Classifications

• • H—ELECTRICITY
  • H04—ELECTRIC COMMUNICATION TECHNIQUE
  • H04W—WIRELESS COMMUNICATION NETWORKS
  • H04W4/00—Services specially adapted for wireless communication networks; Facilities therefor
  • H04W4/06—Selective distribution of broadcast services, e.g. multimedia broadcast multicast service [MBMS]; Services to user groups; One-way selective calling services
  • H04W4/10—Push-to-Talk [PTT] or Push-On-Call services
• • H—ELECTRICITY
  • H04—ELECTRIC COMMUNICATION TECHNIQUE
  • H04W—WIRELESS COMMUNICATION NETWORKS
  • H04W76/00—Connection management
  • H04W76/40—Connection management for selective distribution or broadcast
  • H04W76/45—Connection management for selective distribution or broadcast for Push-to-Talk [PTT] or Push-to-Talk over cellular [PoC] services
• • H—ELECTRICITY
  • H04—ELECTRIC COMMUNICATION TECHNIQUE
  • H04W—WIRELESS COMMUNICATION NETWORKS
  • H04W76/00—Connection management
  • H04W76/20—Manipulation of established connections

Definitions

• the present invention relates to a method for managing a group communication in half-duplex mode between different end devices of a packet switched network.
• IP Internet Protocol
• radiocommunication systems in particular private professional radiocommunication systems, such as those intended for the police or firefighters.
• GSM public switched telephone network
• a mobile station can transmit or receive, but cannot do these two operations at the same time.
• only one mobile station must be authorized to transmit at a given time, the data stream transmitted by this mobile station being retransmitted towards the mobile station (s) participating in the communication (also called call or " call "in English), that is to say to the mobile station concerned if it is an individual call or to all mobile stations participating in the call if it is a call from group.
• multimedia conference mechanisms defined within the framework of Internet protocols, ie protocols for networks operating according to the IP protocol (J. Postel, "Internet Protocol", RFC 791, IETF, September 1981) which has been standardized by the Internet Engineering Task Force (IETF) in the RFC ("Request For Comment") above.
• IP protocol J. Postel, "Internet Protocol", RFC 791, IETF, September 1981
• IETF Internet Engineering Task Force
• RFC Real-Request For Comment
• MCU from the English “Multimedia Conferencing Unit”
• the main Internet protocols have been designed for conventional multimedia applications and do not take into account the specificities of certain applications of professional radiocommunication networks, and in particular the management of the alternation for communications in half-duplex mode.
• a half-duplex management method for communication in half-duplex mode between at least two end devices of a packet-switched transport network in unconnected mode in which an element of indication has the function, when present with a first determined value in packets transmitted from one of said end devices to a central device ensuring communication management, to indicate to said central equipment, on the one hand, that said end equipment acknowledges receipt of the right to transmit which is granted to it by said central equipment and, on the other hand, that it requests that this right to broadcast be maintained.
• This indication element can also have the function, when present with a second determined value in packets transmitted by the central equipment to the end equipment, to indicate to said end equipment that they can ask for the right to broadcast.
• the central equipment can be an MCU, and the frames transmitted on the network can be RTP (Real time Transport Protocol) packets.
• RTP Real time Transport Protocol
• the communication then being established as an RTP / RTCP session (from the English “Real time Transport Control Protocol” ").
• the indication element can then be the marking bit M of the header of the RTP packets, said first value of the indication element being the logical value 1 or 0, and said second value of the indication element being the logical value 0 or 1, respectively.
• the invention also provides a radiocommunication system, in particular a private professional radiocommunication system, comprising base stations and network equipment linked by a packet switched transport network in unconnected mode, in which said base stations comprise means for implementing the method as network end equipment and in which said network equipment comprises means for implementing the method as central equipment.
• a radiocommunication system in particular a private professional radiocommunication system, comprising base stations and network equipment linked by a packet switched transport network in unconnected mode, in which said base stations comprise means for implementing the method as network end equipment and in which said network equipment comprises means for implementing the method as central equipment.
• the invention also provides a base station intended to be used as end equipment in a system as defined above.
• the invention finally provides multimedia videoconferencing equipment intended to be used as central equipment in a system as defined above.
