Abstract:
The present invention relates to a method for selecting another access network when transmitting data from a content server to a terminal in a multi access network environment comprising a discontinuous coverage radio access network. 
     According to the present invention, the method comprises the step of:
       Upon detection of a request from a network selection controller to switch from a discontinuous coverage radio access network to another access network, sending a message to an access controller of said discontinuous coverage radio access network comprising cached data informing it to forward said cached data to said another access network.

Description:
BACKGROUND OF THE INVENTION 
   The invention is based on a priority application EP 04 291 978.7 which is hereby incorporated by reference. 
   The present invention relates to a method for improving access network selection in the context of discontinuous coverage access networks. 
   Discontinuous coverage access networks are radio access networks which do not provide a continuous radio coverage contrary to conventional second or third generation radio access networks as GSM or UMTS. Such discontinuous coverage access networks are constituted by islands of coverage supporting very high bit rates and low power cells. Such networks are commonly referred to as fourth generation access networks. They are best adapted to provide video/audio streaming services and fast download or upload of large data volumes. 
   The islands of radio coverage are nevertheless separated by zones of non-radio coverage so that usual hand over between cells is not to be realized by common techniques. 
   Moreover, if several radio access networks are co-existing in a same area, an advantageous feature is to select the best appropriate radio access network for providing a service requested by a customer while taking into account for example the capabilities of the different access networks, their availability and customer respective operator wishes. 
   The easiest method for such an access network selection would be to provide a central control/switch element above all possible access networks and which triggers the selection or the change of access network. 
   Nevertheless, in the context of discontinuous coverage access networks or in the context of co-existence of continuous and discontinuous coverage access networks such a central control/switch element should be able to cope with caching techniques mandatory in discontinuous coverage radio access networks. Indeed, caches at the different levels of the discontinuous coverage access network architecture may be filled with data during the time when the terminal is in a non coverage area. These cached data should be delivered with delay after the terminal has entered a new zone of radio coverage. If at this moment another access network is selected for the customer all cached data may be lost, if no appropriate measure is taken, rendering a simple access network selection incompatible with a seamless service. 
   A known solution would be that the content server retransmits these data if lost data are detected at the terminal. An appropriate retransmission protocol for example TCP would then take care for the missing data retransmission. However, such retransmissions generate a long latency delay where for example TCP is not adapted to a long latency delay and necessitate an important overhead. This is not adapted to streaming services which cannot cope with long retransmissions due to the high throughput of data to deliver. Moreover, retransmission protocols as the TCP protocols are not fitted for heterogeneous networks mixing fixed or wireless links. 
   A particular object of the present invention is to provide a method for providing access network selection in the context of discontinuous coverage access networks. 
   Another object of the invention is to provide an entity of the radio access network supporting the method. 
   SUMMARY OF THE INVENTION 
   These objects, and others that appear below, are achieved by a method for selecting another access network when transmitting data from a content server to a terminal in a multi access network environment comprising discontinuous coverage access networks according to claim  1 , an access controller according to the independent claim, and a multicast element according to the independent claim. 
   According to the present invention, the method consists in detecting a request from a network selection controller to switch from a discontinuous coverage radio access network to another access network and then to send a message to an access controller of the discontinuous coverage radio access network comprising cached data destined to a terminal and stored in a cache memory of the access controller and forwarding these cached data to the new selected access network. 
   The method according to the present invention presents the advantage to provide a seamless service provision without requiring retransmission of lost cached data. 
   The method according to the present invention further presents the advantage to support continuous streaming services in the context of heterogeneous continuous/discontinuous coverage access networks supporting access network selection. 
   Further advantageous features of the invention are defined in the dependent claims. 

   
     BRIEF DESCRIPTION OF THE DRAWINGS 
     Other characteristics and advantages of the invention will appear on reading the following description of preferred embodiments given by way of non-limiting illustrations, and from the accompanying drawings, in which: 
       FIG. 1  shows a first implementation of the access network selection according to the present invention in the context of mixed continuous/discontinuous coverage access networks; 
       FIG. 2  shows a second implementation of the access network selection according to the present invention in the context of mixed continuous/discontinuous coverage access networks. 
