In this illustration we see two base station systems positioned vertically one on top of the other. Each BSS includes a router labeled "BSC" (identified in a key as a Base Station Controller) and three coverage cells depicted as hexagons. Each hexagon includes a cell tower (identified in a key as Base transceiver stations). The top BSS also includes two cell phones and a car receiving signals from the cell towers (identified as Mobile Subscribers). The bottom BSS includes one cell phone receiving signals from the cell towers. Each BSC is connected to a single router labeled "MSC" (identified as a Mobile Switching Center). The MSC is further connected to a Gateway MSC, labeled G, which is itself connected to a cloud representing the public telephone network.