• Figure 2 a diagram showing a protocol for establishing an individual call involving two base stations of a system according to Figure 1;
• Figure 3 a diagram illustrating the topology of an RTP / RTCP session in the case of an individual communication
• Figure 4 a diagram showing a protocol for establishing a group communication involving three base stations of a system according to Figure 1;
• Figure 5 a diagram illustrating the topology of an RTP / RTCP session in the case of group communication
• - in Figure 8 a flowchart illustrating steps of the method of operating a base station comprising means for implementing the method according to the invention as end equipment; - in Figure 9: a flowchart illustrating steps in the operation of a multimedia video conference equipment (MCU) for the implementation of the method according to the invention as central equipment; and, - in Figure 10: a diagram illustrating the topology of an RTP / RTCP session in the case of a group communication involving several levels of MCU.
• MCU multimedia video conference equipment
• mobile stations 101, 102 and 103 are in the coverage area of base stations 201, 202 and 20 respectively. It will be recalled that the base stations are fixed items of equipment of the radio subsystem of the communication system. radiocommunications, which provide the interface. radio with mobile stations.
• the base stations are connected to an unconnected packet switched transport network 300, such as an IP network.
• an IP network such as an IP network.
• the base stations 201, 202 and 203 are also end devices of an IP network. Packet switching is performed by routers 301, 302 and 303.
• a call server 500 is also connected to the network 300.
• This equipment analyzes calls and establishes multimedia communications on the network 300. It cooperates with a location database 600, which is also connected to the network 300, and which contains information indicating, among other things, the cell under the cover of which the called mobile station is located, thus allowing correct routing of calls.
• Equipment other than that shown in FIG. 1 can naturally form part of the radiocommunication system. Since these devices do not participate in the mechanisms of the method according to the invention, it is not useful to describe them here.
• the different equipment base stations, MCU, call server, etc.
• base stations, MCU, call server, etc. although represented here in the form of separate physical entities for the sake of clarity, can be duplicated, combined or distributed in various ways without departing from the scope of the invention.
• the diagram in FIG. 2 presents a procedure for establishing an individual communication between mobile station 101 and mobile station 102 (here on the initiative of mobile station 101), which uses an application layer signaling protocol such as the SIP protocol (M. Handley et al., “SIP: Session Initiation Protocol”, RFC 2543, IETF, March 1999).
• SIP Session Initiation Protocol
• SIP addresses are similar to e-mail addresses, that is, they are of the form "user @ host", where the field "user” designates, for example, a user name or a telephone, and where the "host” field designates for example a domain name or an address in numerical form.
• the SIP protocol provides methods, including methods called INVITE and ACK, used to initiate a call session between two SIP users.
• the responses to messages sent within the framework of these methods are defined by code classes.
• the call server 500 responds, after consulting the location database 600, with a message indicating a code "302" which means that the mobile station is temporarily under the cover of another station basic (code 302 means "Moved temporarely”).
• This message also indicates in a “Contact” field the address of the MCU processing the communication (here the MCU designated by the address “MCU400”) and, in a “AIso” field, the SIP address of the mobile station 102 under the cover of base station 202 (whose address is “st202” in the example).
• the base station 101 in accordance with the SIP protocol, repeats its INVITE message by addressing it this time to the MCU 400, and in addition mentioning in the "AIso" field the address "mob102 @ st202" of the mobile station 102 under the cover of the base station 202.
• the MCU 400 then sends an INVITE message to the base station 202, mentioning as part of the call the mobile station 102 designated by its address "mob102 @ st202".
• the base station 202 sends as response to the MCU a validation message (code "200 OK") which is acknowledged by the MCU 400 with the aid of an acknowledgment message ACK.
• the call server 500 can also propose an identifier of the mobile stations corresponding, for example, to a temporary number acquired during registration.
• the RTP packets comprise an HD header, and a body of data PL containing the payload (“Payload” in English), that is to say the audio or video data proper.
• FIG. 6 shows the format of the header of a packet according to the RTP protocol (see RFC 1889, mentioned above).
• This header includes the following fields:
• Padding a bit P (“Padding”), which indicates, when it has the logical value 1, the presence of additional bytes at the end of the RTP packet. These additional bytes make it possible to obtain a length having certain characteristics, for example for the purposes of cryptography;
• Extension a bit X (Extension), which indicates when it has logic value 1, the presence of an extension header
• CSRC Count a CC field
• Marker which is a marking bit defined by the profile, that is to say that it can be used according to the needs of the application;
• Payload Type a field PT
• IANA Internet Assigned Numbers Authority
• sequence number the length of which is equal to 16 bits, which is initialized with a random value at the start of the transmission of an RTP packet stream by an end device, and which is incremented by one for each packet sent. This number allows the other or other end devices of the RTP session to reorder the packets or to detect missing packets in the event of loss of RTP packets during their transport through the IP network;
• Time stamp a time stamp
• the time stamp is obtained from a clock whose resolution is sufficient to allow synchronization and jitter calculation (“Jitter” in English).
