Wired LANs: IEEE 802.3 Ethernet

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:

        OSI Layer 3: NETWORK LAYER (IP, ICMP)
  -------------------------------------------------------------
        OSI Layer 2: DATA LINK LAYER
          +---------------------------------------------------+
          | LLC: Logical Link Control (IEEE 802.2)            |
          | - Flow & error control, protocol multiplexing     |
          +---------------------------------------------------+
          | MAC: Medium Access Control                        |
          | - Framing, physical addressing, channel access    |
          | - IEEE 802.3 (Ethernet) / IEEE 802.11 (Wi-Fi)     |
          +---------------------------------------------------+
  -------------------------------------------------------------
        OSI Layer 1: PHYSICAL LAYER (Copper, Fiber, Transceiver)

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

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

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

+----------+-----+---------+---------+-----------+---------------+---------+
| Preamble | SFD | Dest MAC| Src MAC | Type/Len  | Data Payload  |   FCS   |
|  7 bytes | 1 B | 6 bytes | 6 bytes |  2 bytes  | 46-1500 bytes | 4 bytes |
+----------+-----+---------+---------+-----------+---------------+---------+
|<-------------- Header (14 bytes) ------------->|               | Trailer |

Detailed Field Breakdown:

Preamble (7 bytes / 56 bits): Alternating bit pattern 10101010... used to synchronize receiver clocks.
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.
Destination Address (6 bytes / 48 bits): Physical MAC address of the target recipient or broadcast address.
Source Address (6 bytes / 48 bits): Physical MAC address of the transmitting network adapter.
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).
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.
FCS (Frame Check Sequence - 4 bytes): Contains a 32-bit Cyclic Redundancy Check (CRC-32) to detect transmission errors. Corrupted frames are silently dropped.

3. Frame Size Constraints: Why Minimum 64 Bytes?

Minimum Frame Size:

\text{Min Frame} = 6 + 6 + 2 + 46 + 4 = 64\text{ bytes (512 bits)}

Maximum Frame Size:

\text{Max Frame} = 6 + 6 + 2 + 1500 + 4 = 1518\text{ bytes}

(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.

\text{Transmission Time } (T_{tx}) \geq 2 \times \text{Propagation Delay } (T_p)

\frac{\text{Minimum Frame Length}}{\text{Bandwidth}} \geq 2 \times \frac{\text{Max Cable Length}}{\text{Signal Velocity}}

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

\text{Min Length} = 10\text{ Mbps} \times 51.2\text{ }\mu\text{s} = 512\text{ bits} = 64\text{ bytes}

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!

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:

\text{00:1A:2B:3C:4D:5E}

+-----------------------------------+-----------------------------------+
|   24 bits: OUI (Vendor ID)        |    24 bits: NIC Specific Serial   |
+-----------------------------------+-----------------------------------+
      ^
      |-- Bit 0 (LSB of 1st byte): 0 = Unicast, 1 = Multicast
      |-- Bit 1: 0 = Globally Unique (OUI), 1 = Locally Administered

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).

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)

6. Collision vs. Broadcast Domains in LANs

Hub Environment (Repeater):
  [Host A]    [Host B]    [Host C]
      \          |          /
       \         |         /
        +-----------------+
        |   HUB (Layer 1) |  ===> 1 Shared Collision Domain
        +-----------------+       1 Shared Broadcast Domain

Switch Environment (Bridge):
  [Host A]    [Host B]    [Host C]
      |          |          |
    Port 1     Port 2     Port 3
  +-------------------------------+
  |       SWITCH (Layer 2)        |  ===> 3 Dedicated Collision Domains
  +-------------------------------+       1 Shared Broadcast Domain

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.

7. Key Exam Questions & Takeaways

Why is the minimum Ethernet frame size 64 bytes?
- It guarantees that frame transmission duration exceeds the worst-case round-trip propagation delay ( $2 \times T_p$ ) across the maximum network diameter, ensuring stations detect collisions before finishing transmission.
What are the two sublayers of the Data Link Layer in IEEE 802?
- LLC (Logical Link Control, 802.2) for flow/error control and protocol multiplexing.
- MAC (Medium Access Control, 802.3) for framing, physical addressing, and channel access.
How does a switch differ from a hub?
- A Hub (Layer 1) broadcasts every incoming electrical signal to all ports, sharing a single collision domain.
- A Switch (Layer 2) inspects MAC addresses, forwards frames only to the designated port, and creates a separate collision domain per port in full-duplex mode.

LAN: Wired LAN