Chapter 17 – Communications
A communications network provides the means by which information is
exchanged between all the entities and components that comprise a freeway
management and operations program – for example, between freeway practitioners
and other stakeholders; between field devices and a transportation management
center (TMC) of a freeway management system; between the TMC and maintenance
and incident response vehicles; between TMCs within a region; and for
disseminating traveler information to the users of the surface transportation
network. This information may consist of voice, data, video, or some combination
There are multiple communications options (e.g., network architectures,
technologies, standards, implementation strategies) available for meeting
these needs. It is crucial that the most appropriate options be selected
to best support the operational requirements of the freeway management
program and the associated ITS-based systems.
17.1.1 Purpose of Chapter
The Communications Handbook for Traffic Control Systems (Reference
1) treats all aspects of communications networks in depth, and serves
as a comprehensive stand-alone information resource. The "Communications
Handbook" addresses the various technical issues, and provides information
to support planning, design, development, and management of the communications
infrastructure to support a freeway management system (and other traffic
management systems). This chapter of the Freeway Management and Operations
Handbook is intended only as a summary of the information
contained within the Communications Handbook.
17.1.2 Relationship to Other Freeway Management Activities
The communications network of a freeway management system provides the
links by which information is transmitted between a TMC (Chapter
14) and a variety of field elements (i.e., "center-to-field"),
such as ramp meters (Chapter 8), lane control
signals and variable speed limit signs (Chapter 8), changeable message
signs (CMS) and Highway Advisory Radio (Chapter
13), and detectors and cameras (Chapter
15). It also supports the sharing and integration of information between
centers (i.e., "center-to-center") as part of a regional architecture
(Chapter 16) in support of traffic incident
management (Chapter 10), planned special event
management (Chapter 11), emergency management
(Chapter 12), and regional traveler information
dissemination (Chapter 13).
Of course, the importance of "communications" between freeway practitioners
and other stakeholders cannot be overemphasized. As noted in Chapter
2 herein, engaging as many stakeholders as possible in the various
processes that involve or impact freeway management helps to promote a
framework for collaboration and cooperation. This form of communications
may not involve large amounts of data or video (i.e., predominately voice);
and is generally not considered as "state-of-the-art" technology;
but a freeway management and operations program cannot survive without
17.2 Current Practices, Methods, Strategies, and Technologies
Freeway Management Systems (FMS) deployed in the 1970's and 1980's used
communications technologies based on the transfer of voice. All data had
to be converted to something that could be accommodated by an analog voice
based infrastructure. FMS communications systems were based on this technology
because that's what was available. By 1995, developing technologies began
to change the nature of the communications infrastructure. Fiber replaced
copper and digital replaced analog. Departments of Transportation have
begun to take advantage of the new communications technologies as a means
to support the use of new methods and tools in an FMS. The use of fiber
optics supports greater data capacities and the ability to use "real-time"
video imaging. Recently, wireless communications and the Internet have
started to offer effective strategies in support of freeway management
Communications, whether they involve advanced technology or relatively
simple means (e.g., the telephone), are an integral part of any freeway
management program. As noted throughout this Handbook, freeway management
strategies and ITS technologies can assist in reducing congestion, improving
safety, and enhancing mobility. However, without the capability to readily
exchange information – often in "real time" – between the entities and
system components that comprise the freeway management program, the potential
benefits of these strategies and technology systems will not be realized.
To that end, it is not a simple matter to quantify benefits from communications
networks alone, but instead to understand that the benefits realized from
freeway management strategies and ITS technologies are dependent on effective
and reliable communications.
17.2.3 Key Considerations During Freeway Management Program Development
The communications infrastructure is a critical key element of any FMS
(or ITS) system. The communications network typically consumes ten to
twenty-five percent of an overall budget for an ITS-based system. Moreover,
if not adequately implemented, it can inject a serious constraint on the
overall operation. As such, communications considerations and needs should
be an integral part of all aspects of the Systems Engineering process
discussed in Chapter 3 herein.
The Communications Handbook includes a chapter entitled "Developing the
Communications System", which provides a practical approach to the design
and system engineering of a communications network that supports traffic
and transportation requirements. The chapter provides a step-by-step process
that can ultimately result in a communication system requirements analysis
and preliminary design.
