EP0347354B1 - Broadcasting of information messages, especially road information, to fixed or mobile receivers for intermittent listening - Google Patents

Description

The invention relates to the field of broadcasting systems through an imperfect transmission channel and at limited speed, intended for fixed or mobile receivers with intermittent listening.

In a particular application which has motivated the development of the present invention, the broadcasting method relates to messages carrying state information of the attributes of a distribution of objects: in the case of traffic information messages, the objects are for example made up of sections of a road infrastructure, characterized by attributes such as fluidity.

If we stick to the example of traffic information, it will be agreed that the over-the-air broadcasting of information comes up against a certain number of disruptive elements of transmission such as, for example, physical obstacles forming a screen ( buildings, bridges, tunnels ...), or interference (high voltage lines), or to complexifying elements such as for example the fact that the information disseminated must be able to be perfectly meaningful and complete for a very large number of receivers each located in a specific listening configuration (start of random listening, partial and incomplete memory, ...).

Of course, these drawbacks cannot be remedied by a pure and simple increase in the amount of information transmitted at any time, due to the limited processing capacities that can be envisaged for on-board receivers and above all the limited bit rate of the usable transmission channels ( radio transmission).

A number of systems are already known for enabling the dissemination of information messages in this type of condition, and in particular for traffic information.
Thus, the German ARI system, developed by the company BOSCH, consists of add a carrier in an FM channel. The carrier's broadcast selectively corresponds to the moments when a journalist broadcasts, on an agreed radio station, information relating to road traffic. Each adapted receiver is therefore provided with means for switching reception to the frequency of this radio chain, throughout the duration of transmission of the carrier.

This operational system has real efficiency, but its very reduced capacity, which is essentially limited to sending binary information (existence or not of the carrier), does not allow the transmission of road information directly interpretable by memory means. and on-board processing.

The object of the invention is to meet this type of requirement.

According to the invention, these objectives are achieved using a method characterized in that it consists in storing reference information, called equilibrium state information, in said receiver storage means, and in broadcast information messages of deviation from said state of equilibrium, through said broadcast channel,
in that each information message broadcast comprises means of identifying the message distinct from the content of the message, as well as a priori time information of expiry of said message,
in that each of said receivers comprises means for early replacement and / or cancellation of an earlier message upon receipt of a separate subsequent message with the same identifier,
and in that each message is broadcast repeatedly until said a priori expiry time, or until its replacement or early cancellation.

It will be seen that it is the combination of these different characteristics which makes it possible to achieve the objectives set. This will be developed in particular during the description below of a preferred embodiment of the invention.

Advantageously, said identification means include a message version number, which is different for each successive new set of deviation information for the same object or group of objects.

The notion of group of objects covers for example all of the objects successively concerned by the same "phenomenon". In the case of traffic information, a phenomenon is for example constituted by a local traffic jam which extends, over time, over a variable number of road sections.

• lesdits moyens de remplacement et/ou d'annulation anticipés d'un message antérieur comportent des moyens de comparaison des messages reçus, de même identifiants et de même version, de façon à assurer une rectification des erreurs de transmission ;
• dans le cas de l'émission des messages ultérieurs de substitution à un message antérieur de même identifiant, ledit message ultérieur comportant une nouvelle heure a priori de péremption inférieure à l'ancienne heure a priori de péremption du message antérieur :
  • . on diffuse ledit message ultérieur jusqu'à ladite nouvelle heure a priori de péremption
  • . on diffuse un message d'annulation anticipée dudit message antérieur entre ladite nouvelle heure a priori de péremption et ladite ancienne heure de péremption ;
• dans le cas d'un message antérieur multiobjets remplacé au moins partiellement par un message ultérieur de même identifiant, concernant un groupe d'objets recouvrant au moins partiellement le groupe d'objets désigné par ledit message antérieur, lesdits moyens de traitement du récepteur procèdent à l'annulation dudit message antérieur préalablement à la prise en compte du contenu dudit message ultérieur.

