Cooperative intelligent transport systems (C-ITS)
Introduction to C-ITS

Edition 29. June 2020


This introduction to C-ITS provides up-to-date information on global standardization and deployment in the domain of Intelligent Transport Systems (ITS), with a focus on Cooperative Intelligent Transport  Systems (C-ITS). It is the result of many years of standardization and research activities at ESF GmbH.

It serves as a guide for C-ITS standard developers, system designers, and for C-ITS deployment. Especially, it complements the series of Technical Reports TR 21186 and TR 17427, each being a multi-part deliverable.

Cooperative C-ITS - An introduction

Cooperative Intelligent Transport Systems" (C-ITS) is a subset of standards for "Intelligent Transport Systems" (ITS).

ITS aims on improving surface transportation in respect of

  1. safety
    e.g.: crash avoidance, obstacle detection, emergency call, dangerous goods;
  2. efficiency
    e.g. navigation, green wave, priority, lane access control, contextual speed limits, car sharing;
  3. comfort
    e.g. telematics, parking, electric vehicle charging, infotainment;
  4. sustainability,

by applying information and communication technologies (ICT).

ITS specifications are in general developed to address a specific ITS service domain such as public transport, road safety, freight and logistics, public emergency, electronic fee collection.

To support interoperability, C-ITS specifications are developed to exchange and share information ITS applications of a given application domain, and even between application domains.

C-ITS services are based on the exchange of data between vehicles of any category (cars, trucks, buses, emergency and specialized vehicles, ...), the roadside and urban infrastructure (traffic lights, road tolls, variable message signs, ...), control and services centres in the cloud (traffic control centre, service providers, map providers, …), and other road users (pedestrians, cyclists, …).

Some ITS services require cooperation by vehicles with their surrounding environment (other vehicles, other road users, roadside and urban infrastructure, …) while other ITS services require connectivity to remote service platforms (road traffic control centres, map providers, service providers, fleet managers, equipment manufacturers, …).

To support

  1. a large variety of C-ITS services with diverging requirements, and
  2. efficient sharing of information maintained by individual service applications,

it is necessary to combine multiple access technologies and communication protocols with distinct performance characteristics (communication range, available bandwidth, end-to-end transmission delay, quality of service, security, ...).

Combining multiple access technologies and communication protocols requires a common approach to the way communications and data are managed in a secure way. A functional architecture (“the ITS station architecture”) is thus specified to manage security, communications and data related to C-ITS services.

The ITS station architecture is specified in ISO 21217, see Figure 1, and details of functional building blocks of the ITS station architecture are specified in a set of related standards.


Figure  1  — ITS station architecture with some details (from ISO 21217])

Similar to the ISO Open Systems Interconnection (OSI) 7-layer architecture, the ITS station architecture is designed into three independent communication layers (namely the ITS station access layer, the ITS station networking & transport layer, and the ITS station facilities layer) on top of which the ITS Applications entity is located. Additional cross-layer entities in charge of the management activities (management of ITS station units, communications and security) are supporting communications and applications.

An implementation of this ITS station architecture is referred to as "ITS Station Unit) (ITS-SUs); see ISO 21217. The functionalities available in an ITS station unit can be implemented in one or multiple physical units referred to as "ITS Station Communication Units" (ITS-SCUs); even such that the various ITS-SCUs of one ITS-SU are split over a large geographical area, e.g. along a motorway with several 10kms length.

ITS-SUs compliant with ISO 21217 can be deployed in various environments, including vehicles of any kind (vehicle ITS station), on the roadside infrastructure (roadside ITS station), in the cloud (central ITS station) or in nomadic devices (personal ITS station), as illustrated in Figure 2.


Figure  2  — Peer-to-peer communications between ITS station units

As C-ITS deals with safety of human life and property, ITS station units are designed for supporting secure provision of the C-ITS services and secure allocation of resources with prioritized access. Security means cover the two essential operational modes

  1. authentication of the sender of a broadcast message used for information dissemination, and
  2. secure session establishment and maintenance,

and station-internal controlled access to data and station functionalities, e.g. authentication (who is allowed to do what, or who is allowed to access which data and how <read / write / delete>?) and prioritization (who is served first?).

