PCM30
The pulse code modulation (PCM) technology was patented and developed in France in 1938, but could not be used because suitable technology was not available until World War II. This came about with the arrival of digital systems in the 1960s, when improving the performance of communications networks became a real possibility. However, this technology was not completely adopted until the mid-1970s, due to the large amount of analog systems already in place and the high cost of digital systems, as semiconductors were very expensive. PCM’s initial goal was that of converting an analog voice telephone channel into a digital one based on the sampling theorem.
PCM30 describes an application of pulse-code modulation (PCM) in which 30 telephony analog signals are binary coded into a digital signal stream. The term is used today mostly as a synonym for the encoding of 30 channels each with a signalling rate of 64-kbit/s. This rate is also used in the first stage of European PDH technique, so PCM30 is also known as E1.
Originally it described a device in communications technology which converted the 30 analog telephone signals into a digital bit stream of 2048 kbit/s .
At the beginning of the 1960s, the proliferation of analog telephone lines, based on copper wires, together with the lack of space for new installations, led the transmission experts to look at the real application of PCM digitalization techniques and TDM multiplexing. The first digital communications system was set up by Bell Labs in 1962, and consisted of 24 digital channels running at what is known as T1.
In 1965, that permitted the TDM multiplexing of 24 digital telephone channels of 64 kbit/s into a 1.544-Mbit/s signal with a format called T1 (see Figure 11). For the T1 signal, a synchronization bit is added to the 24 TDM time slots, in such a way that the aggregate transmission rate is:
Europe developed its own TDM multiplexing scheme a little later (1968), although it had a different capacity: 32 digital channels of 64 kbit/s (see Figure 11). The resulting signal was transmitted at 2.048 Mbit/s, and its format was called E1 which was standardized by the ITU-T and adopted worldwide except in the U.S., Canada, and Japan. For an E1 signal, the aggregate transmission rate can be obtained from the following equation:
(32channels × 8bit ⁄ channel) ⁄ 125μs = 2.048Mbit/s
The PCM30-base system, the analog switching technology in Europe, and served on the digital transmission of telephone traffic. It has 30 coders, each with a phone channel in an 8-bit digital word (byte) can implement. The 30 bytes are, together with a frame ID word, and a channel identifier, bytewise successively sent (see Multiplexer). Thus, a large 32-byte frame. In receiving direction, the 30 channels are, using a de-multiplexer, converted back to 30 analogue signals. The time slot 0 is used as a frame ID word, and as word detection is used. These are sent alternately. Time slot 16 is a channel identifier for the speech channels. Each voice channel is assigned 4 bits, one after the other in 16, there is an over-under with a frequency of 500 Hz and a length of 16 ⋅ 32 ⋅ 8 # 4096 bits.
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