optional · CSS study notes
Switching
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OSI Layer: Data Link Layer (L2) & WAN Switching
Definition
Switching is the mechanism by which data is moved through intermediate nodes from source to destination in a network of interconnected links.
Circuit Switching
- Dedicated physical path reserved end-to-end for the whole session
- Three phases: setup → data transfer → teardown
- Bandwidth guaranteed; no per-packet addressing needed
- Wasteful during idle periods; setup delay before transfer
- Example: traditional PSTN telephone call
Packet Switching
- Data divided into packets, each carrying header with addressing
- Datagram approach — each packet routed independently, may arrive out of order (IP)
- Virtual circuit approach — logical path set up first, all packets follow it in order (X.25, ATM, Frame Relay, MPLS)
- Efficient statistical multiplexing of shared bandwidth
- Variable delay and possible loss; needs buffering and reordering
Message Switching
- Entire message stored and forwarded node-by-node (store-and-forward)
- No path reservation; requires large storage at each node
- High delay; obsolete (historically telegraph systems)
Comparison
| Basis | Circuit | Datagram Packet | Virtual Circuit |
|---|---|---|---|
| Path | Dedicated | Independent per packet | Fixed logical path |
| Setup | Required | None | Required |
| Bandwidth | Reserved | Shared | Shared |
| Order | Preserved | May be out of order | Preserved |
| Suits | Voice, real-time | Bursty data | Reliable streams |
Numericals
Q1 — Circuit vs packet switching delay
Message = 1 Mb, path has 3 links (2 intermediate nodes), each link 1 Mbps, setup time = 0.5 s, propagation negligible.
- Circuit switching: setup 0.5 s + transmission 1×10⁶ ÷ 1×10⁶ = 1 s → 1.5 s
- Packet switching (message split into 1,000 packets of 1,000 bits): first packet takes 3 hops × 1 ms = 3 ms; remaining 999 packets pipeline at 1 ms each → 3 ms + 999 ms ≈ 1.002 s
- Conclusion: pipelining makes packet switching faster despite no reservation
Q2 — Store-and-forward delay
Message = 7.5 Mb, 2 intermediate routers (3 links), each link 1.5 Mbps.
- Per-hop transmission = 7.5×10⁶ ÷ 1.5×10⁶ = 5 s
- Total = 3 × 5 = 15 s (message switching, no segmentation)
Q3 — Header overhead
Payload 1,000 bytes, header 20 bytes.
- Efficiency = 1,000 ÷ 1,020 = 98.04%; overhead = 1.96%
CSS Exam Angle
- Draw the two diagrams — dedicated path vs independently routed packets
- State clearly: voice historically favours circuit switching; bursty data favours packet switching; modern VoIP proves packet switching can carry voice with QoS
- Virtual circuit is the bridge concept linking this topic to X.25, ATM, and MPLS
Likely Questions
- Differentiate circuit switching and packet switching with examples.
- Compare datagram and virtual-circuit approaches.
- Solve a delay comparison numerical for circuit vs packet switching.
📜 Past-Paper Questions & Structured Answers
Questions are paraphrased; answer focus is suggested guidance, not an official marking scheme.
2016 · Paper-II · Q2(c) · 7 marks
Question (paraphrased): Explain network switching methods.
Answer focus: Definition, three methods, examples and a short comparison.
Switching connects communicating devices through intermediate nodes so every endpoint need not have a direct cable to every other endpoint.
| Method | How it works | Strength / limitation | Example |
|---|---|---|---|
| Circuit switching | Set up an end-to-end circuit, reserve capacity, transfer data, release the circuit | Predictable service once established; reserved capacity is wasted during silence | Traditional circuit-switched telephone call |
| Packet switching | Split data into packets; links are shared and packets are buffered/forwarded | Efficient for bursty traffic; queuing delay and losses can vary | Internet IP datagrams |
| Message switching | Store the entire message at each intermediate node before forwarding | No dedicated circuit; large buffers and delays make it unsuitable for interactive voice | Historical store-and-forward messaging |
Packet-switching forms: datagrams are forwarded independently and may take different routes. Virtual circuits establish a logical route first; packets share physical links while using connection identifiers, as in Frame Relay and ATM. A virtual circuit does not automatically reserve bandwidth.
Choice: circuits suit predictable continuous service; packet switching suits shared data networks. Modern real-time packet applications need capacity planning, scheduling and delay control.
2019 · Paper-II · Q5(c) · 6 marks
Question (paraphrased): Choose and justify switching for real-time voice.
Answer focus: A clear choice, voice requirements, mechanism, example and qualification.
Choice: in the classical comparison, circuit switching is suitable for a real-time voice call.
Requirements: speech needs timely delivery and low variation in delay; a late voice sample can be useless even if eventually delivered. A circuit reserves capacity and follows a fixed path, so competing traffic does not create packet queues on that reserved channel.
Example: a conventional digital telephone call may reserve a 64 kbit/s voice channel for the call’s duration. After setup, samples arrive at a predictable rate; the resources are released when the call ends.
Trade-off: silence still occupies reserved capacity, and new calls may be blocked if circuits are unavailable. Packet-based VoIP is also practical: RTP over UDP, a small jitter buffer, admission control and priority scheduling can meet voice requirements while sharing links. Merely choosing packet switching or a virtual circuit is not a guarantee of low delay.