Ethernet

Ethernet is the dominant link-layer technology for wired local area networks (LANs).

It defines how frames are formatted and transmitted over the physical medium.

History

Ethernet was invented in the mid-1970s by Bob Metcalfe and David Boggs:

• it used a coaxial cable in a bus (line) topology to interconnect nodes
• all devices shared the same communication channel (broadcast LAN)

In the 1980s and early 1990s, Ethernet faced many challenges from other LAN technologies (Token Ring, FDDI, ATM).

Today, Ethernet is the dominant wired LAN technology.

Reasons for Ethernet's success:

• it was the first widely deployed high-speed LAN
• network administrators became familiar with it and reluctant to switch over to other LAN technologies
• competing technologies were more complex and expensive
• Ethernet kept improving to match or exceed their speeds
• because it became so popular, Ethernet equipment became cheap and widely available

By the late 1990s:

• most LANs moved to a hub-based star topology
• hosts and routers are directly connected to a hub with twisted-pair copper wire
• a hub simply repeats incoming signals to all outgoing ports
• thus, Ethernet with a hub-based star topology is also a broadcast LAN

In the early 2000s:

• hubs were replaced by switches
• a switch sends data only to the correct device instead of everyone
• switches prevent collisions
• switches work at layer 2 (link layer), while routers work at layer 3 (network layer)

Ethernet frame structure

Preamble	Destination Address	Source Address	Type	Data (Payload)	CRC
8 bytes	6 bytes	6 bytes	2 bytes	46–1500 bytes	4 bytes

Field	Description
Preamble	used for synchronization starts with 10101010 repeated 7 times: lets the receiver align its clock to the sender's bit timing ends with 10101011: marks the Start Frame Delimiter (SFD), indicating the start of the actual frame needed because Ethernet has no shared clock between sender and receiver: when adapter A sends bits (e.g., 10101010 10101010…), adapter B does not know: where each bit starts how long each bit lasts even a tiny mismatch in clock speed will cause misalignment very quickly
Destination MAC address	identifies the receiving adapter (e.g., BB-BB-BB-BB-BB-BB)
Source MAC address	identifies the sending adapter (e.g., AA-AA-AA-AA-AA-AA)
Data	carries the IP datagram MTU = 1500 bytes (larger datagrams must be fragmented at the IP layer) minimum = 46 bytes: smaller datagrams are padded ("stuffed") IP uses its header length field to determine the actual datagram size and ignore padding
Type	indicates the upper-layer protocol (e.g. IP, ARP, Novell IPX, AppleTalk) allows Ethernet to carry multiple ("to multiplex") network-layer protocols serve to glue a protocol at one layer to a protocol at the layer above, analogous to: the protocol field in the network-layer datagram the port-number fields in the transport-layer segment
CRC	cyclic redundancy check (error-checking code)

Ethernet services

Ethernet provides a connectionless service to the network layer:

• no handshake before sending
• similar to:
  • IP's layer-3 datagram service
  • UDP's layer-4 connectionless service

Ethernet provides an unreliable service to the network layer:

• no acknowledgments (ACK/NACK)
• corrupted or lost frames are simply dropped
• reliability (if needed) is handled by higher layers

This lack of reliable transport at the link layer helps to make Ethernet simple and cheap.

Ethernet standards

Ethernet standards are defined by the IEEE 802.3 (Ethernet) working group and include many variants, such as:

• 10BASE-T, 10BASE-2
• 100BASE-T
• 1000BASE-T (Gigabit Ethernet)
• 1000BASE-LX (Gigabit Ethernet over fiber)
• 10GBASE-T

Ethernet names follow a pattern:

Component	Meaning	Details
First number	Speed	10 = 10 Mbps 100 = 100 Mbps 1000 = 1 Gbps 10G = 10 Gbps
BASE	Baseband transmission	Baseband transmission means that the signal occupies the full bandwidth of the medium as a single channel (no frequency-division): it mainly distinguishes from legacy broadband Ethernet (10BROAD36) almost all IEEE 802.3 standards are for baseband Ethernet
Last part	Physical medium	Ethernet is both a link-layer and physical-layer technology and can run over different media: T = twisted-pair copper cable F / SX / LX / BX = fiber optics (varies by Ethernet generation) earlier versions used coaxial cable (e.g., 10BASE-2)

Early Ethernet (like 10BASE-2 and 10BASE-5) used a shared coaxial bus. All devices shared the same medium, so collisions could happen. These were handled using CSMA/CD (collision detection).

Repeaters were used to extend signal distance by regenerating signals.

Modern Ethernet is different:

• much higher speeds (increased by orders of magnitude)
• switched-Ethernets have become dominant (no collisions, so CSMA/CD is usually unnecessary)

Despite major changes, Ethernet has stayed compatible. The core Ethernet frame format has remained compatible (with extensions such as VLAN tagging).