Abstract:
A method enables mobility in a wireless network consuming less resources than prior art methods. The wireless network contains mobile devices, base stations, and a network server connected to the Internet. The method uses IP headers within the wireless networks, in place of Mobile IP headers. A base station changes mobile device&#39;s home address to a care-of address when the mobile device sends a packet to the network server. Correspondingly, the base station changes the care-of address to the mobile device&#39;s home address when the network server sends a packet to the mobile device.

Description:
RELATED APPLICATIONS  
         [0001]    This application claims the benefit of priority under 35 U.S.C. §120 to PCT application PCT/FI02/00958, filed Nov. 28, 2002 which in turn claims priority to Finnish Patent Application No. 20020061 filed on Jan. 11, 2002.  
           [0002]    1. Field of the Invention  
           [0003]    The present invention generally relates to wireless networks and Internet protocol (IP) communications.  
           [0004]    2. Background of the Invention  
           [0005]    A WLAN (Wireless Local Area Network) is an alternative for a fixed network. The users of a WLAN can move within the network area and use the network services. When users are outside of their home WLAN, they need special arrangements for using network services. “Mobile IP” is a protocol supporting wireless access over the Internet. Mobile IP is outlined in an Internet Engineering Task Force (IETF) proposal entitled “IP Mobility Support (RFC 2002, 1996), C. E. Perkins—Editor. The use of Mobile IP requires that each mobile device is identified by a fixed home address and associated with a home agent. The home agent always knows the location of the mobile device. When a mobile device and a correspondent node communicate via the Internet, packets sent from the correspondent node to the mobile device are directed to the home agent. The correspondent node is any service or any communication device outside of the mobile device&#39;s home network.  
           [0006]    If the mobile device is away from its home network, the home agent forwards packets within a so-called IP-in-IP tunnel to an assigned care-of address registered with the mobile device.  
           [0007]    [0007]FIG. 1 shows a packet with a Mobile IP header. The Mobile IP header includes two Internet peripheral (IP) addresses: a care-of address  101  and a fixed home address  102 . The care-of address  101  discloses the current location of a mobile device. The fixed home address  102  can be used for identifying the mobile device, because each mobile device has a unique fixed home address. The payload of the packet  103  contains an IP header  104 . The IP header includes two IP address fields: a sender address  105  and a receiver address  106 .  
           [0008]    [0008]FIG. 2 depicts the operation of Mobile IP. It uses a so-called two-legged routing scheme for routing packets. In the first leg a packet is routed from a correspondent node to the home agent of a mobile device  201 . In the second leg the packet is routed from the home agent to the mobile device  202 . The mobile device sends its own packet direct to the IP address of the correspondent node  203 . Of course, it is possible that the mobile device communicates with another mobile device. In that case, the home agent of the other mobile device routes the packet from the correspondent node to the current IP address of the other mobile device (not shown).  
           [0009]    Mobile IP provides a reasonably effective framework for macro-mobility, i.e. it allows mobile device users to roam away from a home network without disrupting mobile device applications. However, Mobile IP does not effectively support micro-mobility, i.e. handoffs of a mobile device. Each base station covers a limited geographic area. Each base station has its own IP address and a set of care-of addresses. A base station attaches one of its care-of addresses to a mobile device when the mobile device moves into the base station&#39;s area. A handoff is needed every time the mobile device moves between the geographic areas of two base stations. A handoff changes the mobile device&#39;s care-of address. The handoff also forces the mobile device to notify the home agent of its new care-of address. Therefore the use of Mobile IP results in a lot of signalling between the mobile device and the home agent.  
           [0010]    The prior art methods which support micro-mobility in wireless networks, such as WLANs, suffer from several drawbacks.  
           [0011]    The first drawback is that mobile device handoffs consume a great deal of time and resources within the mobile device&#39;s current network.  
           [0012]    The second drawback is that the handoffs consume the resources of the Internet, because the care-of addresses must be transmitted to the home agents.  
           [0013]    The third drawback is that an additional header, such as a Mobile IP header, increases the need of processing and transmission capacity.  
           [0014]    The fourth drawback relates to a fact that only a few mobile devices and network elements so far have Mobile IP capability, and they are high-priced. Therefore it is profitable to utilize well-tested and low-priced TCP/IP mobile devices and network elements.  
         SUMMARY OF THE INVENTION  
         [0015]    The main objective of the invention is to effectively support micro-mobility in wireless networks. The inventive method uses solely IP headers, thus each packet to be transmitted includes one IP address for identifying a sender and another IP address for identifying a receiver.  
           [0016]    As mentioned above, each mobile device has an IP address which identifies the mobile device. A base station transmits packets between the mobile device&#39;s home address and the network server&#39;s address. For example, the mobile device&#39;s home address could be 194.89.126.17 and the network server&#39;s address could be 194.89.126.4. Each base station has a set of care-of addresses. The set may include, for example, IP address 10.17.6.100.  
           [0017]    A mobile device communicates with the correspondent node via a base station and via a network server. The base station replaces the mobile device&#39;s home address, i.e. the sender address of the packet, with one of its care-of addresses. For example, the base station could replace IP address 194.89.126.17 with 10.17.6.100. Then the base station transmits the packet to the network server that replaces IP address 10.17.6.100 with 194.89.126.17 and transmits the packet to the correspondent node. In response to the transmitted packet, the network server receives a response packet from the correspondent node. The response packet includes IP address 194.89.126.17 placed in the receiver address of the packet. The network server replaces IP address 194.89.126.17 with 10.17.6.100 and transmits it to the base station. The base station replaces the care-of address with the mobile device&#39;s address, i.e. 10.17.6.100 is replaced with 194.89.126.17. After that the base station transmits the packet to the mobile device.  
           [0018]    To summarize: the IP address changes made by a base station and network server result in that a mobile device can move in the area of its home network during a communication. The communication continues in spite of possible handoffs.  
           [0019]    Another objective of the invention is to support mobile devices&#39; mobility between wireless networks. The objectives of the invention are achieved in the patent claims.  
           [0020]    The inventive method can be utilized when implementing a network system composed of a network server and at least one base station. The network server could also be termed a network controller or a handover control unit (HOCU), but it is termed “network server” for shortness and generality.  
           [0021]    The inventive method can be considered as a communications protocol for wireless networks containing base stations and a network server.  
           [0022]    Communication between a mobile device and a base station may be IP-based, but it also may be, for example, Bluetooth-based. Communication between the base station and a network server is IP-based when the communication concerns the IP-packets to be transmitted. In addition, the base station and the network server communicate to transmit a mapping between the mobile device&#39;s home address and its care-of address. The said communication may or may not be IP-based. 
       
