So considering that CSMA/CD is for detecting collisions, mostly in a bus scenario with a lot of stations, then is CSMA/CD still used?
Because right now most of the connections are point to point (for example one router getting connected to another router using cable) and there are no bus stations, and furthermore most of the cables are full duplex (i think?) so is CSMA/CD still used? if not then is there any collision detection in Ethernet?
Also i said mostly because I'm not 100% sure, but are bus networks still a thing in developed countries? i mean why would anyone use it?
Best Answer
Generally, no.
CSMA/CD is required for half-duplex links but these are only possible for 100 Mbit/s and slower. Practically all 100 Mbit/s interfaces support full-duplex mode and autonegotiation, so they negotiate for and link at full duplex.
Half duplex was the only operating mode for links connected by repeater hubs but these have practically all been replaced by switches.
For Gigabit Ethernet, half-duplex mode was defined in the standard but gigabit repeaters didn't surface and there's practically no support for half-duplex mode with GbE on existing devices. Multi-gigabit standards dropped half-duplex operation and repeaters completely and use full duplex exclusively.
So, while early Ethernet was inherently half-duplex and CSMA/CD was mandatory - due to the single-wire shared medium - later generations introduced full-duplex capable media (10BASE-F, 10BASE-T, ...) and subsequently full-duplex concentrators (switches instead of repeater hubs), rendering half duplex and CSMA/CD obsolete.
As a result, to actually see CSMA/CD in action you'd need a 10 or 100 Mbit/s link and either dig up an obsolete repeater hub or force the switch and NIC ports to use half duplex. Practically anything built in the last twenty years is able to use full duplex and will do so unless you don't let it.