JP2008104096A - Radio relay system and its message switching method - Google Patents

Abstract

<P>PROBLEM TO BE SOLVED: To reduce an overhead in message switching by a P-MP system with which a radio base station (BS) and a mobile terminal (MSS) communicate via a relay station (RS). <P>SOLUTION: When the radio base station (BS) and the mobile station (MSS), via the relay station (RS), switch a message whose unique identification (BSID) is registered at the radio base station or the relay station, the BSID registered in the message is simplified by a common rule and the BSID is also rewritten (simplified and restored) at the relay station which directly switches the message with the mobile terminal only for message switching between the radio base station and the relay station or between the relay stations. <P>COPYRIGHT: (C)2008,JPO&INPIT

Description

  The present invention relates to a radio relay system and a message exchange method for exchanging messages between a radio base station and a mobile terminal via at least one relay station, and in particular, between a radio base station and a relay station or between relay stations. The present invention relates to a wireless relay system capable of reducing the amount of messages exchanged between them and a message exchange method thereof.

  Non-Patent Document 1 defines a standard for high-speed wireless data communication (IEEE802.16). In the physical layer of IEEE802.16-2004, single carrier, OFDM (Orthogonal Frequency Division Multiplexing) and OFDMA (Orthogonal Frequency Division Multiple Access) are supported as modulation schemes.

  On the other hand, while Non-Patent Document 1 described above is intended for a stationary SS (subscriber terminal), Non-Patent Document 2 uses the same physical layer and MAC layer as Non-Patent Document 1 By adding several new messages, the standard for high-speed wireless data communication for mobile SS (MSS; Mobile SS) is defined, which is called IEEE802.16e (hereinafter referred to as 802.16e). Hereinafter, IEEE802.16-2004 and IEEE802.16e are generically expressed as IEEE802.16.

  In this IEEE802.16, P-MP (point-to-multipoint) is defined as one of the communication modes. In this P-MP, BS (wireless base station) is used for all MSS (subscriber terminals). Scheduling of transmission and reception opportunities enables efficient communication with guaranteed QoS (Quality of Service).

  Each BS is given a unique identifier of 48 bits called Base Station ID (BSID), and MSS identifies each BS by BSID. The upper 24 bits of the BSID identify the business operator, and each BS is identified by the lower 24 bits within the same business operator.

  FIG. 7 is a diagram showing a frame structure of a MAC (Media Access Control) header defined by IEEE 802.16. A connection ID for uniquely identifying a connection is given to each connection, and a connection for identifying a terminal ID is given to the terminal. A space for this connection ID is prepared in the MAC header of IEEE802.16, and BS and MSS identify the packet by this connection ID.

  The MSS performs a process called ranging in order to adjust the transmission / reception timing and transmission power with the BS when joining the network and periodically after joining. When joining the network, the MSS performs ranging by sending an RNG-REQ (ranging request) message to the BS. If the BS that has received the RNG-REQ needs to adjust the MSS, information such as transmission output, frequency, and transmission timing is sent to the RNG-RSP with a ranging continuation notification (Ranging Status = continue in the RNG-RSP). (Ranging response) Register to the message and respond to the MSS.

  The MSS that has received the RNG-RSP in the ranging continuation notification adjusts the transmission output, frequency, and transmission timing, and then transmits the RNG-REQ to the BS again. When the transmission output of the MSS satisfies a preset value, the BS returns an RNG-RSP with a ranging success notification (Ranging Status = success in the RNG-RSP) to the MSS. Such ranging ends when an RNG-RSP accompanied by a ranging success notification is received by the MSS.

  In IEEE802.16, the current BS (serving BS) broadcasts information (frequency, etc.) of neighboring BSs (neighboring BSs) as neighboring BS advertisement messages, so that MSS can easily scan these neighboring BSs. Is possible. The BS can suck up the MSS scan results (scan report including BSID and CINR (Carrier-to-Interference-Noise Ratio) of neighboring BSs) and can instruct handover to the optimum BS.

  Hereinafter, in the network topology shown in FIG. 8, the procedure when the MSS 2 communicating with the BS 3 hands over to the BS 4 according to the IEEE 802.16 standard will be described along the sequence flow of FIG.

