EC-23.1 Why a Link Needs Correction, Not Just Detection
You can explain the coding gain a redundant code buys in place of transmitter power, distinguish detection from correction and say when each is the right answer, and state what channel capacity promises and what it does not.
Before:S5-3. Digital Signal ProcessingEC-11. Data Communications and Computer Networks
The networks module used a checksum to detect a corrupted frame and ask for it again. That answer fails whenever asking again is impossible or expensive: a deep-space link, a broadcast, a stored disc, a real-time voice call. The alternative is to send enough structured redundancy that the receiver can work out what was sent, and the surprising result is that this is usually cheaper in energy than simply transmitting harder.
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Detection and retransmission against forward correction, and when each fits
Asking for a repeat is simple and costs a round trip, which is unacceptable on a broadcast, a recording or a link with a long delay. Choosing between the two is a system decision made before any code is selected.
NPTEL: Information Theory, Coding and Cryptography (IIT Delhi) · CourseHamming distance, and how much distance buys how much correction
The number of positions in which two valid words differ decides how many errors can be detected and how many corrected. That single quantity is the currency in which every code in this module is priced.
NPTEL: Error Control Coding — An Introduction to Linear Block Codes (IIT Kanpur) · CourseRate, redundancy and coding gain
A code sends more symbols than it carries information, and the reward is that a given error rate is reached at a lower signal-to-noise ratio. Expressing that reward in decibels lets it be compared directly against a bigger amplifier.
NPTEL: Error Control Coding — An Introduction to Linear Block Codes (IIT Kanpur) · CourseChannel capacity, and what the limit does and does not promise
Capacity says a rate below a certain figure can be made as reliable as you like, and says nothing about how long the code must be or how hard it is to decode. Reading it as a practical target rather than a bound is a common error.
NPTEL: Information Theory, Coding and Cryptography (IIT Delhi) · CourseHard and soft decisions, and why the extra information is worth keeping
A demodulator that reports how confident it is gives a decoder much more to work with than one that reports only a bit. The gain from keeping that confidence is typically around two decibels for nothing but arithmetic.
Bursts against random errors, and interleaving as the answer to bursts
Most codes assume errors are scattered, while real channels often destroy a run of consecutive symbols. Spreading a codeword across time before transmission converts a burst back into the scattered errors the code expects.
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