Abstract:
For improving data flow during a handover it is proposed to send downstream traffic ( 6   a,b,c,d ) from a switching center ( 2 ) over at least two paths over base stations ( 3   a,    3   b ) to a mobile terminal ( 4 ) by checking for a change of the base station ( 3   a   , 3   b ) serving the mobile terminal ( 4 ), upon detection of a base station change, adding the new base station ( 3   b ) associated to the mobile terminal ( 4 ) by way of a different path to a forwarding information database of the switching center ( 2 ), and forwarding as many downstream frames ( 6   b,    6   d ) to the mobile terminal ( 4 ) as are according entries in the forwarding information database.

Description:
BACKGROUND OF THE INVENTION  
       [0001]     The invention is based on a priority application EP 05 291 281.3 which is hereby incorporated by reference.  
         [0002]     The present invention relates to a method for sending downstream traffic from a switching center over at least two paths to a mobile node of a L2 network. The present invention relates as well to a switching center for performing this method.  
         [0003]     In the context of mobility, hand-over processes are an important issue. A mobile terminal is serviced by different base stations of a radio network, depending on the actual position of the mobile terminal. In contrast to upstream frames (from the mobile terminal to the network) that are always sent to the next base station and then forwarded inside the network, downstream frames (from the network to the mobile terminal) encounter the problem that during or just after hand-over, the base station through which the mobile terminal can be reached is not necessarily determined. Different base stations are equivalent to different paths to and from the mobile terminal, the mobile terminal being a node of the network.  
         [0004]     In terms of L2 (layer 2 of the OSI model) networks or, in other words, networks on the level of the data link layer, one standard is IEEE 802 defining a logic data control layer and a media access control (MAC) layer constituting the data link layer. Each node of the L2 network is identified by a MAC address permanently assigned to the node, independently of the link or path.  
         [0005]     The forwarding of ingress frames in an 802 MAC layer is standardized in IEEE 802.1d (IEEE 802.1D-2004: Local and metropolitan area networks, Media Access Control (MAC) Bridges). One feature of this forwarding process is, that a unicast frame, whose destination address has been learned, can only be buffered for transmission on one port at any one time and cannot be duplicated.  
         [0006]     In case of mobile terminals, the hand-over is handled by e.g. “MAC in MAC”, VLAN (virtual local area network), or MPLS (multiple protocol layer switching). These are all means to tunnel traffic in L2 networks. The setup and management of tunnels increase both the capital and the operational expenditure concerning the control plane operations of the L2 network as well as cause an overhead penalty.  
         [0007]     It is an object of the present invention to provide a further possibility of forwarding frames causing less expenditure and overhead.  
       SUMMARY OF THE INVENTION  
       [0008]     In a first aspect of the present invention, this object is achieved by a method for sending downstream traffic from a switching center over at least two different paths to a mobile node of a L2 network, said method comprising the steps of: 
        checking the upstream traffic from the mobile node, whether its path is already entered as associated to the mobile node in a forwarding information database of the switching center;     upon detection of a path not yet entered as associated to the mobile node in the forwarding information database, adding the path to the forwarding information database;     forwarding as many downstream frames to the mobile node as paths are entered in the forwarding information database as associated to the mobile node.        
 