These means of identification are justified in a certain number of cases, including in particular:

• said means for replacing and / or canceling an earlier message in advance include means for comparing the messages received, with the same identifiers and the same version, so as to correct the transmission errors;
• in the case of the transmission of subsequent messages replacing a previous message with the same identifier, said subsequent message comprising a new a priori expiration time less than the old a priori expiration time of the previous message:
  • . said subsequent message is broadcast until said new a priori expiration time
  • . an early cancellation message of said previous message is broadcast between said new a priori expiration time and said old expiration time;
• in the case of a previous multi-object message replaced at least partially by a subsequent message of the same identifier, concerning a group of objects at least partially covering the group of objects designated by said previous message, said receiver processing means proceed to the cancellation of said prior message before taking into account the content of said subsequent message.

In other words, any broadcast of a subsequent message with the same identifier as a previous message implicitly means the cancellation of the entire previous message.

• on diffuse un des messages ultérieurs avec le même identifiant que ledit message antérieur;
• on diffuse le ou les autres messages ultérieurs avec un nouvel identifiant, indépendant de l'identifiant dudit message antérieur.

According to another characteristic, in the case of an earlier multi-object message replaced by at least two subsequent messages relating to groups of objects at least partially covering the group of objects designated by said previous message:

• one of the subsequent messages is broadcast with the same identifier as said previous message;
• the subsequent message (s) is broadcast with a new identifier, independent of the identifier of said previous message.

This procedure has the advantage of implicitly signifying the cancellation of the previous message, without at any time the processing of messages does not require an intervention at the level of the designated objects.

• la figure 1 illustre le principe de diffusion d'informations d'une valeur d'écart par rapport à un état d'équilibre, selon l'invention ;
• la figure 2 est un schéma illustrant le processus d'annulation anticipée d'un message antérieur, selon l'invention ;
• la figure 3 illustre le principe de remplacement anticipé d'un message antérieur par un message ultérieur, selon l'invention ;
• la figure 4 illustre le principe de remplacement anticipé d'un message antérieur par deux messages ultérieurs, selon l'invention ;
• la figure 5 illustre le principe de remplacement d'un message antérieur, par un message ultérieur d'heure a priori de péremption plus rapprochée, selon l'invention ;
• la figure 6 illustre le fonctionnement du système RDS, utilisé dans un mode de mise en oeuvre préférentiel dit système de diffusion suivant l'invention ;
• la figure 7 est un schéma illustrant la structure d'un message à longueur fixe, de type "CEMT", tel que diffusé au sein d'un système RDS dans un mode de mise en oeuvre avantageuse de l'invention ;
• la figure 8 illustre une structure possible pour un message d'annulation anticipée d'un message antérieur.

Other characteristics and advantages of the invention will appear on reading the following description of a preferred embodiment of the invention, given by way of illustration and not limitation, and of the appended drawings in which:

• FIG. 1 illustrates the principle of diffusing information with a value of deviation from an equilibrium state, according to the invention;
• FIG. 2 is a diagram illustrating the process of early cancellation of an earlier message, according to the invention;
• FIG. 3 illustrates the principle of early replacement of an earlier message with a later message, according to the invention;
• FIG. 4 illustrates the principle of early replacement of an earlier message with two subsequent messages, according to the invention;
• FIG. 5 illustrates the principle of replacing an earlier message with a later message of a priori expiry time closer, according to the invention;
• FIG. 6 illustrates the operation of the RDS system, used in a preferred mode of implementation called the broadcasting system according to the invention;
• FIG. 7 is a diagram illustrating the structure of a fixed-length message, of the "CEMT" type, as broadcast within an RDS system in an advantageous embodiment of the invention;
• FIG. 8 illustrates a possible structure for an early cancellation message of an earlier message.

The preferred embodiment described below concerns the broadcasting of data to mobiles, using the RDS system (Radio Data System).
However, it is clear that the invention applies to the dissemination of any other type of data, applicable to other type of networks (for example the Franco-German DAB system under development).

The RDS system is a system which is implemented on the basis of sound broadcasting in frequency modulation (FM), as illustrated in FIG. 6.

