A bit-stuffing based framing protocol uses an 8-bit delimiter pattern of 01111110. If the output bit-string after stuffing is 01111100101, then the input bit-string is

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Solution

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Official answer: B) 0111110101

Data Link Layer

In bit stuffing with the flag pattern 01111110 (six consecutive 1s), the transmitter automatically inserts (stuffs) a 0 bit whenever it encounters five consecutive 1s (11111) in the data stream.

Given stuffed output bit-string:

\text{0 1 1 1 1 1 0 0 1 0 1}

Identify five consecutive 1s: 011111
The bit immediately following the five 1s is the stuffed 0, which is removed (destuffed).
Remaining bits after destuffing: 0101.

Combining the bits gives the original input bit-string: 0111110101, corresponding to option (B).

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This Data Link Layer previous year question is Q24 of the GATE CSE 2014 Set 3 paper. Find more GATE CSE PYQs on Data Link Layer in Computer Networks, or browse all GATE CSE PYQs.

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Data Link Layer in GATE CSE

Data Link Layer has 27 GATE CSE PYQs across 17 papers from 2013 to 2026. 5 of them are 1-mark MCQs like this one, and it is the only one in GATE CSE 2014 Set 3.

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Independent practice explanation verified by the GateAI team. GATE is conducted by the IITs; question text follows the official paper.