         [0012]     In essence, the present invention allows for a dual or multiple paths towards the mobile node, i.e. via the old and via the new path(s). Downstream traffic is automatically sent not only to the old location(s), but also to the new location(s).  
         [0013]     As soon as a path change is detected in the upstream traffic, the forwarding information is updated accordingly by adding the new entry or entries concerning the mobile node and its associated paths, i.e. the new entry or entries are appended without immediately deleting the old entry or entries concerning this mobile node and its associated paths. The forwarding process then forwards as many copies of the downstream frame as entries corresponding to the respective mobile node in the forwarding information database exist.  
         [0014]     The method according to the present invention allows for optimal packet flow. integrity and minimum jitter in case of a path change, i.e. due to a hand-over. Since there is no need according to the present invention to setup and maintain any tunnels, the control plane is much simpler and thus operational expenditure is reduced. The capital expenditure is reduced as well due to cheaper network elements, e.g. base station, gateway, since the required protocol stack is minimized.  
         [0015]     Further advantages are, that the data plane performance is better, too. Since there is less overhead in the data plane without the need of en- and decapsulating frames, the throughput is higher. This is particularly important in view of providing real time services.  
         [0016]     Preferably, the mobile node is identified by a MAC address according to IEEE 802.1d.  
         [0017]     The method according to the present invention is advantageously applied in radio network, where the mobile node is a mobile terminal of the radio network and the at least two different paths correspond to a base station each servicing the mobile terminal.  
         [0018]     In preferred embodiments of the present invention, the method further comprises the steps of: 
        checking whether a mobility enhanced switching parameter is enabled;     upon detection of the mobility enhanced switching parameter being enabled, adding the new path associated to the mobile node to the forwarding information database.        
 
         [0021]     In case one wants or use the special forwarding according to the present invention only sometimes, for example to modify the IEEE 802.1d standard temporarily, a mobility enhanced switching parameter is introduced. If this parameter is enabled, forwarding is handled according to the present invention. If this parameter is not enabled, forwarding is handled according to prior art, for example according IEEE 802.1d and, if necessary using one of the above-mentioned tunneling mechanisms.  
         [0022]     Preferably, the method according to the present invention comprises the further step of ageing the former base station entry(ies) in the forwarding information database. This has the advantage that some time after hand-over, the forwarding information database is “cleaned up” and only the entries corresponding to the actual situation are left over. This lapse of time is called “ageing time-out” in IEEE 802.1d. Thus, multiple path forwarding is provided only during hand-over and eventually shortly after hand-over, and then forwarding only over the actual path or respectively to the actual base station(s) is resumed. This avoids using resources for multicasting unnecessarily.  
         [0023]     In a second aspect of the present invention, this object is achieved by a switching center for sending downstream traffic over at least two different paths to a mobile node of a L2 network, with 
        handling means for handling a forwarding information database binding paths and mobile nodes, and in case of occurrence of a path not yet entered in the forwarding information database as associated to the mobile node, adding it as new entry to the forwarding information database;     forwarding means for forwarding as many downstream frames to the mobile node as paths associated to the mobile node entered in the forwarding information database.        
 
         [0026]     The handling means updates the forwarding information database in case of a path change by appending new entries instead of overwriting them. In consequence, the forwarding means forwards downstream frames not only via the new path(s), but also via the old path(s).  
         [0027]     Preferably, the handling means is arranged to identify the mobile node by a MAC address.  
         [0028]     In preferred embodiments, the forwarding means is arranged to forward the downstream frames to a mobile terminal of a radio network over base stations corresponding to a different path each.  
         [0029]     Preferably, the switching center has in addition detecting means for detecting a change of the base station serving the mobile terminal by the switching center itself. It has shown to be advantageous to detect a change in bases stations by receiving upstream traffic from at least one base station not yet associated to the mobile terminal in the forwarding information database. Detection can then be done by the switching center. Other possibilities would be to provide dedicated signals from some network control entity to the switching center. This applies as well to generally forwarding frames to mobile node over various paths.  
         [0030]     In preferred embodiments of the present invention, the switching center has in addition further detecting means for detecting the status of a mobility enhanced switching parameter. This is especially useful, when utilizing the switching center in an IEEE 802.1d environment. 
     
    
     BRIEF DESCRIPTION OF THE DRAWINGS  
       [0031]     A detailed description of the invention is provided below. Said description is provided by way of a non-limiting example to be read with reference to the attached drawings in which:  
         [0032]      FIG. 1  shows a flowchart of an embodiment of the method according to the present invention;  
         [0033]      FIG. 2   a  illustrates the situation of a mobile terminal being serviced by a first base station;  
         [0034]      FIG. 2   b  illustrates the situation of a mobile terminal being serviced by a second base station just after the base station change;  
         [0035]      FIG. 2   c  illustrates the situation of a mobile terminal being serviced by the second base station some time after the base station change;  
         [0036]      FIG. 3  shows a schematic view of an embodiment of the switching center according to the present invention, and  
         [0037]      FIG. 4  shows schematically the structure of a forwarding information database. 
     
