Ethernet Shared Media

IEEE 802.3 family of standards defines physical interface specifications for the wired Ethernet. It Provides asynchronous networking using “carrier sense, multiple access with collision detect” (CSMA/CD) over coax, twisted-pair copper and optical fiber media. Shared media refers to Ethernet designs that use hubs as networking devices or a coaxial cable run in a bus topology . Hubs force the network devices to operate in half-duplex mode and use CSMA/CD to deal with collisions that occur.

Hub

Ethernet hubs operate at half-duplex, which allows a host to either transmit or receive data, but not simultaneously. Meaning, if any two devices connected to a hub send a frame simultaneously, a collision will occur. Thus, all ports on a hub belong to the same collision domain. A collision domain is simply defined as any physical segment where a collision can occur. Multiple hubs that are uplinked together still all belong to one collision domain. Increasing the number of host devices in a single collision domain will increase the number of collisions, which will degrade performance. Hubs also belong to only one broadcast domain – a hub will forward both broadcasts and multicasts out every port but the originating port. A broadcast domain is a logical segmentation of a network, dictating how far a broadcast (or multicast) frame can propagate. Hubs are Layer-1 devices that physically connect network devices together for communication. Hubs can also be referred to as repeaters.

CSMA/CD was used in the now obsolete shared media Ethernet variants (10BASE5, 10BASE2) and in the early versions of  UTP which used hubs/repeater. Modern Ethernet networks, built with switches and full-duplex connections, no longer need to use CSMA/CD because each Ethernet segment, or collision domain, is now isolated. 

Modern networks use switches with Ethernet and permit the creation of point-to-point links that function independently from each other at L1, making collisions extremely rare or even impossible if all the devices are configured properly. It is possible to run a system in full-duplex mode with such a configuration. Systems enjoy sending and receiving data at the same time as other systems in the infrastructure, which makes for a collision-less network. 

Creating a LAN by using Ethernet in this full-duplex, point-to-point manner creates a separate collision domain for each port  on the switch in the LAN and with only a single device (PC, server, AP, phone, printer, etc) in the collision domain (port), there is no chance of collisions.