FR3083946A1 - GROUP ACQUISITION IN A WIRELESS COMMUNICATION NETWORK - Google Patents

Abstract

In a communication system comprising a server, a plurality of terminal devices and at least one collection gateway serving as a relay between the server and the terminal devices, the server: creates at least one group and assigns (601) terminal devices to each group created; defines, for each group created, instances of a group reception window separated by a period P, so as to occur at deterministic times; receives (604, 616) ascending frames from the terminal devices; for a said terminal device assigned to a group, acknowledge the ascending frames received in a descending frame (611) transmitted in point-to-multipoint mode (612a, 612b) in a said instance of the group reception window defined for said group said descending frame including a group acknowledgment for all ascending frames transmitted by terminal devices of the group during the period P preceding said instance of the group reception window; and for a said terminal device which is not assigned to a group, acknowledge (605, 617) the ascending frames received in point-to-point mode.

Description

The present invention relates to a transmission of acknowledgments of ascending frames in a communication system in which predetermined reception windows are used to transmit descending frames acknowledging the ascending frames in question.

The Internet of Things is emerging. The Internet of Things represents the extension of the Internet to things and places in the physical world. While the Internet does not usually extend beyond the electronic world, the Internet of Things represents the exchange of information and data from devices in the real world to the Internet, such as for example collection of statements of electrical consumption or water consumption. The Internet of Things is considered the third evolution of the Internet, called Web 3.0. The Internet of Things is partly responsible for the current increase in the volume of data to be transmitted and stored, and is therefore at the origin of what is called "Big Data". The Internet of Things is universal in designating the communication of objects for various uses, for example in the industrial, agro-food, e-health or home automation fields.

To enable communicating objects, also called terminal devices, to communicate within the framework of the Internet of Objects, collection gateways located in geographically high points are deployed by an operator. Excluding maintenance operations, these collection walkways are typically fixed and permanent. One can for example quote on this model the networks SigFox (registered trademark) or ThingPark (registered trademark). These collection gateways communicate with communicating objects using medium or long-range radio communications, such as, for example, the LoRaWAN technology (“Long Range Wide-Area Network” in English), also known by the short form “LoRa” (“Long Range ”In English) from the name of the alliance promoting LoRaWAN technology. These collection gateways thus serve as relays between communicating objects and a server (core network) configured to process information transmitted by the terminal devices and to send messages (e.g. instructions) to said terminal devices.

To be able to go to sleep as much as possible, the terminal devices apply, in their indirect communications with the server (via one or more collection gateways), a communication mechanism known as "Class A" in the data transmission protocol of LoRaWAN type. This mechanism consists in defining, deterministically for a terminal device considered and the collection gateway which acts as a relay for the server, one or more reception windows during which the terminal device considered listens to the communication medium (two reception windows in the case of the LoRaWAN type data transmission protocol) where downlink communications must be carried out. One or more reception windows start at the end of a period of predefined duration according to an ascending communication initiated by the terminal device in question and also have a predefined duration. A downlink frame which must be addressed to said terminal device is then carried out in one or the other (if any) of said several reception windows, in particular for making an acknowledgment of said ascending frame. It is typically necessary for the terminal device in question to know that the server has actually received the data of the upward frame transmitted by said terminal device. This reception window approach starting at predetermined times in a deterministic manner for the terminal device in question and the collection gateway which acts as a relay for the server allows said terminal device to go into standby in the meantime and thus to preserve its energy autonomy.

However, from the point of view of the management of downlink transmissions, this approach offers few opportunities for transmitting data to said terminal device, unless the number of reception windows is increased or the duration thereof is increased. In addition, this drawback is reinforced by the fact that part of these reception windows serves to formally acknowledge the ascending frame which triggered the definition of said reception windows. Indeed, while the collection gateway in question acknowledges in this way the ascending frame in question, the communication medium on which communications between the terminal devices and the collection gateways are based is not fully free to perform other communications. In other words, either it prevents other communications, or it interferes with other communications.

It is desirable to overcome these drawbacks of the state of the art, which can be found more generally in wireless communication networks operating on such reception windows, such as, for example, in communication systems of the LPWAN type. ("Low-Power Wide Area Network" in English).

The invention relates to a method for transmitting acknowledgments in a communication system comprising a server, a plurality of terminal devices, and at least one collection gateway serving as a relay between the server and the terminal devices, each terminal device communicating with the minus a so-called collection gateway via a wireless communication medium, the method being implemented by the server. The method comprises the following steps: creating at least one group and assigning terminal devices to each group created; define, for each group created, instances of a group reception window separated by a period P, so that the instances of group reception windows occur at deterministic times; receive ascending frames from the terminal devices; for a said terminal device assigned to a group, acknowledge the ascending frames received in a downlink frame transmitted in point-to-multipoint mode in a said instance of the group reception window defined for said group, said downlink frame including a group acknowledgment for all the ascending frames sent by terminal devices of the group during the period P preceding said instance of the group reception window; and for a said terminal device which is not assigned to a group, acknowledge the ascending frames received in point-to-point mode. Bandwidth consumption of the communication system is therefore more efficient thanks to group acknowledgment.

According to a particular embodiment, by assigning a said terminal device to a group, the server transmits to said terminal device a subscription message by which the server indicates to said terminal device that said terminal device has been associated with said group, the message of subscription including index information indicating which index is associated with said terminal device within said group, the index associating with said terminal device a bit position in a field of group acknowledgments.

According to a particular embodiment, the server transmits to said terminal device a subscription message by which the server indicates to said terminal device that said terminal device has been associated with said group, and an index is implicitly associated with said terminal device within said group, the index associating to said terminal device a bit position in a field of group acknowledgments, the index being of value equal to the value of Ng_b> 0 least significant bits among Nd bits used to represent an address assigned to said terminal device for communicate in the communication system.

According to a particular embodiment, the subscription message includes a time reference field representative of an instant at which the next instance of the group reception window starts for said group.

According to a particular embodiment, the time reference field also includes additional information representative of a duration of the period P separating the group reception window instances for said group.

According to a particular embodiment, the time reference field also includes information on the variability of the period P separating the group reception window instances for said group.

According to a particular embodiment, the duration of the period P is variable according to a predefined sequence of durations, and the time reference field includes information indicating where the group is in said predefined sequence of durations, the predefined sequence of durations being then known in advance by the terminal devices or included in the subscription message.

According to a particular embodiment, the group acknowledgment includes a time synchronization field providing a relative time corresponding to the instant at which the next instance of the group reception window occurs for said group.

The invention also relates to a server configured to implement this first method presented above in any of its embodiments.

