Among other things, a communication system comprising remote units, a reference timing source, a controller, a controller clock, and a remote unit clock. The remote units exchange radio frequency (RF) signals with mobile devices. The reference timing source is synchronized with a coordinated universal time (UTC) or a Global Positioning System (GPS). The controller clock is synchronized with the reference timing source and provides timing information to the controller. The remote unit clock is synchronized with the controller clock and provides timing information to a remote unit. The controller and the remote unit are configured to transmit time stamp messages to synchronize the controller clock and the remote unit clock, by avoiding contention between time stamp transmissions and baseband data transmissions or between time stamp transmissions of different remote units to the controller.