Sling Media SlingPlayer User Manual page 35

If you're in the room with a Slingbox, you can tell whether or not someone's using it
by looking at the word "sling" on the front of the Slingbox. If the letter "n" is lit up in
red, then someone's using that Slingbox. (The other two small lights tell you that the
Slingbox is powered on and that it is connected to your network.)
SlingPlayer basics • Page 35