The invention also relates to a method for receiving acknowledgments in a communication system comprising a server, a plurality of terminal devices, and at least one collection gateway serving as a relay between the server and the terminal devices, each terminal device communicating with at least one said collection gateway via a wireless communication medium, the method being implemented by each terminal device. The method includes the following steps: transmitting ascending frames to the server; when said terminal device has received a message from the server assigning the terminal device to a group, schedule a listening of the wireless communication medium during instances of a group reception window separated by a period P, so that instances of group receive windows occur at deterministic times; when said terminal device is assigned to a group, receive in point-to-multipoint mode down frames including acknowledgments of said up frames during instances of the group reception window, each down frame including a group acknowledgment for all up frames emitted by group terminal devices during the period P preceding said instance of the group reception window; and when said terminal device is not assigned to a group, receive in point-to-point mode down frames including acknowledgments of said up frames.

According to a particular embodiment, the method comprises the following steps: receiving from the server a subscription message by which the server indicates to said terminal device that said terminal device has been associated with said group, the subscription message including information index indicating which index is associated with said terminal device within said group, the index associating with said terminal device a bit position in a field of group acknowledgments; and on receipt of a group acknowledgment, determining whether the ascending frame previously transmitted by said terminal device is acknowledged, by checking the value at said bit position in the group acknowledgments field.

According to a particular embodiment, the method comprises the following step: receiving from the server a subscription message by which the server indicates to said terminal device that said terminal device has been associated with said group. In addition, an index is implicitly associated with said terminal device within said group, the index associating with said terminal device a bit position in a field of group acknowledgments, the index being of value equal to the value of Ng_b> 0 bits of low weight among Nd bits used to represent an address assigned to said terminal device for communicating in the communication system. And the method further comprises the following step: on receipt of a group acknowledgment, determining whether the ascending frame previously transmitted by said terminal device is acknowledged, by checking the value at said bit position in the group acknowledgments field .

According to a particular embodiment, the subscription message includes a time reference field representative of an instant at which the next instance of the group reception window starts for said group, and said terminal device schedules listening to the medium. wireless communication based on the time reference field.

According to a particular embodiment, the time reference field also includes additional information representative of a duration of the period P separating the group reception window instances for said group.

According to a particular embodiment, the time reference field also includes information on the variability of the period P separating the group reception window instances for said group.

According to a particular embodiment, the duration of the period P is variable according to a predefined sequence of durations, the time reference field also indicates where the group is in said predefined sequence of durations, the predefined sequence of durations then being known in advance by the terminal device or included in the subscription message.

According to a particular embodiment, the group acknowledgment includes a time synchronization field providing a relative time corresponding to the instant at which the next instance of the group reception window occurs for said group, and the method further comprises the following steps: programming a listening of the wireless communication medium during an instance of group reception window for said group, independently of any upward transmission by said terminal device; and resynchronize the internal clock of said terminal device by determining an offset between an effective time of transmission, or reception, of a group acknowledgment received during the group reception window instance thus programmed and a time previously estimated transmission, or reception, of said group acknowledgment.

The invention also relates to a terminal device configured to implement this second method presented above in any of its embodiments.

The characteristics of the invention mentioned above, as well as others, will appear more clearly on reading the following description of an exemplary embodiment, said description being made in relation to the accompanying drawings, among which:

- Fig. 1 schematically illustrates a communication system in which the present invention can be implemented;

- Fig. 2 schematically illustrates an example of hardware architecture of a communication device of the communication system of FIG. 1;

- Fig. 3 schematically illustrates an example of a protocol architecture implemented in the communication system of FIG. 1;

- Fig. 4 schematically illustrates arrangements of reception windows, in a particular embodiment of the invention;

- Figs. 5A and 5B schematically illustrate fields of messages exchanged in the communication system of FIG. 1, in a particular embodiment of the invention;

- Fig. 6 schematically illustrates the exchange of messages in the communication system of FIG. 1, in a particular embodiment of the invention;

- Fig. 7 schematically illustrates a first algorithm for programming reception windows, in a particular embodiment of the invention; and

- Fig. 8 schematically illustrates a second algorithm for programming reception windows, in a particular embodiment of the invention.

Fig. 1 schematically illustrates a communication system in which the present invention can be implemented.

The communication system includes at least one collection gateway 120,

121, 122, 123 (noted GW, for "GateWay" in English, in Figs.). The collection gateways 120, 121, 122, 123 have respective communication links with a server 130 (denoted SRV in the Figs.) To which said collection gateways 120, 121,

122, 123 are attached. For example, the server 130 is of LNS type (“LoRaWAN Network Server” in English). Four collection gateways are shown in FIG. 1, but the communication system may include a different amount of collection gateways.

In the communication system, messages must be relayed in the form of frames from a set of terminal devices 110, 111, 112 (denoted ED, for “End Device” in English, in Figs.) To the server 130. Three terminal devices are shown in FIG. 1, but the communication system may include a different quantity of terminal devices.

The server 130 has a role of controlling and collecting information available from the terminal devices 110, 111, 112, and the collection gateways 120, 121, 122, 123 have a relay role between the terminal devices 110, 111, 112 and the server 130. Messages, in particular of acknowledgment, can also be transmitted in the form of frames from the server 130 to the terminal devices 110, 111, 112 via the collection gateways 120, 121, 122, 123.

To enable this relay role to be fulfilled, each collection gateway 120, 121, 122, 123 has at least one radio interface allowing said collection gateway to communicate with at least one terminal device 110, 111, 112 in s' pressing on a wireless communication network, and preferably using a communication technology of the LPWAN type. Said radio interface is for example of the LoRa type, thus making it possible to implement, within the communication system, a data transmission protocol of the LoRaWAN type. Said radio interface is such that a terminal device can be within radio communication range of a plurality of collection gateways, depending on the geographical position of said terminal device with respect to the collection gateways 120, 121, 122, 123 and conditions for transmission by radio in the environment of said terminal device and of the collection gateways 120, 121, 122, 123. This is the case, for example, of the terminal device 110 in FIG. 1, which is within radio communication range of the collection gateways 120, 121 and 122 (materialization by dotted lines in FIG. 1). In addition, each collection gateway 120, 121, 122, 123 has at least one other interface allowing said collection gateway to communicate with the server 130. For example, this other interface is a wired interface making it possible to communicate with the server 130 via the Internet.

An example of protocol architecture implemented in the communication system so as to allow the terminal devices 110, 111, 112 and the server 130 to communicate via the collection gateways 120, 121, 122, 123 is described below in relation to the Fig. 3.

