139 lines
4.7 KiB
Plaintext
139 lines
4.7 KiB
Plaintext
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\ / |_
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/ \ e n o n | o u n d a t i o n
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presents:
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\ /
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*------ the \ /
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/ \ files -----*
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/ \
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Fall/1993 Issue: 2
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INTRODUCTION TO BASIC DIGITAL TECHNOLOGY
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Written by: Erik Turbo
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File: 2 of 6
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2. Time Division Multiplexing (TDM)
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2.1 Defining Multiplexing
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Multiplexing basically combines or merges a number of signals into one
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composite signal. The most common type of multiplexer at NET is called
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a TIME DIVISION MULTIPLEXER.
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Time Division Multiplexing
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In the telephone industry, the D Channel Bank Time Division Multiplexer
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is the type most commonly used and the one you will probably use.
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The D Channel Bank Time Division Mutliplexer works by taking twenty four
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(24) voice channels and time division multiplexing them at the near end
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terminal. Then the signals are sent over a pair of wires to the far
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end terminal.
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This smae process is occuring at the far end terminal. The signal from the
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far end terminal is sent over another pair of wires to the near end terminal.
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Each terminal is equipped to restore the signal recieved to its orignal
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form.
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Time Division Multiplexing is used to take low speed information, sample
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it, and then send this information over a high speed data line. Each
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time all twenty four (24) channels have been sampled and sent, a framing
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signal is sent. This framing signal aids the far end terminal in identying
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and reassembling all of the information for each channel. This process is
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known as synchronizing data.
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On the next page you will learn how the sampled signals are quantized and
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encoded by a process called Pulse Code Modulation (PCM) before they are
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transmitted via Time Division Multiplexing to a distant terminal.
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2.2 Multiplexing--Pulse Code Modulation (PCM)
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> At 8,000 samples per second, a single channel is sampled once per 125
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micro-seconds. Each sample uses 5.2 microseconds of time.
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> There are approximately 120 microseconds of idle time between each sample
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on a single channel Pulse Code Modulation (PCM) facility.
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> To make efficient use of the facility, many samples are sent on the same
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path, which is called multiplexing.
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2.3 Pulse Code Modulation (PCM) Frame
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A Frame requires 125 microseconds to transmit and contains once encoded
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sample (8-bit word) for each channel that is multiplexed, plus the framing
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bit.
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The Frame is sub-divided into Time Slots. A Time Slot represents the time
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required the send one 8-bit word.
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The basic Pulse Code Modulation (PCM) bit stream contains 1,544,000 bits/sec.
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2.4 DS1 Bit Rate Computation
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24 CHANNELS
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x 8 BITS/WORD
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192 BIT
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+ 1 FRAMING BIT
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193 BITS/FRAME
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X 8000 TIMES/SEC
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1544000 BITS/SEC
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OR
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1.544 M BITS/SEC
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2.5 Time Division Multiplex (TDM)
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> This diagram shows the overall digital transmission system.
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> The Analog signal is sampled 8000 times a second via a process called
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Pulse Amplitude Modulation (PAM). The Pulse Amplitude Modulation (PAM)
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sample represents the amplitude of the signal at the time of sampling.
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> Each Pulse Amplitude Modulation (PAM) sample is quanitzed and encoded
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to an 8-bit Digital signal via a process called Pulse Code Modulation
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(PCM).
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> The Pulse Code Modulation (PCM) samples from all 24 channels are combined
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via a process called Time Division Multiplexing and transmitted to a
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distant terminal over a common path.
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> At the distant terminal, the Pulse Code Modulation (PCM) samples are
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decoded, demultiplexed and filtered to reconstruct the orignal Analog
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waveform.
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2.6 Conclusion
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Multiplexing is vital to our business because it allows us to take advanage
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of the idle time between each signle channel Pulse Code Modulation (PCM)
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facility. The idle time is used efficiently via multiplexing which allows
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us to send many samples on the same path.
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So multiplexing maximizes efficient use of the facility and reduces idle
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time by sending numerous samples over the same path.
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NOTICE
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Not for use or disclosure outside the NYNEX Corporation
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or any of its subsidiaries except when rightfully stolen.
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------------------------------------------------------------------------------
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EOF ---------------- Xenon Foundation Productions 1993 -------------------EOF
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------------------------------------------------------------------------------
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