Abstract:
A system and method provide seamless data network telecommunication service to a mobile wireless node during mobile wireless call handoff from a first base station to a second base station. Prior to call handoff, communication content addressed to the mobile wireless node is routed to a first address for delivery to the mobile wireless node via the first base station. Upon initiation of the call handoff, the communication content is routed to a second address for delivery to the mobile wireless node via the second base station. At the same time, routing of the same communication content is continued to the first address. Upon completion of the call handoff, communication content routing to the first address is terminated.

Description:
CROSS-REFERENCE TO RELATED APPLICATIONS 
     Not Applicable 
     STATEMENT REGARDING FEDERALLY SPONSORED RESEARCH OR DEVELOPMENT 
     Not Applicable 
     BACKGROUND OF THE INVENTION 
     1. Field of the Invention 
     This invention relates to the implementation of telecommunication service in a data network on behalf of mobile wireless nodes. More particularly, the invention pertains to a system and method for routing data network communication content to mobile wireless nodes that are undergoing call hand off from one radio base station to another. 
     2. Description of the Prior Art 
     Considerable attention has been directed toward the implementation of mobile telecommunication service in computer data networks, and particularly the ability to route communication content to mobile wireless nodes that routinely connect to the data network at different points of attachment, via air interfaces. These include cellular telephones, Personal Digital Assistants (PDAs), laptop computers, and other mobile wireless communication equipment. 
     To facilitate mobile wireless telecommunication service in a data network, it is desirable (although not always possible) to allow mobile wireless nodes to change their link-layer point of network attachment without reassigning a new network address. According to current data network telecommunication standards for mobile equipment in general (e.g., the “Mobile IP” standards promulgated by the Internet Engineering Task Force (IETF) or the General Packet Radio Service (GPRS) standards proposed by the European Telecommunication Standards Institute (ETSI)), one way to provide the desired network address transparency is to employ “mobility agents.” These are network routing nodes that route communication content on behalf of mobile nodes as they move around the network. For example, according to the IETF Mobile IP standards, a mobile node&#39;s mobility agents may consist of a “home agent” routing node and may also include a “foreign agent” routing node. The home agent is a routing node in the mobile node&#39;s subnetwork that maintains a network interface on the link indicated by the mobile node&#39;s “home address,” which is a network address intended to remain assigned to the mobile node for an extended time period. When the mobile node is away from its home subnetwork, the home agent intercepts communication content bound for the mobile node&#39;s home address and tunnels it for delivery to a “care-of” address assigned to the mobile node when the mobile node registers on a foreign subnetwork. The care-of address may be the address of a foreign agent routing node in the foreign subnetwork. 
     Correspondent nodes wishing to communicate with a foreign-registered mobile node are able to address their communication content to the mobile node&#39;s home address. Transparently, the communication content is tunneled to the mobile node&#39;s care-of address and delivered to the mobile node on the foreign subnetwork. Normal routing is used for sending return communication content from the mobile node to the correspondent node. 
     The foregoing routing mechanism can be used for mobile wireless nodes connected to a foreign subnetwork via an air interface. However, a problem may arise if the mobile wireless node is being actively transported while communicating over the data network, and a call handoff is required from one radio base station to another. In that case, the old base station may be linked to one care-of address, while the new base station is linked to another care-of address. Call handoff then requires that the communication tunneling endpoint be transferred from the old care-of address to the new care-of address. 
     This may create gaps that interrupt the timely delivery of call content, which can degrade communication quality, particularly for voice telephony. Such gaps arise from the inability of the data network to coordinate well with the air interface so as to determine the exact time of handoff. Delay can occur between the point of handoff and the point at which the home agent begins routing communication content to the new care-of address. 
     Accordingly, there is a need in a data network telecommunication system serving mobile wireless nodes for improved call handoff without loss of communication content. What is required is a system and method that seamlessly routes communication content during handoff so that the mobile wireless node does not experience noticeable communication content loss other than that caused by the air interface, if any. 
     SUMMARY OF THE INVENTION 
     A system and method in accordance with the invention provides seamless data network telecommunication service to a mobile wireless node during mobile wireless call handoff from a first radio base station to a second radio base station. Prior to call handoff, communication content addressed to the mobile wireless node is routed on behalf of the mobile wireless node to a first address for delivery to the mobile wireless node via the first base station. Upon initiation of the call handoff, the communication content is routed to a second address for delivery to the mobile wireless node via the second base station. At the same time, routing of the same communication content to the first address is continued. Upon completion of the call handoff, communication content routing to the first address is terminated. 