NOTE          ISO 21217 introduced the reference name "Bounded Secured Managed Entity" (BSME) of the ITS station unit to indicate these and further security features.

NOTE          There can be station units for ITS in general, and particularly for C-ITS, that are not conformant with ISO 21217 and the set of related standards from ISO in a strict sense. However, in order to ensure secure communications with ITS-SUs, as a minimum, security means are applied that are conformant with respective security standards.

Due to the diverging requirements from the multiplicity of already known and continuously emerging ITS applications, multiple communication technologies that are fundamentally different can be supported in a specific ITS station unit. Supporting multiple access technologies and communication protocols, also referred to as “hybrid communications”, is a design principle of the ITS station architecture. The ITS station architecture is thus specified with no pre-defined mandatory communication technologies. It can support any type of existing and forthcoming technology to the conditions that

    1. it respects the same design principles;
    2. its integration into the ITS station architecture is specified in a support standard, and
    3. preserves backward compatibility with existing standards.

Presently, specifications have been developed to support a number of access technologies, e.g.:

  1. all kinds of cellular access technologies (e.g. specified at 3GPP with profile standards from other SDOs tailoring them to the ITS station reference architecture);
  2. satellite communications;
  3. other technologies such as infrared, millimetre wave (ultra wideband communications), vehicular WiFi (ITS-G5/US-DSRC/ITS-M5: all profiles of IEEE 802.11 OCB), optical light communications,

and several flavours of communication protocol suites:

  1. GeoNetworking / Basic Transport Protocol from ETSI,
  2. FNTP from ISO,
  3. WSMP from IEEE
  4. the suite of IPv6 protocols from IETF with supporting specifications from ISO.

The ITS station architecture actually combines

  1. localized communications, i.e. communications to nearby stations without involving networking from a source station through nodes of a network to a final destination station – also referred to as "ad-hoc communications" and
  1. networked communications.

NOTE          Whilst networked communications, e.g. cellular communications and access to Internet, can apply the principle of "Technology Neutrality" (allowing simultaneous usage of a mix of incompatible technologies), localized communication between ITS station units has to be based on a specific access technology per service (or service domain) in order to enable interoperability.

EXAMPLE            ITS-M5 (ISO 21215) with FNTP (ISO 29281-1) is an example of a protocol stack for localized communications. Cellular network access to Internet (ISO 17515-1) with IPv6 (ISO 21210) is an example of a protocol stack for networked communications.

Unlike many legacy applications, the choice of the access technology and communication protocol can be made transparent to the applications, i.e. ITS applications are technology-agnostic. This is achieved through a number of functionalities across the ITS station architecture in support of hybrid communications, and is illustrated in Figure 3.


Figure  3  — Path and flow management for technology-agnostic ITS applications

Before transmitting data, applications provide their communication requirements (level of priority, amount of data to be transmitted, expected level of security, expected end-to-end transmission delay, …) to the management entity of the ITS station unit for each type of communication flow. In the meantime, the management entity maintains various information (local regulation enforcing the use of a specific communication profile, existing capabilities of the ITS station unit and their status, characteristics and load of available radio technologies, current load of the ITS station unit, …). Based on the communication requirement and the current view of the management, the uppermost relevant communication profile is selected and ITS station resources are securely committed for identified communication flow.

Standardization activities

C-ITS standards are developed by "Technical Committees" (TCs) of "Standard Development Organizations" (SDOs) dedicated to ITS, e.g. 

  1. TC 204 within the International Organization for Standardization (ISO),
  2. TC 278 within the European Committee for Standardization (CEN) and
  3. TC ITS within the European Telecommunications Standards Institute (ETSI).

Particularly relevant standardization activities are conducted within

  1. ISO/TC 204/WG 1 on architectural issues and globally unique identifiers,
  2. ISO/TC 204/WG 16 on access technologies, communication protocols, and probe data,
  3. ISO/TC 204/WG 17 on personal devices,
  4. ISO/TC 204/WG 18 on cooperative ITS (applications and protocols),
  5. CEN/TC 278/WG 16 on cooperative ITS (joint WG with ISO/TC 204/WG 18),
  6. CEN/TC 278/WG 17 on mobility integration and urban ITS (joint WG with ISO/TC 204/WG 19),
  7. ETSI TC ITS on localized broadcast communications using ITS-G5.

