Abstract:
A data communication system capable of forwarding IP-addressed data to devices as such devices move among networks having different IP addresses is disclosed herein. The system includes first and second networks containing first and second pluralities of nodes. At least one of the first plurality of nodes is adapted to receive data transmissions from an external IP-based network. Upon joining the first network, this device is assigned an IP address which remains with it irrespective of whether it moves beyond the range of the first network. When this device roams into the vicinity of the second network, data addressed to the device which is received at the first network is forwarded via at least one node of the second network to the device.

Description:
CROSS REFERENCE TO RELATED PATENT APPLICATIONS 
     The entire disclosures of copending U.S. Patent Application entitled “Distributed Management of an Extended Network Containing Short-Range Wireless Links” and copending U.S. Patent Application entitled “Implementation of Power Control in a Wireless Overlay Network”, both filed on even date herewith, are hereby incorporated by reference. 
     FIELD OF THE INVENTION 
     The present invention relates generally to communication systems, and more particularly to a method and associated apparatus for allowing a device to continue communication via the Internet Protocol (“IP”) even while moving among networks having different associated IP addresses. 
     BACKGROUND OF THE INVENTION 
     Computer networks allow multiple computers, peripherals and other information storage, retrieval or processing devices to share data. Each device attached to a network is typically referred to as a node on the network, or a node that is part of the network. Local Area Networks (“LANs”) have historically consisted of nodes interconnected by physical telecommunications media (e.g., coaxial cable, twisted pair wire, or fiber optics). Recently wireless LANS, the nodes of which are not connected by means of a physical medium, have started to appear in the market. These wireless LANs communicate by means of infra-red (IR), radio or other signals. One of the benefits of using wireless LANs is that cabling is not required. This is a particularly useful feature for mobile nodes such as laptop and notebook computers, PDAs (personal digital assistants), and the like. If equipped with an appropriate wireless adapter, the mobile nodes can move around within a predefined coverage area and remain connected to the network. 
     In order for the nodes of a particular network to be able to communicate with each other, some sort of common addressing scheme must be implemented. In a traditional wired network, one very common method of communicating between nodes is to utilize Internet Protocol (“IP”) addressing. When a node initially joins a network operating with the Internet Protocol, it is assigned an IP address consisting of at least two portions: a network address and a destination address. IP addresses are typically fixed and remain with the node irrespective of whether the node relocates to another network. In a traditional wired network, the fixed nature of IP addresses is not problematic because nodes are stationary and consequently do not migrate to networks outside of their original home network. Recently, wireless nodes have been added to traditionally hard-wired networks. Like all devices adhering to the Internet Protocol, mobile devices are accorded IP addresses having a network portion and a device portion. The network portion identifies the mobile device&#39;s “home network” while the device portion distinguishes the mobile device from other members of its home network. The routing of data packets through the Internet is generally exclusively based upon the network portion of the destination IP address. Specifically, Internet routers extract the destination network address from a given data packet and then forward that packet to the appropriate network. Once the packet reaches the appropriate network, the network server examines the device portion of the IP address and forwards the packet to the appropriate device. 
     When a mobile device is located in its home network, data packets addressed to a particular mobile device are simply sent to such device upon being received at the home network&#39;s access point. This process is transparent to the user so long as the mobile unit is located within range of its home network. However, when the mobile device travels outside of its home network data packets cannot easily be forwarded thereto and are often “dropped”, which results in loss of data. 
