# Q1(b) · Error control

Subject: Computer Networks

Exam: TEE

Reader: https://deep-learning-study-room.vercel.app/cn/#tee/papers/paper-2025-legacy-reexam/q1b

Canonical: https://deep-learning-study-room.vercel.app/read/cn/tee/papers/paper-2025-legacy-reexam--q1b/

Actual paper; 100 marks attempted. Current-syllabus alignment is stated per question.

### Q1(b). What is error control. What are the different methods defined for error control. [5 marks]

### Answer

A transmission error changes one or more bits. A **single-bit error** flips one bit. A **burst error** affects a span from the first corrupted bit to the last; the bits in between need not all change. Detection identifies an invalid received pattern; correction identifies the intended pattern within the code’s guaranteed capability.

## Block coding

1. Divide data into fixed k-bit datawords. Map each one to an n-bit codeword, with n greater than k.
2. The n−k redundant bits create separation between valid codewords. There are 2k valid codewords, a subset of the 2n possible n-bit patterns, for an injective binary block code.
3. A receiver can reject patterns that are not valid codewords. Nearest-codeword decoding can correct errors only if the received pattern has an unambiguous nearest intended codeword under the error bound.
4. Error-control protocols may combine detection with retransmission (ARQ), or use sufficient coding redundancy for forward error correction (FEC).



$$
R_c=k/n,\quad r=n-k
$$



#### Practical example

A simple even-parity code maps 101 to 1010. The redundant parity bit permits detection of any single-bit flip, but cannot identify which bit changed.

**Yaad rakho:** Detection asks error hua? Correction asks kaunsa correct word?