While all of these groups are developing around the same ITS station architecture, each group has a specific focus.

  1. ISO TC 204 and CEN TC 278 are usually more focused to high level definition, technology-agnostic and generically applicable functionalities (vehicles, roadside infrastructure, personal devices and control centres), whereas
  2. ETSI is focused on vehicle-centric technologies and services using ITS-G5 and its associated set of protocols.

Supported access technologies and communication protocols are mostly developed within ISO/TC 204/WG 16 and ETSI TC ITS WG3 and WG4. Generic facilities (generic messaging, PVT, global LDM, …) and infrastructure-centric messaging (SPaT / MAP / IVI, …) are developed in ISO/TC 204/WG 18 jointly with CEN/TC 278/WG 16. ETSI TC ITS WG1 is developing vehicle-centric localized messages (CAM, DENM, …) and other vehicle-centric facilities (vehicle-LDM, POTI).

ISO/TC 204, CEN/TC 278 and ETSI TC ITS are also developing standards using building blocks developed by other organizations. For instance,

  1. security functionalities developed by IEEE 1609 and IETF are adopted to exchange certificates and sign messages;
  2. Internet Protocols developed by IETF are adopted for end-to-end communications over hybrid communications and
  3. access technologies developed by IEEE and 3GPP are integrated as access technologies of the ITS station architecture.

Pilot deployments of C-ITS services

ISO, CEN and ETSI specifications related to C-ITS have been developed with the strong support of the European Commission since 2006, following its ITS Directive on the framework for the deployment of Intelligent Transport Systems in the field of road transport and for interfaces with other modes of transport, and its 6th and 7th framework research programmes (FP6 / FP7). From 2006 to 2012, the standardization activities at ISO was fuelled by collaborative projects (CVIS, SafeSpot, Coopers, COMeSafety, SeVeCom, Anemone, GeoNet, FOTsis, DriveC2X and ITSSv6) which provided work force and developed proof of concepts and prototypes, in particular CVIS. From 2010 to 2013 the standardization activities were further supported by the Standardization Mandate M/453 given to CEN/TC 278 and ETSI TC ITS to develop C-ITS standards in support of the ITS Directive. A subsequent support was given by the Standardization Mandate M/546 on Urban ITS given to CEN/TC 278.

In the meantime, various national efforts have also been initiated, for instance SCOREF in France and CONVERGE in Germany.

The ITS station architecture serves as a reference for numerous C-ITS services developed around the world, and more particularly in Europe. Early deployments of C-ITS services complying with the ITS station architecture have been initiated in Europe under the framework of the C-ROADS and InterCor initiatives supported by the European Commission. National pilot deployments are underway all across Europe (SCOOP in France, NordicWay in Scandinavia, the C-ITS corridor project between The Netherlands, Germany, and Austria) and in other regions such as Austroads in Australia and New Zealand, and in Israel. These early deployment projects are typically focused on road safety and traffic efficiency services that rely on the exchange of data between vehicles and the roadside infrastructure. Such data exchange is performed through both localized communications and networked communications.

In these European deployments, localized communications, also known as V2X, are performed using the ITS-G5 access technology within the 5.9 GHz frequency band, a WiFi profile designed for vehicular communications, whereas networked communication are typically made through a cellular technology (e.g. LTE). Other technologies can of course be used in the future (e.g. 5G, infra-red, …) as long as they comply with the ITS station architecture and related standards defining technology building blocks.

Early deployments have evidenced the need to deploy C-ITS services using a diversity of access technologies, for instance either ITS-G5 or LTE, or a combination of both. For instance, the French pilot deployment (SCOOP) is using ITS-G5 between vehicle and roadside ITS stations to inform about immediate dangers (CAM, DENM) and LTE is used by patrol vehicles to provide information to road control centres. In Scandinavia, the scarce population has driven NordicWay to deploy roadside ITS stations only at critical locations and to rely on LTE to deliver environmental information (DENM) from  road control centres to vehicles.