     Several methods have been proposed to allow mobile devices to continue. IP-based communication even when moving between networks having different IP addresses. One such technique requires that the mobile unit report changes in its address to the access point or network manager in its home network. A router located in its home network stores each new IP address for the mobile unit and forwards any incoming packets to the mobile device&#39;s new IP address. An alternate approach involves establishing a pseudo presence, also called a “spoofed address,” at the mobile&#39;s home network location. However, when mobile devices frequently move between networks having different IP addresses, these address-mapping and forwarding techniques can result in packets being forwarded to multiple locations, which may cause confusion and loss of data 
     SUMMARY OF THE INVENTION 
     Briefly, therefore, the present invention relates to a data communication system capable of forwarding IP-addressed data to devices as such devices move among networks having different IP addresses. The present invention includes first and second networks containing first and second pluralities of nodes. At least one of the first plurality of nodes is adapted to receive data transmissions from an external IP-based network. Upon joining the first network, this device is assigned an IP address which remains with it irrespective of whether it moves beyond the range of the first network (i.e., the device&#39;s “home” network). When this device roams into the vicinity of the second network, data addressed to the device which is received at the first network is forwarded via at least one node of the second network to the roaming device. 
     In a preferred embodiment, each of the nodes in the first and second networks broadcast messages indicating the services that it offers and the nodes that are within its range. These broadcasts allow the mobile device to determine which devices it can use to create a connection between itself and the local access point in its home network. When the mobile device moves beyond its home network, it reestablishes a connection to its local access point using whatever combination of nodes it deems as being most efficient. Once this connection is made, the local access point is able to forward data to the mobile device. 
     In an alternate embodiment, an internetworking node participates in both the first and second networks, and receives a first set of network information relating to the first network from the first plurality of nodes. In an another embodiment, the mobile device is capable of anticipating when it is about to lose contact with one of the nodes that is providing a connection between it and its home local access point. When the mobile device makes this determination, it attempts to establish an alternative route for the connection to its local access point. If the mobile device is of a type requiring continuous connection to an external network, an additional embodiment allows the mobile device to simultaneously maintain more than one connection between itself and its local access point. This minimizes the risk that data packets addressed to the mobile device will be “dropped”. 
    
    
     
       BRIEF DESCRIPTION OF THE DRAWINGS 
       In the accompanying drawings: 
         FIG. 1  shows a data communication system comprising two wireless networks and a means for forwarding data between the networks. 
         FIG. 2  contains a data communication system consisting of two wireless networks and an internetworking node in accordance with a preferred embodiment of the present invention. 
         FIG. 3  is a block diagram illustratively representing the components of a wireless node and associated software configured in accordance with a preferred embodiment of the present invention. 
     
    
    
     DETAILED DESCRIPTION OF THE INVENTION 
     The present invention is more fully described with reference to  FIGS. 1-3 . An exemplary implementation of the invention is discussed and illustrated with reference to its implementation using wireless networks predicated on the proposed “Bluetooth” wireless communications protocol. This protocol contemplates the grouping of physically proximate wireless nodes, and is described in  Specification of the Bluetooth System , v0.8, Jan. 22, 1999 (and in subsequent revisions thereof). It should be understood that this invention is not limited to such a wireless protocol, and could be similarly implemented using other types of fixed or wireless networks. As is described hereinafter, the present invention discloses a method and apparatus for relaying data addressed to a mobile device nominally associated with a first network when such device becomes associated with other networks. 
       FIG. 1  illustrates a data communication system  10  consisting of a hard-wired network  12  and first and second wireless networks  20  and  22  containing network nodes A 1  and A 2  respectively. The first and second wireless networks  20  and  22  each contain a plurality of wireless nodes. For purposes of clarity, only wireless nodes L 1  and L 3  are depicted in wireless networks  20  and  22 . It should nonetheless be understood that the present invention is equally applicable to implementations in which first and second wireless networks include numerous wireless nodes. The system  10  also contains a relay node L 2  connecting the two wireless networks, a first mobile device D 1 , and a hard-wired device D 2  connected to the hard-wired network  12 . In a typical configuration, D 2  would actually be a network of devices rather than a single device. The device D 2  may comprise either a network of devices or simply a single device. Since the actual number of devices represented by D 2  is irrelevant for purposes of explanation of the present invention, device D 2  will hereinafter be referred to as a single device. 
