ow it to transmit. When it transmits, all other devices on that piece of media must remain in a receive- only state. To ensure the STB transmits without interfering with its neighbors, three prerequisites must be met: All the devices on a shared piece of media must be phase aligned SO that transmission begins and ends at the correct moment after the headend has granted the STB a transmission time-slot. There must be a guard band or guard time between the transmissions from different STBs to ensure transmissions do not talk over the top of each other if there is a small error in phase alignment. The distance from the headend to the STB must be measured (a process known as ranging) to help calculate the time that the signal takes from the transmitter to the headend. Once the STB knows the distance to the headend receiver, it will transmit its data earlier to compensate for the time delay before the headend receives it (in fiber, this advance might be around 5 ns per meter of distance). Figure 2-6 shows the situation where two STBs are transmitting at the correct time SO that the signal arrives at the headend correctly aligned, even when allowing for the longer distance to set-top box #1 compared to set-top boxes #2 and #3. Signal at headend Guard band Internet Distant Midway Closer STB #1 STB #2 STB #3 Figure 2-6 Correctly Synchronized Signals on a Shared Media A side-effect of a system being tightly aligned with accurate range measurements is that the para- meters of the length of transmission and the width of the guard band can be adjusted to increase the effective throughput and responsiveness on the shared cable/media. As one example, the more ac- curate the phase alignment, the smaller the guard band needs to be, because there is a smaller win- dow for a transmission to be early or late. Figure 2-7 shows what might happen when there are large phase errors and range errors on the shared media. Signal at Corrupted signal headend STB #1 transmits too STB #2 transmits too late - range error early - phase error Int