On November 16, 1974, a 1679 digit message was broadcast into space from the Arecibo Telescope. The stream of 0's and 1's was created by shifting the frequencies, or, here in LEGO form by Chris Doyle.
1679 is the product of 23 x 73. If some alien race receives the signal, and then breaks it down into 79 separate sets of 23, then can then assemble these into a picture.
The result would give them information about us - an indication of the elements used in our DNA makeup, an image of the double helix, an image of a human, and the composition of our Solar System.
This was really more of a proof of concept experiment than an actual attempt at communication with others in our universe. Given that the globular star cluster targeted by this message is 25000 light years away, any response would come in about the year 52000 AD.
BTW, Carl Sagan, who was a proponent of the search for extra-terrestrial intelligence (SETI), was a part of the composition of the message. If you read Contact (again, please, don't watch the horrible movie), he spends a great deal of time discussing how messages could be transmitted across the stars and subsequently decoded.