   

   DETAILED DESCRIPTION OF THE INVENTION 
     FIG. 1  a first implementation of the access network selection according to the present invention in the context of mixed continuous/discontinuous coverage access networks.  FIG. 1  illustrates an example of network architecture for co-existing fourth generation radio access network and legacy access network where the present invention may be implemented. 
   The architecture of the whole system comprises a content server  11 , a home agent  12  supporting mobile IP functionality, an access network selection controller  13 , two possible access networks: a first discontinuous coverage radio access network comprising access controllers  141 ,  142 , and radio access points  161 ,  162 ,  163  and a second 3G radio access network  164  providing a continuous radio coverage on the areas covered by radio access points  161 , . . . ,  163 . Additionally, the communication network comprises an IP network  15 , and a terminal  17 . 
   Content server  11 , Home agent  12 , access controllers  141 ,  142 , radio access points  161 , . . . ,  163  and 3G radio access network  164  are connected to IP network  15 . Terminals  17  is accessible from radio access points  161 , . . . ,  163  and from 3G radio access network  164  through two air interfaces. 
   Concerning discontinuous coverage radio access network, each pair (access controller, radio access point) ( 141 ,  161 ), ( 141 ,  162 ) ( 142 ,  163 ) defines a zone of radio coverage. In the example described below, the radio coverage is discontinuous so that there remain areas of non radio coverage between two areas of radio coverage ( 141 ,  161 ), ( 141 ,  162 ) ( 142 ,  163 ). It will be understood by the person skilled in the art that the present invention may also apply to mixed continuous and discontinuous networks. 
   Terminal  17  is under radio coverage area ( 141 ,  162 ) as well as under the radio coverage of 3G radio access network. Nevertheless, it is assumed in this embodiment that terminal  17  is currently connected to content server  11  over discontinuous coverage radio access network ( 141 ,  162 ). 
   It is the task of access network selection controller  13  to detect the best appropriate radio access network for serving terminal  17  and to trigger the change of access network serving terminal  17 . The criterion for changing the access network may be for example the event that terminal  17  currently served by a discontinuous coverage radio access network has its terminal cache memory not filled enough, so that it is advantageous to change the access network to a legacy 3G radio access network providing continuous coverage. Other criteria for change of access network may be dependent on user preferences, operator preferences, required service types . . . . Such criteria are clear for a person skilled in the art and are out of the scope of the present invention. 
   Access network selection controller  13  may communicate with 3G access network  164 , to access controllers  141 ,  142  of discontinuous coverage radio access network as well as to Home agent  12  in order to inform these entity of a need for changing the access network for serving terminal  17 . 
   Access controllers  141 ,  142  comprise a first portion each containing a foreign agent which is addressed by the home agent in the context of mobile IP. Indeed, when sending data content server  11  uses a unique IP address to identify terminal  17 . 
   Additionally, access controllers  141 ,  142  contain a cache memory for storing received data which cannot be immediately delivered to terminal  17  because terminal  17  is currently moving in a non-radio coverage zone or because these data where sent in an anticipated way to access controllers  141 ,  142 . 
   The data not immediately used have to be stored in an appropriate cache memory which should be large enough to ensure that the whole data received during the period where terminal  17  is not under any radio coverage are available in the cache memory. The cache memory should be dimensioned using for example parameters as the average data throughput on the link between access controller  141 ,  142  and radio access points  161 , . . . ,  163  or the average duration of the non coverage period for terminal  17 . 
   Then, the data stored in the cache memory are forwarded to the radio access point  162  under the coverage of which terminal  17  is currently located (arrow C on  FIG. 1 ). 
   In case, now, of a message from the access network selection controller  13  indicating that terminal  17  should no more be server by discontinuous coverage radio access network ( 141 ,  162 ) but by 3G radio access network  164 . The data sent from content server  11  follow following path according to the present invention: 
   Content server  11 —Home agent  12  (arrow A on  FIG. 1 ), then 
   Home agent  12 —Foreign agent of access controller  141  (arrow B on  FIG. 1 ), and then 
   Cache memory of access controller  141 —3G radio access network  164  (arrow C′ on  FIG. 1 ). 
