What is Ethernet?

Ethernet is a set of technologies and standards for building local area networks (LAN), which define:

• how devices on the network exchange frames,
• what format the frames have,
• how the physical transmission of data works (cable, connectors, speed),
• how the network detects collisions and manages access to the medium.

Short wording for an interview:

Ethernet is a technology for wired local networks that defines the rules for transmitting data in frames, methods of accessing the medium, and the physical characteristics of cables. It is the most widespread LAN standard in the world.

---

#What Ethernet defines

#Ethernet includes specifications at the levels:

• Data link layer (L2, MAC)
• Physical layer (L1)

That is, it defines:

• the structure of the Ethernet frame (frame format)
• the media access method (CSMA/CD in older versions)
• cable types (twisted pair, coaxial, fiber)
• connectors (RJ-45)
• speeds (10 Mbit/s → 100 Gbit/s and higher)

---

#The Ethernet frame: the basis of data transmission

The structure of a standard Ethernet frame:

• Destination MAC: the recipient's MAC address
• Source MAC: the sender's MAC address
• EtherType: which protocol is inside (IPv4, IPv6, etc.)
• Payload: the payload (46-1500 bytes per the standard)
• FCS: checksum

Frame size: 64-1518 bytes (without a VLAN tag). With VLAN: up to 1522 bytes.

---

#How Ethernet works (simplified)

1. The device forms a frame.
2. It sends it through the network card (NIC).
3. The switch receives the frame and reads the MAC addresses.
4. The switch forwards the frame only to the needed port (unicast), or to all ports (broadcast).

Routers do not forward Ethernet frames directly: only L3 packets.

---

#The physical part of Ethernet

#1. Cable types:

• Twisted pair (UTP/STP): Cat5e, Cat6, Cat6a, Cat7, Cat8
• Fiber optic (SM/MM): for higher speeds and longer distances
• Coaxial cable was used in the past, now almost never

#2. Connectors:

• RJ-45: for twisted pair
• LC/SC: for fiber

#3. Ethernet speeds:

---

#Switches and Ethernet

Today, Ethernet = switched Ethernet:

• operates in full-duplex, no collisions
• the switch creates a separate collision domain per port
• the MAC address table determines where to send frames

The old variant, Ethernet over hubs, is no longer used.

---

#Media access methods

The historical method: CSMA/CD:

• Carrier Sense
• Multiple Access
• Collision Detection

It was used when devices shared a common medium (coaxial cable or hubs). Today it is not used, since everything operates through switches in full-duplex.

---

#Why Ethernet became the number one standard

• cheap
• simple
• scalable
• fast (growing up to 800Gbit+)
• compatible
• reliable
• there is a huge amount of equipment available

Used:

• at home
• in offices
• in data centers
• at ISPs