     In preferred aspects of the invention, it is contemplated that routing to the first and second addresses will be performed by a network routing node that acts on behalf of the mobile wireless node. If the invention is incorporated in a mobile networking environment as envisioned by the IETF Mobile IP standards, it is contemplated that the routing node will be implemented as a “home agent” that is located on the mobile wireless node&#39;s home subnetwork, and which is configured to route communication content to designated care-of addresses as the mobile wireless node moves between cells served by the first and second base stations. In the IETF Mobile IP embodiment of the invention, it is contemplated that the care-of addresses will correspond to “foreign agents” that are linked to, or reside at, the first and second base stations. 
     Finally, it will be understood that the mobile wireless node may communicate with correspondent nodes located in a data network, such as a LAN or WAN, or an internetwork, such as the public Internet, or communicate with telecommunication equipment connected to a voice network, such as the PSTN. The term “data network” as used herein is not intended to be limited in any way and should be understood as broadly including any system that allows data communication to occur by and between one or more data communication resources linked by one or more communication pathways. 
    
    
     BRIEF DESCRIPTION OF THE DRAWING 
     The foregoing and other features and advantages of the invention will be apparent from the following more particular description of a preferred embodiment of the invention, as illustrated in the accompanying Drawing, in which: 
     FIG. 1 is a block diagram showing a first embodiment of an exemplary data network telecommunication system for routing communication content between a correspondent node in a voice network and a mobile wireless node in the data network, as the mobile wireless node undergoes handoff from one radio base station to another; 
     FIG. 2 is a block diagram showing a second embodiment of an exemplary data network telecommunication system for routing communication content between a correspondent node in a data network and a mobile wireless node therein, as the mobile wireless node undergoes handoff from one radio base station to another; and 
     FIG. 3 is a flow diagram showing communication content routing in accordance with the invention. 
    
    
     DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENT 
     Turning now to the figures, wherein like reference numerals represent like elements in all of the several views, FIGS. 1 and 2 illustrate an exemplary telecommunication system  2  for routing communication content, e.g., voice telephony content, video content, data content, etc., between a mobile wireless node and a correspondent network node. In FIG. 1, the correspondent node is located in a voice network  4 , such as the PSTN, and the mobile wireless node is located in a data network  6 , such as the Internet. Two exemplary correspondent nodes in the voice network  4  are shown by way of example only, as wireline subscriber equipment  8  (e.g., a telephone) and wireless subscriber equipment  10  (e.g., a cellular telephone, a PDA, etc.). In FIG. 2, the correspondent node and the mobile wireless node are both located in the data network  6 . The correspondent node could be implemented, by way of example only, as a computer  11 . 
     In FIG. 1, a PSTN/data network gateway  12  routes communication content from the correspondent nodes  8  and  10  to the data network  6 . With the exception of the programming modifications described below, the gateway  12  is conventional in nature and could be implemented, by way of example only, using the PacketStar™ Gateway 1000 product from Lucent Technologies Inc. 
     In both of FIGS. 1 and 2, the data network  6  includes a mobile wireless node  14 . The mobile wireless node  14  could be a cellular telephone or PDA device. It could also be implemented as a portable (e.g., laptop) computer  16  having a voice interface  18  and a radio transceiver  20 . As is conventional, the mobile wireless node  14  communicates via an air interface to fixed radio base stations. Two such base stations are shown in FIG. 1 by reference numerals  22  and  24 . The mobile node  14  could communicate with the base stations  22  and  24  via analog signaling, or one of the digital transmission standards such as Time Division Multiple Access (TDMA) or Code Division Multiple Access (CDMA). 
     Although not shown, each base station  22  and  24  is located to serve a defined geographic area that is commonly known as a cell. Each cell forms part of a cluster of similarly configured cells that are located so as to be “contiguous” to other such cells, albeit with some overlapping of base station signal coverage. As is well known in the art, this contiguous cell arrangement allows mobile wireless nodes to maintain communication with the data network  6  as they pass from cell to cell. This process, known as “call handoff,” results in a mobile wireless node terminating communication with the base station of the cell that the mobile wireless node is leaving, and initiating communication with the base station of the cell that the mobile node is entering. 
