What are noiseless and noisy channels?

Data link layer protocols are categorized based on whether the transmission channel is noiseless or noisy. This classification determines the complexity of protocols required, with noiseless channels needing simpler protocols and noisy channels requiring sophisticated error control mechanisms.

Noiseless Channels

Noiseless channels are idealized communication channels where no frames are lost, duplicated, or corrupted during transmission. Two primary protocols are designed for such channels:

Simplest Protocol

This is the most basic protocol with the following characteristics:

• Unidirectional communication − Data frames travel only from sender to receiver

• No flow control − Assumes the receiver can process frames immediately

• No error control − Perfect channel assumption eliminates the need for error handling

Stop-and-Wait Protocol

This protocol adds flow control to prevent receiver overflow:

• Send one frame at a time − Sender transmits a frame and waits

• Acknowledgment required − Receiver sends ACK before next frame is transmitted

• Prevents buffer overflow − Ensures receiver can handle incoming frames

Noisy Channels

Noisy channels represent real-world communication channels where frames can be lost, corrupted, or duplicated. Three Automatic Repeat Request (ARQ) protocols handle these error conditions:

Stop-and-Wait ARQ

This protocol extends Stop-and-Wait with error detection and correction:

• Sequence numbering − Uses alternating 0 and 1 sequence numbers to detect duplicates

• Timeout mechanism − Retransmits frames when ACK is not received within timeout period

• Error detection − Uses checksums to detect corrupted frames

Go-Back-N ARQ

This protocol improves efficiency by allowing multiple outstanding frames:

• Sliding window − Multiple frames can be transmitted before receiving ACK

• Sequential numbering − Frames numbered from 0 to 2^m-1 where m is header bits

• Selective retransmission − Retransmits from error point onward

Selective Repeat ARQ

The most sophisticated ARQ protocol that retransmits only corrupted or lost frames, providing the highest efficiency for noisy channels.

Channel Capacity Comparison

Example Calculation

For a 4 kHz channel with binary signaling and 20dB SNR:

Noiseless channel: 2 × 4000 × log?(2) = 8000 bps
Noisy channel: 4000 × log?(1 + 100) = 26,575 bps

Conclusion

Noiseless channels use simple protocols without error control, while noisy channels require sophisticated ARQ protocols with error detection, correction, and retransmission mechanisms. The choice of protocol depends on the channel's noise characteristics and required reliability.