A theme that is repeated throughout the Communications Handbook is that
the design of a communications network to support roadway and transportation
functions is not a stand-alone process. The determination
of functionality and selection of options must be done as an integrated
part of the overall traffic management system design, starting with the
development of requirements. Moreover, it is important to keep in mind
that the communication subsystem is a supporting element of the overall
traffic management system. Accordingly, the communication engineer should
also be fully aware of the vision and the system concept of operations.
A primary axiom that drives the design of a communications system is
–"there are no absolutes"! For most communication systems, there are
usually several ways to achieve the desired results. It is important to
approach the communications system design with the right attitude. There
is a tendency to look at the "gee-wiz" of communications technologies.
Project managers and engineers must avoid this potential trap during the
requirements analysis. The communications networks are designed and implemented
in support of the traffic management system – not vice-versa! At the same
time, it is perfectly acceptable to ask the communication engineer to
look at system options using leading edge technology. This will give the
communication engineer an understanding of project team expectations.
In return, the project team is provided with enough information to make
the right decisions.
The Communications Handbook offers several key points to consider when
developing and designing a communications network for an ITS-based system,
- View the communication system as a part of the overall traffic/transportation
project. There are many examples of adding the communications network
as an afterthought. This eventually causes dissatisfaction with the
overall system, resulting in the need to spend additional money to correct
- Look at the whole system, not just the immediate implementation project.
Many ITS programs are implemented in phases or as part of roadway construction
/ reconstruction projects. These project sections are, in fact, part
of a larger plan. The communications system should be part of the larger
overall plan. The communications network must be analyzed and designed
to serve the long-term traffic management needs (e.g., what will the
ultimate system provide in terms of geographic coverage and functionality).
The potential communication needs of other government entities should
also be considered in the analysis and design.
- Answer the following primary "questions"
- What is the purpose of the proposed transportation
system? Relate the communication requirements to the reason for
the project's existence and its required functionality.
- Where will it be located? Location of the project
and the surrounding conditions has an impact on overall design of
the physical infrastructure, technology selection, and the cost
of construction of a communication network.
- When (over what period of time) will it be deployed?
During a relatively short deployment time frame, project planners
can assume that communication technology will remain stable. The
communication system design team can propose a system without concern
that communication technology and process will change. On the other
hand, long-term projects can expect to see a need to combine current
and legacy equipment into a working system.
- Who will operate and maintain the system? Consider
if the communication system will require that operational personnel
have a need to activate various functions of the communication equipment
and to trouble-shoot communication problems. Answering the question
of who will operate and maintain the system will lead to operator
and maintenance staff qualification requirements.
- Why is the traffic system being deployed? This
may seem redundant to the question of "what" is
being deployed, but at this point the project team will focus on
the specific type of traffic system. The communication engineers
responsible for analyzing and designing the communications system
need to be provided with a good understanding of how various types
of traffic/transportation systems work. This will lead to a design
of the communication system based on the functions of the traffic/transportation
- A variety of How questions, including how it
will be funded, how many devices will be deployed, how much redundancy
is required, how will regional integration requirements be met,
17.2.4 Relationship to National ITS Architecture
The National Architecture for ITS does not provide a lot of detail for
any specific communications technology. The "communications layer" of
the National ITS Architecture provides the "links" between the various
"systems" (e.g., center, vehicle, roadside, and travelers) as shown in
the ITS architecture "sausage diagram".
The National ITS Architecture has identified four communication media
types to support the communications requirements between the nineteen
subsystems. They are wireline (fixed-to-fixed), wide area
wireless (fixed-to-mobile), dedicated short-range communications (fixed-to-mobile), and vehicle-to-vehicle (mobile-to-mobile).
17.2.5 Technologies and Strategies
A primary axiom that drives the design of a communications system is
– "there are no absolutes"! For most communication networks, there are
usually several ways (e.g., architectures, technologies) to achieve the
desired results. The Communications Handbook includes a chapter (i.e.,
"Fundamentals of Communication Technology") that discusses the various
elements of a communication system, including transmission media; signaling
interfaces for voice, data and video; and transmission protocols.