• the initial value of the time stamp is determined randomly, as for the sequence number;
• Synchronization Source identifier an SSRC synchronization source identifier
• This source can be the end device which generates the RTP packet, but it can also be an intermediate device of the network called a mixing entity (or “mixer”, in English), which creates a new stream of RTP packets from RTP packets received from the sources themselves, after modifying the synchronization.
• the SSRC identifier designates the mixing entity
• variable length field containing a list of CSRC contributing source identifiers, each coded on 32 bits, and the number of which is indicated in the CC field mentioned above (there can be between 0 and 15 such codes in the listing).
• These contributing sources are the end devices which generate the payload of the RTP packet.
• the CSRC codes are inserted by the mixing equipment, from the SSRC codes of the contributing sources.
• the first twelve bytes are present in all RTP packets, while the list of CSRC identifiers is only present if it is inserted by one or more mixing entities.
• the format of the payload of an RTP packet conforms to the diagram in FIG. 7.
• the payload of the RTP packet corresponds to the following fields: - an NF field (" Number of Frames ”), coded on 2 bits, which contains a value from which the number of speech frames which are contained in the RTP packet is determined; - a bit C (“Encrypted”), which is set to logical value 1 when information relating to encryption (comprising an algorithm identifier and a key identifier, see below) is contained in the RTP packet;
• Priority field the length of which is equal to 3 bits, which indicates a priority level associated with the speech frames contained in the RTP packet;
• Source Address a source address
• the contributing source code CSRC identifies the end equipment (that is to say here the base station) which generates the RTP packet and not this user;
• Key ID an encryption key identifier
• the base stations are both equipment of the radio subsystem of the radiocommunication system (which provides the radio interface with the mobile stations), and end equipment of the transport network. 300, which send and receive RTP packets.
• the mobile stations 101, 102 and 103 are part of a group communication in half-duplex mode, established according to the SIP session initialization protocol illustrated by the diagram in FIG. 4. More particularly, suppose by example that the mobile station 101 has the right to transmit for the current alternation and is in the process of transmitting.
• the speech frames transmitted by the mobile station 101 on the radio channel are picked up by the base station 201. From there, they are transmitted to the MCU 400, through the IP network, in RTP packets.
• the MCU transmits these RTP packets to base stations 201, 202 and 203.
• These RTP packets contain the CSRC code of base station 201, which is the source selected by the MCU to control the alternation in progress.
• the base stations 202 and 203 in turn transmit them, via respective radio channels, to the mobile stations 102 and 103 respectively.
• the MCU 400 as central equipment, performs arbitration in the event of a conflict between requests for the right to transmit from different mobile stations through the corresponding base stations, and notifies the different base stations of the result of this arbitration. It must also be able to notify without delay the mobile stations in reception phase of the end of the current alternation, which corresponds to the cessation of the transmission of speech frames by the mobile station which had obtained the right to transmit for the work-study program in progress. In this way, these mobile stations in reception phase have the possibility of requesting the right to transmit; To do this, the invention proposes that an indication element, included in the RTP packets, fulfills a certain number of functions for managing the half-day course.
• the indication element may have the function, in combination with the CSRC code, of indicating to the base station selected by the MCU that the right to transmit has been granted to it.
• the indication element indeed has this function when it is present, with a first determined value, in the RTP packets transmitted by the MCU to the base stations 201, 202 and 203.
• the indication element also has the function, when it is present with a second determined value, in the RTP frames transmitted to the MCU from the base station having the right to transmit, (ie, the one whose CSRC code is indicated in the RTP packets transmitted by the MCU) to indicate to the MCU, on the one hand that said base station acknowledges receipt of the right to transmit which has been granted to it by the MCU, and on the other hand that it requests the maintenance of this right to broadcast.
• a second determined value in the RTP frames transmitted to the MCU from the base station having the right to transmit, (ie, the one whose CSRC code is indicated in the RTP packets transmitted by the MCU) to indicate to the MCU, on the one hand that said base station acknowledges receipt of the right to transmit which has been granted to it by the MCU, and on the other hand that it requests the maintenance of this right to broadcast.
