F01 is a digital mode of the Russian 6 operator. It utilizes a custom FSK system compatible with ITA2 RTTY.

F01 messages consist of 5-digit groups. In null messages, only a single "00000" group appears. Actual traffic ends with a group made of a 2-digit serial number + 3-digit group count. The serial number ranges from 01 to 99 inclusive, increments by one with each new message, and wraps around.

On certain schedules, the messages begin with 5-group headers using the same link identifiers as messages sent via two-way communications with Russian diplomatic missions, implying that they are their intended targets.

The regular schedules of F01 consist of 2 or 3 identical transmissions 10 minutes apart. Each successive transmission is 1-3 MHz lower or higher in frequency. When traffic is sent, the three transmissions are usually also made the next day, at the same times and frequencies. Test data is broadcast before each transmission, consisting of "RYRYRY."