257 lines
9.7 KiB
Plaintext
257 lines
9.7 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: 4
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INTRODUCTION TO BASIC DIGITAL TECHNOLOGY
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Written by: Erik Turbo
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File: 4 of 6
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4. Digital Transmission
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4.1 Interoffice Facilities
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Below is a list of interoffice facilities required for digital transmission
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of data.
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1. VF Facilities
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A small number of Special Services circuits use VF, Voice Frequency, pairs
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for the facility between Central Offices.
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a. MFT Equipment
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The Metallic Facility Terminal (MFT) is the AT&T family of plug-in
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equipment developed to provide transmission and/or signaling fuctions
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required for metallic facilities. MFT provides the interfect between
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VF metallic circuits and switching systmes, station equipment, or
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another metallic cable.
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In addition to metallic facilities, micro-wave radio is utilized for both
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digital and analog transmission. Fiber-optic facilites are also utilized
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for digital transmission.
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2. Carrier Facilities
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Carrier facilities, both analog and digital, are pair gain devices. They
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are more economic than using Voice Frequency (VF) metallic cable pairs.
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3. Digital Transmission Systems
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a. Digital Hierarchy
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Digital terminals are connected together by an almost countless number
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of facility and equipment configurations.
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b. Digital Channel Banks
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The most common channel banks used by the BOCs are the AT&T D4 and D5
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terminals. The channel units provide transmission and signalling
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features required to interface with 2 or 4 wire circuits.
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4.2 T1 Carrier Line
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The T1 line carries DS1 signals (1.544 Mb/s) between signal digroup channel
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banks using four-wire bidirectional transmission over standard cable
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pairs.
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4.3 Digital Line Coding
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> Bipolar Coding
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Bipolar coding is the basic line coding procedure used by T-carrier lines.
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A Bipolar code uses alternating polarties for encoding "1"'s.
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> Clear Channel Capability
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Clear Channel Capability is used to describe the capability to transmit
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a DSO or DS1 level signal which contains any mix of ones and zeros,
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including all zeros. The density requirement is no more than 15 zeros.
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If more than 15 zeros are used then ones must be put in or an error
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will result.
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Due to current signaling and maintenance requirements, only 56 kbps of
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the DSO signal is available for use by the customer.
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In order to provide 64 kbps Clear Channel Capability (64 CCC), the Bipolar
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with Eight Zero Substitution (B8ZS), is the method recommended as the North
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American Standard. However, interim methods such as Fractionally Controlled
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Mutliplexing (FCM), or Zero Byte Time Slot Interchange (ZBTSI), can be
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utilized as an expedient.
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> Bipolar Advantages
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- Simplifies error detection
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- Elimates DC components
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- Reduces bandwith requirements
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4.4 Digital Wave Forms
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0 1 1 0 0 1 0 1
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+V ----------- ----- -----
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0V _____|___________|__________|_____|______|_____|_
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-V
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Unipolar Non-return to zero
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0 1 1 0 0 1 0 1
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+V ---- ---- ----- -----
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0V _____|____|_|____|__________|_____|______|_____|_
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-V
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Unipolar Return to Zero
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0 1 1 0 0 1 0 1
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+V ---- -----
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0V _____|____|_________________|_____|______________
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-V |_____| |_____|
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Bipolar non Return to Zero
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0 1 1 0 0 1 0 1
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+V ____ ____
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0V _____|____|_________________|____|______________
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-V |____| |____|
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Bipolar Return to Zero
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Comparison of Digital Waveforms
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4.5 Error Detection
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+ ____ ____ ____
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0 ____| |________ ____| |__________ ___| |________
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- |____| |____|
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/\ < -- Noise Burst
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+ _____/\_______________________________________/____\___________________
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/ \ / \ /\ /\ / \ __ / \
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0 _/________\____/____ \____/____\/____\____/____________\/__\____/_____\
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\/ \/ \/ \/ \
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- _______________________________________________________________________
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Bi-Polar Violation
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+ ____ ____ ____ ____
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0 ____| |________ ____| |___| |_ ___| |________
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- |____| |____|
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Error Detection
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The error detection technique is very simple. Since each successive "1" bit
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is of opposite polarity, and extra pulse will show up as an error. This
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error detection technique is called Bipolar Violation Detection.
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4.6 Analog Switching with Digital Transmission
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__________ 1 001010 2 _______ 3
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_________ | Local | __ __ |Toll | __
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|Telephone|----|-x--x--x--|------|__|----------|__|------|-x-x-x-|---|__|-.
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--------- | Office | |Office | |0
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|__________| |_______| |0
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Analog - Digital |1
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Conversion |0
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|1
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|0
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__________ 6 5 _______ 4 |
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_________ | Local | __ 001010 __ |Toll | __ |
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|Telephone|----|-x--x--x--|------|__|----------|__|------|-x-x-x-|---|__|-.
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--------- | Office | |Office |
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|__________| |_______|
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Key:
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1,2,3,4,5 and 6 are Analog/Digital Conversion switching systems
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001010 is the digital bit-stream representation of the analog signal
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Analog Switching with Digital Transmission diagram
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The diagram shows an Analog signal being converted to Digital for transmission
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and back to Analog for switching. This configuration causes additional
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noise and distortion to be added to the Analog signal.
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Toll Office
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__________ 1 ________
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_________ | Local | __ 01101110 | |
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|Telephone|----|-x--x--x--|------|__|---------------|01101110|-------.
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--------- | Office | | | |
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|__________| |________| |
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Toll Office |
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__________ 2 ________ |
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_________ | Local | __ 01101110 | | |
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|Telephone|----|-x--x--x--|------|__|---------------|01101110|-------.
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--------- | Office | | |
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|__________| |________|
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Digital Switching with Digital Transmission
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> Four points of analog/digital conversion have been elimated from the
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previous example.
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> The digital switch will switch the digital stream directly. Therefor,
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digital to analog conversion is not needed.
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> The introduction of a digital toll office reduces the need for some
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analog/digital conversion.
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> This reduces the cost of providing additional digital transmission
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facilities since no analog/digital conversion will be required at the
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toll office location and this will improve the overall transmission.
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------------------------------------------------------------------------------
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EOF ---------------- Xenon Foundation Productions 1993 -------------------EOF
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------------------------------------------------------------------------------
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