Fig. 2 schematically illustrates an example of hardware architecture of a communication device of the communication system of FIG. 1. Each terminal device 110, 111, 112 and / or each collection gateway 120, 121, 122, 123 and / or the server 130 can be constructed on the basis of such a hardware architecture.

The communication device comprises, connected by a communication bus 210: a processor or CPU (“Central Processing Unit” in English) 201; a random access memory RAM (“Random Access Memory” in English) 202; a Read Only Memory (ROM) 203; a storage unit or storage media reader, such as an SD (Secure Digital) 204 card reader or an HDD (Hard Disk Drive); an IF 1 205 communication interface, and possibly another IF2 206 communication interface.

When the communication device of FIG. 2 shows a terminal device of the communication system, the communication interface IF1 205 is configured to allow said terminal device to communicate with collection gateways of the communication system.

When the communication device of FIG. 2 represents a collection gateway 120, 121, 122, 123 of the communication system, the communication interface IF1 205 is configured to allow said collection gateway to communicate with terminal devices of the communication system, and the other interface IF2 communication module 206 is configured to allow said collection gateway to communicate with the server 130.

When the communication device of FIG. 2 represents the server 130, the communication interface IF1 205 is configured to allow said server 130 to communicate with the collection gateways 120, 121, 122, 123, in a manner compatible with the communication interface IF2 206 of the gateways collection 120, 121, 122, 123.

The processor 201 is capable of executing instructions loaded in the RAM memory 202 from the ROM memory 203, from an external memory, from a storage medium, or from a communication network. When the communication device in question is powered up, the processor 201 is capable of reading instructions from the RAM 202 and executing them. These instructions form a computer program causing the processor 201 to implement all or part of the algorithms and steps described here in relation to the communication device in question.

Thus, all or part of the algorithms and steps described here can be implemented in software form by executing a set of instructions by a programmable machine, such as a DSP ("Digital Signal Processor" in English) or a microcontroller. All or part of the algorithms and steps described here can also be implemented in hardware form by a machine or a dedicated component, such as an FPGA ("Field-Programmable Gate Array" in English) or an ASIC ("ApplicationSpecific Integrated Circuit" in English). In general, the communication device in question comprises electronic circuitry adapted and configured to implement the algorithms and steps described here in relation to the communication device in question.

FIG. 3 schematically illustrates an example of a protocol architecture implemented in the communication system of FIG. 1. Fig. 3 illustrates the protocol architecture in a distributed manner between the terminal device 110, the collection gateway 120 and the server 130.

The terminal device 110 comprises a top layer 311 and a bottom layer 313, as well as an intermediate layer 312 making the link between the top layer 311 and the bottom layer 313. The top layer 311 is a client application. The terminal device 110 can thus include several client applications. The intermediate layer 312 implements the exchange protocol, for example of the LoRaWAN type, between the terminal device 110 and the server 130 via the collection gateway 120. The bottom layer 313 is the physical layer (PHY) of the radio interface of the terminal device 110, for example of the LoRa type, which allows said terminal device 110 to communicate with the collection gateways of the communication system, such as for example the collection gateway 120.

The server 130 comprises a high layer 331 and a low layer 333, as well as an intermediate layer 332 making the link between the high layer 331 and the low layer 333. The high layer 331 is a server application. The server 130 can thus include several server applications. It should be noted that the representation of FIG. 3 is a logical representation, and that the server applications can be executed on respective equipment distinct from the server 130, or that the server 130 can be seen as an assembly made up of a plurality of interconnected machines which collaboratively implements the features described here.

The intermediate layer 332 implements the exchange protocol, for example of the HTTPS type (“HyperText Transfer Protocol Secure” in English), between the server 130 and the collection gateways 120, 121, 122, 123. The low layer 333 is the physical layer (PHY) of the server interface 130 which makes it possible to communicate with the collection gateways 120, 121, 122, 123.

The collection gateway 120 comprises a first low layer 322 and a second low layer 323, as well as an adaptation module 321. The first low layer 322 is the physical layer (PHY) of the radio interface of the collection gateway 120 which makes it possible to communicate with terminal devices of the communication system, such as for example the terminal device 110. The second low layer 323 is the physical layer (PHY) of the interface of the collection gateway 120 which makes it possible to communicate with the server 130. The adaptation module 321 is configured to convert the messages received via the first low layer 322 into messages adapted to the second low layer 323, and vice versa. During this conversion, the collection gateway 120 can enrich said message with additional information, such as for example an indication of signal level received RS SI determined by said collection gateway 120 on receipt of said message.

The protocol architecture shown in FIG. 3 is such that the intermediate layer 312 of the terminal device 110 communicates with the intermediate layer 321 of the collection gateway 120, by relying on the respective lower layers of the terminal device 110 and of the collection gateway 120. The protocol architecture shown in FIG. 3 is also such that the upper layer 311 of the terminal device 110 communicates with the upper layer 331 of the server 130, by relying on the respective intermediate layers 312, 321, 332 of the terminal device 110, of the collection gateway 120 and of the server 130. The same server application can thus communicate with respective client applications executed on as many terminal devices, and the same terminal device executing a plurality of client applications can thus communicate with as many server applications executed on the server 130.

The layers and protocol modules represented can be supplemented, in particular by protocol stacks allowing the server 130 to exchange with the collection gateway 120, in particular to allow the server 130 to configure the collection gateway 120.

Thanks to the creation of at least one group and the assignment of several terminal devices to said group, the server 130 is able to acknowledge within the same downlink frame several ascending frames respectively transmitted by separate terminal devices, or by separate client applications (whether or not within the same terminal device). By virtue of the definition of the group and of an associated group reception window, the server 130 makes an appointment with the terminal devices of the group at predetermined times at which said terminal devices can listen to the communication medium (between said terminal devices and the collection gateways) to determine whether their ascending frame is acknowledged in a group acknowledgment transmitted at that time by a collection gateway on behalf of the server 130.

Fig · 4 schematically illustrates arrangements of reception windows, in a particular embodiment of the invention. Consider by way of illustration that the terminal device 110 transmits an ascending frame 501, which is relayed by one or more collection gateways to the server 130. Let us further consider that the terminal device 110 (or, at least, a client application of the terminal device 110) belongs to a group for which a reception window RW2 503, called the group reception window, is defined at predetermined times. The occurrence of instances of the reception window RW2 503 occurs at deterministic times for the server 130, as well as for the terminal devices belonging to the group in question. Fig. 4 shows two successive instances of the group reception window RW2 503, separated by a period P of time T.

Preferably, the instances of the reception window RW2 503 occur at substantially regular instants (substantially fixed duration of the period P). In a particular embodiment, the instances of the reception window RW2 503 occur at regular times (fixed duration of the period P).

