Summary of "Lec-35: Carrier Sense Multiple Access in Computer Network || CSMA || Computer Networks"

Main ideas / lessons about CSMA (Carrier Sense Multiple Access)

CSMA is a MAC-layer protocol (typically discussed after Aloha). It is heavily emphasized in competitive exams and Computer Networks courses.
“Multiple Access” means many nodes share the same communication medium/channel (e.g., a shared wireless/wired link).

Before sending data, each node performs carrier sensing:

A node (e.g., Node A) checks whether the medium is busy by sensing whether a signal/carrier is present.
If the medium is busy, the node holds its data to avoid collisions.
If the medium is idle, the node can transmit.

If two nodes transmit simultaneously while the channel is busy, a collision occurs.
Collisions waste bandwidth and reduce throughput.

A node does not sense the entire path end-to-end.
It senses only around its own location—i.e., whether a signal has reached it.
Because of propagation delay, a node may not detect another node’s transmission until the signal arrives at it.

Like checking whether a vehicle is in front of your house: you can’t detect vehicles somewhere else on the whole street until they reach you.

Rule
- Sense the channel.
- If the channel is idle → transmit immediately.
- If the channel is busy → keep sensing continuously until it becomes idle.
Behavior
- “Persistent” means tenacious/continuous checking.
- When the channel finally looks idle, it sends right away.
Consequence (worst case)
- If multiple nodes are waiting, then when the channel becomes free, all may transmit immediately, leading to higher collision probability.

Rule
- If the channel is idle → transmit immediately.
- If the channel is busy → wait for a random backoff time, then attempt again.
Behavior
- Uses random waiting/backoff to reduce simultaneous transmissions.
- Example described:
  - After detecting busy:
    - N1 might wait 5 minutes
    - N2 might wait 15 minutes
    - N3 might wait 30 minutes
  - These different delays stagger attempts.
Consequence
- Collision probability is less than 1-persistent, due to desynchronization from randomness.
Trade-off / issue
- Even if the channel becomes free shortly after a node starts waiting, it may still wait unnecessarily until its timer ends → causing extra delay.

Rule (hybrid approach)
- Nodes continuously sense like 1-persistent.
- If the channel is busy → keep sensing.
- When the channel becomes idle:
  - transmit with probability P
  - otherwise (probability 1 − P) keep waiting and continue sensing.
Probability constraints
- P must be in [0, 1].
- Extreme cases:
  - P = 0 → behaves like 0-persistent (more deferral)
  - P = 1 → behaves like 1-persistent (immediate transmission when idle)
Expected outcome
- Intermediate P values (e.g., 0.5, 0.1) aim to:
  - reduce collisions
  - improve throughput
Real-world mention
- Wi‑Fi commonly uses P-persistent CSMA.

The video notes that numerical problems are common for these topics in exams.
Next concept mentioned for later videos:
- CSMA/CD = Collision Detection
- CSMA/CA = Collision Avoidance (These are presented as key next-step modifications to CSMA.)