   This mechanism enables it to first send over arrow C′ the cached data received from content server  11  at access controller  141  but which where not forwarded immediately to terminal  17  and then to forward the new data received from content server  11  after the change of access network. Since the cached data as well as the new data are transitiing through the cache memory of access controller  141 , a continuous data flow is ensured at terminal  17 . 
   It will be understood by those skilled in the art that the 3G radio access network used to illustrate the present invention in  FIG. 1  can be any other type of access network either fixed or wireless. 
     FIG. 2  (?) shows a second implementation of the access network selection according to the present invention in the context of mixed continuous/discontinuous coverage access networks. 
   The architecture of the whole system comprises a content server  11 , a multicast element  12  (instead of a Home agent  12  of  FIG. 1 ), and an access network selection controller  13 , two possible access networks: a first discontinuous coverage radio access network comprising access controllers  141 ,  142 , and radio access points  161 ,  162 ,  163  and a second 3G radio access network  164  providing a continuous radio coverage on the areas covered by radio access points  161 , . . . ,  163 . Additionally the communication network comprises an IP network  15 , and terminal  17 . 
   Content server  11 , multicast element  12 , access controllers  141 ,  142 , radio access points  161 , . . . ,  163  and 3G radio access network  164  are connected to IP network  15 . Terminals  17  is accessible from radio access points  161 , . . . ,  163  and from 3G radio access network  164  through two air interfaces. 
   Terminal  17  is under radio coverage area ( 141 ,  162 ) as well as under the radio coverage of 3G radio access network. Nevertheless, it is assumed in this embodiment that terminal  17  is currently connected to content server  11  over discontinuous coverage radio access network ( 141 ,  162 ). 
   It is the task of access network selection controller  13  to detect the best appropriate radio access network for serving terminal  17  and to trigger the change of access network serving terminal  17 . The criterion for changing the access network may be for example the event that terminal  17  currently served by a discontinuous coverage radio access network has its terminal cache memory not filled enough, so that it is advantageous to change the access network to a legacy 3G radio access network providing continuous coverage. 
   Access network selection controller  13  can communicate with 3G access network  164 , to access controllers  141 ,  142  of discontinuous coverage radio access network as well as to multicast element  12 . 
   Multicast element  12  is a central element in the communication network which controls the transmission of data from content server  11  to terminal  17 . Multicast element  12  is also involved in the proper execution of access network modification. 
   Access controllers  141 ,  142  comprise a first portion containing control functions which may be addressed by multicast element  12  or by access network selection controller  13 . Additionally, access controllers  141 ,  142  contain a cache memory for storing received data which cannot be immediately delivered to terminal  17 . 
   Then, the data stored in the cache memory are forwarded to radio access point  162  under the coverage of which terminal  17  is currently located (arrow F on  FIG. 2 ). 
   In case now of a message from the access network selection controller  13  indicating that terminal  17  should no more be served by discontinuous coverage radio access network but by 3G radio access network, and according to the present invention, the following steps will be executed: 
   Multicast element  12  first asks serving access controller  141  for data stored in its cache memory (arrow E in  FIG. 2 ) in order that no data already sent by content server  11  but not already received by terminal  17  are not lost. These data are sent back to multicast element  12  (arrow F′ of  FIG. 2 ). Then, multicast element  12  sends these data to 3G access network  164 . 
   An advantage of the present embodiment compared to the first embodiment illustrated in  FIG. 1  is that access controllers  141 ,  142  of the discontinuous coverage radio access network have a reduced complexity since they need not to remain the anchor point for data transmission during the whole time spent by terminal  17  under the control of 3G radio access network  164 . On the contrary, in the present embodiment it is the task of multicast element  12  to ask for cached data stored in access controller  141 ,  142  and to take care for transmitting these cache data before the data received after the access network change has happened. 
   It will be clear for a person skilled in the art that the change of access network from a 3G access network to a discontinuous coverage access network can also be performed using the method according to the present invention.