in dvb - h mode , the different multiplexed services ( s 1 , s 2 , s 3 , s 4 ) of a digital data stream are broadcasted by periodic bursts ( s i ). for above mentioned energy saving reasons , the mobile equipment is only activated at the time of the reception of one or several services . for example , in fig1 , the equipment is only active during the duration ( l 1 ) of a service ( s 1 ) which is received during time periods ( p 1 ), whereas it will be on stand - by mode during the intervals separating the bursts ( s i ) of the service ( s 1 ). the services multiplexing can form again a continuous and uninterrupted stream transmitted with a constant throughput when additional service bursts ( s i ) are transmitted during the interruption periods . fig2 shows an example where the intervals between s 4 and s 1 of fig1 are filled with supplementary services s 5 and s 6 . this type of stream is received in an intermittent way by the mobile equipment that synchronizes to the bursts ( s i ) of one or several selected services and switches to stand - by mode when other services are transmitted . the ratio between the interruption period of the bursts ( s i ) and the activity period of the mobile equipment during reception of selected services gives a measure of the energy saving of the dvb - h mode . the duration ( l ) of a burst is approximately some hundred of milliseconds while the intermediary interruption period ( p ) can last for a few seconds . the ratio between these durations can thus reach more than 90 % depending on the configuration of the bursts ( s i ) of the broadcasted signal and on the number of selected services . the method of the invention illustrated in fig4 showing that a control message ecm is inserted into each burst of a service . according to one preferred embodiment , the control message is placed at the start of the burst following the identifier id and before the encrypted or useful content ( c 1 ) of the burst . fig3 shows an enlarged view of a burst of the service s 1 containing a control message ecm 1 after the identifier id 1 of the burst . this identifier id 1 is used by the mobile equipment to synchronize on the selected service and to determine the activation and stand - by periods . the control message ecm 1 contains at least one control word allowing the decryption of the data of the content ( c 1 ) of the service s 1 to which it is associated . in a stream of bursts ( s i ) of several multiplexed services such as schematized in fig4 , the control messages , respectively the control words that are included in there can change from one burst of the same service to another or of a group of bursts to another . for example , for the service s 1 , the control message ecm 1 a remains constant during three bursts in order to change for the further burst ( ecm 1 b ). it is also possible to change the control word for each burst as is the case for the service s 2 of the example where the control messages ( ecm 2 a , ecm 2 b , ecm 2 c . . . ) inserted at the start of the bursts ( s i ) are never identical . the schematic of fig6 shows a data stream ( fd ) comprising bursts ( s i ) of two services ( s 1 , s 2 ) and a stream ( fm ) of ecm control messages associated to the services ( s 1 , s 2 ) of the stream ( fd ). the control messages ( ecm 1 a , ecm 1 b , ecm 1 c , . . . ) and ( ecm 2 a , ecm 2 b , ecm 2 c , . . . ) used to decrypt respectively the encrypted data of the services ( s 1 ) and ( s 2 ) are stored in a memory ( m ), than extracted in order to be inserted into the corresponding bursts ( s i ) of each service ( s 1 , s 2 ). a same message ecm is inserted in several consecutive bursts ( s i ) of a service until the reception of a new message ecm concerning this service . in the example of fig6 , the messages ecm 1 a et ecm 1 b are repeated in two consecutive bursts of the service ( s 1 ) and the message ecm 1 c is introduced into the further burst ( s ) until reception of the following message and so on . in a similar way , the control messages ( ecm 2 a , ecm 2 b , ecm 2 c , . . . ) of the service ( s 2 ) are introduced into the respective bursts . according to a preferred configuration , the messages are placed at the head of the bursts ( s i ) in order to reduce the time for accessing the content of the bursts ( s i ) during zapping ( fast passing from service ( s 1 ) to service ( s 2 ) and reversely ). it has to be noted that the ecm messages stored in the memory ( m ) are erased as new messages associated to a given service arrive . in other words , an ecm message is kept as long as it is used to be introduced into the bursts ( s i ) and this until reception of a new message , for example the message ecm 1 a is replaced by the message ecm 1 b . the document of the standards etsi tr 102 377 “ dvb - h implementation guidelines ”, does not foresee this method of transmission of ecm messages in the bursts ( s i ) of the services ( s 1 , s 2 , . . . ). they are transmitted in an independent stream ( fm ) in parallel to the stream of the content bursts ( s i ). according to this document , the transmission of the bursts and of the messages is then simplified because it requires neither memory nor device for memory reading and for introducing messages into the appropriate bursts . furthermore , the configuration of the mobile reception equipment is also different for allowing a permanent reception of the messages stream while being synchronized with the bursts of the services which content has to be decrypted . the insertion of the control message at the beginning of the burst allows a control word to be obtained immediately when the mobile equipment switches from one service to another . in fact , after having read the identifier ( id 1 , id 2 ), it extracts the necessary control word from the control message ecm for decrypting the data from the content that follows in the burst . the extracted control word can be stored in a temporary memory of the mobile equipment in order to be used for the next bursts ( s i ) when a certain number of the latter contain the same control message as in the case of the service s 1 in the example . in an audio / video embodiment for example , where the different services each represent a television channel and thanks to the control message preceding the encrypted data , it becomes easy to pass from one channel to another and vice - versa without a blackout of the image or the sound . the control word for decrypting the audio / video data is thus immediately available to the decryption unit of the mobile equipment . a control message ecm can contain several control words as schematized in fig5 . in order to increase the processing speed of the bursts ( s i ), the mobile equipment stores several control words ( cw 11 , cw 12 , cw 13 ) related to a given service s 1 and corresponding to a series of bursts of this service . according to the hardware and software configuration ( decryption algorithm ) of the mobile equipment , it is faster to extract a certain number of control words from one or several messages and store them than to extract them each time from the message transmitted with the bursts ( s i ). the mobile equipment extracts the control words from a certain number of bursts ( s i ) for storing them , when these stored words become obsolete ; it extracts and stores new words from further bursts ( s i ) and so on . the control message can also contain , in addition to the current control word for decrypting the burst of current data , the control words of previous and following bursts ( s i ). this alternative is useful when the mobile equipment is provided with a mass storage unit suitable for recording all or part of the broadcasted data . in fact , the implementation of these particular control words allows a fast forward or back browsing of recorded encrypted data .