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U.S. DOT Guidance on C2C Communications StandardsInformation Level C2C Communications StandardsNTCIP's Center-to-Center Naming Conventions (NTCIP 1104) should be used for standardizing a common naming hierarchy. Although it is not the focus of this advisory, dialog (message sequences) 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 ITS Standards such as the Message Set for External Traffic Management Center Communications (MS/ETMCC), Incident Management (IM), Advanced Traveler Information Systems (ATIS), and Transit Communications Information Profiles (TCIP), which will be covered individually in future advisories. Application Level C2C Communications StandardsDATEX-ASN (NTCIP 2304) was adapted 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 is a defined computer-friendly format, analogous to the preface of a dictionary that specifies how boldface and italics are used to interpret dictionary entries. DATEX-ASN works in conjunction with the Internet (Transport Level) protocols TCP/IP and UDP/IP, and Point-to-Point (PPP) protocols, and is well suited for communications that require real-time, efficient, fast data exchange with limited communications bandwidth. DATEX-ASN provides services such as publish-subscribe and single publish-subscribe (a form of request-response) for coordinated C2C communications. This standard does not typically use native object-oriented solutions. Standardized DATEX-ASN has been in use for the longest amount of time. When using the DATEX-ASN Application Level Protocol, the NTCIP Octet Encoding Rule (NTCIP 1102) should be employed if bandwidth is limited; alternatively, the other option is to utilize the ISO/IEC 8825-1 Basic Encoding Rules (BER), which results in less efficient data packets but a flexible method of coding. NTCIP 1102 has been utilized in a number of ITS deployments; however, NTCIP 1104 is still relatively immature and needs to be field tested. 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 and publish-subscribe for coordinated C2C communications. WSDL/SOAP/XML is 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 (e.g. less CPU cycles, memory, and time) to interpret messages than XML, and has an installed base of users with a complete published standard profile. Based on its pervasiveness in the IT industry, XML should be considered for new interfaces to ITS centers with careful consideration for center-to-center processing requirements. 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. 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), assuming 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.  Copies of the standards can be obtained through:
NTCIP 2306 also includes a simple file-based information sharing mechanism called XMLDirect, which leverages the Internet's File Transfer Protocol (FTP) standard 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 and accept files. Regardless of the C2C profile chosen, centers can also use NTCIP 2303, File Transfer Protocol (FTP) Application Profile, which provides a C2C profile-neutral way to exchange files over the Internet. Functional C2C installations can be developed using a combination of these strategies. In fact, the above description of NTCIP 2306 is simplistic; there are more options for exchanging XML over HTTP, HTTPS, and FTP. Each transportation agency will need to weigh the pros and cons of each protocol choice according to its particular needs and the constraints of its environment. Legacy systems may also need to be accommodated. ITS deployments currently utilizing CORBA may elect to continue to do so but are encouraged to build WSDL/SOAP/XML or DATEX-ASN interfaces when integrating new systems and using XMLDirect or FTP for simple file transfers. Transport Level C2C Communications StandardsNTCIP 2202 defines a NTCIP Transport Level Profile using the Internet standards relating to TCP/IP and UDP/IP. The DATEX-ASN profile can use either TCP/IP or UDP/IP depending on message transport requirements. The WSDL/SOAP/XML profile only uses the TCP/IP transport mechanism. This profile leverages the industry standardized Internet-based TCP, UDP, and IP protocols, which are proven protocols for system deployment. Subnetwork Level C2C Communications StandardsNTCIP 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 communications may use serial communication for low-bandwidth applications. This profile leverages the industry standardized Internet-based PPP protocol, which is a proven Internet protocol for system deployment. 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. This profile leverages the previously standardized Internet-based Ethernet protocol which is a proven protocol for system deployment. US DOT Guidance SummaryClearly, C2C communications is complex. As with any project, be aware of the functions and interfaces that you may wish for today as well as those desired for the future.
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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