METHOD FOR DATA TRANSMISSION TO SLAVE EXCHANGES The present invention relates to a method of a data transmission to several slave exchanges in a telecommunication system, in which method the data is transmitted to the slave exchanges and an acknowledgement is ex- pected from the slave exchanges of a data reception. The method also relates to a telecommunication system comprising a master exchange and slave ex- changes having a data transmission connection with the master exchange, and in which system the master exchange comprises transmitter means for transmitting data to the slave exchanges and receiver means for receiving ac- knowledgement messages from the slave exchanges, the slave exchanges comprise receiver means for receiving data transmitted by the master ex- change and transmitter means for transmitting the acknowledgement mes- sages to the master exchange in response to the data reception.

The invention relates particularly to the data transmission between exchanges of mobile communication systems, although the invention can also be applied in other connections. In the following, the invention is described by way of example with reference to the mobile communication systems in par- ticular.

In mobile communication systems where several exchanges offer services to the same subscriber stations there is the need to transmit for ex- ample subscriber station information from one exchange to the other. Such a need for transmitting subscriber station information occurs for example in situations where the operator, or, alternatively, the subscriber himself changes settings regarding his subscriber station in the system. Thus, it must be possi- ble to transmit said setting change to all the exchanges maintaining data con- cerning the settings of said subscriber station. If the data transmission to all the exchanges requiring the information fails, there is a risk of mutually incon- sistent data concerning individual subscriber stations in databases in different exchanges.

In a previously known solution, the master exchange is arranged to transmit the changed data to all the slave exchanges requiring said data by utilizing a telecommunication connection between the master exchange and each slave exchange. Thus, the slave exchanges update the data in their da- tabases to correspond with the new data received from the master exchange and acknowledge the reception of the new data to the master exchange. The most significant disadvantage of the known solution is that if the data trans-

mission connection between the master exchange and one of the slave ex- changes is temporarily cut off, the updated data cannot then be transmitted to said slave exchange. Thus, the data in said slave exchange is inconsistent in regard to the data in other slave exchanges, and thus, different parameters may be stored for said subscriber station in different exchanges.

The object of the present invention is to solve the above mentioned problem and provide a method which is more efficient and reliable than previ- ous methods for the data transmission to the slave exchanges. The object is achieved by the method of the invention characterized by commanding, in case one of the slave exchanges does not acknowledge the data reception, at least one slave exchange from which an acknowledgement is received to transmit said data to the slave exchange from which no acknowledgement is received if said at least one slave exchange has a connection with the slave exchange from which no acknowledgement is received.

The invention is based on the idea that when a telecommunication connection to one of the slave exchanges is cut off, other slave exchanges can be used as alternative distribution routes by transmitting data through some other exchange to the slave exchange from which no acknowledgement is received. The method of the invention can be applied without a prior knowl- edge about the manner in which the exchanges are connected with each other. Thus, the most significant advantages of the invention are that the data to the slave exchanges is delivered more reliably than before and that the use of the other slave exchanges as alternative distribution routes does not require a prior knowledge about the exchanges which are connected with each other in the system, since in accordance with the invention, all the slave exchanges can be commanded for example by turns to transmit the data to the slave ex- change from which no acknowledgement is received until one of the slave ex- changes finally succeeds in the data transmission. This enables the utilization of the slave exchanges in a very flexible manner, since for example when new exchanges are added to the system, the alternative distribution routes need not to be redefined.

The invention also relates to a system wherein the method of the invention can be applied. A telecommunication system of the invention is characterized in that the transmitter means of the master exchange are ar- ranged to send a predetermined control message to at least one slave ex- change having a data transmission connection with the master exchange if the

master exchange does not receive an acknowledgement message from one of the slave exchanges to which it has transmitted the data, and that the system comprises at least one slave exchange being responsible to the control mes- sage received from the master exchange for transmitting the data received from the master exchange to said slave exchange from which the master ex- change has not received the acknowledgement message if said at least one slave exchange has a telecommunication connection with the slave exchange from which the master exchange has not received the acknowledgement mes- sage.

