test signals for transmission by transmitter 312 over pair a of data cable 18 the test signal returns through filter 18 and pair b of data cable 18 receiver 314 to detector 324 if the network device 12 u2032 is a self powered dte device as shown in fig2 b or if there is an open circuit detector 324 does not detect a return signal as a result power is not supplied by network device 10 to network device 12 u2032 over data cable 18 referring to fig3 a an exemplary test signal generated by signal generator 322 comprising plural pulses is illustrated an initial pulse having a magnitude of u2212 1 volt is applied for 752 ns followed by positive pulses 1 volt having a width of about 152 ns negative u2212 1 volt pulses having a width of approximately 72 ns and ending with a negative u2212 1 volt pulse having a width of 304 ns successive test signals are spaced by at least 156 ms it will be appreciated that the test signal in fig3 a is shown for illustrative purposes only and other appropriate test signals may be utilized fig4 a illustrates the test signal received by detector 324 when the length of the data cable is approximately zero meters and network device 10 is connected to cable powered dte device 12 the received test signal comprises a peak to peak voltage of about 1 25 volts fig5 a illustrates the.