===== Introduction =====

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:

<code>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</code>
    
or

<code>        Configuration Options for Node CLASS3

        [0]     Exit this procedure

        [1]     Perform an entire configuration
        [2]     Change naming information
        [3]     Configure Devices on this machine
        [4]     Configure Transports
        [5]     Configure Timezone Differential Factor
        [6]     Configure Event Dispatcher
        [7]     Configure Application database
        [8]     Configure MOP Client database
        [9]     Configure Cluster Alias
        [10]    Replace MOP Client configuration
        [11]    Configure satellite nodes
        [12]    Configure cluster script locations

* Which configuration option to perform?                 [1] : 3
%NET$CONFIGURE-I-SCANCONFIG, scanning device configuration - please wait
* Data Link name to use for EWA0 (DExxx/TULIP)?   [CSMACD-0] :</code>

So what does it mean?

===== Details =====


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.