The preferred embodiments of the method and telecommunication system are disclosed in the dependent claims 2 to 4 and 6 to 10.

In the following, the invention will be described in more detail by means of a preferred embodiment of the invention with reference to the ac- companying figures, of which Figure 1 shows a flow diagram of a first preferred embodiment of the invention, and Figure 2 shows a block diagram of a first preferred embodiment of the telecommunication system of the invention.

Figure 1 shows a flow diagram of a first preferred embodiment of the method of the invention.

In block A in Figure 1, updated data is transmitted to all the slave exchanges who require said data. A timer is switched on at the same time.

In block B, acknowledgements of the received data are received from slave exchanges.

In block C it is checked if the timer shows the expiration of the time reserved for the reception of the acknowledgements, i. e. if the time limit Tref is exceeded.

If the time reserved for the reception of the acknowledgements is exceeded, the routine proceeds to block D where it is checked if the acknow- ledgement is received of the data reception from all the slave exchanges to which the data was sent. If the acknowledgement is received from all the slave exchanges, the data transmission has been successful, and the routine pro- ceeds to block K.

In contrast, if there is no acknowledgement from one of the slave exchanges, the routine proceeds through block E to block F where the data is

retransmitted to said slave exchange (or slave exchanges) from which no ac- knowledgement is received.

When the data has been retransmitted, the routine proceeds through block G to block H where one of the slave exchanges to which a data transmission connection exists, i. e. exchange n, is commanded to transmit the data to the slave exchange from which no acknowledgement is received. In accordance with the invention, it is not necessary to know in advance whether the slave exchange being commanded to transmit the data has a telecommu- nication connection with the slave exchange from which no acknowledgement is received. If no connection exists, the data transmission naturally fails, i. e. the acknowledgement is still not received.

In block I it is checked if the acknowledgement is received from the slave exchange from which no acknowledgement was received. If so, the rou- tine proceeds to block K. If there is still no acknowledgement, the routine pro- ceeds to block J where it is checked if all the slave exchanges to which a con- nection exists are commanded to transmit the data to the slave exchange from which no acknowledgement is received. If no, the routine returns to block F where the data is retransmitted after which the slave exchange next in turn to which a connection exists is commanded to transmit the data to the slave ex- change from which no acknowledgement is received.

When all the slave exchanges to which a connection exists have been commanded once to transmit the data to the slave exchange from which no acknowledgement is received, the process is repeated, i. e. the first slave exchange is commanded to retransmit the data to the slave exchange from which no acknowledgement is received. According to the flow diagram in Fig- ure 1, the process is repeated until the acknowledgement is received from all the slave exchanges. Alternatively, the data transmission can be attempted for a given period of time and if the data transmission fails, an alarm is given to the system operator.

In the flow diagram in Figure 1 it is assumed that the slave ex- changes are commanded to transmit the data by turns to the slave exchange from which no acknowledgement is received and, at the same time, an attempt is made to transmit the data directly between each command. This is naturally only one alternative to apply the method of the invention. Alternatively, for ex- ample an attempt can be made to transmit the data first directly for a prede- termined number of times to the exchange from which no acknowledgement is

received, after which all the exchanges to which a connection exists are com- manded to transmit the data substantially simultaneously to the slave ex- change from which no acknowledgement is received.

Figure 2 shows a block diagram of a preferred embodiment of the telecommunication system of the invention. The part of the telecommunication system presented in Figure 2 can be for example a part of the TETRA network (Trans-European Trunked Radio system), although the invention can also be applied in other connections.

A master exchange DXT4 in Figure 2 has a data transmission con- nection with slave exchanges DXT1 to DXT3 through data transmission con- nections L1 to L3. The master exchange DXT4 and the slave exchanges DXT1 to DXT3 can be totally identical TETRA network exchanges.

