The analyzer also can demodulate the channel's signals and produce constellation diagrams of the 64 QAM and 256 QAM downstream signals or the 16 QAM and QPSK upstream signals. Each of perhaps more than two dozen downstream transmitters and more than 100 upstream receivers in a CMTS must transmit data to and from hundreds of cable modems. Therefore, each cable modem sends data in bursts, then goes quiet until the next scheduled time slice. The CMTS must detect the beginning and end of each burst. An oscilloscope set to trigger on the length of a burst verifies that the CMTS receives data sent with the proper burst length.
Traffic generators do more than transmit data packets.
They also perform MAC-layer measurements for data throughput, packet loss, and packet latency. The possible combinations of tests on RF channels, power levels, modulations, forward error correction, and others are nearly limitless. Although they can't test for every possible combination, engineers still need to make hundreds of measurements. Figure 3 diagrams a typical in-house HFC network that engineers use for interoperability and stress testing. The network includes all the components that make up a network from the CMTS to subscribers' homes.