Multiple access and Ethernet
When many nodes share one medium, they need rules for taking turns. Ethernet began as a shared cable with random access and evolved into today's switched, full-duplex networks.
The multiple access problem
On a broadcast link, such as a shared cable or a radio channel, any transmission reaches every node, and two simultaneous transmissions collide and garble each other. Multiple access protocols coordinate who transmits when. Ideally a single active node gets the full rate R, and N active nodes each average about R / N, without central control.
Channel partitioning
One family divides the channel. Time-division multiplexing (TDM) gives each node fixed time slots; frequency-division multiplexing (FDM) gives each a frequency band; code-division multiple access (CDMA) gives each a code. These are collision-free and fair but waste capacity when few nodes are active, since unused slots or bands sit idle.
Random access: ALOHA
Random access protocols let nodes transmit at full rate whenever they have data and recover from collisions by retransmitting after random delays. ALOHA, developed by Norman Abramson at the University of Hawaii in 1970 for radio links between islands, was the first. Slotted ALOHA, which synchronises transmissions to time slots, achieves at most about 37% (1/e) efficiency; pure ALOHA about 18%.
CSMA/CD
Carrier sense multiple access (CSMA) adds listening before talking: a node does not transmit if it hears another transmission. With collision detection (CSMA/CD), a node that detects a collision while sending stops, sends a jam signal, and retries after a random backoff. Classic Ethernet used binary exponential backoff: after the nth collision, wait a random number of slot times between 0 and 2^n - 1, up to a limit.
Taking-turns protocols
Polling protocols have a master invite each node in turn, as in Bluetooth; token-passing protocols circulate a special frame giving permission to transmit, as in Token Ring and FDDI. Cable internet systems (DOCSIS) combine approaches: the head-end assigns upstream time slots in response to requests from modems.
Ethernet
Ethernet was invented at Xerox PARC in 1973 by Robert Metcalfe and David Boggs and standardised by the IEEE as 802.3. Speeds have risen from 10 Mbit/s to 100 Mbit/s, 1 Gbit/s, 10 Gbit/s, and 400 Gbit/s and beyond in data centres, over twisted-pair copper and fibre. An Ethernet frame has a destination MAC, source MAC, EtherType field (such as 0x0800 for IPv4 or 0x86DD for IPv6), payload of 46 to 1,500 bytes, and CRC.
From shared to switched Ethernet
Early Ethernet used a shared coaxial bus and later hubs, which simply repeat bits to every port, so all nodes shared one collision domain. Modern Ethernet connects each device to its own switch port over full-duplex links, where sending and receiving happen simultaneously without collisions. CSMA/CD is therefore rarely used today, though the frame format remains.
Worked example
Binary exponential backoff: after a first collision, a node picks K from {0, 1} and waits K slot times; after a second, from {0, 1, 2, 3}; after a third, from {0, ..., 7}. Classic 10 Mbit/s Ethernet used a slot time of 512 bit times, 51.2 microseconds, so after a third collision a node might wait up to 7 x 51.2 = 358.4 microseconds. Growing ranges spread retries out when many nodes compete.
Key takeaways
Shared broadcast links need multiple access protocols to avoid and recover from collisions. Channel partitioning (TDM, FDM, CDMA) is fair but wastes idle capacity. Random access began with ALOHA (1970), with slotted ALOHA at most about 37% efficient. CSMA/CD listens before sending, detects collisions, and uses binary exponential backoff. Polling and token passing take turns. Ethernet (Metcalfe and Boggs, 1973; IEEE 802.3) grew from 10 Mbit/s to hundreds of Gbit/s; its frames carry MAC addresses, EtherType, a payload of up to 1,500 bytes, and a CRC. Modern switched, full-duplex Ethernet has no collisions.
Flashcard
Shared medium heard by all nodes
Check your understanding
Why do modern switched Ethernet networks rarely need CSMA/CD?