Old Ethernet vs New Ethernet

Old  -  half-duplex shared media with CSMA/CD MAC Layer
New  -  full-duplex dedicated media, structured wiring without CSMA/CD MAC Layer

The original Ethernet concept was revolutionary in that it was the first network to share a bus with CSMA/CD.   

Pre-Ethernet

Earlier LAN networks were mostly experimental, shared a common medium, but relied on either transmitting only at certain times to make sure so other station was also transmitting, or they simply retransmitted whenever the receiving station failed to acknowledge the data.  In other-words, a station could transmit data without listening first, and just hope that it makes it - if it doesn't then it retransmits until it does.  So long as traffic loads were not too heavy, this actually worked out surprisingly well.

Old Ethernet  -  CSMA/CD, MAC Layer, Shared Media, Half-Duplex

Old Ethernet relied on CSMA/CD, which added an intelligent structure that maximized efficient use of the shared network cable (the bus).  This was what the MAC layer was based on  .  .  .   MAC = CSMA/CD !!!  The protocol was also half-duplex, since no two stations could successfully transmit simultaneously without a collision.  It required turn-taking, and turn-taking is half-duplex.  There are still many old Ethernet networks in operation today !!

Old Ethernet will have either a shared bus or a passive hub, which simply connects cables together.  , usually in a Star topology.  The star network may seem to be using dedicated media, but it can be either dedicated or shared.  A passive hub connects all of the separate cables that run to the workstations together, physically and logically - so that electrically it is identical to a bus topology !!!  For example, we could use thinnet with T couplers to connect 4 computers together with a single bus (Ethernet segment), or use a passive hub for a Star topology.

IMPORTANT - even though the passive hub with twisted pair connected stations is a Star topology, the hub acts as a short bus, and all transmiited data from every station is sent out to all - same as a bus, electrically and logically !!! :

Old Ethernet  -  Shared Media,CSMA/CD with Half-Duplex

The world had been waiting for this . . . full duplex with dedicated media !!  This doubled the overall maximum possible speed of communications.  In many instances, though, it did not have a major effect on speed, because the applications themselves are usually written for half-duplex communication.  For example, if your PC is communicating with a host, the client makes a request, the host asnwers, etc.  Or if you download a large file - you make a request, the file is sent, and you acknowledge that it has been received.

But speed is not the issue - the important thing, is that each station is immune to the traffic of all other stations !!  With shared medium, all stations are fighting for their own piece of the pie.  With dedicated media, they all have their own, private collision domain.