Rockwell Automation Allen-Bradley ControlNet 1734-ACNR User Manual page 151

ControlNet Overview B-3
A ControlNet link's most important function is to transport time-critical
control information, such as I/O data and control interlocking. Other
information (non-time-critical messages such as program uploads and
downloads) is also transported but does not interfere with time-critical
messages because of ControlNet's transmission of scheduled and unscheduled
data.
On a ControlNet link, information is transferred between nodes by
establishing connections. Each message sent by a producer contains a
Connection ID (CID). Nodes that have been configured to recognize the CID
consume the message, therefore becoming consumers.
Media access to the network is controlled by a time-slice access algorithm,
Concurrent Time Domain Multiple Access (CTDMA), which regulates a
node's opportunity to transmit in each network update interval (NUI). You
configure how often the NUI repeats by selecting a network update time
(NUT) in milliseconds.
Publication CNET-UM001C-EN-P - November 2005