     Without intending to limit the scope of the invention, it is contemplated that the telecommunication system and method disclosed herein will in most cases be incorporated in a data network that implements the Mobile IP standards promulgated by the IETF, the GPRS standards proposed by the ETSI, or some variant of one of these standards. In that case, a home agent  26  will be assigned to the mobile wireless node  14 . As described by way of the Background set forth above, the home agent  26  is a router located in the mobile wireless node&#39;s home subnetwork that allows correspondent nodes, such as the nodes  8 ,  10  and  11 , to communicate with the mobile wireless node  14  using the latter&#39;s home address. It does this by maintaining a network interface on the network link indicated by the mobile wireless node&#39;s home address. As also described in the Background set forth above, when the mobile wireless node  14  is registered on a foreign subnetwork, communication content from correspondent nodes is tunneled to an assigned care-of address, which would be the address of a foreign agent. 
     In FIGS. 1 and 2, and with additional now to FIG. 3, it is assumed that the mobile wireless node initially communicates through an air interface to the base station  22 , which is identified as the “Old BSS” (Old Base Station System). As is conventional, when the mobile wireless node  14  first establishes communication with the Old BSS  22 , it undergoes an initial registration procedure whereby a care-of address is assigned and reported to the home agent  26 . This is shown by step S 1  in FIG.  3 . This initial registration procedure could be initiated by either the mobile wireless node  14  or the Old BSS  22 . In the exemplary case illustrated herein, the care-of address is that of a foreign agent  28 , which is identified in FIGS. 1 and 2 as the “Old Foreign Agent.” The Old Foreign Agent  28  is preferably a router in the data network that maintains a link layer connection to the Old BSS  22 . The Old Foreign Agent  28  can be physically placed at any suitable location. For example, it could be implemented as part of Old BSS  22  itself The home agent  26  implements the initial registration by binding the home address of the mobile wireless node  14  with the care-of address of the Old Foreign Agent  28  in a process known as mobility binding. This allows the home agent  26  to intercept communication content addressed to the mobile wireless node&#39;s home address and tunnel it to the Old Foreign Agent  28 . 
     During the time period that the mobile wireless node  14  is communicating with the Old BSS  22 , communication content from correspondent nodes, such as the nodes  8 ,  10  and  11 , is routed (via the gateway  12  for nodes  8  and  10 ) to the home agent  26 . This is shown by step S 2  in FIG.  3 . Using an appropriate conventional packet encapsulation protocol, the home agent  26  tunnels the communication content to the Old Foreign Agent  28 , where it is received, unencapsulated, and delivered to the Old BSS  22  (e.g., via link layer addressing). The tunneling path from the home agent  26  to the Old Foreign Agent  28  is shown by the pathway labeled  30  in FIGS. 1 and 2. For return communications from the mobile wireless node  14  to the correspondent nodes, the home agent  26  is not used. Rather, normal routing occurs over the pathway labeled  32  between the Old Foreign Agent and either the gateway  12  (FIG. 1) or a correspondent node in the data network  6  (e.g., the node  11  in FIG.  2 ). 
     It is assumed now that the mobile wireless node  14  is traveling across a geographic area from the cell served by the Old BSS  22  to the cell served by the base station  24 , which is identified in FIGS. 1 and 2 as the “New BSS.” As this occurs, the process of hand-off negotiation is initiated in the mobile wireless system. This is shown by step S 3  in FIG.  3 . As is known in the art, this process typically includes negotiation by and between several base stations. During the handoff negotiation period, it is assumed that the New BSS  24  is selected as the new base station that will serve the mobile wireless node  14 . It is further assumed that the New BSS  24  connects to the data network  6  via a different link than the Old BSS  22 , and that a new foreign agent is warranted for the mobile wireless node  14 . This new foreign agent is shown by reference numeral  34  in FIGS. 1 and 2, and is identified as the “New Foreign Agent.” It is preferably identical in construction and function as the Old Foreign Agent  28 , except that it maintains a link layer connection to the New BSS  24 . 
