Published: July 2006

Center-to-Center Communications Profiles

ITS Standards Advisories provide the ITS transportation community with information and guidance on the consideration and use of ITS standards.

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C2C Background

Center-to-Center (C2C) communications spans the entire ITS domain, covering the exchange of data between computers physically located in different transportation management center facilities (e.g., traffic management centers, transit management centers, public safety, parking management centers, and so forth). C2C standards enable this data exchange, specifying what information is exchanged, how and when it is exchanged, and the underlying transport mechanisms. C2C standards can be divided into two categories: (1) the message and data content and (2) the rules for exchanging the messages and data. The focus of this ITS standards advisory is the latter category, which we refer to in this advisory as C2C communications profile standards. (Other ITS standards such as the Standards for Traffic Management Center-to-Center Communication, commonly known as the TMDD and MS/ETMCC, define the former category of specific message/data content). The two categories of standards work together to successfully exchange meaningful ITS-related information.

Consensus-based working groups from AASHTO, ITE, and NEMA are chartered with developing a set of underlying C2C communications standards for the transportation community, called the National Transportation Communications for ITS Protocol (NTCIP) standards. NTCIP organizes its standards using a layered approach as shown in Figure 1, similar to the ISO Open System Interconnection (OSI) framework model. While message/data standards address the Information Level, NTCIP C2C communications standards primarily address the Applications, Transport, and Subnetwork Levels, although there are several NTCIP encoding standards at the Information Level. At these levels, ITS and NTCIP leverage existing computer and telecommunications industry base standards, and specify what is mandatory and optional where alternatives are available for ITS applications. In some cases, the NTCIP standards have extended the industry base standard to meet the specific needs of the transportation community. In this manner, the NTCIP C2C communications standards are "profiles" of base standards and other standards.

Description of diagram is in paragraph immediately preceding graphic.

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C2C Communication Standards

The full list of C2C communications profile standards and their current statuses are listed in the back of this advisory. These standards are described in more detail in this section.

Information Level C2C Communications Standards

Information Level standards define the messages and data, and the type of message encoding. Although message set and data dictionary standards are outside the scope of this advisory, encoding schemes are part of the NTCIP Family of C2C standards. These include Octet Encoding Rules (OER) which provide an efficient means to encode messages, and Center-to-Center Naming Conventions which define a standardized way to create names for centers, devices, etc. These standards follow the NTCIP numbering convention of "1XXX".

Applications Level C2C Communications Standards

Applications Level standards define the rules and procedures for exchanging information data. In the OSI model, these standards roughly equate to the Session, Presentation, and Applications Layers. In the NTCIP C2C standards family, Applications Level standards include Web Services Description Language/Simple Object Access Protocol (WSDL/SOAP) using eXtensible Markup Language (XML) encoding, Data Exchange ASN (DATEX-ASN) using Abstract Syntax Notation One (ASN.1) with Basic Encoding Rules (BER) or Octet Encoding Rules (OER), and FTP (File Transfer Protocol). Committee working draft standards were explored for a fourth communication protocol, CORBA; however, based on feedback from the transportation community, the FHWA and SDOs have decided not to pursue further CORBA standards development. These application level standards follow the NTCIP numbering convention of "23XX".

Transport Level C2C Communications Standards

Transport Level standards define the rules and procedures for exchanging the Application data between two points on a network, including routing and switching. In the OSI model, these standards roughly equate to the Transport and Network Layers. In the NTCIP C2C standards family, Transport Level standards include the User Datagram Protocol (UDP) over Internet Protocol (UDP/IP) for connectionless transport services standard and the Transmission Control Protocol over Internet Protocol (TCP/IP) for connection-oriented transport services standard. These standards may be used with any of the above Applications Level standards, and follow the NTCIP numbering convention of "22XX".