     For purposes of illustration, it is assumed that certain of the wireless nodes depicted in  FIG. 1  are mobile relative to one another. In the network architecture of  FIG. 1 , each node within the first network  20  is within the wireless coverage area of network access node A 1  and each node within the second network  22  is within the wireless coverage area of network access node A 2 . The network access nodes A 1  and A 2  serve as conduits to the external network  12  for the first and second wireless networks  20  and  22 , respectively. The access nodes A 1  and A 2 , as well as a relay node L 2 , are also disposed for wireless communication with the nodes L 1  and L 3 . 
     Because the nodes within the wireless networks  20  and  22  are not necessarily within transmission range of all the other wireless nodes within the same networks, each wireless node may not be able to monitor all of the traffic within its network. For example, wireless node L 1  may be able to “listen” to wireless node L 2  but may not be able to monitor transmissions from wireless node L 3 . In a preferred implementation each wireless node transmits an advertisement identifying its address and the services it offers. Each such advertisement also incorporates the address of all other wireless nodes from which advertisements are received by the node transmitting a given advertisement. Based on this information, the mobile device D 1  can determine all nodes participating in the networks and the service offered by each. As used herein the term “services” encompasses, without limitation, the capability of a given node to relay message information to and from one or more outside networks. Accordingly, the advertisement issued by each wireless node may also identify the other networks within which the wireless node is capable of communication and the services offered by each. 
     Referring to  FIG. 1 , when D 1  first seeks to join the first wireless network  20  it transmits a signal to the network access node A 1  via wireless node L 1  so that it can be assigned an IP address by the Internet Service Provider (“ISP”) providing network access via node A 1 . Through this transmission, device D 1  establishes the first wireless network  20  as its home network; that is, it designates the network address of the first wireless network  20  as its network address. Because device D 1  is not hard-wired to node A 1 , it is necessary for device D 1  to access node A 1  via a wireless communication link. In the exemplary implementation of  FIG. 1 , device D 1  determines that it can establish a connection to node A 1  through node L 1  by listening to the periodic advertisements issued by nodes A 1  and L 1 . As was mentioned above, these broadcasts contain network connectivity information pertinent to of all of the devices within range of A 1  and L 1 . For example, A 1 &#39;s broadcast would indicate that it is connected to the hard-wired network  12 , and is capable of communication with nodes L 1  and L 2 . Similarly, node L 1 &#39;s broadcast would reveal that it is within range of, and capable of communication with, nodes A 1 , L 2  and D 1 . 
     Data may be routed to node D 1  in a conventional manner when node D 1  is within its home network  20 . In this situation data destined for node D 1  is sent to the IP address of node A 1  from the external network  12 . On the basis of the advertisements broadcast by L 1 , node A 1  determines that node L 1  has established a connection to D 1 . Node A 1  then sends the data received from the external network  12  to node D 1  via node L 1 . 
     Routing data to D 1  using IP addressing becomes problematic when D 1  moves outside of its home network  20 , since node D 1 &#39;s IP address does not automatically change to reflect its new location within a different network. As a consequence, all the data intended to be sent to node D 1  will continue to be routed over the external network  12  to node A 1 . If node A 1  does not have a separate connection to device D 1  when data packets arrive, node A 1  will either drop the data packets or queue them for retransmission on the assumption that D 1  will establish a new connection with A 1 . 
     Referring to  FIG. 1 , when D 1  relocates to a position within the wireless coverage area of network  22  it listens to the connectivity information broadcast by the nodes within its range (device D 1  is shown in phantom in network  22 ). If device D 1  becomes located sufficiently near node L 3 , it may listen to its connectivity advertisement and determine a path over which data may be routed from node A 1 . After making this determination, D 1  requests a connection to node A 1  via a node along the chosen path (e.g., node L 3 ). In order to establish a connection between nodes D 1  and A 1 , node L 3  establishes a connection to node L 2  which in turn establishes a connection to node A 1 . Once these connections have been made, node A 1  will forward any data packets addressed to D 1  that it has received over the hard-wired network  12 . 