This service was developed because of the problems posed by the short range of FM transmitters, and therefore the need to set up a network of transmitters with distributed frequencies to offer a service with large coverage. This technique makes it possible to receive, at any point in the geographical area covered, a given broadcast service, provided that the receiver is tuned to the frequency of the corresponding local transmitter. This causes discomfort for the mobile user (typically the motorist) when he reaches the coverage border of the local transmitter to which it is tuned: he must indeed manually search for another frequency when the reception becomes poor.

The RDS system has endeavored to solve this problem by offering an automatic tuning function.

• un premier signal 61 correspondant à l'addition des voix stéréophoniques (D + G)/2 (voix droite plus voix gauche) ;
• une sous porteuse 62 à 19 KHz, qui caractérisé le fait que le canal est stéréophonique
• un signal 63 correspondant à la soustraction des voix (D - G)/2;
• le signal 60 correspondant au signal RDS, centré sur 57 KHz.

As shown in FIG. 6, the principle consists in adding a digital subcarrier 60 to the basic sound signal.
The total spectrum of the FM channel therefore includes

• a first signal 61 corresponding to the addition of stereophonic voices (D + G) / 2 (right voice plus left voice);
• a subcarrier 62 at 19 KHz, which characterizes the fact that the channel is stereophonic
• a signal 63 corresponding to the subtraction of the voices (D - G) / 2;
• signal 60 corresponding to the RDS signal, centered on 57 KHz.

The amplitude of the subcarrier 60 is modulated by "biphase" representation of the data bits.

The digital channel thus obtained represents a capacity of 1187.5 bits per second organized in frames of 104 bits each. About 11 of these frames are transmitted by an FM / RDS transmitter every second. Each frame is identified as belonging to a certain type: 16 types are planned.
All the frames contain a 16 bit word called PI (Program Identification), which characterizes by a code the program broadcast by the considered transmitter. Certain frames (of type 0) contain the list of so-called alternative frequencies and correspond schematically to the central frequencies of the surrounding transmitters which broadcast the same program.

• le récepteur acquiert la liste des fréquences alternatives ;
• il balaie ces fréquences et décide, selon une stratégie qui lui est propre, de retenir la meilleure à chaque instant.

The principle of automatic tuning is then as follows:

• the receiver acquires the list of alternative frequencies;
• it scans these frequencies and decides, according to its own strategy, to select the best at all times.

• codage de l'horaire des émissions ;
• radiotexte ;
• synchronisation d'horloges ;
• "paging" ou appel de personnes ;
• messages d'informations routières.

From this digital broadcasting system, different applications have been proposed. Specific types of frames have been dedicated to each of these applications, in particular:

• coding of the program schedule;
• radio text;
• clock synchronization;
• "paging" or calling people;
• traffic information messages.

It is to this latter class of application that the preferred embodiment of the invention presented below relates.

Given the operating constraints of the FM / RDS network not detailed here, the useful bit rate available, per transmitter, for an application of the type: "traffic information message" hardly exceeds one hundred bits per second.

Besides the principles of cellularization of the diffusion and modulation of the speed of distribution, (not developed here because independent of the present invention), the inventors endeavored to carry out a coding as compact as possible of the messages, in particular in supporting, for certain configurations of road information messages, the work of the ECMT (European Conference of Ministers of Transport).

Compared to this work (Madrid 1987 version), the invention provides the essential new notion of message identification.

A complete example of an information message is shown in FIG. 7. This standard message takes up the CEMT standard with a few modifications, notably concerning the addition of an identifier.

The message presented belongs to the category of messages called "user" (and therefore intended to provide the user with road information), unlike "system" messages of the type of message deletion messages as will be seen in relation to figure 8.

The standard message presented in FIG. 7 comprises on the one hand a header 71, on 17 bits, followed by an information train 72 on 47 bits.
The header includes the following fields: CYCLE 73, IDENTIFIER 75, and TYPE OF NOTIFICATION 74.

The CYCLE 73 field (on 1 bit) specifies a message delivery speed which is not relevant to the subject of the present invention.
The NOTIFICATION TYPE 74 field (on 1 bit) specifies whether the user must be notified of the receipt of the message.

