --[S1]======[S2]======[S3]-- B setup dedicated path release ``` - Space-division or time-division (TDM) switches are used. - **Advantages:** fixed, small delay; no congestion once connected; data arrive in order; good for real-time voice. - **Disadvantages:** setup delay; channel capacity is wasted during silence; both ends must use the same rate; a busy network blocks new calls. #### 2. Message switching There is no dedicated path. The **whole message** is sent with a destination address to the next node, which **stores** it, checks it and **forwards** it when the next link is free (**store-and-forward**). Used in early telegraph and e-mail relay systems. ```text A --msg--> [N1 store] --msg--> [N2 store] --msg--> B ``` - **Advantages:** good line efficiency (links shared); no blocking (messages queued); priorities and code/speed conversion possible; broadcast possible. - **Disadvantages:** long and variable delay; nodes need large storage (disk); not suitable for interactive or real-time traffic. #### 3. Packet switching The message is divided into small **packets** (e.g. up to 1500 bytes) each with a header. Packets are stored and forwarded through the network; links are shared statistically. Packets are small, so they can be held in memory and **pipelined** over successive links, giving much less delay than message switching. **(a) Datagram approach** - Each packet is routed **independently**; no setup. - Packets may take different routes, arrive out of order or be lost; the destination reorders them. - Example: IP / the internet. - Robust to node failure, no setup delay; but variable delay and no guaranteed order. **(b) Virtual circuit approach** - A logical route (**virtual circuit**) is set up first with a call-request/call-accept exchange; each packet carries a short **VC identifier**. - All packets follow the same path in order; the circuit is cleared at the end. - Examples: X.25, Frame Relay, ATM, MPLS (label switched paths). - Allows QoS and in-order delivery; but setup delay and failure of a node breaks the VC. ```text Datagram: 1 -> R1-R2-R4 2 -> R1-R3-R4 Virtual circ.: 1,2,3 -> R1-R2-R4 (all same path) ``` #### Delay comparison (sending a long message over 3 hops) ```text Circuit: |setup|=========data========| Message: |==msg==|==msg==|==msg==| Packet: |p1|p2|p3| |p1|p2|p3| |p1|p2|p3| (pipelined) ``` Packet switching delivers sooner than message switching because packets overlap on successive links. #### Comparison of techniques | Point | Circuit | Message | Packet (datagram / VC) | |---|---|---|---| | Dedicated path | yes | no | no | | Setup phase | yes | no | no / yes | | Unit sent | continuous bits | whole message | packets | | Storage at nodes | none | large (disk) | small (memory) | | Delay | small, fixed after setup | large, variable | small, variable | | Line efficiency | low | high | high | | Order of delivery | in order | in order | not guaranteed / in order | | Overload effect | calls blocked | delay increases | delay increases, loss | | Typical use | PSTN voice | telegraph, e-mail relay | internet, X.25, ATM | #### Choice of technique - Real-time, constant-rate traffic (voice) suits **circuit switching**. - Bursty computer data suits **packet switching**, the basis of today's networks. - Modern networks combine both ideas: e.g. VoIP carries voice in packets but MPLS virtual circuits give it a guaranteed path. ## Check yourself The answer's key points as multiple-choice questions, with the correct option and why. 1. Which sequence correctly gives the three phases of circuit switching? A. Packet division, routing, reassembly B. Store, check, forward C. Circuit establishment, data transfer, circuit disconnect D. Data transfer, circuit establishment, disconnect Answer: C. A dedicated path is set up first, used for the whole call, then released. (Key point: Circuit switching phases) 2. How does a node in message switching handle an incoming message? A. It forwards each bit instantly over a reserved path B. It rejects it if the next link is busy C. It splits it into 1500-byte packets and routes each one D. It stores the whole message, checks it and forwards it when the next link is free Answer: D. Message switching is store-and-forward of the whole message, so there is no blocking but delay is long and variable. (Key point: Message switching) 3. Why does packet switching give less delay than message switching for a long message? A. Packets are small and are pipelined, overlapping on successive links B. Packets skip the header C. Packets use a dedicated circuit D. Packets need no storage at nodes at all Answer: A. Small packets are forwarded while later packets are still being sent, so transmission on successive links overlaps. (Key point: Packet vs message delay) 4. Which statement about packet switching approaches is correct? A. Both need a call set-up phase B. Neither can deliver packets out of order C. Datagram packets all follow the same path; virtual circuit packets are routed independently D. Datagram packets are routed independently and may arrive out of order; virtual circuit packets follow one set-up path in order Answer: D. Datagram has no set-up and independent routing; a virtual circuit is set up first and keeps order. (Key point: Datagram vs virtual circuit) ## Related in Telecommunications - [Flow control mechanisms in data communication](https://www.bctengineer.com/question.md?key=semester_8%2Ftelecommunications%3Aflow-control-mechanisms) — Telecommunications, chapter 8 - [Digital switching system and its functions](https://www.bctengineer.com/question.md?key=semester_8%2Ftelecommunications%3Adigital-switching-functions) — Telecommunications, chapter 4 - [Short note: Routing algorithm in data communication](https://www.bctengineer.com/question.md?key=semester_8%2Ftelecommunications%3Arouting-algorithm-data-comm) — Telecommunications, chapter 8, asked 2× - [Flow control in data communication network](https://www.bctengineer.com/question.md?key=semester_8%2Ftelecommunications%3Aflow-control-network) — Telecommunications, chapter 8 - [Digital carrier system and multiplexing techniques](https://www.bctengineer.com/question.md?key=semester_8%2Ftelecommunications%3Adigital-carrier-multiplexing) — Telecommunications, chapter 3 - [Space switch and why TST is used](https://www.bctengineer.com/question.md?key=semester_8%2Ftelecommunications%3Aspace-switch-why-tst) — Telecommunications, chapter 4 ## Related in other subjects - [Data reduction strategies](https://www.bctengineer.com/question.md?key=semester_7%2Fdata_mining%3Adata-reduction) — Data Mining, chapter 2 - [Data transfer techniques for slow I/O](https://www.bctengineer.com/question.md?key=semester_5%2Finstrumentation_ii%3Aio-speed-mismatch) — Instrumentation II, chapter 1, asked 3× - [Data compression techniques](https://www.bctengineer.com/question.md?key=semester_8%2Fmultimedia_system%3Adata-compression-techniques) — Multimedia System, chapter 5, asked 2× - [I/O functions used with data files](https://www.bctengineer.com/question.md?key=semester_1%2Fcomputer_programming%3Afile-io-functions) — Computer Programming, chapter 10, asked 2×