Due to the transmission of the ascending frame 501, an instance of a reception window RW1 502, called the individual reception window, begins at the end of a period of predefined duration after the transmission of the ascending frame 501. Several instances of the reception window RW1 502 can thus be created in a predefined manner.

The individual reception window RW1 502 is intended to allow the server 130 to make downlink communications in point-to-point mode, via a collection gateway selected by the server 130, intended for the terminal device 110 It is the same communication mechanism known as “Class A” in the LoRaWAN type data transmission protocol, except that, when the terminal device 110 belongs to a group, the acknowledgment of the ascending frame 501 is carried out in the following group reception window RW2 503 rather than in the individual reception window RW1 502. In fact, the server 130 not necessarily having information or commands to be transmitted specifically to the terminal device 110 at this time, perform a downlink transmission in an instance of the individual reception window RW1 502 to simply acknowledge the ascending frame 501 generates there is an unnecessary overhead of bandwidth. This is called group acquittal. This aspect is detailed below in relation to FIG. 6. If however the server

130 has information or commands to be transmitted specifically to the terminal device 110 at this time, the server 130 can perform a downlink transmission in an instance of the individual reception window RW1 502 and acknowledge said ascending frame within the descending frame in addition to said information or orders. We find here the behavior which is for example called "piggy-back" in the LoRaWAN protocol.

It is considered that the terminal device 110 is prohibited from transmitting more than one ascending frame corresponding to a given group, to which said terminal device 110 belongs, between two instances of the group reception window RW2 503 associated with said group. The rate of the instances of the RW2 group reception window 503 is therefore defined in order to meet the application needs between the terminal devices 110, 111, 112 and the server 130.

In a particular embodiment, the terminal device 110 is prohibited from transmitting ascending frames during certain time sub-periods of the period P. For example, the terminal device 110 is prohibited from transmitting ascending frames during a margin period M which precedes (and adjoins) the start time of an instance of the RW2 group reception window 503, as shown in FIG. 4. This avoids creating uncertainty as to the instance of the group reception window RW2 503 where the acknowledgment of the ascending frame in question is supposed to be.

In a particular embodiment, the terminal device 110 ensures that the transmission of the ascending frame 501 does not only result in an instance of the individual reception window RW1 502, which is defined according to the time of transmission of the ascending frame 501, overlaps an instance of the RW2 group reception window 503. In other words, the terminal device 110 ensures that the transmission of the ascending frame 501 is such that any instance of the individual reception window RW1 502 is distinct from any instance of the group reception window RW2 503 defined for the group to which said terminal device 110 belongs (or its client application considered).

It should be noted that the description is made for terminal devices which can receive downlink frames in individual reception windows, as is the case in "Class A" in the LoRaWAN type data transmission protocol. The principle of group reception windows can however be applied to terminal devices which have the possibility of using additional reception windows defined in beacons transmitted by the collection gateways 120, 121, 122, 123, as is the case in “Class B” in the LoRaWAN type data transmission protocol, where several terminal devices can listen to the medium in the same time window. The principle of group reception windows can however also be applied to terminal devices which constantly listen to the communication medium, as is the case in “Class C” in the LoRaWAN type data transmission protocol, because the gain in bandwidth remains valid.

Figs. 5A and 5B schematically illustrate fields of messages exchanged in the communication system of FIG. 1, in a particular embodiment of the invention.

Fig. 5A more particularly represents fields used in a SUB-RQ message by which the server 130 indicates to a terminal device that said terminal device has been associated with a group by the server 130. In other words, the SUB-RQ message is a message announcing the subscription to said group.

In a particular embodiment, the same terminal device can be associated by the server 130 with several groups. In particular, the server 130 can associate different client applications of said terminal device with different respective groups. Thus, a GID field (for “Group IDentifier” in English) provides a group identifier allowing the terminal device in question to know which group is associated with it by the SUB-RQ message. When there is only one possible group in the communication system or when there is no ambiguity on the group designation, this GID field can be omitted.

In addition, an indexing field IND preferably indicates to said terminal device which index (also called "rank") is associated with said terminal device (or the client application in question) within said group.

The index in question can be assigned by the server 130 to the terminal device in question by taking an index from a set of available indexes. For example, the server 130 allocates the indexes in ascending or descending order among said set of available indexes. According to another example, the server 130 allocates the indexes by random or pseudo-random selection from among said set of available indexes. In a preferred variant, the index in question can be implicitly found by the terminal device and it is therefore not necessary to indicate it explicitly. To do this, the server 130 uses the address assigned to the terminal device in question to communicate in the communication system of FIG. 1. It is indeed customary to dynamically assign addresses to the terminal devices of a communication system during an authentication phase where said terminal devices declare themselves to (a server) of the communication system. In the LoRaWAN protocol, this address is called DevAddr.

This address is typically, but not necessarily, represented on a larger number of bits than required by the maximum capacity of the group. For example, this dynamically assigned address is on 32 bits, while the maximum capacity of the group is 256 terminal devices (this strictly requires only 8 representation bits).

Consider that the maximum capacity of the group is Ng, which requires only Ng_b> 0 representation bits and that the addresses assigned to the terminal devices in the communication system of FIG. 1 strictly require Nd> Ng_b representation bits. The law of large numbers ensures that the least significant Ng_b bits among the Nd bits of said addresses are sufficiently diverse among the terminal devices of the group in question to avoid conflicts of values. Each terminal device is then assigned the index within the group which corresponds to the least significant Ng_b bits among the Nd bits of the address assigned to it for communicating in the communication system of FIG. 1. This allows the server 130 and each terminal device to determine this index independently. It is thus possible to do without the IND field mentioned above. The server 130 can however still notify the index thus obtained in said field IND.

Returning to FIG. 5 A, a time reference field NW (for “Next Window” in English) furthermore provides said terminal device with information representative of the time at which the next instance of the group reception window RW2 503 for said group starts. This information can be an absolute or relative time.

In the case where the communication system ensures synchronism of the internal clocks of the various constituent elements of said communication system, said information can be representative of an absolute time. However, it is more in terms of implementation that said information is representative of a relative time (that is to say a delay from a deterministic moment).

In the case where the information is a relative time, the server 130 calculates this relative time taking into account the instant of occurrence of the instance of the individual reception window RW1 502 in which the SUB-RQ message is transmitted. . Said information contained in the time reference field NW can be supplemented by additional information representative of a duration of the period P separating the group reception window instances RW2 503 for said group, which can itself be supplemented by a variability information for the period P separating the group reception window instances RW2 503 for said group. The duration of the period P is for example made variable according to a predefined sequence of durations. Note that this variability can be represented by hazards signed around a nominal value of duration of period P. The time reference field NW then indicates where the group is in said predefined sequence of durations (eg by an index at within said sequence of durations), the predefined sequence of durations then being known in advance by the terminal devices (eg by configuration at the installation) or entered in the time reference field NW.