• T 76, pour le type de message ("usager", ou "système") ;
• Thème 77, sur 3 bits, correspondant à 8 thèmes possibles (trafic, météo, ...) ;
• Portée P 78, sur 1 bit, (message global ou local) ;
• Numéro 79, sur 10 bits, dont les deux derniers sont réservés à l'expression d'un numéro de version (modulo 4) dans le cas de messages dits "évolutifs", c'est-à-dire susceptible de donner lieu à la diffusion de versions successives.

The identifier 75 is represented on four sub-fields:

• T 76, for the type of message ("user", or "system");
• Theme 77, on 3 bits, corresponding to 8 possible themes (traffic, weather, ...);
• Scope P 78, on 1 bit, (global or local message);
• Number 79, on 10 bits, the last two of which are reserved for the expression of a version number (modulo 4) in the case of so-called "evolving" messages, that is to say likely to give rise to the dissemination of successive versions.

The information stream 72 of the message presented in FIG. 7 successively comprises a LOCALIZER sub-field 81, supplying in particular an item of item designation information and attributes to which the message relates; two fields CAUSE 82 and EFFECT 83 indicating the new value assigned to the attributes attached to the object, as well as a STOP-TIME field 84 relating to a time information a priori of expiry of the message and a COUNCIL field 85.

The messages broadcast can also be messages of variable length.

• T : Type sur 1 octet (identification du champ), découpé en deux sous-champs T1.T2. (T1: nature du ou des attributs; T2: mode de codage de la valeur prise par le ou les attributs dans le sous-champ V);
• L : Longueur du message de taille variable, sur 1 octet, et exprimée en octets ;
• V : Valeur du message, à savoir son contenu informatif proprement dit, sur L octets.

In this case, the header 71 of the message may be slightly different, for example by providing 5 additional bits for the THEME subfield 77. The message stream 72 can then be replaced by fields of the "TLV" type:

• T: Type on 1 byte (field identification), divided into two sub-fields T1.T2. (T1: nature of the attribute (s); T2: coding mode of the value taken by the attribute (s) in subfield V);
• L: Length of the message of variable size, on 1 byte, and expressed in bytes;
• V: Value of the message, namely its informative content proper, on L bytes.

For example, the messages relating to urban traffic can be of variable length, and organized in the following manner.

The urban network is modeled as a set of sections.

Each section is identified by an 8-bit number. Here is described the fluidity type message which is maintained by the information source and informs the terminals in real time of the traffic fluidity.

Coding of the "TLV" message:

T = T1.T2
T1 = fluidity: '0000'
T2 = O coding: '0000'
L = 3.N (in binary on a byte)
V = S1S2 ... SN where each Si occupies 3 bytes and describes the fluidity of a section according to the following format.

If = SEC-ID, STATUS, TREND, TFIN,
or
SEC-ID = section number on 8 bits

000:

fluide (équilibre)

001:

ralenti

010

très ralenti

011

bloqué

111:

indéterminé

STATUS = 3 bits coding the fluidity state of the section. Among the 8 values only 5 are defined:

000:

fluid (balance)

001:

slow motion

010

very slowed down

011

blocked

111:

undetermined

00:

stationnaire

01:

fluidité décroissante

10:

fluidité croissante

11:

indéterminé

TREND: 2 bits indicating a trend in fluidity evolution:

00:

stationary

01:

decreasing fluidity

10:

increasing fluidity

11:

undetermined

TFIN = 11 bits which give the moment in the day when this observation can be considered obsolete. These 11 bits give in clear the number of the minute considered during the day.

Examples:

TFIN

= 0 00H00MN

TFIN

= 600 10H00MN

TFIN

= 1312 21H52MN

• la notion d'état d'équilibre, par rapport auquel sont transmises des informations d'écart (fig. 1) ;
• la notion d'heures a priori de péremption du message ;
• la gestion des annulations et/ou remplacements anticipés des messages (fig. 2, 3, 4, 5, et 8).

The implementation of the invention, for the dissemination of messages of the types described above, as for the dissemination of other types of messages, involves the following three principles:

• the concept of equilibrium state, with respect to which deviation information is transmitted (fig. 1);
• the notion of a priori hours of expiry of the message;
• management of early cancellations and / or replacements of messages (fig. 2, 3, 4, 5, and 8).

