such as a Bell System C-4 channel, will maintain a response of -2 dB to +3 dB from 500 Hz to 3000 Hz and -2 dB to +6 dB from 300 Hz to 3200 Hz. Considering tandem operation, the deterioration of amplitude response is arithmeti- cally cumulative when sections are added. This is particularly true at band edge in view of channel unit transformers and filters that account for the upper and lower cutoff char- acteristics. Figure 10.11 illustrates a typical example of attenuation distortion (amplitude response) across carrier equipment back-to-back. Attenuation distortion, phase distortion, and noise were introduced in Section 3.3. 10.9.3.3 Noise. Another important consideration in the transmission of data is noise. All extraneous elements appearing at the voice channel output that were not due to the input signal are considered to be noise. For convenience, noise is broken down into four categories: (1) thermal, (2) crosstalk, (3) intermodulation, and (4) impulse. Thermal DATA COMMUNICATIONS Frequency in Hz Figure 10.11 Typical attenuation distortion across a voice channel, with carrier equipment back-to-back. noise, often called "white noise," Johnson noise, or "resistance noise," is of a Gaussian nature meaning completely random. Any system or circuit operating above absolute zero will display thermal noise. The noise is caused by the random noise of discrete electrons in the conduction path. Crosstalk is a form of noise caused by unwanted coupling of one signal path into another. It may be caused by direct inductive or capacitive coupling between conductor or between radio antennas. Intermodulation noise is another form of unwanted coupling, usu- ally caused by signals mixing in nonlinear elements of a system. Carrier and radio systems are highly susceptible to intermodulation noise, particularly when overloaded. Impulse noise can be a primary source of error in the transmission of data over PSTN and similar networks. It is sporadic and may occur in bursts or discrete impulses, which are called "hits." Some types o