220.127.116.11 Transmission Media
Transmission media are those elements that provide communication systems
with a path on which to travel. Alternatives include the following:
- Twisted Pair: Twisted pair is the ordinary copper
wire that provides basic telephone services to the home and many businesses.
In fact, it is referred to as "Plain Old telephone Service" (POTS).
The twisted pair is composed of two insulated copper wires twisted around
one another. The twisting is done to prevent opposing electrical currents
traveling along the individual wires from interfering with each other.
This interference is called "crosstalk". A broad generalization is that
twisted copper pair is in fact the basis for all telecommunication technology
and services today. Even the basis for 10-Base-T Ethernet is twisted
pair. For some application, twisted pair is enclosed in a shield that
functions as a ground. This is known as shielded twisted pair (STP).
Twisted pair comes with each pair uniquely color-coded when it is packaged
in multiple pairs. There is an IEEE standard for color-coding of wires,
wire pairs, and wire bundles. The color-coding allows technicians to
install system wiring in a standard manner.
- Coaxial Cable: Coaxial cable is a primary type of
copper cable used by cable TV companies for signal distribution between
the community antenna and user homes and businesses. Coaxial cable is
called "coaxial" because it includes one physical channel that carries
the signal surrounded (after a layer of insulation) by another concentric
physical channel, both running along the same axis. The outer channel
serves as a ground. Many of these cables or pairs of coaxial tubes can
be placed in a single outer sheathing and, with repeaters, can carry
information for a great distance.
- Fiber Optic Cable: Fiber optic (or "optical
fiber") refers to the medium and the technology associated with
the transmission of information as light impulses along a strand of
glass, and referred to as fiber. Fiber optic strand carries much more
information than conventional copper wire and is far less subject to
electromagnetic interference (EMI). Most telephone company long-distance
lines are now fiber optic. Transmission over fiber optic strands requires
repeating (or regeneration) at varying intervals. The spacing between
these intervals is greater (potentially more than 100 km) than what
is normally required for copper based systems. The fiber cable is constructed
in several layers. The core is the actual glass, or fiber, conductor.
This is covered with a refractive coating that causes the light to travel
in a controlled path along the entire length of the glass core. The
next layer is a protective cover that keeps the core and coating from
sustaining damage. It also prevents light from escaping the assembly,
and has a color-coding for identification purposes. The core, coating
and covering are collectively referred to as a "strand". Fiber strands
are produced in two basic varieties: Multi mode and Single mode. Each
variety is used to facilitate specific requirements of the communication
- Multi mode is optical fiber that is designed
to carry multiple light rays or modes concurrently, each at a slightly
different reflection angle within the optical fiber core. Multimode
fiber transmission is used for relatively short distances because
the modes tend to disperse over longer lengths (this is called modal
dispersion). For longer distances, single mode fiber is used.
Multimode fiber has a larger core than single mode.
- Single mode is optical fiber that is designed
for the transmission of a single ray or mode of light as a carrier
and is used for long-distance signal transmission. For short distances,
multimode fiber is used. Single mode fiber has a much smaller core
than multimode fiber. Single mode fiber is produced in several variations.
The variations are designed to facilitate "very long reach (distances)",
and the transmission of multiple light frequencies within a single
- CDPD: CDPD (cellular digital packet data) is an analog
data overlay that has been operation since 1993. This service provides
data throughput at 9.6 KBps, and is an overlay to the analog cellular
telephone system. CDPD is being used by a number of communities as a
wireless communication link to control traffic signal systems. As the
analog cell systems are converted to digital, CDPD is being phased out.
The wireless carriers are not providing a substitute.
- Microwave: Microwave is a fixed point-to-point service
that provides connectivity between major communication nodes. Telephone
and long distance companies use the service to provide backup for their
cabled (wireline) infrastructure and to reach remote locations. Public
Safety agencies use microwave to connect 2-way radio transmitter sites
to a central location. The frequencies allocated for this service are
in the 6 and 11 gigahertz ranges. All users are required to obtain a
license for use from the FCC (Federal Communications Commission). Frequency
licenses are granted on a non-interfering (with other users) basis.
Systems can be designed to operate over distances of about 20 miles
between any two points. Other frequencies available in the 900-megahertz,
2 and 23-gigahertz range do not require a license. Because these frequencies
do not require a license it is up to users to resolve any interference
problems without support from the FCC. As with all microwave, the FCC
permits only point-to-point uses.
