Switching Types
Switching is the fundamental mechanism of moving data from an input port to an output port. The history of networking is the battle between two philosophies: Circuit Switching (Telephone Network) and Packet Switching (Internet).
1. Circuit Switching (The Old Way)
In a circuit-switched network, a dedicated physical path is established between two nodes before they can communicate. This dedicated path is guaranteed, meaning the bandwidth is exclusively yours for the duration of the connection.
- Setup Phase: You dial a number. The switches reserve a path end-to-end. If no paths are available, the call is blocked (busy signal).
- Data Phase: You talk. The bandwidth is 100% yours. No one else can use it, even if you are silent.
- Teardown Phase: You hang up. The path is released and the resources are returned to the pool.
Multiplexing in Circuit Switching
A single physical wire can carry multiple circuits simultaneously using Multiplexing:
- FDM (Frequency-Division Multiplexing): The frequency spectrum of a link is divided into distinct bands, each dedicated to one connection (like radio stations).
- TDM (Time-Division Multiplexing): Time is divided into frames of fixed duration, and each frame is divided into a fixed number of time slots. Each connection gets one slot in every frame.
[!WARNING] The Problem: If you stop talking for 10 seconds, the line is silent, but the resources are still reserved. Nobody else can use them. This is incredibly inefficient for data traffic, which is “bursty” (e.g., loading a webpage takes 1s, reading it takes 60s). Data is characterized by short bursts of high activity followed by long periods of silence.
2. Packet Switching (The Internet Way)
In a packet-switched network, data is chopped into small chunks called Packets. There are no dedicated paths or guaranteed reservations. Each packet fights for survival and is sent independently.
- No Reservation: You just send packets when you have data.
- Store-and-Forward Transmission: A router must receive the entire packet before it can begin transmitting the first bit of the packet onto the outbound link. If a packet is L bits and the link rate is R bits/sec, the transmission delay is L/R seconds per link.
- Statistical Multiplexing: Resources are shared dynamically based on demand. If 10 users share a 100Mbps link, and only 1 is active, they get 100Mbps. If all 10 are active, they get 10Mbps.
Queuing Delays and Packet Loss
Because there are no reservations, packets from multiple sources can arrive at a router faster than the router can forward them.
- Queuing Delay: Packets must wait in a buffer (queue) until the outbound link is free.
- Packet Loss: If the buffer becomes full, incoming packets have nowhere to go and are dropped (lost). The network does not guarantee delivery.
[!TIP] Why it won: Packet switching allows us to oversubscribe the network. An ISP can sell 1Gbps plans to 100 people on a single 10Gbps line because statistically, they won’t all use it at the exact same millisecond. It perfectly matches the bursty nature of data traffic.
3. Interactive: The Multiplexing Simulator
Visualize why Packet Switching handles “Bursty” traffic better.
- Scenario: 4 Users sharing 1 Wire.
- Circuit: Each user gets a dedicated slice (Slot). If they don’t use it, it’s wasted.
- Packet: Users share a queue. If one is silent, others use the space.
4. Comparison
| Feature | Circuit Switching | Packet Switching |
|---|---|---|
| Path | Dedicated (Physical) | Dynamic (Routing) |
| Bandwidth | Fixed per user | Shared (Multiplexed) |
| Utilization | Low (Silence is wasted) | High (Fill every gap) |
| Congestion | Blocked Call (Can’t dial) | Queuing Delay (Lag) |
| Example | PSTN (Phone Network) | TCP/IP (Internet) |
5. Real-World Context: The Transition to VoIP
The difference between these two philosophies is perfectly illustrated by the death of the traditional landline.
Historically, the PSTN (Public Switched Telephone Network) was a massive, global circuit-switched network. It guaranteed voice quality because the path was dedicated, but it was expensive to build and maintain parallel to the rising data networks (the Internet).
Today, almost all voice calls—even traditional “landlines”—are converted into data packets and sent over the internet via VoIP (Voice over IP). We abandoned the guaranteed quality of Circuit Switching because Packet Switching became so fast and high-bandwidth that the occasional lost packet or slight queuing delay (jitter) is unnoticeable to the human ear. The economic efficiency of a single, unified, packet-switched network for all data types (web, video, voice) ultimately destroyed the dedicated circuit model.