       FIG. 2  depicts an alternate embodiment of the present invention in which a wireless node L 2 ′ exists as a member of both first and second wireless networks  20 ′ and  22 ′. The node L 2 ′ facilitates internetworking between the networks  20 ′ and  22 ′ in the manner described in the aforementioned copending Patent Application entitled “Distributed Management of an Extended Network Containing Short-Range Wireless Links”. The wireless node L 2 ′, or “internetworking node”, understands the protocols of both the first and second wireless networks  20 ′ and  22 ′ and is capable of transparently relaying data traffic therebetween. 
     Referring again to  FIG. 2 , once device D 1 ′ begins moving outside of its home network  20 ′ it may anticipate the loss of its connection to the network access node A 1 ′. This is because device D 1 ′ monitors the signal strength of the advertisements being transmitted from L 1 ′, and will take notice when this signal strength drops below a predefined threshold. Once device D 1 ′ determines that a dropped connection between itself and node L 1 ′ is imminent, it will “listen” to the advertisements being broadcast by other wireless nodes within its range to determine if it can establish a simultaneous connection to node A 1 ′. In the exemplary implementation of  FIG. 2 , device D 1 ′ determines that it may establish communication with node A 1 ′ via node L 2 ′. Accordingly, device D 1 ′ then drops its connection with L 1 ′ after establishing a connection with node L 2 ′. In this way device D 1 ′ endeavors to minimize the risk of loss of data packets by briefly maintaining a simultaneous connection to two distinct wireless nodes. In applications where preventing any loss of data packets is very important, device D 1 ′ may maintain such a simultaneous connection continuously or for substantially longer periods of time. 
       FIG. 3  is a block diagram illustratively representing the components of a wireless node  100  and associated software configured in accordance with a preferred embodiment of the present invention. Referring to  FIG. 3 , the wireless node  100  may be in the form of an electronic device (e.g., a laptop or desktop computer, hand-held electronic organizer, or printer) containing a first wireless adapter card  104  and a first RF transceiver  106  disposed for communication in accordance with a first network communication standard. The wireless node  100  may also optionally include a second wireless adapter card  108  and a second RF transceiver  110  for communicating in accordance with a second network communication standard. The second wireless adapter card  108  and the second RF transceiver  110  may be included within the wireless node  100  when, for example, it is anticipated that the wireless node may participate in networks operative in accordance with different network standards. For example, internetworking node L 2 ′ would preferably be configured with different wireless transceiver and associated adapter cards to the extent the first network  20 ′ and second network  22 ′ were governed by differing network communication standards. The wireless node  100  may also include a LAN adapter card  112  for facilitating communication with a wired LAN, or alternately a wireline modem for effecting communication through the PSTN. 
     Each wireless adapter card and transceiver is controlled by a CPU  114  operative to execute program instructions of the various software routines  122  stored in memory  126 . Within wireless nodes, a network resources table  130  is updated in response to internet management broadcasts by transmitting nodes of the individual wireless networks. The network resources table  130  stores the network address and services offered by each wireless node. In the case of internetworking node L 2 ′, these advertised services may include the services provided by a wireless node from a neighboring network in communication with the internetworking node. 
     Included among the software routines  122  within internetworking nodes is a forwarding routine  138  for forwarding messages to the wired LAN via the LAN adapter card  112 , or to another wireless node via one of the wireless transceivers  106 ,  110  and associated wireless adapter card  104 ,  108 . An advertisement generating routine  140  operates to generate the advertisements issued by the wireless node  100  which specify its network address and services offered. 
     Although the above application has been described primarily with reference to specific embodiments, one skilled in the art can readily appreciate that the teachings of the present invention may be applied in other communication contexts. For example, in certain embodiments the first and second wireless networks described above could instead comprise networks of fixed devices linked by hard-wired local area networks (“LANs”). Assuming a wireless or other communication link could be established between a pair of devices within the different networks, data could be forwarded among such networks in accordance with the invention upon movement of a device from one network to the other. Thus the application is meant only to be limited by the scope of the appended claims.