Understanding the network.pdf - Back to Home

transmission system that could reduce cable densities and provide reliable transport
over longer distances. To meet this requirement, AT&T developed the T-carrier
digital transmission system.
Digital transmission is not affected by the line transmission noise and the
amplification problems associated with analog transmission, because digital
information is transmitted as Binary Coded Decimal (BCD) words instead of actual
sine-wave signals. When a sine-wave signal collects noise during transmission, it is
interpreted to the receiver as part of the signal. When a transmitted pulse code
signal gathers noise, it is not significant enough to alter the "code" from being
properly interpreted at the receiving end. Of course, over a long enough distance, a
pulse code signal can become distorted enough so it is no longer recognizable. To
overcome this problem, digital transport systems use signal regenerators at specific
distances along the transmission path. The regenerator takes the incoming signal,
regenerates a clean signal, and then transmits it out.
Pulse Code Modulation
Pulse code modulation (PCM) is the process used to convert analog signals into
digital signals. There are two PCM sampling methods: linear and non-linear. PCM
works by segmenting the available analog frequency range into steps, where each
step is a certain frequency level and has a specific voltage level associated with it.
The sampling quality is dictated by the size of the steps and the number of available
voltage increments that can be used to represent those steps. With linear PCM, the
analog signal is segmented into equal voltage increments. The problem with linear
PCM is that in limited bandwidth scenarios the steps are too large because there are
not enough voltage steps to accurately represent the signal. Consequently, certain
linear PCM samples come out distorted and full of noise. To improve the sample
quality and still accommodate the limited available bandwidth, non-linear PCM was
developed. Non-linear PCM algorithms use variably sized steps. Broad steps are
used for frequencies that are not important to the accurate reproduction of the
signal source, and ones that are finer are used for the important ones. There are two
non-linear PCM algorithms used for digital signal transmission: µ-Law and A-Law. In
the United States, µ-Law is used for PCM encoding. In Europe, A-Law is used for
PCM encoding. The actual analog-to-digital conversion is accomplished through a
three-step process using an analog-to-digital (A-to-D) codec. The three steps are
described here:
• Sampling—The first step in the conversion is to take the analog signal and
sample it. This results in a Pulse Amplitude Modulated (PAM) signal. The
sampling rate is determined using Nyquist's Theorem, which states that a
sampling rate needs to be at least two times the highest frequency in the
analog signal. For analog voice transmission, which is based on FDM, the