- Spread Spectrum: Spread spectrum radio is a technology
that "spreads" the transmission over a group of radio frequencies. Two
techniques are used. The most common is called "frequency hopping" The
radio uses one frequency at a time but at pre-determined intervals jumps
to another frequency to help provide a "secure" transmission. The second
system actually spreads the transmission over several frequencies at
the same time. The method helps to prevent interference from other users.
These systems are generally used for distances of less than 2 air miles
(put in note about air miles vs. land miles).
- 2-Way Radio: 2-Way Radio systems have been in common
use since the 1930's. Originally used by the military, various federal
agencies, police, fire and ambulance, and local governments, its use
has expanded to include almost every aspect of our social infrastructure,
including individual citizens using "Ham Radio" systems. Most commonly
used frequencies are in the 30, 150, 450-512 and 800 megahertz ranges.
Coverage is usually expressed in terms of "air mile radius". Systems
in the 150 MHz band can typically cover 15 to 30 air miles in radius
from a single transmitter location. The FCC has been encouraging the
use of regional systems that incorporate all state, county and municipal
agencies into a single group of radio channels. The available radio
spectrum is being re-allocated to accommodate these systems. Today,
many Departments of Transportation are joining forces with public safety
agencies to create a common radio communication system. This allows
for easier coordination of resources to resolve traffic incidents.
- Free Space Optics: Free Space Optics (FSO) is another
wireless system being used today. Instead of using radio frequencies,
this system uses a LASER transmitted through the air between two points.
The LASER can be used for transmission of broadcast quality video. These
systems are limited to an effective range of 3 air miles.
18.104.22.168 Transmission / Signaling Interfaces
Data can be transmitted in either an analog or digital format. Private
line systems (leased from a Carrier) are always point-to-point. Analog
Private-line circuits are normally referred to as 3002 or 3004. The 3000
designation refers to available bandwidth. The 2 and 4 refer to the number
of wires in the circuit.
Digital Private-line service is DDS (Digital Data Service), T-1/T-3,
DS-1/DS-3, Fractional T-1, and SONET. DDS are digital voice channel equivalents.
T-1 service is channelized to accommodate 24 DDS circuits. The terms T-1
and DS-1 are often used interchangeably, but each is a distinctly different
service provided by telephone companies and carriers.
- T-1 service is channelized with the carrier providing
all equipment. The customer is provided with 24 DS-0 interfaces. Each
DS-0 interface has a maximum data capacity of 56 kbps (or can accommodate
one voice circuit). The customer tells the carrier how to configure
the local channel bank (multiplexer).
- DS-1 service allows the customer to configure the
high-speed circuit. The customer provides (and is responsible for maintaining)
all local equipment. The carrier provides (and maintains) the transmission
path. The customer can channelize the DS-1 to their own specifications
as long as the bandwidth required does not exceed 1.536 mbps, and the
DS-1 signal meets applicable AT&T, Bellcor and ANSI standards.
- Customers may purchase fractional service to save
money. In this case, they don't pay for a full T-1 or DS-1. However,
the economies for this type of service are only realized for longer
distances. The local links for Fractional T-1/DS-1 are still charged
at the full service rate.
- T-3 and DS-3 services are essentially higher bandwidth
variants of T-1 and DS-1. The T-3 provides either 28 T-1s or 28 DS-1s,
and the DS-3 provides about 44 mbps of contiguous bandwidth. DS-3s are
used for Distance Learning and broadcast quality video. They are also
used in enterprise networks to connect major office centers.
- DSL (Digital Subscriber Loop) services are DS-1 and
Fractional DS-1 variants that use existing P.O.T.S. service telephone
lines to provide broadband services at a substantially lower cost. The
primary difference between the services is that DS-1 is setup to connect
to user locations and is private. DSL service is typically used to provide
broadband Internet connectivity.
