Appendix A
E1 background
The invention of the telephone in 1876 (A.G. Bell) has completely changed the
world. Today it would be difficult for us to imagine our life without the
telephone.
Even these days with the ever-growing number of computer networks and the
booming Internet, voice communication still forms the major part of the total
volume of the communication traffic.
In the early days each telephone connection required a dedicated link all the way
between two users. Shortly after the steadily growing number of subscribers led
to the development of various methods and technologies, which enabled several
telephone connections to be transmitted over a single cable. Most of the systems
at that time used Frequency Division Multiplexing (FDM) technology. Here the
idea was to modulate each telephone channel with a different carrier frequency
to shift the signals into different frequency ranges.
With ever increasing demands for higher transmission rates with better quality
and the advent of semiconductor circuits other techniques were developed. In
the 1960s, digital systems started to appear. Here the telephone channels are
separated by time using a new type of transmission method known as Pulse
Code Modulation (PCM).
The analogue signal (speech) from the telephone is first converted to a Pulse
Amplitude Modulated (PAM) signal using a process called sampling. Then
using quantization and encoding this sampled analogue (PAM) signal is
converted to a digital PCM signal. Based on the principle above the
Plesiochronous Digital Hierarchy (PDM) has been developed.
Here the 300-3400 Hz band-limited analogue signal is sampled at 8000 Hz.
Then the PAM signals are quantized using a 13-segment compression
characteristic known as A-law (in T1 systems 15-segment
µ
-law characteristic
is used). Finally the signal is encoded using an 8 bit code word format. This
source coding produces 8-bit code words at a rate of 8 kHz, giving 64 kbps data
rate. To improve the utilization of the transmission medium, the signals are
transmitted by time division multiplexing, where the code-words are interleaved
and contained in a PCM frame.
A primary frame consists of 32 code words called timeslots, which are
numbered 0 to 31. A PCM31 frame comprises of 31 timeslots used for traffic
and 1 timeslot used for synchronization.
