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Wired LANs: IEEE 802.3 Ethernet

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1. Overview and IEEE 802 Architecture

A Local Area Network (LAN) is a computer network that interconnects computers within a limited geographical area such as a residence, school, laboratory, university campus, or office building.

The IEEE 802 Committee split the OSI Data Link Layer into two distinct sublayers to support multiple physical media under a unified interface:

1. LLC (Logical Link Control — IEEE 802.2): Provides a uniform interface to the network layer, independent of the underlying transmission technology.
2. MAC (Medium Access Control — IEEE 802.3): Controls physical framing, MAC addressing, and access to the shared medium.

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2. Standard Ethernet Frame Format (IEEE 802.3 / DIX II)

Ethernet transmits data encapsulated in frames with standardized headers and trailers:

Detailed Field Breakdown:

1. Preamble (7 bytes / 56 bits): Alternating bit pattern 10101010... used to synchronize receiver clocks.
2. SFD (Start of Frame Delimiter - 1 byte): Exact sequence 10101011. The last two consecutive 1s alert the receiver that the next bit is the start of the destination address.
3. Destination Address (6 bytes / 48 bits): Physical MAC address of the target recipient or broadcast address.
4. Source Address (6 bytes / 48 bits): Physical MAC address of the transmitting network adapter.
5. Length / Type (2 bytes):
   • If value $\leq 1500$, it specifies the payload length in bytes (IEEE 802.3 standard).
   • If value $\geq 1536$ ($0x0600$), it defines the higher-layer protocol (Ethernet II format, e.g., 0x0800 for IPv4, 0x0806 for ARP).
6. Data (Payload - 46 to 1500 bytes):
   • Encapsulated network layer packet (MTU = 1500 bytes).
   • If user data is less than 46 bytes, padding bytes (0x00) are appended to maintain the minimum frame size.
7. FCS (Frame Check Sequence - 4 bytes): Contains a 32-bit Cyclic Redundancy Check (CRC-32) to detect transmission errors. Corrupted frames are silently dropped.

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3. Frame Size Constraints: Why Minimum 64 Bytes?

• Minimum Frame Size:

• Maximum Frame Size:

(Excludes Preamble and SFD)

The Physical Reason for Minimum Frame Size:

In CSMA/CD, a sender must continue transmitting long enough to detect a collision from the furthest station on the cable before completing transmission.

For classic $10\text{ Mbps}$ Ethernet spanning $2.5\text{ km}$ with repeaters, the round-trip propagation time is $\approx 51.2\text{ }\mu\text{s}$.

If a frame were shorter than 64 bytes, a collision occurring at the far end would bounce back after the sender finished transmitting, preventing the sender from detecting the collision!

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4. Ethernet MAC Addressing

A MAC address is a permanent hardware address burned into the Network Interface Card (NIC) ROM.

• Size: 48 bits (6 bytes), represented as 12 hexadecimal digits separated by colons or hyphens:

• Unicast: Directed to a single host (e.g., 00:1A:2B:3C:4D:5E).
• Multicast: Directed to a group of subscribed hosts (1st byte odd, e.g., 01:00:5E:00:00:01).
• Broadcast: Delivered to all hosts on the local network (FF:FF:FF:FF:FF:FF).

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5. Evolution of Ethernet

Generation	Standard	Data Rate	Cable Types	Access Method / Mode
Standard Ethernet	IEEE 802.3	$10\text{ Mbps}$	10Base5 (Thick), 10Base2 (Thin), 10Base-T (Cat 3 UTP)	Half-Duplex (CSMA/CD)
Fast Ethernet	IEEE 802.3u	$100\text{ Mbps}$	100Base-TX (Cat 5), 100Base-FX (Fiber)	Full-Duplex / Switched
Gigabit Ethernet	IEEE 802.3z / ab	$1\text{ Gbps}$	1000Base-T (Cat 5e/6), 1000Base-SX/LX (Fiber)	Full-Duplex Switched
10G Ethernet	IEEE 802.3ae / an	$10\text{ Gbps}$	10GBase-T (Cat 6a), 10GBase-SR/LR (Fiber)	Strictly Full-Duplex (No CSMA/CD)

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6. Collision vs. Broadcast Domains in LANs

• Collision Domain: A network segment where packet collisions can occur when two devices transmit simultaneously. A switch port isolates each connection into its own collision domain.
• Broadcast Domain: The set of all devices that receive a broadcast frame (FF:FF:FF:FF:FF:FF). All switch ports belong to the same broadcast domain unless separated by VLANs or Routers.

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