- SONET (Synchronous Optical Network) is the first
fiber optic based digital transmission protocol/standard. The SONET
format allows different types of transmission signal formats to be carried
on one line as a uniform payload with network management. A single SONET
channel will carry a mixture of basic voice, high and low speed data,
video, and Ethernet. All of these signals will be unaffected by the
fact that they are being transported as part of a SONET payload. The
SONET standard starts at the optical equivalent of DS-3. This is referred
to as an OC-1 (Optical Carrier 1). The optical carrier includes all
of the DS-3 data and network management overhead, plus a SONET network
management overhead. In North America, the following SONET hierarchy
is used: OC-3; OC-12; OC-48; OC-96; OC-192. The number indicates the
total of DS-3 channel equivalents in the payload.
- Asynchronous Transfer Mode (ATM) is a widely deployed
communications backbone technology. This standards-based transport medium
is widely used within the core--at the access and in the edge of telecommunications
systems to send data, video and voice at high speeds. ATM uses sophisticated
network management features to allow carriers to guarantee quality of
service. Sometimes referred to as cell relay, ATM uses short, fixed-length
packets called cells for transport. Information is divided among these
cells, transmitted and then re-assembled at their final destination.
ATM services are offered by most carriers. A number of DOTs are using
this type of service – especially in metropolitan areas – to connect
CCTV cameras (using compressed video), traffic signal systems, and dynamic
message signs to Traffic Operations Centers.
Electro-mechanical interfaces for data transmission and signaling normally
fall under the following standards: RS-232; RS-422; RS-423; RS-449; RS-485.
Each of these standards provides for the connector wiring diagrams and
electrical signaling values for communications purposes. These standards
were developed by the EIA (Electronic Industries Alliance) and the TIA
(Telecommunications Industry Association).
Ethernet was invented by the Xerox Corporation in 1973
to provide connectivity between many computers and one printer. The original
Xerox design has evolved into an IEEE standard (802.3XX) with many variations
that include 10Base-T, Fast-Ethernet (100Base-T), and GigE (Gigabit Ethernet).
The Ethernet system consists of three basic elements:
- physical medium used to carry Ethernet signals between computers,
- set of medium access control rules embedded in each Ethernet interface
that allow multiple computers to fairly arbitrate access to the shared
- Ethernet frame that consists of a standardized set of bits used to
carry data over the system
Ethernet works by setting up a very broadband connection to allow packets
of data to move at high speed through a network. This assures that many
users can communicate with devices in a timely manner. The Ethernet is
shared, and under normal circumstances, no one user has exclusivity. Ethernet
uses a protocol called CSMA (carrier sense multiple access).
In this arrangement, the transmitting device looks at the network to determine
if other devices are transmitting. The device "senses" the presence of
a carrier. If no carrier is present, it proceeds with the transmission.
22.214.171.124 Video Transmission
Video is transmitted in either an analog or digital format. Video transmitted
in an analog format must usually travel over coaxial cable or fiber optic
cable. The bandwidth requirements cannot be easily handled by twisted
Video can be transmitted in a digital format via twisted pair. It can
be transmitted in a broadband arrangement as full quality and full motion,
or as a compressed signal offering lower image or motion qualities. Via
twisted pair, video is either transmitted in a compressed format, or sent
frame-by-frame. The frame-by-frame process is usually called "slow-scan
Digital video requires that the analog video be converted to digital
"data". This is accomplished via a CODEC (coder-decoder). The process
is very similar to the conversion of voice from analog to digital, but
is substantially more complex. Several different types of video CODECs
are available to serve a wide variety of communication needs. The CODEC
provides two functions. First, it converts the analog video to a digital
code. Second, it "compresses" the digital information to reduce the amount
of bandwidth required for transmission. In the process of converting from
analog to digital and back to analog, the video image loses some quality.
Also the compression process injects a loss of video quality. Each of
the following CODECs has its own set of video image quality loss characteristics.
- H.261 CODECs are used primarily for video conferencing. The analog
to digital process sacrifices motion for video and audio quality. They
typically use POTS (or DDS) services to reduce total cost of operation
and are designed to provide simultaneous multiple connections for group
conferencing. However, they can use T-1 and "fractional T-1" circuits
for better image quality.
- DS-3 CODECs were developed for use in distance learning systems, providing
full motion, full video and audio quality for the classroom situation.
Communication is accomplished via broadband links. The communication
links can be leased DS-3 service, or privately installed copper or fiber
- JPEG (Joint Photographic Group Experts) and Motion JPEG are some of
the most widely used CODECs for video surveillance purposes. However,
they were primarily developed for the purpose of storing images electronically.