    
     DETAILED DESCRIPTION OF THE INVENTION  
       [0038]     The present invention will be exemplarily explained with respect to a radio network with a mobile terminal serviced by one or more base stations.  
         [0039]      FIG. 1  shows a schematic flowchart of an embodiment of the method according to the present invention. In the present example, as first step  101  it is checked, whether the MAC address, respectively the mobile terminal identified by it, is already known. If not, the switching continues in a conventional way.  
         [0040]     If the MAC address is already known, a change of the base station servicing a mobile terminal is detected (step  103 ). The base station change is detected inside the switching center with a forwarding information database by comparing upstream traffic coming from the mobile terminal via a certain base station, i.e. via a certain path, with the entries in the forwarding information database binding the MAC address of the mobile terminal with certain paths corresponding to base stations, this having the function of an egress interface to the switching center. If the actual combination of mobile terminal, i.e. MAC address, and base station, i.e. egress interface or path over base station, is not to be found in the forwarding information database, a change of the base station servicing the mobile terminal has occurred.  
         [0041]     If such a change has been detected, the status of the mobility enhanced switching (MES) parameter is checked (step  105 ). The MES parameter indicates, whether forwarding to several base stations is allowed or not. This parameter may be set by a L2 network control entity, and may differ, e.g. for various mobile terminals or various base stations in heterogeneous networks with hardware of different kinds.  
         [0042]     In case both a base station change has occurred and the MES parameter is enabled, the forwarding information database is updated by appending the new pair of mobile terminal address and egress interface (step  107 ) without overwriting former pairs concerning this specific mobile terminal. Thus, immediately after a base station change, i.e. during or shortly after a hand-over, at least two entries concerning the same mobile terminal are in the forwarding information database, the one using the former base station and the one using the latter base station.  
         [0043]     If there is subsequent downstream traffic to the mobile terminal, the downstream frames are forwarded (step  109 ) according to the entries of the forwarding information database. In the present example, two copies are forwarded, one via the former base station and one via the latter base station. This makes sure that the downstream traffic really reaches the mobile terminal and is not lost because of being sent only to a base station not servicing the mobile terminal in the present moment.  
         [0044]     The forwarding in case of terminal mobility according to the present invention allows for optimal packet flow integrity and minimum jitter in case of a hand-over. Since there is no need according to the present invention to setup and maintain any tunnels, the control plane is much simpler and thus reduced operational expenditure. The capital expenditure is reduced as well due to cheaper network elements, e.g. base station, gateway, since the required protocol stack is minimized.  
         [0045]     Further advantages are, that the data plane performance is better, too. Since there is less overhead in the data plane without the need of en- and decapsulating frames, the throughput is higher. This is particularly important in view of providing real time services.  
         [0046]     The situation before, just after and sometime after hand-over is illustrated in  FIGS. 2   a - c . In  FIG. 2   a , the mobile terminal  4  sends upstream traffic  5   a,b,c  to the base station controller  1 , that will process the traffic according to its kind, via base station  3   a  and switching center  2 . Downstream traffic  6   a,b,c  is sent from the base station controller  1  to the switching center  2 . There, it is looked up in the forwarding information database of the switching center  2 , via which base station the ingress frames are to be sent to the specific mobile terminal  4 . As for the moment the mobile terminal  4  is quasi-fixed, the forwarding information database contains exactly one entry showing that the mobile terminal  4  is service by base station  3   a , and the ingress frames are forwarded to base station  3   a  by the switching center in a conventional way, for example according to IEEE 802.1d.  