These three aspects will now be detailed.

First of all, the proposed system is based on the assumption that any dynamic attribute (likely to vary) admits a rest value or equilibrium value (10) (Fig 1). When the current value observed corresponds to this equilibrium value (10), there is obviously no particular message to send; on the other hand, when the attribute is removed from its equilibrium position (and the phenomenon is detected), a message will come to indicate this "abnormal" state (11, 12).

The messages broadcast are "commands" addressed to on-board computers.

Each on-board terminal includes a processing and on-board storage.

Advantageously, the storage means can be either in the form of a list management system, or preferably in the form of an on-board database.

The on-board database is advantageously a cartographic base constituted by a set of objects; a set of attributes is attached to each object. Objects, when they are of a cartographic nature, can be addressed by a message field generally called "localization" or "localizer" (see Fig. 7, reference 81).

A typical object is a road segment, a section of road framed by two crossroads; the attributes of this object for example: the length of the segment, the category and the name of the road axis to which it belongs, the number of lanes, the direction of traffic, the fluidity, etc ... certain attributes are very static ( for example the length), others are dynamic (for example the fluidity).

• jusqu'à l'instant t₁, la circulation est normale (10) (à l'état d'équilibre); de t1 à t2, elle est difficile (11) ;
• de t2 à t3, elle est impossible (12) ;
• de t3 à t4, elle est à nouveau difficile (11) ;
• à partir de t4, elle est à nouveau normale (10).

Thus, using the example of road traffic information, on a given road segment, the figure illustrates the following successive situations:

• until time t₁, the circulation is normal (10) (in a state of equilibrium); from t1 to t2, it is difficult (11);
• from t2 to t3, it is impossible (12);
• from t3 to t4, it is again difficult (11);
• from t4, it is normal again (10).

Figure 1 illustrates the need for the system to diffuse the abnormal states all the time they last and not only during the transitions. Indeed, if only transition messages were broadcast at times t1, t2, t3 and t4, a user "listening" at time T would never be warned that the current state is "difficult"; as the instant T can be arbitrary, the message must be broadcast throughout the duration of the disturbance.

• . information valable "jusqu'à nouvel ordre",
• . information valable jusqu'à une certaine date ou heure a priori de péremption ("stop-time") figurant dans le message,
• . information valable pendant une durée de temps conventionnelle et implicite.

The temporal scope of messages can be defined in three ways:

• . valid information "until further notice",
• . information valid until a certain a priori expiry date or time ("stop-time") appearing in the message,
• . information valid for a conventional and implicit period of time.

• c'est bien une intention de la source,
• ou c'est un problème de transmission (sortie intermittente de la zone de couverture par exemple).

The first method should be avoided in an unconnected environment; in this case, in fact, the absence of a "new order" can have two origins that the receiver cannot always distinguish:

• it is an intention of the source,
• or it is a transmission problem (intermittent exit from the coverage area for example).

The second method ("stop-time" explained in the message) is functionally well suited and allows unambiguous management. This will be illustrated in particular below, in the commentary associated with FIG. 5.

The third method (validity of the message during a conventional period) is in practice a variant of the previous one in which the "STOP-TIME" is defined, but by default and at a constant value.
When a phenomenon, described by a message already transmitted, experiences an anticipated end (compared to the a priori expiry time, or "STOP-TIME" of the message), the explicit identifier 75 makes it possible to define a command-message very simple of the type: "deletion says message n0 i". This situation is illustrated in Fig. 2.

A first message M (I) is first broadcast with t'1 as the a priori expiration time at t2 the phenomenon has disappeared prematurely; a SUP (I) deletion message must then be broadcast from t2 to t'1.
Each message has an expiration date in a field; when this date is reached, the stored message is automatically destroyed by the system terminals.

It may happen that it is necessary to destroy them "prematurely", that is to say before the expiration date with which they are disseminated, in particular following a new "photograph" of the phenomenon that generated the message and that makes it prematurely obsolete.

