Ethernet (IEEE 802.3) — CSMA - CD, frame format

4.3.5 · Coding › Computer Networks

WHY does CSMA/CD exist?

WHAT problem: Classic (half-duplex) Ethernet mein, kai stations ek broadcast medium share karte hain. Ek waqt mein sirf ek hi frame wire par ho sakta hai. Agar do stations simultaneously transmit karein, unke signals overlap ho jaate hain → garbage → ek collision. Hume ek rule chahiye kaun kab bolta hai bina kisi central controller ke.

WHY this approach: Koi manager turns nahi de raha. Isliye hum har station ko ek hi decentralized etiquette follow karaate hain. Naam is etiquette ko decode karta hai:

CS = Carrier Sense → pehle suno; transmit tabhi karo jab medium idle ho.
MA = Multiple Access → sab stations ek hi medium equally share karte hain.
CD = Collision Detection → bhejna jaari rakhte hue bhi suno; agar collision detect ho, turant abort karo taaki wire waste na ho.

HOW does a station know a collision happened? — The key constraint

Maano $T_{p}$ = one-way propagation delay (end to end). Round-trip worst case hai $2 T_{p}$ . Sender ko tab bhi transmit karte rehna chahiye jab collision wapas aaye:

$T_{t r an s mi t} \geq 2 T_{p}$

Minimum frame size ko first principles se derive karo. $L$ bits ke frame ka transmission time bandwidth $B$ (bits/s) par $T_{t r an s mi t} = L / B$ hai. Substitute karo:

$\frac{L}{B} \geq 2 T_{p} ⟹ L_{min} = 2 \cdot T_{p} \cdot B$

Binary Exponential Backoff — WHY randomized & exponential

$n$ -wein collision ke baad, $K$ ko ${0, 1, \dots, 2^{n} - 1}$ se uniformly random choose karo, phir $K \times$ slot time wait karo (slot time $= 2 T_{p}$ , worst-case round trip).

$K \in {0, 1, \dots, 2^{m i n (n, 10)} - 1}, wait = K \times (slot time)$

$n$ ko 10 par cap kiya gaya hai (window $0..1023$ par badhna band ho jaati hai).
16 collisions ke baad → abort.

Ethernet (IEEE 802.3) Frame Format

Figure — Ethernet (IEEE 802.3) — CSMA - CD, frame format

Definition Fields (sizes in bytes)

Field	Bytes	Purpose (WHY)
Preamble	7	`10101010`×7 — receiver clock ko bit timing ke saath synchronize karne deta hai
SFD (Start Frame Delimiter)	1	`10101011` — do `1` kehte hain "frame ab shuru hota hai"
Destination MAC	6	kaun receive karta hai
Source MAC	6	kisne bheja
Length / Type	2	≤1500 = payload length; ≥1536 (0x0600) = EtherType (e.g. IPv4)
Data + Pad	46–1500	payload; 46 tak padded taaki frame 64-byte minimum poori kare
FCS (CRC-32)	4	frame par error detection

Min data = 46 kyun? Frame minimum (DA+SA+Len+Data+FCS) 64 bytes hona chahiye: $6 + 6 + 2 + data 46 + 4 = 64 bytes (the CSMA/CD requirement!)$ Max data = 1500 kyun? Ek design tradeoff: efficiency ke liye kaafi bada, buffer/latency bound karne ke liye kaafi chhota. Max frame (no preamble/SFD) $= 6 + 6 + 2 + 1500 + 4 = 1518$ bytes.

Flashcards

CSMA/CD ke letters kya stand karte hain?

Carrier Sense, Multiple Access / Collision Detection

Ethernet mein minimum frame size kyun hoti hai?

Taaki sender tab bhi transmit kar raha ho jab worst-case collision echo wapas aaye (

T_{t x} \geq 2 T_{p}

), detection guarantee karne ke liye.

Minimum frame length ka formula?

L_{min} = 2 T_{p} B

= (round-trip time) × bandwidth.

Minimum aur maximum Ethernet frame sizes (DA→FCS)?

64 bytes aur 1518 bytes.

Minimum data field 46 bytes kyun hota hai?

6+6+2+46+4 = 64 = CSMA/CD ki required minimum frame size.

Jam signal kya hai aur use kyun bhejte hain?

Ek 32-bit signal jo collision detect hone par bheja jaata hai taaki SABHI stations ko collision ka zaroor pata chale.

n-wein collision ke baad Binary exponential backoff range?

K uniformly {0,...,2^min(n,10)−1} se choose hota hai, wait = K × slot time.

Kitne collisions ke baad station give up karta hai?

16 attempts.

Preamble vs SFD ka purpose?

Preamble (7B, 10101010…) receiver clock sync karta hai; SFD (1B, 10101011) exact frame start mark karta hai.

2-byte field mein Length aur Type mein distinguish kaise karein?

≤1500 = length; ≥1536 (0x0600) = EtherType.

Kya preamble 64-byte minimum mein count hota hai?

Nahi — sirf DA se FCS tak count hota hai.

Recall Feynman: 12-saal ke bachche ko explain karo

Socho ek walkie-talkie channel poori class share kar rahi hai. Rule hai: pehle suno — agar koi baat kar raha hai, wait karo. Agar quiet hai, baat karo. Lekin awaaz door wali wall tak pahunchne mein time lagti hai, isliye do bachche usi waqt bina ek doosre ko sune shuru ho sakte hain → unke words noise mein crash ho jaate hain. Yeh pakka karne ke liye ki aapko crash pata chale, aapko itna lambe waqt tak bolte rehna chahiye ki garble aapke paas wapas aaye — isliye har message ki ek minimum length hoti hai. Jab crash ho, dono "STATIC!" chillaate hain (the jam) taaki sab ruk jaayein, phir har koi dice roll karta hai kitna chup rehna hai. Dobara crash? Bada dice roll karo. Bahut zyada crashes ke baad, give up karo.

Connections

CSMA-CA — wireless variant (802.11): collisions avoid karta hai kyunki radio par listen-while-sending nahi ho sakta.
ALOHA and Slotted ALOHA — carrier sensing ke bina ancestor; CSMA isse improve karta hai.
Propagation Delay vs Transmission Delay — $L_{min} = 2 T_{p} B$ ke peeche core timing.
CRC and Error Detection — FCS field kaise kaam karta hai.
MAC Addresses — 6-byte addresses ki structure.
Switched vs Shared Ethernet — full-duplex switches modern LANs mein CSMA/CD ko obsolete bana dete hain.