Transmission of uplink messages by user devices in 5G and 6G involves substantial delays and numerous complex steps. For lower latency and simpler operation, procedures are disclosed by which a user device can transmit an uplink message in an unscheduled but monitored channel, allocated for at-will transmissions. The user device can transmit a header including, for example, the identity code of the user device, the size of a subsequent data message, and the destination address, followed by the data message. To reduce unnecessary delays, the user device can calculate when the data message is expected to be ready to transmit, and can start transmitting the header at an earlier time, calculated to be finished when the data message will be ready. The data message is then transmitted with zero delay. When low latency is required, the disclosed procedures can reduce delays substantially. Many additional aspects are disclosed.