Circuit Switching vs Packet Switching
| Aspect | Circuit Switching | Packet Switching |
|---|---|---|
| Path | Dedicated end-to-end circuit | No dedicated path — packets routed independently |
| Resource use | Reserved even when idle | Shared — efficient for bursty traffic |
| Setup | Connection established before communication | Packets sent immediately (no setup) |
| Delay | Constant (once connected) | Variable (queuing at each hop) |
| Example | Traditional telephone network | The Internet |
The Internet uses packet switching because computer traffic is bursty. A web page loads in bursts with idle periods between clicks. Packet switching lets many users share the same link efficiently.
Error Detection — CRC (Cyclic Redundancy Check)
The sender appends an FCS (Frame Check Sequence) to each frame. The receiver recomputes it and discards the frame if it doesn’t match.
CRC is far better than simple checksums at detecting burst errors (multiple consecutive corrupted bits). A 32-bit CRC (CRC-32) detects all single-bit errors, all double-bit errors, and any odd number of errors.
Flow Control
Prevents the sender from overwhelming the receiver.
| Method | How it works | Efficiency |
|---|---|---|
| Stop-and-Wait | Send one frame, wait for ACK, send next | Low — cable idle while waiting |
| Sliding Window | Send up to N frames before requiring an ACK | High — continuous flow |
The number of frames a sender can transmit before receiving an ACK is called the window size. TCP’s sliding window is adaptive — it grows when the network is good and shrinks when congestion is detected.
Q: Why is Packet Switching preferred for the Internet?
A: Computer traffic is bursty — active periods of transmission followed by idle time. Packet switching shares the link among many users, maximizing utilization. Circuit switching would waste bandwidth on idle connections.
Q: What is CRC?
A: Cyclic Redundancy Check — an error-detecting code. The sender divides the data by a predetermined polynomial and appends the remainder. The receiver performs the same division — if the remainders differ, a bit error was detected. Highly effective against burst errors.
Q: What is the goal of flow control?
A: To match the sender’s transmission rate to the receiver’s processing capacity. Without flow control, a fast sender can overflow the receiver’s buffer, causing packet loss and retransmissions.
Q: How does sliding window flow control differ from Stop-and-Wait?
A: Stop-and-Wait sends one frame and waits — throughput is limited by latency. Sliding Window allows multiple outstanding frames, keeping the link fully utilized. TCP uses a sliding window.
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