What is CSMACD? If you've been working with DECnet-Plus (aka DECnet Phase V or DECnet/OSI on older versions of OpenVMS), you've seen the letters CSMACD in a number of places. Such as:
NCL> show csma-cd port * Node 0 CSMA-CD Port CSMACD$EWA_0001 at 2019-02-07-12:26:28.074-07:00Iinf Identifiers Name = CSMACD$EWA_0001
Configuration Options for Node CLASS3  Exit this procedure  Perform an entire configuration  Change naming information  Configure Devices on this machine  Configure Transports  Configure Timezone Differential Factor  Configure Event Dispatcher  Configure Application database  Configure MOP Client database  Configure Cluster Alias  Replace MOP Client configuration  Configure satellite nodes  Configure cluster script locations * Which configuration option to perform?  : 3 %NET$CONFIGURE-I-SCANCONFIG, scanning device configuration - please wait * Data Link name to use for EWA0 (DExxx/TULIP)? [CSMACD-0] :
So what does it mean?
CSMACD includes three important concepts when dealing with communication on Ethernet. It originated in a time before twisted pair. During this time, the network was “Thickwire” (10Base5) or “Thinwire”. It was a shared medium, which meant that machines can talk at the same time. Thus, like conversations in a crowded room, you have to take turns talking in order for communication to be effective for you.
Carrier Sense (CS) - listen to make sure the network is quiet before beginning to send a message.
Multiple Access (MA) - Ethernet is a shared access medium which may be used by many hosts.
Collision Detect (CD) - In the event that multiple hosts begin talking at the same time, each host must detect this collision and stop transmitting. They can retry their communication after a random period of waiting.