Abstract:
A method is described for routing of data in hierarchical mobility management systems with tiered mobile anchor points. Routing of data is switched from old to new communications nodes and regulated, by a controlling mobile anchor point having both communications nodes in its domain, to occur synchronously with the expiry of a determined period following its receipt of a handover initiation signal. The main determinant of the period is the remaining time of attachment of the old communications node.

Description:
This invention relates to mobile communications, and in particular the handing over of a mobile node from one static communications node to another whilst the mobile node is receiving data, especially packet data. 
     BACKGROUND 
       FIG. 1  shows a standard configuration for enabling mobile communications between a Mobile Node MN, such as a portable telephone, and a corresponding node, not shown, via the Internet I. The Mobile Node MN is wirelessly connected to the network via the Access Points, AP 1 , AP 2 , AP 3  etc and the Access Routers AR 1 , AR 2  are connected to the Internet via a Router R. 
     Initially, with the Mobile Node MN in the position shown, signals are exchanged between the Mobile Node MN and the Internet via access point AP 2 , Access Router AR 1  and Router R. When the Mobile Node is to be handed over from Access Point  2  to Access Point  3 , for example because it is travelling from one towards the other, handover initiation and handover acknowledgement messages are exchanged between Access Routers AR 1  and AR 2 . The message sent from the new Access Router AR 2  to the old Access Router AR 1  will include a new care of address (nCoA) to be used by the Mobile Node once it is under the control of Access Router AR 2 . This new address is then communicated back to the Mobile Node MN. At a specified time after this event, data packets intended for the Mobile Node MN are no longer sent from AR 1  to AP 2  but are instead “tunnelled” from AR 1  to AR 2  in preparation for the Mobile Node MN being handed over from AP 2  to AP 3 . At this point it is necessary for packets to be buffered in AR 2  pending handover of the MN from AP 2  to AP 3  i.e. layer  2  (L 2 ) handover. Once layer  2  handover has been completed, signals are exchanged between the MN and the AR 2  via the AP 3  so that packets can be directed from AR 2  to AP 3  and then the MN. Finally, a binding update message BU is sent from the MN to the Router R. At this point the router knows that the MN is in communication with AR 2  so that it can send packets direct to AR 2  without tunnelling from AR 1  to AR 2 . 
     In the above example, there is a single Router R dealing with all handover requests. This Router needs to store and keep updated the care of addresses used by the Mobile Nodes as they move from place to place. In practice, having only one Router R would require too much processing time to manage a large number of Mobile Nodes (MNs). 
     More recently, so-called “mobile anchor points” have been proposed. These are discussed in many documents including U.S. patent publication US 2001/0046223A1 (described here as mobility agents). A mobility agent or mobile anchor point can perform the functions of a home agent including maintaining a list which provides a mapping between a home address of a Mobile Node and its corresponding current care of address. Mobile anchor points may be included at different levels of the network hierarchy (tiered mobile anchor points) and this can reduce signalling delay when a Mobile Node changes point of attachment to the network. 
     SUMMARY OF THE INVENTION 
     The aim of the present invention is to provide fast handover procedures by exploiting the benefits from using concepts related to standard:
         i) hierarchical handover procedures, such as the adoption of one or more extra degrees of localisation between an Access Router and the top Mobile Anchor Point (MAP) in the MAP hierarchy.   ii) anticipated handover procedures       

     The invention provides a method for routing of data in hierarchical mobility management systems with tiered mobile anchor points in which routing of data is switched from old to new communications nodes and regulated, by a controlling mobile anchor point having both communications nodes in its domain, to occur synchronously with the expiry of a determined period following its receipt of a handover initiation signal, where the determinant of the period is the remaining time of attachment of the old communications node. 
     The invention may be applied to real-time multi-media applications in packet networks. Effective routing of data packets and fast handover mechanisms reduce potential handover delays and loss of data at the receiver 
     It will be appreciated from the foregoing that there is no tunnelling between Access Routers in the method of the invention. Because of the extra degree of localisation achieved by the MAP hierarchy, it is practical in terms of time to redirect packets via a new route involving one or more MAPs once a MAP having the old and new communications nodes in its domain knows the new care of address to be used by the Mobile Node. 
     The process involves what might be called a “partial anticipated handover” in that packets are directed to the new point of attachment in readiness and (prior to) the arrival of the Mobile Node. 
     Preferably, the new care of address does not have to be reported at every level in the MAP hierarchy, but only as far as the lowest level MAP having both old and new communications nodes (Access Routers) in its domain. 
     As will be explained in more detail below, in the preferred implementation of this invention, for the purpose of synchronisation, messages are exchanged between the old and new communications nodes that effectively define the duration of the handover, and from this other components of the system will derive timing signals. 
    