         [0047]     In  FIG. 2   b , the mobile terminal  4  has moved to the service region of base station  3   b . The first time upstream traffic  5   a,b,c  is sent via base station  3   b , the forwarding information database of the switching center  2  is updated without deleting the old entry concerning base station  3   a , eventually only after checking, whether a MES parameter is enabled. In consequence, the ingress frames of downstream traffic  6   a,b,c ,d arriving at switching center  2  are forwarded to both base station  3   a  (arrow  6   d ) and base station  3   b  (arrow  6   b ) to avoid loosing ingress frames. The uncertainty on the level of the switching center  2  on which base station  3   a ,  3   b  is servicing the mobile terminal  4  is symbolized by the dashed-line terminal  4  with dashed-line connections to the former base station  3   a.    
         [0048]     To economize resources, the old entry in the forwarding information database is aged, i.e. after a predefined time the entry is deleted from the database. The ageing time is predefined such that it is made sure, that hand-over is completed and the mobile terminal  4  is serviced only by the new base station anymore. Ageing is also a part of the normal operation of IEEE 802.1d. Thus, ageing is not a hindrance to embedding the forwarding according to the present invention in an IEEE 802.1d environment.  
         [0049]      FIG. 3  shows more in detail the switching center  2 . As an ingress frame  61  arrives at the switching center  2 , the first detecting means  21  checks, whether a change of base station has occurred, and the second detecting means  22  checks, whether the MES parameter is enabled. Both detecting means  21 ,  22  can be designed to check only upstream traffic, if e.g. the base station change is to be detected as explained before. In configurations, where the information for updating the forwarding information database  25  comes from the control plane, the actual information may be part of downstream or upstream traffic at the switching center  2 .  
         [0050]     Depending on the information contained in the downstream traffic, handling means  23  updates the forwarding information database  25 , either conventionally or according to the present invention, or only reads out the forwarding information database  25  and gives the information on whereto to forward ingress frame  61  to the forwarding means  24 . In case MES was enabled and the mobile terminal is involved in a hand-over, i.e. the former entry has not yet been deleted due to ageing, the forwarding means receives two paths through which to reach the mobile terminal. Thus, the forwarding means  24  generates two copies of the ingress frame  61  and forwards one to the former base station (egress frame  62   a ) and one to the latter base station (egress frame  62   b ).  
         [0051]      FIG. 4  shows schematically the structure of forwarding information database  25 . The mobile terminals are identified by MAC addresses (column  251 ) and associated with the different paths over different base stations (column  252 ). For example, for the mobile terminal corresponding to the address MAC 1, there are two entries, path over base station A and path over base station B. This could reflect the situation of an actual hand-over between base station A and base station B. Or it could reflect the more complex situation, that the mobile terminal MAC 1 is serviced continually by both base stations A, B.  
         [0052]     Concerning the mobile terminal identified by MAC n, there are even four entries. This could, too, reflect the situation of several base stations T, U, V, W servicing mobile terminal MAC n, continually and/or due to hand-overs. In particular, this could reflect the situation that the mobile terminal is moving faster than expected, when the ageing time was set, the ageing time being too long to react to the fast changes of base stations.  
         [0053]     Although having described several preferred embodiments of the invention, those skilled in the art would appreciate that various changes, alterations, and substitutions can be made without departing from the spirit and concepts of the present invention. The invention is, therefore, claimed in any of its forms or modifications with the proper scope of the appended claims. For example various combinations of the features of the following dependent claims could be made with the features of the independent claim without departing from the scope of the present invention. Furthermore, any reference numerals in the claims shall not be construed as limiting scope.  
       LIST OF REFERENCE NUMERALS  
       [0000]    
       
           1  base station controller  
           2  switching center  
           21  first detection means  
           22  second detection means  
           23  handling means  
           24  forwarding means  
           25  forwarding information database  
           251  mobile terminal column  
           252  path column  
           3   a,b  base station  
           4  mobile terminal  
           5   a,b,c  upstream traffic  
           6   a,b,c,d  down stream traffic  
           61  ingress frame  
           62   a,b  egress frame  
           101  step  
           103  step  
           105  step  
           107  step  
           109  step