Full Duplex/Half Duplex - DTX NSM

One of the most common configuration errors we see is the mixing of full and half duplex ports. Running the Network Connectivity test will allow you to see how the port has been configured and whether is supports both full and half duplex.
Half vs. Full Duplex
100BASE-TX uses two pairs to transmit (1,2-3,6). In half duplex mode, one pair is used to transmit, the other to receive. With full duplex, each pair can transmit and receive at the same time, just as with 1000BASE-T.
The DTX-NSM will show the port capability on the results screen, shown below: 

DTX NSM Result Screen

The alternating arrows just below 10/100 indicate that the port is half duplex. If this link were full duplex, you would see two solid arrows. With Negotiation Details highlighted, pressing ENTER reveals more detail; shown below:

DTX NSM Highlighted Negotiation Details

This port is not capable of supporting full duplex communications.
Mixing full / half duplex
You will see a collision rate exceeding 5% and errors when mixing full and half duplex links. On a full duplex connection, there should be 0% collisions.