  Procedure 1: MSS2 periodically receives neighboring BS information from BS3, which is a serving BS.

  Procedure 2: MSS2 scans adjacent BSs as needed or as directed by BS3.

  Procedure 3: BS3 determines the necessity of MSS2 handover (HO). Here, a case where it is determined that a handover from BS3 to BS4 is necessary will be described as an example.

  Procedure 4: BS3 sends HO advance notification message to BS4. This message includes the identifier of the MSS2 requesting handover, the parameters of the connection held by the MSS2, the bandwidth and QoS parameters required for the MSS2.

  Procedure 5: BS4 that has received the HO advance notification message returns to BS3 with an HO advance notification response message. This message includes information such as whether QoS necessary for MSS2 can be supported.

  Procedure 6: BS3 that has received the HO advance notification response message transmits an HO confirmation message for handover to BS4 and a BSHO request message to MSS2. FIG. 10 is a diagram showing an example of the format of this BSHO request message, in which a “recommended BS number” field in which the number of BSs that can be recommended as a handover destination is registered, and a 48-bit BSID that identifies the recommended BS are registered. A number of fields including “adjacent BSID field” are reserved. In the illustrated example, since the recommended number of BSs is “2”, two “adjacent BSID fields” (BSID1, BSID2) are reserved.

  Procedure 7: The MSS 2 that has received the BSHO request message transmits an HO indication message to the BS 3 to start handover. This message includes a handover start time and the like.

Procedure 8: The MSS 2 completes the handover by executing a high-speed ranging process with the BS 4 as the handover destination.
IEEE Standard for Local and metropolitan area networks-Part 16: Air Interface for Fixed Broadband Wireless Access Systems, IEEE 802.16 -2004, Oct, 2004. Draft IEEE Standard for Local and metropolitan area networks-Part 16: Air Interface for Fixed and Mobile Broadband Wireless Access Systems, IEEE P802.16e / D5a, Dec. 02, 2004.

  In wireless data communication, there is a dead zone where the MSS cannot communicate with the BS even though the MSS is in the service area due to the influence of radio wave attenuation caused by shielding by obstacles located between the transmitting and receiving antennas. In order to perform high-speed wireless data communication, it is necessary to use a high frequency. However, since the wavelength is short and the straightness of radio waves increases, it is expected that the dead zone will increase. As a method for eliminating such a dead zone, a method is considered in which a relay station (Relay Station) is installed between the BS and the MSS, and the BS and the MSS communicate via the RS.

  However, in the wireless relay system, data addressed to the MSS and data from the MSS are wirelessly relayed by the RS, so the data between the BS and the RS or between the RS and the RS becomes overhead compared to when the BS and the MSS are directly connected.

  An object of the present invention is to solve the above-described problems of the prior art, in a P-MP wireless relay system in which a wireless base station (BS) and a mobile terminal (MSS) communicate via a relay station (RS). It is an object of the present invention to provide a radio relay system and a message exchanging method thereof in which messages exchanged between RS / RS and between RS / RS can be shortened, and overhead associated with relay can be kept low.

  In order to achieve the above object, the present invention provides a wireless base station that is directly connected to a mobile terminal or indirectly through at least one relay station, and an identifier BSID is uniquely assigned to the wireless base station and each relay station. In a message exchange method in a P-MP wireless relay system in which a wireless base station and a mobile terminal exchange messages registered with the BSID, the wireless base station and each relay station use the BSID based on a predetermined rule. Whether the base station has a common BSID conversion function that reversibly converts to a simple BSID with a short bit length and restores it, and whether the mobile terminal that is the destination of the message is connected to the mobile station via its own relay station And a procedure for rewriting the BSID of the message to a simple BSID and transmitting the message to the relay station when the mobile terminal is connected via the relay station. If the destination of the sage is a mobile terminal connected to its own station, the simple BSID of the message is restored to the BSID and sent to the mobile terminal, and the BSID of the message received from the mobile terminal is rewritten to the simple BSID and relayed And a procedure for performing.

