Traffic Control Systems Handbook: Chapter 9. Communications
Traffic control systems have traditionally used wireline communications to transfer information between field controllers and a traffic management center (TMC) or a field master controller. These systems have consisted of owned cable or leased line facilities. While these alternatives still remain, additional alternatives have become available in the last few years. The functional requirements for surface street traffic control have expanded considerably in the last few years and currently include CCTV, variable message signs and transit priority.
This chapter provides a broad overview of requirements and technologies for communications. A more detailed treatment is available in the Telecommunications Handbook for Transportation Professionals (1).
9.2 Functions of a Communication System
A communications system to support surface street traffic control may be required to provide communications to facilitate the following functions:
- Synchronization of timing among controllers in a traffic control section. A common time reference is required to provide a common cycle length and establish the appropriate offsets.
- Upload and download of timing plans and other parameters between the field controller and the TMC.
- Monitoring of field equipment status and reporting of equipment malfunctions.
- Selection of timing plans by the TMC or field master controller.
- Provision of any communications between the TMC and the field controller or between field controllers that may be necessary to support adaptive control algorithms.
- Communication of video information to the TMC and control of cameras by the TMC.
- Upload of logs developed by emergency vehicle signal preemption equipment.
- Support of signals required for transit priority.
- Support of traffic probe based incident detection equipment.
9.3 Alternative Communication Technologies
Among the possible communication functions, only the transmission of a considerable number of CCTV signals requires the use of broad-band communication technology. Broad-band technology supports higher data rates than usually provided by modems using standard telephone channels (e.g. 56 kilobits/second). In many cases, particularly where a number of functions are to be supported by the same communication system, it may be most practical to use broadband technology. The types of technologies that may be used are shown in Table 9-1.
|Agency Owned Technologies|
|Wireline||Fiber optic cable technology. T1 or SONET techniques possibly including ATM are commonly used where multiple functions are required. Fiber optic based Ethernet are gaining in popularity at this time. Good quality CCTV capability and data capability.||
|Leased Channel Service||Dial Up Data Service||Dial Up Channel Service|
|T1, fractional T1, POTS (plain old telephone service), DSL, cable service. Service may range from capability of good quality CCTV and data capability to no CCTV capability.||CDPD, GPRS, dial up internet, ISDN, pager.
Data capability, CCTV capability poor or unavailable. Dial up service is only useful for limited traffic signal systems and other functions.
This section is adapted from the Freeway Management and Operations Handbook (2).
The National Transportation Communications for ITS Protocol (NTCIP) suite of standard communications protocols and data definitions has been designed to accommodate the diverse needs of various subsystems and user services of the National ITS Architecture. NTCIP standards are intended to handle these needs in the following two areas:
The first type of communications is between a management system or center and multiple control or monitoring devices, such as a traffic control system communicating with intersection controllers, dynamic message signs, and control of CCTV cameras. This type is referred to as center-to-field (C2F) communications.
The second type of communication involves messages sent between two or more central management systems, such as a freeway management system and a traffic signal control system. This type of communication is referred to as center-to-center (C2C) communications. Even if two or more of the various center subsystems are located within the same "center" or building, they are still considered logically separate. C2C involves peer-to-peer communications between any number of system computers in a many-to-many network. This type of communication is similar to the Internet, in that any center can request information from, or provide information to, any number of other centers.NTCIP provides the mechanism whereby interchangeability and interoperability among the various components of transportation systems can be achieved, where "interchangeability" is defined as the capability to exchange devices of the same type (e.g., a signal controller from different vendors) without changing the software; and "interoperability" is defined as the capability to operate devices from different manufacturers, or different device types on the same communications channel. Specific NTCIP standards are discussed in more detail in subsequent chapters. Additional information regarding NTCIP may be found on the NTCIP website (www.ntcip.org), including the NTCIP Guide (3).
1. Leader, S. "Telecommunications Handbook for Transportation Professionals: The Basics of Telecommunications." Federal Highway Administration Report No. FHWA-HOP-04-034, September 2004.
2. Neudorff, L.G., J.E. Randall, R. Reiss, and R. Gordon. "Freeway Management and Operations Handbook." Federal Highway Administration Report No. FHWA-OP-04-003, Washington, DC, September 2003.
3. "The National Transportation Communications for ITS Protocol Online Resource." AASHTO, ITE, NEMA. June 2003 (http://www.ntcip.org).