Introduction to Communications Access for Land Mobiles

Set of communication standards for Co-operative ITS

Quite often we were asked "What is CALM?".

The question is good, and the answer can be given from different perspectives. The most useless answer would be, although it is correct, to say: "It is a set of standards for C-ITS" developed at ISO TC204 WG16. There are much more standards related to C-ITS as just those from WG16. Nevertheless WG16 did something very special, which is explained below.

In any case, usage of the term "CALM" in the standards from ISO TC204 WG16 is deprecated, as it would be misleading. CALM is not referring to a closed system.

The ITS station reference architecture originally developed for CALM:

ITSSarchitectureThe set of CALM communciation standards is build on the basis of the well-known layered OSI model, which was simplified and extended in order to define the ITS station reference architecture [ISO 21217], which consists of six parts (Applications, Management, Access, Networking & Transport, Facilities and Security).

The OSI communication layers are

     Access: OSI layers 1 and 2
     Networking & Transport: OSI layers 3 and 4
     Facilities: OSI layers 5, 6 and 7.

The Security entity provides means to secure communications and the ITS station.

The Managmenet entity provides means to manage the ITS station.

These layers and entities are interconnected via interfaces, which usually are Service Access Points (SAPs) [ISO 24102-3].

All of this is used by Applications, which contains ITS-S applications. Applications connect to the communications tools via an Application Programming Interface (API), which itself provides the functionality of the MA-SAP, FA-SAP and SA-SAP.

The term ITS station (ITS-S) identifies the functionality provided by these six blocks. CALM standards do not specify any ITS-S application. However now it is clear, why CALM goes beyond communications - it considers ITS-S applications as part of the Bounded Secured Managed Domain (BSMD).

Implementations of ITS stations:

In order to understand better, what is meant with BSMD, we have to look on the usage of this ITS station reference architecture, i.e. we have to identify possible implementations and operations. Let us start with details of a single instance of an ITS station.

Simple vehicle implementationIn a "simple" implementation of an OBE (On Board Equipment), as e.g. used in the CVIS project, the whole functionality of the ITS-S is implemented in two physical units, which are interconnected via the IST station-internal network. In this example, distinction is made between an ITS-S Host and an ITS-S Router with some functionality of a host.

An ITS-S Router [ISO 21217] contains the communication interfaces and protocols needed to connect to another ITS-S unit. An ITS-S Host contains ITS-S applications.

There is station-internal management communications between these two units of the OBE as specified in [24102-3]. As the ITS station-internal network is observable, this management communications needs to be secured in order to protect the ITS-S from unauthorized manipulations, which might have an impact on the whole ITS.

RSEA more complex implementation of an RSU (Road Side Unit) contains several ITS-S Routers, one or several ITS-S Hosts, and ITS-S Gateway towards sensors and actors, and an ITS-S Border Router to connect to external networks, e.g. the Internet. These units are interconnected via the ITS station-internal network.

There is an unlimited number of possible implementations. All of them follow the same principles given by the concept of the BSMD.


Peer-to-peer communications between ITS sub-systems:

All communications between entities of ITS stations is peer-to-peer, disregard of the context of usage of the ITS-S, and disreaged of the networking type between instances of ITS-S. Four contexts are identified as illustrated below.

ITS sub-systems


Networks connected to an ITS station:

TopLevelNWAn ITS station may connect to different networks as illustrated on the left-hand side. The grey-shaded networks are ITS-specific networks. The other networks are legacy networks.

There may also be an ITS station-internal network, which is not illustrated here, and which in the uppermost simple implementation of an ITS-S may be reduced to a node only.

Communications between ITS stations, and between an ITS-S and a legacy station, may be via any combination of these networks. This is illustrated in the four following figures.

ITStoITSsingleSingle-hop communications between two units of an ITS-S.

ITStoLegacySingleSingle-hop communications between an ITS-S unit and a legacy unit.

ITStoITSnetworkMulti-hop communications between two units of an ITS-S.

ITStoLegacyNetworkMulti-hop communications between an ITS-S unit and a legacy unit.