  According to the present invention, when a radio base station and a mobile terminal exchange messages registered with an identifier (BSID) unique to a radio base station or a relay station via a relay station, the radio base station and the relay station Only for messages exchanged with and exchanged between relay stations, the BSID registered in the message is simplified by common rules, while the relay stations exchange messages directly with mobile terminals. Since BSID rewriting (simplification and restoration) is performed at the mobile terminal, message exchange is performed without causing the mobile terminal to recognize the BSID rewriting, in other words, without changing the mobile terminal. The overhead can be reduced.

  DESCRIPTION OF THE PREFERRED EMBODIMENTS Hereinafter, the best embodiment of the present invention will be described in detail with reference to the drawings. FIG. 1 is a diagram showing an example of a network configuration of the P-MP system to which the present invention is applied. Within a radio wave reach of a radio base station (BS1), two relay stations (RS1-1, RS1- 2) is located, and the relay station (RS2-1) is located within the radio wave reach of the radio base station (BS2).

  FIG. 2 is a diagram showing the configuration of the BSID assigned to the BS and each RS in the present embodiment, and the upper 24 bits of the BSID expressed by all 48 bits is an operator ID, and this operator ID is excluded. Of the remaining 24 bits, the domain is represented by the upper 21 bits, and BSs or RSs having the same upper 21 bits are defined as “in the same BS domain” in which a plurality of RSs are connected in a tree shape with the BS as a vertex. Is done. The lower 3 bits of the BSID assigned to the BS are all “0”, and the lower 3 bits of the BSID assigned to each RS are other than that. BSID assignment to each RS is performed automatically by the BS or manually by the operator.

  A BS (for example, BS1 in FIG. 1) whose lower 24 bits excluding the operator ID of BSID is [0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 1 0 0 0] For the first RS connected to the station (for example, RS1-1 in FIG. 1), BSID [0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 whose address value is incremented by “1”. 0 0 1 0 0 1] is assigned, and this address is stored as an assigned address in a table or the like. Next, when the RS (for example, RS1-2 in FIG. 1) is connected, the table is referred to, and the latest assigned address value is further incremented by “1” BSID [0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 1 0 1 0] is assigned.

  When the BS transmits a MAC management message including the BSID to the mobile terminal (MSS) under the RS, a MAC management message including only the lower 3 bits of the BSID as an identifier (simple BSID) is created. However, such rewriting of the BSID is permitted only when the simplified BSID can be restored to the original BSID by each subordinate RS, that is, when the BS can reversibly rewrite the BSID. In this embodiment, when the upper 45 bits of the BSID registered in the message matches the upper 45 bits of the BSID assigned to the local station, it is determined that the BS can reversibly rewrite the BSID. The

  An example of a BSHO request message that is one of the MAC management messages is shown in FIG. Compared to the conventional BSHO request message described with reference to FIG. 10, it can be seen that the message size is significantly reduced as the number of BSID bits is reduced from 48 bits to 3 bits.

  The RS that has received the BSHO request message transfers the message as it is if the transfer destination is another RS, and if the transfer destination is the MSS, 45 bits including 24 bits of the operator ID and 21 bits indicating the BS domain are added. Is added before 3 bits of the simple BSID to restore the original 48-bit BSID, and is then transmitted to the MSS. Therefore, the BSHO request message transmitted from the RS to the MSS is as shown in FIG.

  On the other hand, the RS that has received the MAC management message including the BSID from the MSS first compares the operator ID of the BSID registered in this message and the 21 bits representing the BS domain with that of its own BSID. If they are identical, it is determined that this BSID can be reversibly rewritten into a simple BSID. Then, 24 bits corresponding to the operator ID and 21 bits representing the BS domain are deleted, and only the 3-bit simple BSID is registered and transferred in the BSID field. On the other hand, if they are not identical, the message is transferred as it is, that is, with the 48-bit BSID registered.

  In addition, the BS that has received the MAC management message in which the BSID is represented by the simple BSID with 3 bits, 45 bits including the operator ID 24 bits and the BS domain 21 bits are added to the 3-bit simple BSID in the BSID field. To restore the original 48-bit BSID. Then, this is replaced with the BSID field of the MAC management message and transferred to a predetermined entity.

  Next, in the network topology shown in FIG. 1, a message exchange procedure when MSS2 communicating with RS1-1 hands over to RS1-2 will be described along the sequence flow of FIG.

  Procedure 1: MSS2 periodically receives a neighbor BS advertisement message from the connected RS1-1.

