Ethernet Basics & CSMA/CD

Ethernet (IEEE 802.3) is the dominant LAN technology. It defines wiring, signaling, frame formats, and media access rules. Over 90% of wired local networks use Ethernet.

Ethernet Frame Structure

Field	Length	Purpose
Preamble	7 bytes	Synchronization
SFD	1 byte	Start Frame Delimiter
Destination MAC	6 bytes	Target address
Source MAC	6 bytes	Sender address
EtherType	2 bytes	Protocol type (e.g., IPv4 = 0x0800)
Payload	46-1500 bytes	Actual data
FCS	4 bytes	CRC-32 error check

Maximum frame size: 1518 bytes (without preamble). Minimum: 64 bytes (to ensure collision detection works).

CSMA/CD (Carrier Sense Multiple Access with Collision Detection)

On shared Ethernet (hubs), all devices share the same medium. CSMA/CD prevents chaos:

1. Carrier Sense — listen before transmitting. If the cable is busy, wait.
2. Multiple Access — many devices share the same medium.
3. Collision Detection — while transmitting, listen for collisions. If a collision is detected:
   • Transmit a jam signal (ensures all devices know a collision occurred)
   • Wait a random backoff time (exponential backoff)
   • Retry

Important: CSMA/CD is Obsolete on Modern Switches

Modern Ethernet switches operate in full-duplex mode. The switch connects each device to its own dedicated port with separate transmit and receive paths. Collisions are physically impossible — no CSMA/CD needed.