• the indication element also has the function when it is present, with a third determined value, in an RTP packet transmitted by the MCU to the base stations, to indicate to the base stations that they can ask for the right to broadcast. The one of them which will be selected by the MCU, will then take control of the next alternation.
• the indication element finally has the function, when it is present with a fourth value determined in an empty RTP packet which is transmitted to the MCU from the base station having the right to transmit, to indicate to the MCU that said base station renounces its right to transmit. This occurs when the current alternation is finished, that is to say when the mobile station which had obtained the right to transmit for the current alternation, stops transmitting speech frames.
• the indication element can be a field of any length, which codes the aforementioned determined values.
• this indication element can advantageously be reduced to one bit, since it has two distinct functions when it is present in an RTP packet transmitted to the base stations from the MCU (depending on its value among said first and said third determined different values), and two distinct functions when it is present in an RTP packet transmitted from a base station to the MCU (again depending on its value among said second. and said third values determined different).
• bit M of the header of the RTP packets in relation to the fundamental operating mechanisms of the MCU as an RTP mixing entity.
• Said first value and said second value of the indication element are then, for example, the logical value 1, while said third and said determined fourth values are logical value 0.
• the flow diagram of FIG. 8 illustrates the operation of a base station as end equipment according to the invention.
• the base station 201 suppose more particularly the example of the base station 201.
• the mobile station 101 manifests its intention to transmit by appropriate signaling to the base station 201. In practice, this occurs when the mobile station user 101 presses the PTT (Push-To-Talk) button and speaks into the microphone of the mobile station.
• PTT Push-To-Talk
• the base station 201 continues to transmit the frames on the air interface. of voice received in the packets received from the MCU and the mobile station 101 remains in the reception phase. It will be noted that the priority associated with the request from the mobile station 101 can be transmitted by the above-mentioned signaling or be calculated by the base station 201 according to an ad-hoc method. In addition, the priority associated with the current alternation is indicated in the RTP packets received by the base station 201 (in the aforementioned PRIO field).
• step 302 the base station 201 deduces from this that it can grant (at least temporarily) the right to transmit to the mobile station 101 which has started to transmit speech frames.
• M the logical value 0
• the identifier CSRC is different from the identifier of the base station 201, the latter deduces therefrom, in a step 305, that it has not been selected by the MCU, that is to say that the right to transmit was not granted to it by the MCU, or, in other words, that control of the half-day was granted by the MCU to another base station.
• a step 306 it interrupts the transmission of RTP packets to the MCU and notifies the base station 101 that it has no right to transmit.
• Step 306 is equivalent to the aforementioned step 303.
• the identifier CSRC of the RTP packets transmitted by the MCU is that of the base station 201
• the latter deduces therefrom, in step 305, that it can continue the transmission to the MCU of the RTP packets containing the speech frames transmitted by the mobile station 101 on the radio channel.
• the mobile station 101 can stop transmitting speech frames on the radio channel connecting it to the base station 201, if the user releases the PTT button. This event is monitored by the base station 201 in a step 308. If the mobile station 101 continues to transmit speech frames, RTP packets containing these frames are generated by the base station 201 and transmitted to the MCU. The process continues by repeating the above-mentioned step 305. If, conversely, the mobile station stops transmitting speech frames, then, in a step 309, the base station transmits, to the MCU, the last RTP packets with the bit M set to logic value 0 These latter packets contain the last speech frames transmitted by the mobile station 101 (and timed by passing through a buffer memory of the base station).
• the bit M with the logical value 0 then has the function of indicating to the MCU that the base station 201 renounces its right to transmit. In this way, the MCU is informed of the forthcoming cessation of the transmission of RTP packets by the base station 201 even before these packets are not issued.
• the MCU can then alert the other base stations by transmitting these latter RTP packets with the bit M set to the logical value 0 to indicate that the end of the half-cycle in progress is near, and that it will soon be able to request (and, for one of them, obtain) the right to broadcast.
• the base station 201 when it has transmitted the last speech frames in RTP packets with the bit M set to zero (step 309), it transmits, in a step 310, a certain number (for example three) of RTP packets empty, that is to say without payload, and whose bit M is at logical value 0. It transmits several such packets in order to minimize the risks of non-reception by the MCU, which can occur if the network lose packets due to overloaded routers. It is recalled that empty packets are characterized by an NF field containing the value zero. These empty packets have the function of really signaling the end of the current alternation.
