"...Both the orbiter and ISS have two channels that can be used to communicate between the vehicle and the ground. For the orbiter, these are called Air-To-Ground 1 and Air-To-Ground 2. For ISS, these are called Space-To-Ground 1 and Space-To-Ground 2. During docked missions, we configure these channels so that Air-To-Ground 1 and Space-To-Ground 1 are connected. This means that when people talk to either vehicle on this "Big Loop," both vehicles hear the calls. The Air or Space-To-Ground 2 loops are used for the ISS team to talk just to the ISS or for the shuttle team to talk just to the shuttle.
The crew and ground usually call out which loop they are calling on so that the other side knows which channel to reply on."