Chapter 3 · Watch, then practise
Data Link Layer
Use PPP as a concrete link protocol to understand framing, protocol identification and corruption detection. Trace bit stuffing to see how payload is distinguished from a frame boundary.
3 questions · 3 with related videos. Matches are based on playlist titles; broader background matches are labeled.
What to study
- Framing and encapsulation
- Bit stuffing
- Frame check sequence
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Notes
Point-to-Point Protocol (PPP)
Neso Academy · 6:12
Supplementary PPP lecture for understanding the frame and its protocol field.
1. Why identify a protocol?
If one link carries both IPv4 and IPv6, how can PPP deliver each payload to the correct network protocol?
PPP includes a Protocol field identifying the encapsulated protocol. The receiver first recognizes the frame, then uses this field to dispatch the information field. A link need not be dedicated to a single network-layer protocol.
Bit Stuffing
Neso Academy · 4:34
Supplementary bit-stuffing explanation; the exact PPP payload is worked in the answer.
2. Worked bit stuffing
For bit-synchronous PPP framing, stuff the payload bits 01111110. What does the receiver do?
Insert a zero after five consecutive ones: 011111010. The receiver removes that inserted zero after the run of five ones, recovering 01111110. This prevents data from imitating the flag pattern; stuffing is performed after computing the frame check sequence.
Error Detection
Neso Academy · 8:08 · Background lecture
Error-detection background for understanding what a failed frame check can establish.
3. Detection versus repair
A received PPP frame fails its frame check sequence. Does the FCS repair the damaged payload?
No. The FCS detects many corruption patterns so that an invalid frame can be discarded. It does not identify and repair arbitrary damaged bits. Retransmission, when required, needs a separate reliability mechanism.