• the MCU receives these packets, it selects, in a step 702, one of the base stations according to an ad-hoc selection algorithm.
• this algorithm selects this source.
• the selection algorithm can involve, for example, the priority, the identity of the sender or any other criterion.
• the MCU transmits the received RTP packets to all the base stations participating in the group communication (namely, in the example, the base stations 201, 202 and 203) of the selected base station (namely, in the example, the base station 201), after having set the bit M to the logical value 1 and having placed its own identifier in the field SSRC and that of the selected source in the CSRC field (the value of the CC field of the RTP packet is then equal to
• the MCU When, in a step 711, the MCU then receives a new packet
• the MCU first checks, in a step 704, that this packet indeed comes from the selected source. For this purpose the CSRC identifier of the packet is used.
• the MCU verifies, in a step 705, if the base station requests the maintenance of its right to transmit. This is the case if the RTP packet has an M bit with the logical value 1. If so, this packet is transmitted as indicated above (return to step 703 above). If on the contrary the bit M has the logical value 0, then, in a step 706, the MCU checks whether it is an empty packet. If the packet is empty (that is to say if it does not contain any speech frame), it means that the source indicates the end of the alternation.
• the last RTP packets (those which remain in the buffer of the MCU) are transmitted as indicated above (with reference to step 703 above) but with the bit M set to the value logic 0, in order to indicate to the base stations the end of the alternation.
• the MCU then returns to the standby state 700. If, on the contrary, the packet is not empty (that is to say that it contains at least one speech frame), the RTP packet is transmitted with the bit M in logic state 1 (we return to step 703). If, contrary to the assumption made above for the test of step 704, the RTP packet received in step 711 does not come from the selected source, two cases can arise. They are examined in a step 708.
• the source of the received RTP packet is selected as the new selected source.
• the received RTP packet is then transmitted, going back to step 703 with, in the CSRC field, the identifier of the new selected source. Otherwise, the packet is purely and simply rejected, in a step 710, and the MCU waits for the reception of a new packet (return to step 711).
• step 709 when, at step 709, the MCU selects a new source while a source is already transmitting RTP packets, the test of step 305 is checked for the latter source, so that it stops transmitting to RTP packets on the IP network and stops the transmission of speech frames by the mobile station concerned.
• the technique presented above therefore makes it possible both to ensure an arbitration of the work-stay requests by the base stations, a preemption of the communication when the right to transmit is requested by a base station with a higher priority, and anticipation of the end of the current alternation, in order to prepare the next alternation as soon as the end of the current alternation is announced by the transmission by the selected base station of empty RTP packets with the bit M set to logic value 0.
• the terms “previous” and “first” used above obviously refer to the order of RTP packets as indicated by the sequence number contained in the header of RTP packets (see Figure 6) .
• the operation described by the flowcharts in FIGS. 8 and 9 may require guard times for the equipment of the IP network, namely the end equipment (base stations ) and the central equipment (MCU), are protected against connection cuts, whether these have a physical origin or come from a failure or overload of the routers.
• an MCU 801 (or master MCU) ensures the connection and arbitration of the half-cycles between sub-conferences managed by the MCU 802 and 803 (or slave MCUs), the latter performing the alternation arbitration and the connection of base stations respectively 804 to 806, and 807 to 809.
• MCU 801 master MCU
• the flowcharts of. operation of the base stations and the master MCU are identical to those presented previously with regard to FIGS. 8 and 9 respectively, while the operation of the slave MCUs 802 or 803 is in accordance with the flow diagram of FIG.
• the slave MCU 802 transmits at the start of the half-day RTP packets received from a base station such as 804 with the bit M at the logic value 0, leaving the bit M at the logic value 0 for RTP packets transmitted to the master MCU, while the M bit is set to logical value 1 for the RTP packets retransmitted to the various base stations 804 to 806 participating in the communication.
• the respective logical values of the marking bit M assigned to the different functions of this bit according to the invention can naturally be inverted.

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• Multimedia (AREA)
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• Data Exchanges In Wide-Area Networks (AREA)

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  • 2001-03-29 FR FR0104241A patent/FR2823038B1/fr not_active Expired - Fee Related
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  • 2002-03-26 EP EP02722377A patent/EP1374607A1/de not_active Withdrawn
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