  Procedure 2: MSS2 scans neighboring BSs and neighboring RSs as needed or in response to instructions from the BS. Here, the RS 1-1 and BS 1 acquire information on the neighboring BS and neighboring RS by receiving a scan report including the neighboring BS and the BSID and CINR of the neighboring RS from the MSS 2.

  Procedure 3: BS1 determines the necessity of MSS1 handover. Here, the description will be continued assuming that a handover from RS1-1 to RS1-2 is necessary.

  Step 4: BS1 determines whether the requested bandwidth and QoS of MSS2 can be supported by the handover destination BS (that is, itself) or the RS, and if handover is possible, the MSS identifier etc. is transferred to the handover destination RS1-2. Send HO confirmation message including. When the handover destination is the BS itself, this HO confirmation message need not be transmitted. The BS also sends a BSHO request message to MSS2. In this message, only the lower 3 bits are registered as the simple BSID as the BSID of the handover destination (RS1-2).

  Procedure 5: RS1-1 that received the BSHO request message has MSS2 as the forwarding destination of this message. Therefore, the upper 45 bits of its own BSID, that is, the 24 bits of the operator ID, are added before the 3-bit simple BSID in the message. 21 bits representing the BS domain are added to restore the 48-bit BSID and register it in the message.

  Procedure 6: RS1-1 sends a BSHO request message with BSID restored to 48 bits to MSS2.

  Procedure 7: The MSS2 that has received the BSHO request message transmits an HO indication message including a handover start time to the BS1 via RS1-1. The BSID registered in this message is 48 bits.

  Procedure 8: RS1-1 that has received the HO indication message compares the operator ID and BS domain ID of its BSID with those of its own BSID. Since the two match here, RS1-1 deletes the upper 45 bits of the BSID and updates and registers only the lower 3 bits as a simple BSID in the message.

  Procedure 9: Send HO indication message with BSID 3 bits to BS1.

  Step 10: The BS1 that has received the HO indication message adds the upper 45 bits of its own BSID before the 3-bit simple BSID, that is, the 24-bit operator ID and the 21-bit indicating the BS domain, and then the 48-bit BSID. And send it to a given entity.

  Procedure 11: MSS2 executes high-speed ranging processing with the handover destination RS1-2, and the handover is completed.

  5 and 6 are diagrams schematically showing a BSID allocation method for each RS when data communication between the BS and the MSS is performed via a plurality of RSs. Of the 48-bit BSID, the upper 18 bits of the remaining 24 bits excluding the upper 24 bits of the operator ID are used for identifying the BS domain, and the lower 6 bits are used for identifying the RS. The lower 6 bits of the BSID assigned to the BS are all “0”.

  The BSID assigned to the RS located in the first hop from the BS is that all the lower 3 bits of the lower 6 bits are all “0”, and only the upper 3 bits are unique values according to the connection order to the BS. Become. In the BSID assigned to the RS located at the second hop from the BS, the upper 3 bits of the lower 6 bits are the same as the upper 3 bits of the lower 6 bits of the BSID of the RS of the first hop in the BS direction. Allocation of BSID to these RSs is also performed automatically by BS.

  For example, when considering a BS in which the remaining 24 bits excluding the BSID operator ID are [0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 1 0 0 0 0 0 0] (for example, BS1 in FIG. 5). ), BSID [0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 1 0 0 1 0 0 0] is assigned to the RS first connected to BS1 (for example, RS1-1 in FIG. 5). BSID [0 0 0 00 0 0 0 0 0 0 0 assigned to the next connected RS (for example, RS1-2 in FIG. 5) is obtained by incrementing the upper 3 bits of the lower 6 bits by “1”. 0 0 0 0 1 0 1 0 0 0 0] is automatically assigned.

  Also, in the RS connected to the RS (for example, RS1-1-1 connected to RS1-1 in FIG. 5), the upper 3 bits of the lower 6 bits are the same as the connected RS (RS1-1). BSID [0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 1 0 0 1 0 0 1] in which the lower 3 bits are incremented by “1” is assigned. Similarly, BSID [0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 1 0 1 0 0 0 1] is assigned to RS1-2-1 connected to RS1-2 in FIG.