Fig. 5B more particularly represents fields used in a group acknowledgment message MACK ("Multicast ACKnowledgement" in English), by which the server 130 performs a group acknowledgment for a set of terminal devices belonging to the same group. The transmission of any MACK group acknowledgment is carried out in point-to-multipoint in an instance of the RW2 group reception window 503. The GID field already described identifies, if necessary, the group concerned.

In addition, an ACK field identifies which are the indexes of the terminal devices for which the MACK group acknowledgment is intended among the terminal devices of said group. In Fig. 5B, eight sub-fields of index 0 to 7 are shown in a purely illustrative manner (for example, the ACK field could contain 256 bits, thus making it possible to constitute groups of 256 terminal devices or client applications). Preferably, groups of maximum capacity equal to Nb terminal devices, where Nb is a power of 2, are formed.

When a first predefined value (for example "1") is placed at the index associated with a terminal device, said MACK group acknowledgment message then includes an acknowledgment intended for the terminal device in question; and when a second predefined value (for example “0”) is placed at the index associated with the terminal device in question, said group acknowledgment message MACK then does not include an acknowledgment intended for the terminal device in question.

In addition, a time synchronization field TS (for “Time Synchronization” in English) is preferably included in the group acknowledgment message MACK. The TS time synchronization field provides information indicating a relative time representative of the instant at which the next instance of the group reception window occurs. This time synchronization field TS has the same function as the time reference field NW, but the time synchronization field TS is relative, where the time reference field NW could be absolute.

In the case where the period P separating the instances of group reception window is variable, the TS field then also indicates where the group is in the predefined sequence of durations mentioned above.

Fig. 6 schematically illustrates the exchange of messages in the communication system of FIG. 1, in a particular embodiment of the invention. It is seen in Fig. 6 that the terminal device 110 (or a client application of the terminal device 110) is assigned to a group and that the server 130 decides to also assign the terminal device 112 (or a client application of the terminal device 112) to this same group.

When there are several possible groups in the communication system, the server 130 assigns the terminal devices to such or such group according to the client applications of said terminal devices. According to another criterion, the server 130 assigns the terminal devices to such and such a group as a function of the collection gateways which relay the ascending frames of said terminal devices, or which statistically most often relay the ascending frames of said terminal devices. According to yet another criterion, the server 130 assigns the terminal devices to such and such a group as a function of the geographic location of said terminal devices. Combinations of criteria including at least one of the above can be achieved.

When a first group has reached a maximum quantity of terminal devices authorized in said first group (in particular because the MACK message has a maximum capacity for simultaneous acknowledgments), at least a second group is created to subsequently include other devices therein. terminals that meet the criteria for inclusion in the first group. It is then possible, if necessary, to balance the groups thus created, by disassociating one or more terminal devices from a group and then associating them with another group. As a variant, when a group has reached the maximum quantity of terminal devices authorized in said group, it is possible to replace one or more terminal devices of said group with other terminal devices.

In a step 601, the server 130 transmits a downlink frame including a SUB-RQ message to the terminal device 112 via the collection gateway 121. The server 130 transmits said SUB-RQ message in response to an ascending frame (not shown in Fig. 6) transmitted by the terminal device 112, so that the collection gateway 121 can transmit said SUB-RQ message, as well as an individual acknowledgment (in point-to-point mode) of said ascending frame, in a instance of the individual reception window RW 1 502 which is defined according to the transmission time of said ascending frame. In addition to header fields for ensuring communications according to the protocol architecture described in relation to FIG. 3, the SUB-RQ message includes fields, as described in relation to FIG. 5A, enabling the terminal device 112 to be informed that the server 130 associates the terminal device 112 with the group in question.

Preferably, the server 130 transmits the frame which includes the SUBRQ message in response to an ascending frame transmitted by the client application communicating with a server application with which the group in question is associated. Thus, the terminal device 112 knows which client application (if there is ambiguity) corresponds to the group in question.

In a step 602, the collection gateway 121 transmits the SUB-RQ message to the terminal device 112 in said instance of the individual reception window RW1 502. The announcement of subscription to the group in question is thus made in point mode- medium rare.

In a step 603, the terminal device 112 transmits an ascending frame including a SUB-RSP message by which the terminal device 112 confirms the reception of the SUB-RQ message. With the SUB-RSP message, the terminal device 112 has the possibility of accepting or refusing the subscription. For example, the terminal device 112 can already belong to one or more groups and have reached a maximum limit (e.g. 4) of simultaneous groups authorized. According to another example, accepting to subscribe to a group entails a slightly greater energy expenditure for the terminal device 112, since the terminal device 112 will have to wake up more frequently to listen to the communication medium. The terminal device 112 can for example refuse to subscribe to a group when the level of energy reserves of the terminal device 112 is below a predefined threshold. Consider here that the terminal device 112 accepts the subscription offered by the server 130.

In a step 604, the collection gateway 121 has received the ascending frame transmitted in step 603 by the terminal device 112 and therefore relays the SUB-RSP message to the server 130, which is then informed that the terminal device 112 has accepted the subscription to the group in question. The ascending frame transmitted in step 603 may have been relayed to the server 130 by several gateways within radio range of the terminal device 112, then in charge of the server 130 to ensure the deduplication of frames.

In a step 605, the server 130 acknowledges, by a UACK message (“Unicast ACK” in English) the ascending frame transmitted by the terminal device 112 in step 603. This individual acknowledgment is relayed to the terminal device 112 by the collection gateway 121 in a step 606. The transmission of this individual acknowledgment is therefore carried out in point-to-point mode, that is to say in an instance of the individual reception window RW1 502 which is defined according to an instant of transmission of the ascending frame of step 603.

Subsequently, in a step 607, the terminal device 110 transmits an ULF ascending frame (for “UpLink Frame”) to the server 130. This ULF ascending frame is picked up by the collection gateway 121 which relays it, in a step 608, to the server 130. Here again, this ULF ascending frame may have been relayed to the server 130 by several gateways within radio range of the terminal device 112, then in charge of the server 130 to ensure the deduplication of frames.

And in a step 609, the terminal device 112 transmits another ascending ULF frame to the server 130. This ascending ULF frame is picked up by the collection gateway 121 (and / or by another collection gateway) which relays it , in a step 610, to the server 130.

