Circuit Switching — Dedicated Paths and Why They Mattered
Learn about circuit switching — dedicated paths and why they mattered
The Core Idea: Resource Reservation
In Lesson 1, we saw how central offices connected subscribers by physically patching circuits together. Circuit switching formalizes this idea: when you make a call, a dedicated communication path is established end-to-end, and resources are reserved exclusively for that call for its entire duration — whether you're talking, silent, or on hold.
This is the fundamental difference between circuit switching and packet switching (Lesson 10). In a circuit-switched network, bandwidth is allocated, not shared. You get a fixed-capacity pipe, and nobody else can use it until you hang up.
To understand why this matters, think about what it guarantees:
- Zero jitter: The path is fixed; every sample traverses the same route with the same delay
- Zero packet loss: There are no packets to lose — it's a continuous stream
- Fixed latency: Propagation delay is constant because the path doesn't change
- Guaranteed bandwidth: Your 64 kbps channel is always available
These are exactly the properties that voice communication needs, and exactly the properties that packet switching struggles to provide. Every QoS mechanism in modern VoIP (Lesson 11) is an attempt to approximate what circuit switching gave us for free.
The tradeoff? Efficiency. When you're silent (which is roughly 60% of a typical phone call), your reserved bandwidth goes completely unused. Nobody else can use it. This waste is what made packet switching revolutionary — but we're getting ahead of ourselves.
Time-Division Multiplexing and the DS0
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