  In this way, in this embodiment, the hierarchical connection state between the BS and each RS is expressed hierarchically as part of the BSID, so if the BSID is included in the BS / RS message, the common part is reduced. By doing so, the message size can be greatly reduced. In addition, referring to the BSID of each RS, the positional relationship between the BS and each RS can be easily recognized. Furthermore, even when the BS designates the MSS handover destination, it is possible to preferentially select an RS closer to the BS or the BS itself as a connection destination by referring to each candidate BSID.

  In the above-described embodiment, among the 48-bit BSIDs, the remaining 24 bits, excluding the operator ID, are used for the upper 18 bits for BS domain identification, and the lower 6 bits are used for relay station (RS) identification. Although described above, the present invention is not limited to this, and the number of bits allocated to the identification of the BS domain and the relay station may be appropriately changed according to the scale of the network, the number of accommodated terminals, and the like. Of the 48-bit BSID, the remaining 24 bits, excluding the operator ID, may be used for identifying the RS, and the lower (24-N) bits may be used for identifying the BS domain.

  The present invention has been described above with reference to preferred embodiments. However, the present invention is not necessarily limited to the above-described embodiments, and various changes can be made without departing from the concept of the present invention.

  FIG. 11 is a block diagram showing a configuration of a main part of a message exchange unit to which the present invention is applied with respect to the BS and each RS. Here, illustrations of components unnecessary for the description of the present invention are omitted. Yes.

  The message exchange unit 101 exchanges messages between the backbone and the accommodating station (RS or MSS) in the case of BS, and between the accommodating stations in the case of RS. The BSID extraction unit 102 extracts the BSID or simple BSID registered in the message to be exchanged. The BSID conversion unit 103 includes a rewriting unit 103a that rewrites a BSID to a simple BSID and a restoration unit 103b that restores the simple BSID to a BSID. The BSID memory 104 stores a BSID (48 bits) assigned to the own station. The conversion control unit 105 permits or restricts rewriting of the BSID by the BSID conversion unit 103 under a predetermined condition.

  In such a configuration, the conversion control unit 105 compares the BSID extracted from the message with the BSID of the local station stored in the BSID storage unit 104, and the upper 45 bits of both, that is, the operator ID of 24 If the bit matches 21 bits representing the BS domain, the extracted BSID is passed to the BSID conversion unit 103 under a predetermined condition to request rewriting.

  At this time, if it is a BS, if the message destination terminal (MSS) is indirectly connected to the local station via the RS, the BSID is rewritten to a simple BSID on the condition that the BSID matches, while the MSS and the self If the station is directly connected, the BSID cannot be rewritten. On the other hand, in the case of RS, the BSID is set on the condition that the upper 45 bits of the BSID registered in the message received from the MSS directly connected to the own station matches the upper 45 bits of the BSID of the own station. Rewritten to simple BSID.

  When a simple BSID is extracted from the message to be exchanged by the BSID extraction unit 102, the conversion control unit 105 extracts the upper 45 bits of its own BSID stored in the BSID storage unit 104. The original BSID is restored by concatenating to the simplified BSID under the specified conditions.

  If it is BS at this time, the simple BSID of the message received from RS will be restored to BSID. If it is RS, the simple BSID of the message addressed to the MSS directly connected to the local station is restored to the BSID and transferred to the MSS.

1 is a diagram showing a configuration of a P-MP network to which a handover system according to the present invention is applied. FIG. It is the figure which expressed typically the structure of BSID allocated to BS and each RS. It is the figure which showed the format of the BSHO request message in which BSID is simply expressed by 3 bits. 5 is a sequence flow showing an example of a handover procedure according to the present invention. It is the figure which showed typically the allocation method of BSID to each RS in case the data communication between BS and MSS is performed via several RS. It is the figure which showed typically the allocation method of BSID to each RS in case the data communication between BS and MSS is performed via several RS. FIG. 3 is a diagram illustrating a frame structure of a MAC header defined by IEEE 802.16. It is the figure which showed an example of the network topology to which IEEE802.16e is applied. It is the sequence flow which showed the handover procedure by IEEE802.16e. It is the figure which showed the format of the BSHO request message in which BSID is represented by 48 bits. It is the block diagram which showed the structure of the message exchange part of BS and RS to which this invention is applied.