Consider that these two ascending ULF frames were transmitted during the same period P which separates two instances of group reception window RW2 503 for the group associated with the terminal devices 110 and 112 (or their respective client applications in question).

In a step 611, the server 130 transmits a group acknowledgment MACK for the ascending ULF frames transmitted in the steps 607 and 609. In the field

ACK, the values associated with the indexes of the terminal devices 110 and 112 indicate that the server 130 acknowledges the uplink transmissions ULF of steps 607 and 609. This group acknowledgment MACK is relayed, in a step 612, by the collection gateway 121 in the instance of the group reception window RW2 503 which follows the period P during which the ascending ULF frames were transmitted in steps 607 and 609. In FIG. 6, step 612 is broken down into two sub-steps 612a and 612b to show that the transmission of the MACK group acknowledgment is carried out in point-to-multipoint mode. Since the terminal devices 110 and 112 listen to the communication medium in the instance of the group reception window RW2 503 which concerns them, the terminal devices 110 and 112 receive the group acknowledgment MACK relayed by the gateway collection 121. The terminal devices 110 and 112 then analyze the group acknowledgment MACK to determine whether their own index (or rank) in the ACK field indicates that the ULF ascending frames which they previously transmitted in steps 607 and 609 are acknowledged by the server 130. Otherwise, the unacknowledged ULF ascending frame must subsequently be retransmitted by the terminal device concerned.

So that the collection gateway 121 knows when to transmit the group acknowledgment MACK, the server 130 indicates in an encapsulation header of the group acknowledgment MACK information representative of the time at which the collection gateway 121 is supposed to relay the MACK group acknowledgment to the terminal devices. The collection gateway 121 can moreover then transmit several successive copies (e.g. 3 copies) of the MACK group acknowledgment to increase the chances of reception on the side of the terminal devices concerned.

Note that with regard to the descending frames to be transmitted in the instances of individual reception window RW1 502, the collection gateway 121 keeps track of the instant of reception of the upward frame concerned and the server 130 indicates, in a header encapsulating said downlink frame, a relative time after which the collection gateway 121 is assumed to relay the downlink frame. The collection gateway 121 then relays said downlink frame after expiration of said relative time from the moment of receipt of the upstream frame concerned. As a variant, the collection gateway 121 can determine for itself when each instance of the individual reception window RW1 502 starts from the instant of reception of the upward frame concerned, by having prior knowledge of the time. predefined relative to separate said instance of the individual reception window RW1 from the instant of reception of the upward frame concerned.

To disassociate a terminal device from a group, a procedure similar to that used to associate the terminal device with the group is implemented. For example, the server 130 may want to disassociate said terminal device from a group in order to replace it with another terminal device whose presence in said group is more suitable, for example because said other terminal device transmits more frequently or because said other terminal device is geographically located more close to the other terminal devices of said group.

Consider the case of a disassociation of the terminal device 112. Thus, in a step 613, the server 130 transmits a downlink frame including a UNSUB-RQ message to the terminal device 112 via the collection gateway 121. The server 130 transmits said UNSUB-RQ message in response to an ascending frame transmitted by the terminal device 112 so that the collection gateway 121 can transmit said UNSUB-RQ message, as well as an individual acknowledgment (in point-to-point mode) of said frame ascending, in an instance of the individual reception window RW1 502 which is defined according to an instant of transmission of said ascending frame. In addition to header fields for ensuring communications according to the protocol architecture described in relation to FIG. 3, the UNSUB-RQ message includes the GID field allowing the group in question to be identified, if necessary.

In a step 614, the collection gateway 121 transmits the message UNSUBRQ to the terminal device 112 in said instance of the individual reception window RW1 502. The announcement of disassociation to the group in question is thus made in point-to-point mode point.

In a step 615, the terminal device 112 transmits an ascending frame including a UNSUB-RSP message by which the terminal device 112 confirms the reception of the UNSUB-RQ message. By the UNSUB-RSP message, the terminal device 112 confirms that it has taken into account the group's unsubscription, which means that the acknowledgments of the ascending frames sent by the terminal device 112 (or by the client application concerned) are then made in instances of individual reception windows RW1 502.

In a step 616, the collection gateway 121 has picked up the ascending frame transmitted in step 615 by the terminal device 112 and therefore performs a relay of the UNSUB-RSP message intended for the server 130, which is then informed that the terminal device 112 confirmed having taken into account the group's unsubscription. Here again, the upward frame in question may have been relayed to the server 130 by several gateways within radio range of the terminal device 112, then in charge of the server 130 to ensure the deduplication of frames.

In a step 617, the server 130 acknowledges, by a UACK message, the ascending frame transmitted by the terminal device 112 in step 615. This individual acknowledgment UACK is relayed to the terminal device 112 by the collection gateway 121 in a step 618. The transmission of this individual acknowledgment UACK is therefore carried out in point-to-point mode, that is to say in an instance of the individual reception window RW1 502 which is defined according to an instant of transmission of the ascending frame of step 615.

Fig. 7 schematically illustrates a first algorithm for programming reception windows, in a particular embodiment of the invention. The algorithm in Fig. 7 is implemented by the terminal devices 110, 111, 112. Consider by way of illustration that the algorithm of FIG. 7 is implemented by the terminal device 110.

In a step 701, the terminal device 110 transmits an ascending frame, for reception by at least one collection gateway responsible for relaying it to the server 130.

In a step 702, the terminal device 110 consequently programs a listening of the communication medium in at least one instance of an individual reception window RW1 502.

In a step 703, the terminal device 110 checks whether said terminal device 110 belongs to a group (or whether the client application of the terminal device 110 which is concerned with the sending of said upward frame belongs to a group). If this is the case, a step 704 is carried out; otherwise, a step 705 is carried out.

In step 704, the terminal device 110 consequently schedules a listening of the communication medium in the next instance of the group reception window RW2 503 associated with said group. It is recalled here that, when the terminal device 110 receives the SUB-RQ message, the terminal device 110 receives from the server 130 in the time reference field NW information representative of the instant at which a next instance of the reception window will start. group RW2 503. Furthermore, it is recalled here that the terminal device 110 is aware of the durations of the periods P which separate the instances of group reception windows

RW2 for the group to which said terminal device 110 (or the relevant client application) belongs. Thus, the terminal device 110 is aware of the instants at which each said instance of the group reception window RW2 503 in question is supposed to occur. Then step 705 is carried out.

In step 705, the terminal device 110 waits for the next scheduled reception window instance. The terminal device 110 can go to sleep until the next scheduled reception window instance occurs.

Fig. 8 schematically illustrates a second algorithm for programming reception windows, in a particular embodiment of the invention. Consider for illustrative purposes that the algorithm of Fig. 8 is implemented by the terminal device 110.