Subnetwork Level C2C Communications Standards

Subnetwork Level standards define the rules and procedures for encoding and decoding the information at the bit level to be transmitted and received. In the OSI model, these standards roughly equate to the Data Link and Physical Layers. In the NTCIP C2C standards family, Subnetwork Level standards include the high-bandwidth Ethernet and Point-to-Point Over RS232 Protocol standards, although other industry standards such as ATM and SONET could also be used. These application level standards follow the NTCIP numbering convention of "21XX".

Summary

For each C2C interface, deployers must select a protocol or profile (combination of protocols and base standards) from each level of the NTCIP framework. For more detailed information, please see NTCIP 9001 — NTCIP Guide.

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U.S. DOT Guidance on C2C Standards

Information Level C2C Communications Standards

When using the DATEX-ASN Applications Level Protocol, the NTCIP Octet Encoding Rule (1102) should be employed if bandwidth is limited; alternatively, the other option is to utilize Basic Encoding Rules (BER) which result in less efficient data packets. NTCIP's Center-to-Center Naming Conventions (1104) should be used for standardizing a common naming hierarchy.

Dialog and message standards should be used instead of proprietary or extended messages (and custom dialogs) to support interoperability between centers built by different vendors. Dialog and message standards are defined in the Message Set for External Traffic Management Center Communications (MS/ETMCC), Incident Management (IM), Advanced Travler Information Systems (ATIS), and Transit Communications Information Protocol (TCIP) which will be covered individually in future advisories.

Applications Level C2C Communications Standards

DATEX-ASN, based on an ISO standard, was developed specifically for ITS C2C applications and uses predefined message sets formatted in the ASN.1 syntax to exchange information in peer-to-peer networks between computers. ASN.1, or Abstract Syntax Notation One, is a defined format, similar to the preface of a dictionary which specifies how boldface and italics are to be used in entries that a computer will understand. DATEX-ASN works in conjunction with the Internet (Transport Level) protocols TCP/IP and UDP/IP, and is best suited for communications that require real-time, efficient, fast data exchange with limited communications bandwidth. This standard does not typically use native object-oriented solutions. Standardized DATEX-ASN has been in use for the longest amount of time.

WSDL/SOAP/XML is a newer arrival among the ITS communications standards and is used throughout the information technology industry. As a result, there are many XML tools available, such as XML parsers and XML editors, and a large base of industry XML experts. WSDL/SOAP/XML is an extension of the methods of encoding and formatting data used by the World Wide Web (WWW). As such, this standard promises flexibility and relative simplicity. XML is a format for messages and data; SOAP defines packaging of messages and a framework for passing of the messages over a transport protocol similar to the way DATEX-ASN encodes ASN.1-formatted data; and WSDL, in conjunction with SOAP and XML, provides services such as request-response and publish-subscribe for coordinated C2C communications. WSDL/SOAP/XML are best suited for systems requiring data exchanges over communications links with sufficient bandwidth, and processors with sufficient processing time available. Before deciding to implement an XML-based interface, consider that the ITS standard to support XML communications is currently under development. The draft NTCIP 2306, Application Profile for XML Message Encoding and Transport in ITS Center-to-Center Communications (C2C XML), is available for review and comment at http://www.ntcip.org.

XML seems to be positioned best to accommodate future technical advances and system enhancements, given its flexibility and extensibility. However, DATEX-ASN also has advantages for efficient communications, requires less computing processing resources to interpret messages than XML, and its installed base of users with a complete published standard profile. XML may be considered for new, complex, and large-scale implementations that have large bandwidth and powerful computer processing resources available. To learn more about considerations when using XML-based technologies for ITS, please see the "NTCIP 9010 — XML in ITS Center-to-Center Communications" report. The document is available at http://www.ntcip.org.

WSDL/SOAP/XML is increasingly being used in C2C development projects. In practice, multiple protocols (DATEX-ASN and WSDL/SOAP/XML) may be implemented on a single network (or a series of interoperable networks), given that translators exist at the interfaces between computer systems using different protocols. Many of the ITS data/message set standards include both the ASN.1 and XML formats for the messages and data elements.