     Following initiation of the handoff procedure, but prior to actual handoff of the mobile wireless node  14  to the New BSS  24 , registration is invoked (by either the mobile wireless node  14  or the NEW BSS  24 ) to establish the address of the New Foreign Agent  34  as a new care-of address. This is shown by step S 4  in FIG.  3 . As is in the case of the initial registration procedure described above, the home agent  26  establishes a mobility binding between the mobile wireless node&#39;s home address and the new care-of address corresponding to the New Foreign Agent  34 . However, unlike prior art systems, the home agent  26  does not immediately terminate (de-register) the old mobility binding between the mobile wireless node&#39;s home address and the old care-of address corresponding the Old Foreign Agent  28 . Rather, the home agent  26  continues to maintain both mobility bindings. Moreover, it duplicates the communication content received from the correspondent nodes, such as the nodes  8 ,  10  and  11 , and tunnels the communication content simultaneously to the Old Foreign Agent  28 , via the pathway  30 , and to the New Foreign Agent  34 , via a new pathway labeled  36  in FIGS. 1 and 2. This is shown by step S 5  in FIG.  3 . Again, a separate pathway  38  is used to route return communication content, without using the home agent  26 , between the New Foreign Agent  34  and either the gateway  12  (FIG. 1) or a data network correspondent node (e.g., the node  11  in FIG.  2 ). 
     When handoff of the mobile wireless node  14  actually occurs in the mobile wireless system, the communication content from the home agent  26  is immediately available at the New BSS  24 . This is in contrast to prior art systems where the data network  6  must be aware of the air interface managed by the mobile wireless system, and needs to time the routing of communication content to the Old Foreign Agent  28 , or to the New Foreign Agent  34 , to coincide with the precise moment of handoff. In the telecommunication system  2 , the data network  6  does not need to be concerned with the exact time of handoff. The communication content is provided to both the Old BSS  22  and the New BSS  24 , where it can be transmitted no matter which of the two base stations is currently communicating with the mobile wireless node  14 . This multi-route tunneling preferably continues for as long as necessary to ensure that the handoff procedure has completed. The Old Foreign Agent  28  is then deregistered by the home agent  26 , and the home agent  26  reverts to single route tunneling. This is shown by steps S 6  and S 7  in FIG.  3 . 
     Although FIGS. 1 and 2 illustrate a call handoff involving two base stations, some mobile systems, most notably those using CDMA encoding, may involve more than two base stations in a handoff negotiation. Because of the inherent flexibility of CDMA, a CDMA mobile node can communicate with multiple base stations simultaneously. This allows for the sampling of radio signal quality relative to multiple base stations, so that an informed decision can be made about which base station is the best one to accept the call handoff. It will be appreciated that the invention can be readily adapted to handle any number of base stations engaged in call handoff negotiation; namely, by adding a new foreign agent for each base station and routing the same communication content to all foreign agents (old and new) until the call handoff is completed. 
     The foregoing procedure can be advantageously implemented as a new mobile service protocol implemented in the data network  6 . For example, the procedure described herein can be implemented as an enhancement to the IETF Mobile IP standards described by way of the Background set forth above, with the home agent  26  being programmed to perform communication content duplication and multi-route tunneling, and to implement registration and de-registration requests from the mobile wireless node  14  or the Old BSS  22  and New BSS  24 . 
     Embodiments of the present invention can be used with known mobile wireless systems, such as analog and digital cellular telephone networks, and including TDMA and CDMA networks. However, because the handoff procedures for TDMA and CDMA systems vary, somewhat different implementations of the invention may be required in each case. For example, when a CDMA system implements handoff, a condition may occur where the Old BSS  22  and the New BSS  24  simultaneously receive the same communication content from the mobile wireless node  14 . This communication content will then be duplicately routed over the return pathways  32  and  38  to either the gateway  12  (FIG. 1) or to a correspondent node in the data network  6  (e.g., the node  11  in FIG.  2 ). If additional foreign agents are involved, as may occur in a CDMA system, additional duplication of communication content would result. To accommodate this situation, the packet processing software in the gateway  12 , or in the node  11 , can be programmed to selectively discard the duplicate communication content packets. By way of example only, packets arriving on the pathways  32  and  38  (and possibly additional pathways) could be processed on a first-come/first-served basis. For each duplicated packet arriving on multiple pathways, the first packet arriving on any pathway could be processed and later arriving duplicate packets could be discarded. Alternatively, packet selection could be based on some measurable indication of quality. 
     Accordingly, a system and method for seamless data network communication service during mobile wireless call handoff has been described. While various embodiments have been disclosed, it should be apparent that many variations and alternative embodiments could be implemented in accordance with the invention. It is understood, therefore, that the invention is not to be in any way limited except in accordance with the spirit of the appended claims and their equivalents.