- 2071 Shrawan · 7 marks
Switching, circuit vs packet, X.25 layers
Short answer
Switching transfers data through intermediate nodes without direct links. Circuit switching reserves a dedicated path, constant delay, good for voice. Packet switching shares links on demand, uses packets with headers, variable delay, suitable for data. X.25 has three layers: physical, data link (LAPB), and packet (PLP), with virtual circuits.
Define switching. Compare circuit and packet switching. Draw the X.25 layers and data formats.
Switching
Switching is the technique of transferring data from a source to a destination through intermediate nodes (switches) in a network, so that every device does not need a direct link to every other device. Types: circuit switching, message switching and packet switching.
Circuit vs packet switching
| Point | Circuit switching | Packet switching |
|---|---|---|
| Path | Dedicated end-to-end path | Shared links, no dedicated path |
| Setup | Required before transfer | Not needed (datagram) |
| Bandwidth | Reserved, wasted when idle | Used on demand |
| Delay | Setup delay, then constant | Variable queuing delay |
| Data unit | Continuous stream | Packets with headers |
| Failure | Call is lost | Packets re-routed |
| Best for | Voice (PSTN) | Data (Internet) |
X.25 layers
DTE DCE (network)
+------------------+ +------------------+
| 3 Packet (PLP) |<------->| 3 Packet (PLP) |
| 2 Link (LAPB) |<------->| 2 Link (LAPB) |
| 1 Physical (X.21)|<------->| 1 Physical (X.21)|
+------------------+ +------------------+
Data formats
LAPB frame:
| Flag | Address | Control | Info (packet) | FCS | Flag |
| 8 | 8 | 8 | variable | 16 | 8 |
X.25 packet inside the info field:
| GFI 4 | LCN 12 | Type 8 | User data |
GFI = general format identifier, LCN = logical channel number. LAPB gives error-free frame transfer; the packet layer sets up virtual circuits and does flow control and multiplexing.
✓ Answer checked by Claude Sonnet 5.5
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