Each still image is converted to an electronic data image and transmitted.
The still images are assembled at a receive decoder and displayed at
a rapid rate to provide motion. They can be used with POTS communication
circuits, fixed low speed data circuits, or broadband copper and fiber
optic communication links. They are also used in wireless applications
such as spread spectrum radio, or CDPD cellular.
- MPEG (Moving Picture Experts Group) CODECs were developed to provide
a better quality motion image compression. There is less image quality
lost in the conversion and compression processes. However, the primary
purpose of MPEG CODECs is to provide "real-time like" motion pictures
via the Internet (also called Streaming Video). The overall process
creates a storage buffer so that there is always a slight delay between
the request to view and the start of the motion picture. For the average
user of the Internet, this is not a problem. CODEC manufactures using
the MPEG-2 standard for traffic surveillance purposes have adapted this
standard to create a real-time video transmission. However, this does
have a minimal impact on final image quality. The MPEG-4 standard was
developed for Internet streaming video, but is also being adapted for
"real-time" surveillance purposes.
17.2.6 Emerging Trends
The "Freeway Management State-of-the-Practice White Paper" (Reference
2) identifies the following area as the "state-of-the-art (Note:
Defined in the reference as "innovative and effective practices
and the application of leading edge technologies that are ready for deployment
in terms of operating accurately and efficiently, but are not fully accepted
and deployed by practitioners"): "to transmit data using wireless
communication media where wireline communication is either too expensive
or is not yet available".
Another emerging trend (from the perspective of freeway management systems)
is the Internet, which is the focus of Chapter 9 of the Communications
Handbook. That chapter provides a basic understanding of the composition
of the Internet, the World Wide Web (WWW), how it works, and how it
can be used as part of an overall communications and operational strategy
for Traffic Signal, FMS, and ITS systems. Many DOTs are using the Internet
as part of an overall public information strategy. A few have begun
to make it part of their internal operational programs.
The Internet Protocol (IP) is the basic software used to control an Internet.
This protocol specifies how gateway machines route information from the
sending computer to the recipient computer. Another protocol, Transmission
Control Protocol (TCP), checks whether the information has arrived at
the destination computer and, if not, causes the information to be resent.
The overall protocol is referred to as TCP/IP – Transmission Control Protocol/Internet
Protocol. Recent advances in traffic management systems are utilizing
IP for communications with field controllers, and streaming video for
The last chapter of the Communications Handbook (Future) provides some
insight on the general future of communications systems and provide a
listing of current standards efforts that may have an impact on the use
of communications systems for traffic and transportation purposes.
17.3 Implementation and Operational Considerations
The Communications Handbook includes material that focuses on the design,
construction and installation of media (both wireline and wireless) for
a communication network. As many ITS systems are deployed in stages, it
is important that the user agency maintain a consistent design and construction
philosophy. This chapter provides useful guidelines on how to maintain
consistency in the overall process.
No communications plan is complete without consideration of system operation
and maintenance. All communication systems require some degree maintenance
and upgrades. The Communications Handbook also addresses these issues.
One of the key issues is who will maintain the communications network
and associated equipment – internal staff or outsourced services; and
what types of personnel are required and their qualifications. The answer
can vary depending on the technology, complexity, and size of the system.
Another important consideration is that of risk assessment. This should
be performed during system design as a consideration of redundancy needs,
and will also have a direct impact on the maintenance requirements. The
communication system is, in most respects, the least failure prone element
of an overall system, but potentially has a high risk of being disrupted
by outside forces.
Planned system updates will also likely be required. Communication equipment
manufacturers will offer firmware updates, and occasionally revise the
physical design of the equipment. Very often, these updates are not critical
to existing operations and systems. However, agencies should budget for
occasional updates, especially if the manufacturer offers a major firmware
update. Upgrades to equipment may also be required due to addition of
Several examples are provided in Reference 1.
1. FHWA; Communications Handbook (Still under development.
To be published in early 2004)
2. "Freeway Management and Operations: State-of-the-Practice
White Paper"; Prepared for Federal Highway Administration, Office of
Travel Management; March 2003