In addition to the possibility of receiving MACK group acknowledgments, the terminal device 110 can use the instances of the RW2 group reception window 503 to synchronize in time in the communication system. Indeed, since the terminal device 110 has the possibility of going to standby, its internal clock can drift over time and a regular time resynchronization is then necessary. The terminal device 110 can then rely on the actual time of reception of the MACK messages to carry out such a time resynchronization. Thus, when the terminal device 110 has transmitted an ascending frame, the terminal device 110 can take advantage of the learning of the instant of reception of the descending frame transmitted in the instance of the following group reception window RW2 503 to perform time resynchronization at the same time as verifying that the server 130 actually acknowledges the ascending frame in question. It is also possible that said terminal device 110 does not have to transmit an ascending frame for a certain time, so that its internal clock can drift too much before the instance of the group reception window RW2 503 which follows its next upward frame transmission. Then, in a particular embodiment, the terminal device 110 will seek to listen to the communication medium in an instance of the group reception window RW2 independently of its transmissions of ascending frames, in order to remain time synchronized in the communication system.

Thus, in a step 801, the terminal device 110 monitors the time synchronization of its internal clock. In other words, the terminal device 110 monitors the theoretical maximum drift of its internal clock since the last time synchronization carried out by the terminal device 110, for example since the last time that the terminal device 110 has resynchronized its internal clock thanks to the learning the instant of transmission of an MACK group acknowledgment. For example, the theoretical maximum drift of the internal clock of the terminal device 110 is of the order of 10 ppm, that is to say less than one second per day more or less.

In a step 802, the terminal device 110 determines whether there is a risk that its internal clock has drifted too much in relation to the rest of the communication system and in particular of the server 130 and / or in relation to a predefined threshold. For example, the terminal device 110 is programmed to wake up from 100 milliseconds before the theoretical start of any reception window. The terminal device 110 can then seek to resynchronize temporally every 2 hours. If there is a risk that the internal clock has drifted too much compared to the rest of the communication system and in particular of the server 130 and / or compared to a predefined threshold, a step 803 is performed; otherwise, step 801 is repeated.

In step 803, the terminal device 110 consequently schedules a listening of the communication medium in the next group reception window RW2 503, as already described in relation to FIG. 7. It will then determine the effective time of transmission (or reception) of the MACK group acknowledgment, although the latter does not concern it. This allows it to know the difference between the actual time of transmission (or reception) of the MACK group acknowledgment and the time at which said terminal device 110 had previously estimated that the time of transmission (or reception) should occur. ) the acknowledgment of the MACK group from its point of view (due to its internal clock). Determining this offset therefore allows it to reset its internal clock to remain synchronized. Then step 801 is repeated.

Claims (22)

