ntermodulation products that very quickly take on power figure (i.e. dBmOp). An additional weighting factor is in- the characteristics of gaussian noise. cluded when the full channel width of 4kHz is considered. This noise source is referred to as intermodulation or cross modu- The weighting factor for a 4kHz channel considered over the full lation noise. 4kHz is 2.5 + 10log(4/3.1) or 3.6dB. The initial problem is to define the power presented to the system by speech traffic. 20.7.3 Thermal noise 20.7.4.1 Single channel load This is sometimes called Browian or Gaussian noise and is noise that is associated with the random movement of electrons at tem- The average power in each channel of a transmission band carrying peratures above absolute zero. normal 'busy hour' speech traffic referred to a OdBr point of meas- For the FDM range of frequencies the noise power that is gener- urement is Lo (dBmO) as in Equation 20.7 (Bell, 1971), where Pvo ated is given by Equation 20.3, where P is noise power in watts, k is the long term average power in a single talker dBm0, g is the is Boltzmann's constant, T is the absolute temperature in degrees standard deviation of the distribution of talker volumes and t is the Kelvin, and B is bandwidth in Hz. traffic activity factor. P=kTB watts (20.3) Lo = Pvo+0.115 g + = log dBmO (20.7) If T = 310°K and B = 4000Hz then the available noise power is -137.5dBm. The absolute thermal noise power at the output of a circuit Single talker volume (Pvo) element of gain GdB and noise figure FdB within a 4kHz channel is given by Equation 20.4. This is the power in dBmO of a continuous individual talker measured using a long term averaging meter (10 seconds) at a OdBr 137.5 + G + F dBm point. This includes power due to signalling frequencies. (20.4) The precise value for has been the subject of much measure- ment and study over the years. The UK figure used for trunk If the absolute power at the transmission level point T1 is known networks is -12.9dBmO. In the Bell System a figure of -13.9dBmO (T