    
     
       BRIEF DESCRIPTION OF THE DRAWINGS 
       An embodiment of the invention will now be described by way of example only and with reference to the accompanying drawings in which: 
         FIG. 1  is a schematic diagram showing a Mobile Node moving between access points for the purpose of describing known handover procedures (above); 
         FIG. 2  illustrates a possible topology configuration using mobile anchor points; 
         FIG. 3  illustrates the timing of signals exchanged between a Mobile Node and higher layers in the topology in a handover method according to the invention; and 
         FIGS. 4 ,  5 ,  6 ,  7 ,  8 ,  9 ,  10 ,  11 ,  12 ,  13 ,  14  and  15  are block diagrams that illustrate the successive steps in a handover method according to the invention. 
     
    
    
     DETAILED DESCRIPTION 
     As noted above, the configuration of the presented proposal is based on the principle that in a MIPv6 (Mobile Internet Protocol version 6) compatible system, a number of mobility agents or mobile anchor points can be included on different levels of the network hierarchy. A suitable configuration is shown in  FIG. 2 . 
     In  FIG. 2 , a Corresponding Node CN (fixed or mobile) is communicating with Mobile Node MN. The media path between CN and MN is via Router R associated with the corresponding node, the Internet, a hierarchical structure of mobile anchor points MAP, a layer of points of attachment or Access Routers AR 1 , AR 2 , AR 3 , AR 4  and a number of Access Points geographically coincident with the Access Routers that are not shown. When a Mobile Node enters a domain, such as the domain controlled by MAP 2 , it will register with each mobility agent between it and the Internet using a different localised care of address. When a packet is sent to the Mobile Node from a corresponding node, the packet arrives at the top level mobility agent. The top level mobility agent will encapsulate the packet and the next mobility agent—below it in the hierarchy—will decapsulate that packet and encapsulate it again prior to passing it down to the next mobility agent in the hierarchy. The process goes on in a similar fashion until the Mobile Node MN receives the data packet. 
     The process of handing over the MN from AR 1  to AR 2  occurs in a number of distinct steps whose timing is shown in  FIG. 3 . Each step is now described in detail. 
     Step 1 
     Referring now to  FIG. 4 , Mobile Node MN is moving away from AR 1  towards AR 2 . Initially, data packets flow from MAP 2  to MN via AR 1 . 
     Step 2 
     As with the prior method described above, a handover is triggered (in a manner which does not form part of this invention) following which handover initiation HI and handover acknowledgement HACK signals are exchanged between current Access Router AR 1  and the new Access Router AR 2  to which the MN is about to be handed over. The message passed back to AR 1  will include a care of address associated with AR 2  to be used by MN once it has been handed over. In this particular example, AR 2  is triggered (see  FIG. 5 ) to send a handover initiation HI message to AR 1  (see  FIG. 6 ), which includes the new care of address.  FIG. 7  shows the HI signal arriving at AR 1 . 
     Step 3 
     The HI signal contains:
     1. The IP address of AR 2 .   2. The potential CoA to be used by MN.   3. An indication of the amount of time AR 1  needs to continue to unicast the MN   

     Items 1 and 2 are standard procedure but item 3 is important to the preferred embodiment of the present invention as will be described below. 
     Step 4a 
     When the AR 1  receives the HI message it must reply with a Handover Acknowledgement message (HACK). The HACK message contains:
     1. The MN&#39;s home address   2. The MN&#39;s old Care of Address   3. The MN&#39;s link layer address   4. The amount of time (dt) the AR 1  is willing to continue unicasting the MN   

     Step 4b 
     At the same time, as shown in  FIG. 9 , AR 1  sends a Proxy Router Advertisement (PrRtAdv) to the MN. The PrRtAdv message contains:
     1. The IP address of AR 2  (the new Access Router)   2. The Care of Address proposed by AR 2  (the new Access Router).   

