Photos of cars on freeway, speeding sign

Freeway Management and Operations Handbook

Chapter 17 – Communications

17.1 Introduction

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 thereof.

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 such communications.

17.2 Current Practices, Methods, Strategies, and Technologies

17.2.1 Overview

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 and operations.

17.2.2 Benefits

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, including:

  • 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 communication problems.
  • 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 equipment.
    • 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, etc.

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. 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 system.
    • 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 light ray.
  • 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. 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 channel,
  • 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. 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 pair configurations.

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 video".

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 optic networks.
  • 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 video transmission.

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 new segments.

17.4 Examples

Several examples are provided in Reference 1.

17.5 References

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