Wi-Fi (IEEE 802.11) — CSMA - CA, bands — Hinglish notes

4.3.6 · Coding › Computer Networks

WHY karta hai Wi-Fi ko ek special MAC protocol ki zaroorat?

WHY nahi use kar sakta Wi-Fi Ethernet ka CSMA/CD?

CSMA/CD = "baat karte waqt suno; agar collision suno, ruk jao." Iske liye sender ko simultaneously transmit aur receive karna hota hai.
Ek Wi-Fi radio jo full power par transmit kar raha hai, nahi kar sakta simultaneously kisi door ke station ki faint colliding signal sense — uska apna transmitter uske receiver ko saturate kar deta hai. Isliye collision detection air mein impossible hai.
Aur bhi bura, hidden terminal problem hai (neeche): do stations receiver par collide kar sakte hain jabki dono ek doosre ko bilkul nahi sun sakte.

Toh Wi-Fi ka jawab hai: avoid karo collisions ko wait karke, sense karke, aur random backoff + (optional) handshakes use karke.

HOW CSMA/CA kaam karta hai — step by step derive kiya hua

Step 1 — Carrier Sense (do tarah ka)

Physical CS: channel par energy measure karo. Busy → defer karo.
Virtual CS: har frame header ek Duration field announce karta hai. Baaki ek timer set karte hain jise NAV (Network Allocation Vector) kehte hain = "channel time T tak busy hai, sense karne ki koshish bhi mat karo." Power bachata hai aur hidden terminals ko handle karta hai.

Step 2 — Interframe Spaces (IFS): waiting time se priority

Channel idle hone ke baad, kuch bhi karne se pehle ek gap wait karna hota hai. Chhota wait = zyada priority.

IFS	Length	Kisliye use hota hai
SIFS	sabse chhota	ACKs, CTS — replies jo jeetne chahiye
DIFS	sabse lamba (data ke liye)	backoff se pehle normal data

Step 3 — Random Backoff (collision avoider)

DIFS wait karne ke baad, station ek random backoff counter pick karta hai: $backoff = Random (0, C W) \times slotTime$ jahan $C W$ contention window hai. Counter sirf tab decrement hota hai jab channel idle ho, aur freeze hota hai jab channel busy ho jaye. 0 par pahunche → transmit karo.

Step 4 — ACK & Exponential Backoff

Sender transmit karta hai, phir SIFS wait karta hai; receiver ACK bhejtaa hai.
ACK nahi mila ⇒ collision/loss assume karo. Retransmit karo, lekin pehle contention window double karo (binary exponential backoff): $C W_{new} = min (2 \cdot (C W_{old} + 1) - 1, C W_{m a x})$ $C W$ double karna retries ko spread out karta hai jab network congested ho.

Step 5 (optional) — RTS/CTS handshake (hidden terminal solve karta hai)

Fix: bade frame se pehle, sender ek chhota RTS (Request To Send) bhejtaa hai; AP CTS (Clear To Send) se reply karta hai. Jo bhi RTS ya CTS sunta hai wo apna NAV set karta hai aur chup rehta hai. Ab hidden C bhi CTS sunta hai aur back off karta hai.

Figure — Wi-Fi (IEEE 802.11) — CSMA - CA, bands

Bands — WHERE Wi-Fi rehta hai

Band	Range (approx)	Wi-Fi gens	Trade-off
2.4 GHz	2.400–2.4835 GHz	b/g/n, ax	Lamba range, better wall penetration, lekin crowded aur slow
5 GHz	~5.15–5.85 GHz	a/n/ac/ax	Zyada channels, faster, chhota range
6 GHz	~5.925–7.125 GHz	Wi-Fi 6E / 7	Bahut bada clean spectrum, sabse kam range

Worked Examples

Common Mistakes (Steel-manned)

Active Recall

Recall Quick self-test (answers chhupa lo!)