       FIG. 9  shows the Proxy Router Advertisement message reaching the MN and the handover acknowledgement message HACK-reaching AR 2 .  FIG. 9  also shows the Handover Initiation transmission from AR 2  to MAP 1  once the HACK message has reached AR 2   
     Step 4c 
     The MN continues being serviced by AR 1  for time (dt) as shown in  FIG. 10 . At the same time the Handover Initiation message travels from MAP 1  to MAP 2  (see  FIG. 11 ), which according to the invention is provided with a timer. It is important to note that the HI message that reaches MAP 2  contains the actual service time (dt) that was agreed between AR 1  and AR 2  during step 4a. The specified interval (dt) can be adjusted accordingly by the MAP 2  to take into account any additional delays imposed by the wired and the wireless links (transmission delays). 
     Step 5 
     MN starts to perform L 2  handover without any need to inform AR 1 . This happens after the elapse of time dt. In normal anticipated handover procedures the MN informs the AR 1  when Layer  2  handover is about to take place by sending a Fast Binding Update signal to AR 1 . In our proposal F-BU transmission is not required. As  FIG. 12  shows, the MN performs Layer  2  handover over a period of dt 2 , during which time the MN cannot receive any data packets. 
     At the same time, once time (dt) has expired, the mobile anchor point  2  (i.e the MAP having both the AR 1  and AR 2  in its domain) is triggered and starts re-directing packets to MAP 1 . 
     The MAP 2  timer plays a critical role in the re-routing of data packets between MAPs. The timer is based on the agreed time (dt) that was passed to MAP 2  via the reception of the Handover Initiation message with adjustments for the delays imposed by the wired link that connects AR 2 , MAP 1  and MAP 2  together and the wireless link between the Access Point and the Mobile Node. The re-routing takes place once MAP 2  timer expires. 
     Packets received by MAP 1  are decapsulated and encapsulated once again and are sent to AR 2  by the MAP 1 . Packets are cached (buffered) in Access Router  2  and/or MAP 1  ready to be sent to the MN once it has signalled successful L 2  handover to AR 2 . The solid areas and the triangular symbol within MAP 2  indicate possible buffering and the ‘timer’ respectively. This stage can be regarded as partial anticipated handover. 
     The invention does not rely on tunnelling between the involved Access Routers, as is the case in normal anticipated procedures. 
     Step 6 
     As shown in  FIG. 13 , once layer  2  handover has finished, the MN sends a Fast Neighbour Advertisement (F-NA) to Access Router  2  indicating that the MN has arrived. The Fast Neighbour Advertisement contains the link layer address of the MN in order to be recognised by AR 2 . Once the Access Router  2  has received the F-NA ( FIG. 14 ) it can start delivering buffered packets to the MN as shown in  FIG. 15 . 
     Binding update transmissions do not need to be sent from MN to AR 2  and then to MAP 1  informing MAP 1  that layer  3  handover will take place (as is the case in standard anticipated and hierarchical handover procedures) since MAP 2  and MAP 1  are already aware of the new subnet prefix that needs to be used and the data packets have already being re-directed from MAP 2  to MAP 1  (i.e Layer  3  handover has been achieved already). 
     It is important that Step 5 is completed before the Mobile Node completes the layer  2  handover and decides to send the F-NA as a trigger mechanism to inform Access Router  2  of its arrival. Otherwise, there will be a delay when the Mobile Node has arrived at Access Router  2  with no packets ready for it to receive. 
     As a result, certain decisions need to be made as follows:
         for how long data needs to be buffered in AR 2  (and in MAP 1  if required)   when the re-routing mechanism, that will redirect packets to MN from the route MAP 2  to AR 1  to the route MAP 2  to MAP 1 , should start taking place in order to guarantee that data packets will already be buffered at AR 2  and waiting to be delivered to MN once layer  2  handover has been completed (As presented above, the “switching” process begins once Step 5 has been completed. It might be required to delay the re-routing process for example if the Mobile Node is moving too slowly.)   the size of the buffer needed to compensate for the likelihood of a Mobile Node moving too slowly.   The foregoing presents an alternative method of performing fast handovers. The method takes advantage of the extra degree of localisation that can be achieved by employing more than one mobile anchor point and also introduces the concept of “partial anticipated” handover without any tunnelling between the involved Access Routers. An examplary configuration of the method is shown in  FIG. 2  where a re-routing mechanism is implemented in the upper MAP. In this example data packets are re-directed (“switched”) once the timer within the mobile anchor point, which is at the top of the hierarchy for this particular configuration, expires. The timer within the aforementioned Mobile Anchor Point takes into account any signalling delays imposed by intervening wired and wireless links and starts running once a “trigger” from the lower mobile anchor point is received. The re-directed packets can subsequently be buffered totally in AR 2  or the buffering load may be split between the involved Access Routers and mobile anchor points (i.e. between AR 2  and MAP 1  in this example). The size of the buffer needs to be adjusted according to the speed of the Mobile Node.       

     It is a feature of the method that Binding Update (BU) transmissions are not needed for the re-direction of data packets since the involved mobile anchor point entities are already informed of the new subnet prefix (that is indicated in the proposed new CoA) that needs to be adopted. The data packets will have already been re-directed once the Mobile Node has reached the new Point of Attachment. (i.e, AR 2 ). 
     A further feature is that Layer  2  handover starts once the agreed time period (dt) has expired. This means that no Fast Binding Update (F-BU) transmissions need to be sent to the old Point of Attachment (i.e, AR 1 ) prior to Layer  2  handover, as is the case in standard anticipated handover procedures.