1) Method for transmitting acknowledgments in a communication system comprising a server (130), a plurality of terminal devices (110, 111, 112), and at least one collection gateway (120, 121, 122, 123) serving relay between the server (130) and the terminal devices (110, 111, 112), each terminal device (110, 111, 112) communicating with at least one said collection gateway (120, 121, 122, 123) via a wireless communication medium, the method being implemented by the server (130), characterized in that the method comprises the following steps: - creating at least one group and assigning (601) terminal devices (110, 112) to each group created; - define, for each group created, instances of a group reception window (503) separated by a period P, so that the instances of group reception windows (503) occur at deterministic times; - receiving (608, 610) ascending frames from the terminal devices (110, 111,112); - for a said terminal device (110) assigned to a group, acknowledging the ascending frames received in a descending frame (611) transmitted in point-to-multipoint mode (612a, 612b) in a said instance of the group reception window (503 ) defined for said group, said downlink frame including group acknowledgment for all ascending frames (ULF) transmitted by terminal devices (110, 112) of the group during the period P preceding said instance of the group reception window (503 ); and - for a said terminal device (111) which is not assigned to a group, acknowledge (605, 617) the ascending frames received in point-to-point mode. 2) Method according to claim 1, characterized in that, by assigning a said terminal device (110, 112) to a group, the server (130) transmits (601) to said terminal device (110, 112) a subscription message whereby the server (130) indicates to said terminal device (110, 112) that said terminal device (110, 112) has been associated with said group, the subscription message including index information (IND) indicating which index is associating said terminal device (110, 112) within said group, the index associating with said terminal device (110, 112) a bit position in a field (ACK) of group acknowledgments. 3) Method according to claim 1, characterized in that the server (130) transmits (601) to said terminal device (110, 112) a subscription message by which the server (130) indicates to said terminal device (110, 112) that said terminal device (110, 112) has been associated with said group, and that an index is implicitly associated with said terminal device (110, 112) within said group, the index associating with said terminal device (110, 112) a bit position in a group acknowledgment (ACK) field, the index being of value equal to the value of Ng_b> 0 least significant bits among Nd bits used to represent an address assigned to said terminal device (110, 112) to communicate in the communication system. 4) Method according to any of claims 2 and 3, characterized in that the subscription message includes a time reference field (NW) representative of an instant at which the next instance of the reception window starts for said group. group (503). 5) Method according to claim 4, characterized in that the time reference field (NW) further includes additional information representative of a duration of the period P separating the group reception window instances (503) for said group . 6) Method according to claim 5, characterized in that the time reference field (NW) further includes variability information of the period P separating the group reception window instances (503) for said group. 7) Method according to claim 6, characterized in that the duration of the period P is variable according to a predefined sequence of durations, and the time reference field (NW) includes information indicating where the group is in said predefined sequence of durations, the predefined sequence of durations then being known in advance by the terminal devices (110, 112) or included in the subscription message. 8) Method according to any one of claims 1 to 7, characterized in that the group acknowledgment includes a time synchronization field (TS) providing a relative time corresponding to the instant at which the next instance of the window occurs. group reception (503) for said group. 9) Method for receiving acknowledgments in a communication system comprising a server (130), a plurality of terminal devices (110, 111,112), and at least one collection gateway (120,121,122,123) serving as a relay between the server (130) and the terminal devices (110, 111, 112), each terminal device (110, 111, 112) communicating with at least one said collection gateway (120, 121, 122, 123) via a wireless communication medium, the method being implemented by each terminal device (110, 111, 112), characterized in that the method comprises the following steps: - transmit (607, 609) upward frames to the server (130); - when said terminal device (110, 112) has received from the server (130) an assignment message from the terminal device (110, 112) to a group, schedule (704) a listening of the wireless communication medium during instances a group reception window (503) separated by a period P, so that the instances of group reception windows (503) occur at deterministic times; - when said terminal device (110, 112) is assigned to a group, receive (612a, 612b) in point-to-multipoint mode of downlink frames including acknowledgments of said ascending frames during the instances of the group reception window (503 ), each downlink frame including group acknowledgment for all ascending frames transmitted by terminal devices (110, 112) of the group during the period P preceding said instance of the group reception window (503); and - when said terminal device (111) is not assigned to a group, receive (606, 618) in point-to-point mode down frames including acknowledgments of said up frames. 10) Method according to claim 9, characterized in that the method comprises the following steps: receiving (602) from the server (130) a subscription message by which the server (130) indicates to said terminal device (110, 112) that said terminal device (110, 112) has been associated with said group, the subscription message including index information indicating which index is associated with said terminal device (110, 112) within said group, the index associating with said terminal device (110, 112) a bit position in a field (ACK) of acknowledgments of group; and on reception (612a, 612b) of a group acknowledgment, determining whether the ascending frame previously transmitted by said terminal device is acknowledged, by checking the value at said bit position in the (ACK) field of group acknowledgments. 11) Method according to claim 9, characterized in that the method comprises the following step: receiving (602) from the server (130) a subscription message by which the server (130) indicates to said terminal device (110, 112) that said terminal device (110, 112) has been associated with said group, in that a index is implicitly associated with said terminal device (110, 112) within said group, the index associating with said terminal device (110, 112) a bit position in a field (ACK) of group acknowledgments, the index being of value equal to the value of Ng_b> 0 least significant bits among Nd bits used to represent an address assigned to said terminal device (110, 112) for communicating in the communication system, and in that the method further comprises the following step : on reception (612a, 612b) of a group acknowledgment, determining whether the ascending frame previously transmitted by said terminal device is acknowledged, by checking the value at said bit position in the group acknowledgments (ACK) field. 12) Method according to any one of claims 10 and 11, characterized in that the subscription message includes a time reference field (NW) representative of an instant at which the next instance of the window starts for said group. group reception (503), and said terminal device (110, 112) schedules listening to the wireless communication medium as a function of the time reference field. 13) Method according to claim 12, characterized in that the time reference field (NW) further includes additional information representative of a duration of the period P separating the group reception window instances (503) for said group . 14) Method according to claim 13, characterized in that the time reference field (NW) further includes variability information of the period P separating the group reception window instances (503) for said group. 15) Method according to claim 14, characterized in that the duration of the period P is variable according to a predefined sequence of durations, and the time reference field further indicates where the group is in said predefined sequence of durations, the sequence predefined durations then being known in advance by the terminal device (110, 112) or included in the subscription message. 16) Method according to any one of claims 9 to 15, characterized in that the group acknowledgment includes a time synchronization field (TS) providing a relative time corresponding to the instant at which the next instance of the window occurs. group reception (503) for said group, and the method further comprises the following steps: when said terminal device detects (802), programming (803) a listening of the wireless communication medium during a group reception window instance (503) for said group, independently of any upward transmission by said terminal device (110, 112); and resynchronize the internal clock of said terminal device by determining an offset between an effective time of transmission, or reception, of a group acknowledgment received during the group reception window instance (503) thus programmed and a previously estimated instant of transmission, or reception, of said group acknowledgment. 17) Method for transmitting and receiving acknowledgments in a communication system comprising a server (130), a plurality of terminal devices (110, 111, 112), and at least one collection gateway (120, 121, 122, 123) serving as a relay between the server (130) and the terminal devices (110, 111,112), characterized in that the server (130) implements the method according to one of claims 1 to 8 and several terminal devices (110, 111 , 112) implement the method according to one of claims 9 to 16. 18) computer program comprising a set of instructions causing the execution, by a processor (201), of the method according to any one of claims 1 to 8 or the method according to any one of claims 9 to 16, when said computer program is executed by said processor. 19) Information storage medium storing a computer program comprising a set of instructions causing the execution, by a processor (201), of the method according to any one of claims 1 to 8 or the method according to any one of claims 9 to 16, when said computer program is executed by said processor. 20) Server (130) intended to be included in a communication system further comprising a plurality of terminal devices (110, 111, 112) and at least one collection gateway (120, 121, 122, 123) serving as a relay between the server (130) and the terminal devices (110, 111, 112), each terminal device (110, 111, 112) communicating with at least one said collection gateway (120, 121, 122, 123) via a wireless communication medium, characterized in that the server (130) comprises: - means for creating at least one group and assigning (601) terminal devices to each group created; - Means for defining, for each group created, instances of a group reception window (503) separated by a period P, so that the instances of group reception windows (503) occur at deterministic instants; - means for receiving (608, 610) ascending frames from the terminal devices (110, 111, 112); - for a said terminal device (110, 112) assigned to a group, means for acknowledging (611) the ascending frames received in a descending frame transmitted in point-to-multipoint mode (612a, 612b) in a said instance of the group reception window (503) defined for said group, said downlink frame including group acknowledgment for all ascending frames (ULF) transmitted by terminal devices (110, 112) of the group during the period P preceding said instance of the group reception window (503); and - for a said terminal device (111) which is not assigned to a group, means for acknowledging (605, 617) the ascending frames received in point-to-point mode. 21) Terminal device (110) intended to be included in a communication system comprising a server (130), at least one other terminal device (111, 112), and at least one collection gateway (120, 121, 122, 123 ) serving as a relay between the server (130) and the terminal devices (110, 111, 112), the claimed terminal device (110) communicating with at least one said collection gateway (120, 121, 122) via a communication medium wireless, characterized in that the claimed terminal device (110) comprises: - means for transmitting (607) upward frames to the server (130); - When the claimed terminal device (110) has received from the server a message assigning the claimed terminal device (110) to a group, means for programming (704) listening to the wireless communication medium during instances of a group reception window (503) separated by a period P, so that instances of group reception windows (503) occur at deterministic times; - when the claimed terminal device (110) is assigned to a group, means for receiving in point-to-multipoint mode (612b) down frames including acknowledgments of said up frames during instances of the group reception window (503 ), each downlink frame including a group acknowledgment for all the ascending frames transmitted (607, 609) by terminal devices (110, 112) of the group during the period P preceding said instance of the group reception window (503); and - when the claimed terminal device is not assigned to a group, means for receiving in point-to-point mode downward frames including acknowledgments of said upward frames. 22) Communication system comprising a server (130), a plurality of terminal devices (110, 111, 112), and at least one collection gateway (120, 121, 122, 123) serving as a relay between the server (130) and the terminal devices (110, 111, 112), characterized in that the server is according to claim 20 and several terminal devices (110, 111, 112) are according to claim 21.