The purpose of upstream capture is to measure plant response and view the underlying noise floor, by capturing at least one OFDMA symbol during a scheduled active or quiet probe. An active probe provides the partial functionality of a network analyzer, since the input is known and the output is captured. This permits full characterization of the linear and nonlinear response of the upstream cable plant. A quiet probe provides an opportunity to view the underlying noise and ingress while no traffic is being transmitted in the OFDMA band being measured. The PNM server selects an active CM to analyze by specifying its MAC address, or performs a quiet probe measurement. When enabled to perform the capture, the CMTS selects a specified transmitting CM, or quiet period when no CMs are transmitting, for the capture. The CMTS sets up the capture as described in MULPIv3.1, selecting either an active SID corresponding to the specified MAC address or the idle SID, and defining an active or quiet probe. The active probe symbol for this capture normally includes all non-excluded subcarriers across the upstream OFDMA channel, with pre-equalization on or off as specified in the MIB. The quiet probe symbol normally includes all subcarriers, that is, during the quiet probe time there are no transmissions in the given upstream OFDMA channel. For the quiet probe, the CMTS captures samples of at least one full OFDMA symbol including the guard interval. The CMTS begins the capture with the first symbol of the specified probe. The sample rate is the FFT sample rate (102.4 Msps). The CMTS reports the list of excluded subcarriers, the cyclic prefix length, and the transmit window rolloff period in order to fully define the transmitted waveform. The CMTS also reports the index of the starting sample used by the receiver for its FFT. For possible comparison with other events, the CMTS reports the timestamp corresponding to the beginning of the probe. In the case where the P-MAPs for the OFDMA upstream being analyzed are being sent in an OFDM downstream, the timestamp reported is the extended timestamp, while in a case with OFDMA upstream channels but no OFDM downstream channels, the reported timestamp is the D3.0 timestamp. For an active probe, the CMTS reports the contents of the Probe Information Element (P-IE) message describing that probe.