Garmin 010-00566-00 GTM 21 Traffic Receiver The GTM 21 is an integrated FM TMC (Traffic Message Channel) traffic receiver, antenna and cigarette lighter adapter. In most areas, depending on the service provider, TMC traffic broadcasts are continuous  there's no waiting for scheduled traffic news updates or random alerts. The GTM 21 is a fully portable, plug-and-play design that uses a 12-volt cigarette lighter adapter. No professional installation required. Just use the suction cups to attach the antenna to your vehicle's windshield. Traffic broadcasts are received via a "silent" FM data channel, you can still listen to music or news programming on your car radio without interference from incoming FM traffic data transmissions. With the GTM 21, you see traffic developments in real-time, as they happen, so you can make the right decision sooner.