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OCR for page 345
A.2 ITS Communication System Design
Communication Systems for rrs are designed to cost-effectively transmit data, video, and/or
voice from sources to destinations (sinks) to meet user and application requirements. No single
combination of communication architecture, topology, or mediums is optimum for aU
requirements. Thus, con~nun~cations system design involves considerations, evaluations, and
trade-offs of:
I. User and application requirements;
2. Data loads (or equivalently composite data rate) between sources and destinations;
3. Geographic area and commun~cabon infrastructure topology;
4. Multimedia requirements: voice, data, and video;
5. Communication media alternatives;
6. Link budget analysis;
7. Communication Architectural requirements: TOC backup, data sharing, etc.;
8. Standards;
9. Reliability, maintainability, and availability requirements;
10. Operational and maintenance support requirements; and
~ I. Costs and available budgets.
Section A.3 win present representative suburban, urban, metropolitan, and rural system designs
wad cost estimates. Section A.4 win address reliability, maintainability, and availability, as weD
as estimating requirements.
A.2.1 Analog versus Digital Communication System
Table A.2.1-! summarizes generic analog and digital communication systems and Weir
comparative charactenstics. The principle advantage of digital communication channels is the
complete reconstruction of the signal at repeaters (not amplifiers) and He availability of modern
error control techniques. Figure A.2.1-1 illustrates He comparison of analog and digital
communication links.
t;`NCHRPPhase~rpti N~3-51 · IF
A2-1
OCR for page 346
Table A.2.~1
Characteristics of Digital and Analog Communication Systems
Characteristic Analog Digital
Channel Type
Description Transmits replica of source Creates digital version of
waveform and reconstructs source waveform or data at
waveform at receiver. transmitter, transmits digital
data, and reconstructs
source waveform or data at
receiver.
Measures of Bandwidth Bit rate
performance Signal-to-Noise (SNR) Bit-Error-Rate (BER) as a
function of SNR
Principle Impairments Channel Distortions Channel Distortions
Noise* Noise*
Non Linear amplifier Non Linear amplifier
Disto rtions Distortions
Non Linear Digitization
D slovens
Principle Advantages Requires less processing Complem y n~=cu~s Sony
Graceful, but cumulative, at digital repeaters, thus
signal degradation minimal degradation above
Usually less Bandwidth SNR threshold.
Requirements than Digital Digital Error Control is
Equivalent available
Low cost digital components
Principle Limitation Noise and Distortions: Abrupt Signal Degradation
Cumulative even at below SNR threshold.
repeaters becoming so Bandwidth Requirements often
severe that SNR degrades greater than Analog
below threshold. Equivalent (improves with
Higher cost analog compression)
components Higher Cost and availability of
A/D and D/A Components
as source bandwidth
Examples with Voice: 3 kHz (telephone) Voice: 64 kbps
Analog: Bandwidth (Uncompressed)
Digital: Bit rate 4-32 kbps
(Compressed)
Video: 6 MHz (NTSC) Video: 3 Mbps (Compressed)
Data: digital modems Data: 300 bps to over 10 Gbps
AM Radio: 10 kHz~
FM Radio: 200 kHz~
* Usually expressed as SNR compared to desired source
** Currently no digital equivalent
/
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OCR for page 347
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