Further on, at the early stage of deployment of C-ITS services, the density of vehicle ITS stations equipped with ITS-G5 capabilities is scarce, whereas roadside ITS stations are only deployed in critical areas. Similarly, many areas anywhere in the world do not have the benefit of sufficient cellular network coverage. While some time critical road safety C-ITS services are best served by localized communications (e.g. notification of immediate danger requiring emergency breaking), there are not always vehicles equipped with the ITS-G5 technology or roadside equipment in the vicinity able to relay the notification immediately to nearby vehicles. In such a situation, using networked communications (e.g. cellular) to provide the information to road control centres, and then from them back to vehicles in a specific are prevents the successive occurrence of road accidents.

All these experiences, gained through early deployments, demonstrate that it is not possible to provide the same level of services to all vehicles in all locations. The type of service and the performance of the service depends on national decisions, the local road environment, the density of population, the density of vehicles equipped, cellular coverage, and numerous other factors. In addition, and importantly, the roadside infrastructure equipment and vehicles have a life expectancy that far exceeds the innovation cycle of new radio and communication technologies, therefore equipment at the roadside and in vehicles is likely to have to accommodate new communications technologies during its lifetime.

Enhanced functionalities in the ITS station architecture

Flexible and future-proof management of data and messages is a pre-requisite for success of C-ITS. New messages – even if efficiently defined in ASN.1 and encoded with Unaligned Packed Encoding Rules - can be made available during run-time of C-ITS simply by providing message configuration information. This allows for faster introduction of new or modified services and increases communications efficiency. An approach to achieve this is known under the title "Global Transport Data Management" (GTDM) framework with specifications provided in TS 21184.

Many C-ITS services depend on accurate information on the kinematics status, e.g. location and speed of a station at a given time, of the ITS station units. Thus, a general "Position-Velocity-Time" (PVT) service, as specified in TS 21176, is a pre-requisite for successful deployment of such services. The basic estimation technology used for the PVT service is the technology known under the name of "Global Navigation Satellite System" (GNSS), namely GALILEO from the European Union, GPS from the United States of America, GLONAS from the Russian Federation. However, in addition to this basic technology, a large variety of complementary technologies are known to improve accuracy and reliability of the PTV service.

Hybrid communication support in the ITS station architecture

The ITS station architecture specified in ISO 21217 and its functionalities in support for hybrid communications provides an answer to these concerns and enables a future-proof and sustainable deployment of C-ITS services.

Due to the diverging requirements from the multiplicity of already known and continuously emerging ITS applications, different communication technologies are to be supported in a specific ITS station unit. This need was envisioned from the start of the development of the ITS station architecture ISO 21217 back to 2002 when ISO/TC 204/WG 16 was founded. As the diversity of ITS applications (ranged into three categories: “road safety”, “traffic efficiency” and “comfort”) with diverging communication requirements cannot be met by a single type of access technology and communication protocol, the ITS station architecture was designed to combine multiple access technologies and access protocols. 

A first proof of concept of this approach was demonstrated as part of the EU funded CVIS project. In 2015, this need was labelled with the term "Hybrid Communication" in the reports of the German CONVERGE project. As of 2019, the need for hybrid communication solutions is expressed in pilot deployment of C-ITS services, particularly in the context of C-Roads in Europe.

Hybrid communications can basically be defined as the composition of multiple access technologies and protocols with different characteristics combined to provide complementary or redundant communication channels. This can arise in multiple situations:

  1. When localized communications, i.e. communication to nearby stations without involving support of an infrastructure network, is combined with networked communications, i.e. communications using support of an infrastructure network, for instance when the V2X communication stack from ETSI is combined with TCP/IP;
  2. When technology-agnostic applications are developed and deployed in a communication system equipped with multiple access technologies with dynamic determination of the most appropriate communication profile;
  3. When safety critical communications, e.g. for platooning, requires physically independent redundant communication channels such that at least one of these channels provides the necessary information.

For deployment of ITS, especially C-ITS and the emerging Urban ITS and "Mobility as a Service" (MaaS) paradigms, consistent and complete sets of standards and profiles of standards including necessary parameterisation are necessary in support of the targeted ITS services to be provided by means of ITS applications. Such sets are referred to as "C-ITS Release". This document presents the C-ITS standards landscape, describes a concept of Releases, and exemplifies this concept with a "C-ITS Release 2".