Room 40 turns German radio traffic into warning

Radio interception offered three kinds of intelligence: the contents of encoded messages, the bearings of their transmitters, and changes in traffic volume. Only the first required breaking the code. The other two could still reveal where forces were concentrating and when an operation began. A burst of traffic could warn that preparations were under way, while sudden silence could indicate that the force had sailed under radio silence. Direction finding narrowed the likely port or operating area, which reconnaissance could then check. Germany's habit of transmitting at high power gave away operational clues even before Room 40 considered what the messages said.