EC-11.2 The Link: Encoding, Framing and Catching Errors
You can explain how bits are represented on a wire and how a receiver stays in step, say where one frame ends and the next begins, compute a check value, and choose between detecting an error and correcting it.
This is the layer where the communication module and the networking module meet, and it is the one most often skipped by readers who go straight to addresses and routing. It is worth the time because everything above it assumes a link that mostly works and occasionally does not, and the occasionally is what the rest of the design is shaped around.
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Line codes, and keeping a receiver in step
A long run of identical bits gives a receiver nothing to synchronise on. Line codes guarantee transitions, at the cost of bandwidth or of extra bits, and that trade is why several of them exist.
NPTEL: Data Communication (IIT Kharagpur) · CourseFraming, and finding the start of a message
A receiver sees a stream of bits and must decide where one frame begins. Length fields, delimiter patterns with escaping, and coding violations are the three approaches, each with a failure mode.
NPTEL: Data Communication (IIT Kharagpur) · CourseParity and checksums, and what they miss
A single parity bit catches any odd number of flipped bits and misses every even number. A checksum is stronger and still misses errors that cancel, which is why neither is used where corruption is likely.
NPTEL: Computer Networks and Internet Protocol (IIT Kharagpur) · CourseThe cyclic redundancy check, and why it is standard
The frame is treated as a large number and divided by a chosen constant, with the remainder sent along. It catches all short bursts of errors, is cheap in hardware, and is what almost every link actually uses.
NPTEL: Data Communication (IIT Kharagpur) · CourseDetect and retransmit against correct in place
Detecting an error and asking again is cheap and needs a return path and time. Correcting it in place costs extra bits on every frame and is the only option when asking again is impossible.
NPTEL: Computer Networks and Internet Protocol (IIT Kharagpur) · CourseSliding windows, and keeping a link busy
Sending one frame and waiting for an answer wastes the link for a whole round trip. Allowing several frames to be outstanding fills that time, and the window size needed grows with the delay.
NPTEL: Computer Networks and Internet Protocol (IIT Kharagpur) · Course
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