NTCIP 2306 also includes a simple file-based information sharing mechanism called XML Direct which leverages the Internet's File Transfer Protocol (FTP) standard (2303) to send and receive files. This profile is not a substitute for WSDL/SOAP/XML or DATEX-ASN; rather it complements these standards as a means to periodically deliver files.

Functional C2C installations can be developed using either or both strategies. Each transportation agency will need to weigh the pros and cons of each protocol according to its particular needs and the constraints of its environment. Legacy systems may need to be accommodated. ITS deployments currently utilizing CORBA may elect to continue to do so but are encouraged to build XML/WSDL/SOAP or DATEX-ASN interfaces when integrating new systems.

Subnetwork Level C2C Communications Standards

NTCIP 2103 defines an NTCIP Subnetwork Profile for communications using a serial or Point-to-Point Protocol (PPP) over RS-232, or dial-up modem communications links. Some center-to-center communication may use serial communication for low-bandwidth applications.

NTCIP 2104 defines an NTCIP Subnetwork Profile for Ethernet. Specifically, it calls for the use of the IEEE LLC and MAC protocols over a 10Base5, 10Base2, 10BaseT, or 10BaseF physical interface. This configuration represents what is used in a typical center-to-center communications environment.

Summary

Clearly, C2C Communications is complex. As with any project, be aware of the functions and interfaces that you may wish today as well as those desired for the future.

• Get to know the range of vendors available for developing C2C projects. The selection of a system integrator is even more critical in C2C projects than in other ITS projects. Even though standards may be applied, each vendor may be associated with a particular proprietary toolkit that works with one or more of the protocols. Thus, vendor choice influences (and in many cases, determines) protocol choice. Furthermore, vendor choice influences the "flavor" and/or version of the protocol that will be used.
• Talk with practitioners and ITS program staff (as indicated in the Standards Resources section below) who are working with the protocols.
• Talk with neighboring and/or peer jurisdictions about their present and future C2C implementations.
• Contact the relevant SDO committees.

The potential benefits of using ITS NTCIP C2C communications standards include reducing the need to build data translators, facilitating system expansion, and easing the ability to interoperate with other transportation management centers. ITS deployers should consider the NTCIP C2C communications, message set, and functional area data dictionary standards, and track the maturity levels for each such that migration to the new ITS standards can be systematically planned as they mature.

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C2C Application Level Standards in Use

Region-wide distribution of transportation management information is a key function at the core of a transportation agency's ability to generate benefits from ITS. Agencies are executing C2C projects across the country using the two communications protocols.

Using WSDL/SOAP/XML

The North Texas Regional Comprehensive ITS Program, implemented by the Texas Department of Transportation and the North Central Texas Regional Council of Governments, provides regional traffic management and other services. This project is changing from DATEX-ASN to WSDL/SOAP/XML. For information on this project, see the web site http://www.nortex-its.org/.

Using DATEX-ASN

TRANSCOM is a coalition of 18 transportation and public safety agencies in the New York/New Jersey/ Connecticut metropolitan region. The TRANSCOM project, Implementation of Regional Architecture (Note: This is different from the Regional ITS Architecture), has installed regional information servers for up to 47 centers at its member agencies to distribute information from roadside devices region wide. TRANSCOM is considering XML messaging over DATEX and WSDL for future web applications. See the web site http://www.xcm.org/.

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Relevance to U.S. DOT Research Initiatives and the National ITS Architecture

U.S. DOT Research Initiatives
C2C communications support the following US DOT Research Initiatives:

• Emergency Transportation Operations
• Integrated Corridor Management Systems
• Clarus
• Mobility Services for All Americans
• Vehicle Infrastructure Integration (VII)

Relevance to the National ITS Architecture

The NTCIP C2C communications standards are common across all Center-to-Center interfaces in the National ITS Architecture.

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C2C Application Level Communication Standards

The following section lists C2C ITS communication standards. For further information on C2C standards, their most recent status, how to obtain a copy of the standards, and the standards development process, see http://www.standards.its.dot.gov/.