Kyun CA nahi CD Wi-Fi mein? → transmit karte waqt collisions detect nahi kar sakte (half-duplex radio).
Kya prove karta hai ki frame pahunchi? → SIFS ke baad ACK.
Kyun backoff counter freeze karo reset ki jagah? → fairness / line mein apni jagah rakho.
Hidden terminal problem kya solve karta hai? → RTS/CTS + NAV.
Kyun 2.4 GHz 5 GHz se zyada door tak pahunchta hai? → lamba wavelength better penetrate/diffract karta hai.

Recall Feynman: 12-saal ke bacche ko samjhao

Socho ek bilkul andheri room mein bacchon ka group hai jo sab baat karna chahte hain, lekin koi apni awaaz sunate waqt khud ko nahi sun sakta. Isliye woh rules banate hain: pehle suno; agar quiet hai, maan mein ek random number count karo; jo pehle counting khatam kare wo bole. Agar koi aur bolne lagta hai, tum counting pause karo aur baad mein continue karo (taaki apni turn mat kho). Bolne ke baad, listener ko "got it!" kehna chahiye — yahi ek tarika hai ki tumhare words kisi aur ke saath gadbad nahi hue. Kuch bacche ek doosre ko nahi sun sakte lekin ek hi teacher ke paas khade hain, isliye baat karne se pehle woh chillate hain "main baat karun?" aur teacher chillata hai "haan, baaki sab chup raho!" — yahi hai RTS/CTS. Neeli awaazein (2.4 GHz) walls ke through carry hoti hain; oonchi awaazein (5 GHz) clear hain lekin dampen ho jaati hain.

Flashcards

Why does Wi-Fi use CSMA/CA instead of CSMA/CD?

Radio half-duplex hai aur transmit karte waqt collisions sense nahi kar sakta; hidden terminals detection impossible banate hain, isliye yeh collisions avoid karta hai.

What is the NAV in 802.11?

Network Allocation Vector — ek virtual-carrier-sense timer jo frame ke Duration field se set hota hai, station ko batata hai ki channel time T tak busy hai.

What is the role of SIFS vs DIFS?

SIFS sabse chhota gap hai (ACK/CTS ke liye taaki replies jeetein); DIFS lamba gap hai jo station data + backoff se pehle wait karta hai.

How does a station pick its backoff?

Random(0, CW) × slotTime; sirf idle slots par count down karta hai aur busy hone par freeze ho jata hai.

What happens to CW after a collision (no ACK)?

Yeh binary exponential backoff se double ho jaata hai: CW_new = min(2(CW_old+1)−1, CW_max).

Why freeze the backoff counter instead of resetting it?

Fairness rakhne ke liye — jo station zyada wait kar chuka hai wo apni earlier position in line rakhta hai.

What proves a unicast frame was received in Wi-Fi?

Data ke baad SIFS mein bheja gaya ACK; ACK nahi mila ⇒ collision/loss assume karo ⇒ retransmit karo.

What is the hidden terminal problem?

Do stations dono AP tak pahunch sakte hain lekin ek doosre ko nahi, isliye dono idle sense karte hain aur AP par collide karte hain.

How does RTS/CTS solve hidden terminals?

AP se CTS saare nearby stations (hidden wale bhi) sun lete hain, jo apna NAV set karte hain aur chup rehte hain.

Why does 2.4 GHz reach farther than 5 GHz?

Lamba wavelength obstacles ke around diffract karta hai aur kam attenuation ke saath penetrate karta hai; 5/6 GHz zyada data carry karta hai lekin zyada absorb hota hai.

How many non-overlapping 20 MHz channels does 2.4 GHz typically have?

Teen (channels 1, 6, 11), isliye yeh congested hai.

Which band do Wi-Fi 6E/7 add and why?

6 GHz — chhote range par zyada throughput ke liye bahut saara clean, uncrowded spectrum.

Connections

CSMA-CD and Ethernet — wired cousin jo collisions detect karta hai
ALOHA and Slotted ALOHA — random-access MAC ke ancestors
Hidden and Exposed Terminal Problems
Frequency, Wavelength and Attenuation — band trade-offs ke peeche ki physics
MAC Layer and ARQ / ACK — acknowledgements & retransmission
Bandwidth vs Throughput