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