Databases DB in the exchanges DXT1 to DXT4 maintain data nec- essary for the operation of the system, such as group information. The group information contains information regarding one or several groups of subscrib- ers who, for example, often need to communicate with each other. Said sub- scribers have been defined as members of the same group in the system.

Thus, it is very easy to make for example group calls, i. e. calls that all the group members can hear. The group information for a single group can in- clude for example: -group identity -a list of group members (for example a list of subscriber identities), and -group area specification, i. e. information indicating the cells in the area of which the network supports the operation of the group.

The group information is always stored in the master exchange (i. e. in the home mobile telephone exchange of the group), and in addition, the in- formation is copied to all the slave exchanges containing even a single cell in their area where the net supports the operation of the group.

In the case in Figure 2, the group information, i. e. the data, can be updated either in the master exchange DXT4 or in the slave exchanges DXT1 to DXT3 transmitting the updating to the master exchange DXT4 which, in turn, transmits the updating onward to other exchanges requiring said data. in connection with the data updating, a transceiver unit 1 is ar- ranged to transmit the data through the data transmission connections L1 to L3 to the slave exchanges DXT1 to DXT3 in the case in Figure 2. The trans- ceivers 1 in the slave exchanges receive the data and store it in the databases

DB and acknowledge the data reception by an acknowledgement message to be sent to the master exchange.

If for example a telecommunication connection L3 is cut off in the system shown in Figure 2 when the master exchange DXT4 transmits the data to the slave exchanges DXT1 to DXT3 shown in Figure 2, the slave exchange DXT3 does not receive the data addressed to it. The transceiver unit 1 in the master exchange detects this by not receiving the acknowledgement from the exchange DXT3 within a predetermined time. The transceiver unit 1 in the master exchange then repeats the data transmission to the slave exchange DXT3, and in addition, the transceiver unit 1 in the master exchange sends a predetermined control message to the slave exchange DXT1 commanding by the control message the slave exchange DXT1 to transmit the data received from the master exchange to the slave exchange DXT3. Depending on the case, the data the slave exchange DXT1 must transmit to the slave exchange DXT3 can be included in the control message, or the slave exchange DXT1 can retrieve said data from its database DB on the basis of the control mes- sage.

Figure 2 shows that there is not a direct telecommunication connec- tion between the slave exchange DXT1 and the slave exchange DXT3, whereby the slave exchange DXT1 cannot transmit the data received from the master exchange to the slave exchange DXT3. In accordance with the inven- tion, the master exchange does not need information about the way the slave exchanges are connected with each other, but it systematically goes through all the slave exchanges with which it has a telecommunication connection, and commands them to transmit the data to that slave exchange from which it has not received the acknowledgement. In the above described case, the slave exchange DXT1 does not have a telecommunication connection with the slave exchange DXT3, and, thus, the data transmission fails. After a predetermined time from the transmission of the control message to the exchange DXT1, the master exchange detects that the acknowledgement from the slave exchange DXT3 is still not received. Then, the transceiver unit 1 in the master exchange DXT4 repeats the data transmission to the slave exchange DXT3, and in addi- tion, the transceiver unit 1 in the master exchange sends the control message to the slave exchange DXT2 commanding by the control message the slave exchange DXT2 to transmit the data received from the master exchange to the slave exchange DXT3. Since the slave exchange DXT2 has a telecommunica-

tion connection L4 with the slave exchange DXT3 the data transmission to the slave exchange DXT3 is this time successful.

When a transceiver unit 1 in the slave exchange DXT3 has received the transmitted data it transmits the acknowledgement message by using the same route through which it received the data, i. e. through the slave exchange DXT2 to the master exchange DXT4. When the master exchange has re- ceived said acknowledgement message it stops data transmission attempts to the slave exchange DXT3.

It is obvious that the above description and the related figures are only intended to illustrate the present invention. Telecommunication systems also comprise other information besides the above mentioned subscriber in- formation and group information presented by way of example, and to the transmission of which the present invention can be applied. A variety of modi- fications and variations will be apparent to those skilled in the art without devi- ating from the scope and spirit of the invention disclosed in the appended claims.