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Wisconsin Department

Intelligent Transportation Systems (ITS)

of Transportation

Design Manual

December, 2000













CHAPTER 9


COMMUNICATION SYSTEMS


Wisconsin Department

Intelligent Transportation Systems (ITS)

of Transportation

Design Manual

December, 2000


9-1
9. Communication
Systems

Communication system design is typically a highly complex process. The
telecommunications industry is technologically dynamic, with new technologies and
enhancement of existing technologies constantly evolving. This chapter sets forth some basic
information on communication systems in general. Emphasis is placed on communication
conduit infrastructure and wireless spread spectrum design issues. Most applications
involving the design of wire line or wireless communication systems will require additional
information that is not currently found in this manual. However, for the design of basic
communication infrastructure, such as conduit systems or spread spectrum infrastructure, this
chapter provides the designer with fundamental guidelines to use in the design of these
systems.


Prior to final design of communication system elements, a strategic communication plan
must be developed for the region, indicating uses, communication types, configuration,
topology, equipment, and other issues beyond the scope of this document. This strategic plan
will provide the blueprint for how the overall system communicates, and will provide direction
to the designer when implementing various types of communication infrastructure.


Aside from the basic physical components of a communication system (such as cable,
modems, etc.), how an intelligent transportation system communicates between various
components revolves around issues such as element protocols and formats. Older systems may
have strict communications protocol guidelines (as defined by existing system software) that
must be followed. Newer systems require communication design following NTCIP
standards. NTCIP stands for the National Transportation Communications for ITS Protocol. It
establishes an array of standards that provides:

the rules for communicating (called protocols), and
the vocabulary (called objects) necessary to allow electronic traffic control
equipment from different manufacturers to operate with each other as a system.


The NTCIP is the first set of standards for the transportation industry that allows traffic
control systems to be built using a "mix and match" approach with equipment from different
manufacturers. Therefore, NTCIP standards reduce the need for reliance on specific equipment
vendors and customized one-of-a-kind software. Bringing together representatives from
equipment manufacturers and system users, NTCIP is a joint product of the National
Electronics Manufacturers Association (NEMA), the American Association of State Highway
and Transportation Officials (AASHTO), and the Institute of Transportation Engineers (ITE).
9.1.
Introduction and Usage

An intelligent transportation system is comprised of many different elements field
components such as variable message signs, detector stations, ramp meters, and CCTV cameras;
central equipment such as computers, workstations and monitors; and the human element (i.e.,
system operators and maintenance personnel). For the system to function properly, it will be
necessary for each of these components to exchange information with other system elements. It is
the communications network that provides the connecting link for this information.

Wisconsin Department

Intelligent Transportation Systems (ITS)

of Transportation

Design Manual

December, 2000


9-2

The communications network is an integral part of any ITS design in that it will affect
(and be affected by) system architecture, configuration, and the operational strategies.
Moreover, if thought of as a single expense, the communications network will likely be the
costliest item in the vast majority of ITS related systems.


The most important consideration in designing a large communications network is that
it must provide reliable service for 10 20 years or more to ensure economic viability. At this
period in the communications industry and continuing into the foreseeable future, the extent of
technological change and market restructuring presents both difficulties and opportunities.
The difficulties are in the real possibility of equipment obsolescence. As with computers, the
communications industry is going through a rapid evolution of available equipment. The
opportunities may involve partnering with the many new communications companies that the
deregulated environment is producing.


Although the design of the communication elements within individual projects may not
involve system assessments and large scale concepts as indicated above, it is important that
individual designers be aware of potential changes in communications network equipment and
structure brought about by either of the above difficulties and/or opportunities.

9.2.
Communication System Types & Fundamentals

Communication network equipment for intelligent transportation systems can be
divided into two different categories: analog and digital. Analog technology conveys data as
electronic signals of varying frequency or amplitude that are added to carrier waves of a given
frequency. Broadcast and phone transmission has conventionally used analog technology.
Digital describes electronic technology that generates, stores, and processes data in terms of
two states: positive and non-positive. Positive is expressed or represented by the number 1 and
non-positive by the number 0. Thus, data transmitted or stored with digital technology is
expressed as a string of 0's and 1's. Each of these state digits is referred to as a binary digit, or
bit in short. A string of bits that a computer can address individually as a group is a byte.


Within each of these categories are Voice typically radio communications, but can
include PBX telephone type systems between centers, Data - elements from system detector
stations, ramp meters, dynamic trailblazer assemblies, and variable message signs, which do
not require large bandwidth (i.e., small packages of data). Video - elements require
transmission of full-motion video for incident verification and traffic surveillance, such as
closed-circuit television cameras or local agency video (large bandwidth/transmission
requirements). The majority of ITS equipment requires data or video transmission
requirements. Hence, these communication system elements will be the primary focus of this
chapter.


There are numerous types of carrier technologies. This ranges from regular telephone
service, one of the most basic forms of communication, to optical carrier (OCx) levels up to OC-
48. A sampling of various communication types, data rates, and media are discussed in this
section and summarized in
Figure 9-1
. In discussions of carrier systems, the following
definitions are presented:

Wisconsin Department

Intelligent Transportation Systems (ITS)

of Transportation

Design Manual

December, 2000


9-3
T-Carrier - The T-carrier system, introduced by the Bell System in the U.S. in the 1960s,
was the first successful system that supported digitized voice transmission. The original
transmission rate (1.544 Mbps) in the T-1 line is in common use today in Internet service
provider connections to the Internet. Internet service providers also commonly use
another level, the T-3 line, providing 44.736 Mbps. Another commonly installed service
is a fractional T-1, which is the rental of some portion of the 24 channels in a T-1 line,
with the other channels going unused. The T-carrier system is entirely digital, using
pulse code modulation and Time-Division Multiplexing. The system uses four wires
and provides duplex capability (two wires for receiving and two for sending at the same
time). The T-1 digital stream consists of 24 64-Kbps channel that are multiplexing. (The
standardized 64 Kbps channel is based on the bandwidth required for a voice
conversation.) The four wires were originally a pair of twisted pair copper wires, but
can now also include coaxial cable, optical fiber, digital microwave, and other media. A
number of variations on the number and use of channels are possible.


Synchronous Optical Network - SONET is the U.S. (American National Standards
Institute) standard for synchronous data transmission on optical media. The
international equivalent of SONET is synchronous digital hierarchy (SDH). Together,
they ensure standards so that digital networks can interconnect internationally and that
existing conventional transmission systems can take advantage of optical media
through tributary attachments. SONET provides standards for a number of line rates up
to the maximum line rate of 9.953 gigabits per second (Gbps). Actual line rates
approaching 20 gigabits per second are possible. SONET is considered to be the
foundation for the physical layer of the broadband ISDN (Broadband Integrated
Services Digital Network). SONET defines a base rate of 51.84 Mbps and a set of
multiples of the base rate known as "Optical Carrier levels (OCx)." Asynchronous
transfer mode (ATM) runs as a layer on top of SONET as well as on top of other
technologies.


Optical Carrier Levels (O