  I compare three methods for transmitting signals over interstellar distances:
radio transmitters, lasers and artificial transits. The quantitative comparison
is based on physical quantities depending on energy cost and transmitting time
L, the last parameter in the Drake equation. With our assumptions, radio
transmitters are the most energy-effective, while macro-engineered
planetary-sized objects producing artificial transits seem effective on the
long term to transmit an attention-getting signal for a time that might be much
longer than the lifetime of the civilization that produced the artifact.