Explanation of symbols

BS ... Radio base station
RS ... Relay station
SS: Subscriber terminal
MSS ... mobile terminal

Claims (10)

The radio base station is directly connected to the mobile terminal or indirectly through at least one relay station, and an identifier BSID is uniquely assigned to the radio base station and each relay station. The radio base station and the mobile terminal In a message exchange method in a P-MP wireless relay system for exchanging registered messages,
The radio base station and each relay station has a BSID conversion function that reversibly rewrites and restores a BSID to a simple BSID with a short bit length based on a predetermined rule,
The radio base station is
A procedure for determining whether or not the mobile terminal that is the destination of the message is connected to the mobile station via a relay station;
When the mobile terminal is connected via a relay station, the message BSID is rewritten into a simple BSID and transmitted to the relay station,
Each of the relay stations
If the destination of the message is a mobile terminal connected to the local station, the procedure to restore the simple BSID of the message to the BSID and send it to the mobile terminal,
A method for exchanging messages in a wireless relay system, comprising a step of rewriting and relaying a BSID of a message received from a mobile terminal to a simple BSID.   2. The message exchange method for a radio relay system according to claim 1, further comprising a procedure for the radio base station to restore a simple BSID of a message received from the relay station to a BSID. The radio base station further comprises:
Including a procedure for determining whether the BSID registered in the message to be transmitted can be reversibly converted by the BSID conversion function;
3. If conversion is possible, the BSID registered in the message is rewritten to a simple BSID and transmitted. If conversion is impossible, the BSID is registered in the message and transmitted. Message exchange method for wireless relay system. The relay station further comprises:
Including a procedure for determining whether the BSID registered in the message to be relayed can be reversibly converted by the BSID conversion function,
4. If conversion is possible, the BSID registered in the message is rewritten and relayed to a simple BSID, and if conversion is impossible, the message is relayed as it is. Message exchange method for wireless relay system. The BSID is composed of a bit string of N bits, and the value of the M (<N) bit at the predetermined position is common to each station in the same domain where a plurality of relay stations are connected in a tree shape with the radio base station as a vertex. And the value of the remaining (N−M) bit part, with the M bit part omitted from N bits, is unique to each station,
The BSID conversion function generates a simple BSID by omitting an M bit part of N bits of a BSID, and restores the BSID by adding the M bit part to the simple BSID. 5. A message exchange method for a wireless relay system according to any one of 1 to 4.   The radio base station and each relay station determine that the BSID of the message can be reversibly converted if the M bit part of the BSID registered in the message matches the M bit part of the BSID of the local station. The message exchange method for a wireless relay system according to claim 5. The radio base station further comprises:
Including a procedure to assign a BSID to each relay station,
6. The message exchange method for a radio relay system according to claim 5, wherein the radio base station assigns a value specific to the number of hops from the own station to each relay station to the bit string of (N−M) bits. . The radio base station is directly connected to the mobile terminal or indirectly via at least one relay station, and an identifier BSID is uniquely assigned to the radio base station and each relay station. The radio base station and the mobile terminal In a P-MP wireless relay system that exchanges registered messages,
The radio base station and each relay station are
Means for exchanging messages;
Means to extract BSID or simple BSID from messages to be exchanged;
Storage means for storing the BSID assigned to the own station;
A BSID conversion means for reversibly converting and restoring a BSID to a simple BSID with a short bit length based on a predetermined rule,
The wireless relay system, wherein the BSID conversion means rewrites the BSID extracted from the message into a simple BSID, and restores the simple BSID extracted from the message to the original BSID based on the BSID of the local station. The radio base station further comprises:
Means for determining whether the BSID registered in the message to be transmitted can be reversibly converted by the BSID conversion function;
9. The wireless communication according to claim 8, wherein if conversion is possible, the BSID registered in the message is rewritten to a simple BSID and transmitted, and if conversion is impossible, the BSID is registered in the message and transmitted. Relay system. The relay station further comprises:
Means for determining whether the BSID registered in the message to be relayed can be reversibly converted by the BSID conversion function;
The wireless relay system according to claim 8 or 9, wherein if the conversion is possible, the BSID registered in the message is rewritten and relayed to a simple BSID, and if the conversion is impossible, the message is relayed as it is. .

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