A solution could then consist simply in ceasing to broadcast the message in question; this solution is not retained because it involves managing a time-out (time after which, if the message has not been received, this will be interpreted as voluntary destruction) very difficult to implement, especially in situations of rapid variation in the number of messages to be broadcast or poor reception conditions.

Cycle (73)

: quelconque

Typ (76)

: système

"Thème" (77)

: 000 (suppression)

Portée (78)

: quelconque

Numéro (79)

: X X X X X X X X X X
(message non évolutif)

Notification (74)

:0
(pas de notification)

We therefore explicitly broadcast a deletion message. An example of the structure of such a message is presented in FIG. 8. It is a "system" type message characterized by a header 17 to 17 bits, followed by an identification 92 of the message to be deleted. If we refer to the example, the header advantageously includes the following information:

Cycle (73)

: any

Typ (76)

: system

"Theme" (77)

: 000 (delete)

Range (78)

: any

Number (79)

: XXXXXXXXXX
(non-evolving message)

Notification (74)

: 0
(no notification)

• . à t1 le phénomène est décrit par un message M1 : localisation L1, durée prévue jusqu'à t'1 ; pour le terminal, la prise en compte de ce message peut être représentée par le rectangle A1B1C1D1.
• . à t2 le phénomène est décrit par le message M2 : localisation L2, durée prévue jusqu'à t'2, représentation dans le terminal A2B2C2D2.

M2 est en fait une nouvelle version dit message M1 ; cette situation est clairement décrite par un couple (identification, version), l'identification étant la même pour M1 et M2 distingués par un numéro de version.In the case of FIG. 3, there is no longer an early cancellation, but an earlier replacement of an earlier message M1 with a later message M2.

• . at t1 the phenomenon is described by a message M1: location L1, duration expected until t'1; for the terminal, taking this message into account can be represented by the rectangle A1B1C1D1.
• . at t2 the phenomenon is described by the message M2: location L2, expected duration up to t'2, representation in the terminal A2B2C2D2.

M2 is in fact a new version called M1 message; this situation is clearly described by a pair (identification, version), the identification being the same for M1 and M2 distinguished by a version number.

FIG. 5 translates a slightly different replacement situation, in which the photograph at time (n + 1) has an a priori expiration time prior to that of time n.

The first M1 message translates as A1B1C1D1

The second message M2 is translated as A2B2C2D2.

de t1 à t2

: M1, ou plus généralement M(I) Vn

de t2 à t'2

: M2, ou plus généralement M(I) Vn+1

de t'2 à t'1

: SUP(1), ou plus généralement SUP(I,Vn);

   avec i: identifiant du message, et Vn: nème version du message d'identifiant i.It is clear that from t'2 to t'1 it will be necessary to broadcast a SUP (I) message. Diffusion can be organized as follows:

from t1 to t2

: M1, or more generally M (I) Vn

from t2 to t'2

: M2, or more generally M (I) Vn + 1

from t'2 to t'1

: SUP (1), or more generally SUP (I, Vn);

with i: identifier of the message, and Vn: nth version of the message with identifier i.

This last broadcast SUP (I, Vn) is necessary to take into account the terminals which would have acquired the message M (I) between t1 and t2 then would no longer have been listening for the whole duration (t2 t'2) of the message M (I) Vn + 1.

More generally, it may happen that a phenomenon, described at an instant by a message M1 of location L1, evolves and occupies in a second time a double location L2 and L3, requiring the dissemination of two new messages (fig 4) ; it is therefore useful that the protocol indicates that the two messages in question represent the update of the first.

Let I, J, K be the respective identifications of these messages called M (I), M (J) and M (K); let SUP (I) be the message to delete M (I).

The sequence M (J) - M (K) - SUP (I) indicates by convention that M (I) is deleted and replaced by M (J) and M (K). Chronologically, the information SUP (I) is advantageously broadcast after the messages M (J) and M (K): this makes it possible to delimit the logical module of the messages "I, J, K", and implicitly means that the deletion of M (I) must intervene before taking into account the content of the messages M (J) and M (K).

More specifically, the procedure adopted makes it possible to avoid sorting objects, when processing information messages for updating the information stored in the receiver.

