FDD), whereby the uplink (transmit from handset) and downlink (receive from tower) used differ- ent bands-frequency is used to divide the duplex channels. This system needed only the normal frequency synchronization, although some applications running on top of FDD radios may need phase synchronization. Guard time Uplink band Guard band Downlink band Figure 2-5 FDD VS. TDD Radios The other option is known as time division duplex (TDD), whereby the uplink and downlink chan- nels share the same band, but both ends of the link take turns either transmitting or receiving-time is used to divide the duplex channels. Because TDD involves time, it is no surprise that this system requires phase synchronization. TDD is the predominant radio type for 5G deployments. 2.1.1 Cable and PON There are several transport technologies that require phase synchronization to efficiently function. Examples include the hybrid fiber coax (HFC) cable and PON systems used to provide consumer broadband services to residential households. The design of these systems reflects the fact that most broadband subscribers download much more traffic than they upload, and SO the traffic flows are inherently asymmetric. As a result of this requirement, these systems commonly transmit data in the downstream direction differently than they transmit data in the upstream direction. For example, the broadband router might broadcast the same downstream signal to every receiving set-top box (STB) but implement a unicast transmission technique in the upstream direction. Because the consumer's STB is sharing the cable or fiber with numerous other subscribers, it requires exclusive access to the cable while transmitting. Therefore, there needs to be arbitration between competing upstream transmitters to ensure that they don't transmit over the top of each other's signals. While details vary somewhat depending on the specific implementation of the transport, in shared media broadband, the STB must bid for permission to use a time-slot (or similar resource) to all