• dans le cas d'un message antérieur multiobjets remplacé au moins partiellement par un message ultérieur de même identifiant, concernant un groupe d'objets recouvrant au moins partiellement le groupe d'objets désigné par ledit message antérieur, lesdits moyens de traitement du récepteur procèdent à l'annulation dudit message antérieur préalablement à la prise en compte du contenu dudit message ultérieur;
• dans le cas d'un message antérieur multiobjets remplacé par au moins deux messages ultérieurs concernant des groupes d'objets recouvrant au moins partiellement le groupe d'objets désigné par ledit message antérieur:

• . on diffuse un des messages ultérieurs avec le même identifiant que ledit message antérieur;
• . on diffuse le ou les autres messages ultérieurs avec un nouvel identifiant, indépendant de l'identifiant dudit message antérieur.

As already mentioned, the procedure for broadcasting alternative messages can be summarized as follows:

• in the case of a previous multi-object message replaced at least partially by a subsequent message of the same identifier, relating to a group of objects at least partially covering the group of objects designated by said previous message, said receiver processing means proceed to the cancellation of said prior message before taking into account the content of said subsequent message;
• in the case of an earlier multi-object message replaced by at least two subsequent messages concerning groups of objects at least partially covering the group of objects designated by said previous message:

• . one of the subsequent messages is broadcast with the same identifier as said previous message;
• . the subsequent message (s) is broadcast with a new identifier, independent of the identifier of said previous message.

On this last point, it should be noted that the implicit cancellation of the previous message resulting from the fact that one of the subsequent messages has the same identifier, can be replaced by the explicit broadcasting of a message to cancel the previous message, followed by the dissemination of subsequent messages carrying an identifier independent of the identifier known as the canceled previous message.

Claims (7)

1. Method of broadcasting information messages, especially information messages on the state of the attributes of a distribution of objects, consisting, for example, of the segments of a road infrastructure, over a channel with imperfect transmission and a limited throughput, destined for intermittently listening fixed or mobile receivers, the said receivers comprising means for memory storage and processing of the said broadcast information, a method characterised in that it consists in storing reference information, called steady-state information, in the said memory storage means of the receivers, and in broadcasting information messages on deviations with respect to the said steady state, over the said broadcasting channel, in that each information message broadcast comprises message identification means (75) which are distinct from the contents of the message, as well as an item of original expiry time information (84) for the said message, in that each of the said receivers comprises means for early replacement and/or cancellation of a previous message on reception of a subsequent, separate message with the same identifier, and in that each message is repeatedly broadcast up to the said original expiry time (84), or until its early replacement or cancellation.
2. Method according to Claim 1, characterised in that the said identification means (75) comprise a message version number, different for each new successive set of deviation information for the same object or group of objects.
3. Method according to Claim 1, characterised in that the said original expiry time (84) is calculated on the basis of a fixed, standard validity duration for the corresponding message.
4. Method according to Claim 2, characterised in that the said early replacement and/or cancellation means for a previous message comprise means of comparison of the received messages with the same identifiers and the same versions, so as to provide transmission error corrections.
5. Method according to Claim 1, characterised in that, in the event of broadcasting of a subsequent message to be substituted for a previous message with the same identifier, the said subsequent message comprising a new original expiry time less than the former original expiry time of the previous message,

   - the said subsequent message is transmitted until the new original expiry time

   - an early cancellation message for the said previous message is transmitted between the said new original expiry time and the said former original expiry time.
6. Method according to Claim 1, characterised in that, in the case of a previous multiobject message replaced at least partially by a subsequent message with the same identifier, concerning a group of objects overlapping at least partially the group of objects designated by the said previous message, the said processing means of the receiver carry out cancellation of the said previous message prior to taking account of the contents of the said subsequent message.
7. Method according to Claim 1, characterised in that, in the case of a previous multiobject message replaced by at least two subsequent messages concerning groups of objects overlapping at least partially the group of objects designated by the said previous message:

   - one of the subsequent messages is broadcast with the same identifier as the said previous message;

   - the other subsequent message or messages is or are transmitted with a new identifier, which is independent of the identifier of the said previous message.

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