    
    
     BRIEF DESCRIPTION OF THE DRAWINGS  
       [0023]    The invention is described more closely with reference to the accompanying drawings, in which  
         [0024]    [0024]FIG. 1 shows a header used in Mobile IP,  
         [0025]    [0025]FIG. 2 depicts the operation of Mobile IP,  
         [0026]    [0026]FIG. 3 shows an example of a wireless network,  
         [0027]    [0027]FIG. 4 shows use examples of the method,  
         [0028]    [0028]FIG. 5 shows an example of a mobile device list,  
         [0029]    [0029]FIG. 6 shows four embodiments of updating a care-of database,  
         [0030]    [0030]FIG. 7 depicts communication when a mobile device locates in a foreign network,  
         [0031]    [0031]FIG. 8 shows two mobile device lists with a visitor address field,  
         [0032]    [0032]FIG. 9 shows two embodiments of updating care-of databases when a mobile device logs into a foreign network, 
     
    
     DETAILED DESCRIPTION OF THE INVENTION  
       [0033]    The method uses solely IP headers, thus there is less data to be processed and transmitted compared to Mobile IP communications. In addition, only few mobile devices and network elements have Mobile IP capability and they are high-priced, thus it is technically and economically profitable to choose the method that operates with or without Mobile IP.  
         [0034]    [0034]FIG. 3 shows an example of a wireless network in which the method can be utilized. The wireless network contains a network server, base stations, and mobile devices. The network server can also be termed a domain router because Internet traffic passes through it. When a mobile device  301  sends a packet to a correspondent node, the packet is transmitted via a base station  302 , via the network server  303 , and via the Internet  304  to the correspondent node  305 . The base station  302  and base stations  306  and  307  compose a home network of the mobile device  301 . Each base station  302 ,  306 , and  307  has its own transmission area in which the base station serves mobile devices. In this example one of the base stations, i.e. base station  307 , communicates with the network server via the Internet.  
         [0035]    The transmission areas of base stations are termed sub-networks. Thus, in FIG. 3, the home network of the mobile device  301  is composed of three sub-networks.  
         [0036]    [0036]FIG. 4A shows a first use example of the method. The example concerns a mobile device  401 , a base station  402 , a network server  403 , and a correspondent node  404 . In the first use example the mobile device  401  is located in the sub-network area of the base station  402  all the time when the mobile device  401  communicates with the correspondent node  404 . The mobile device  401  has IP-address 194.89.189.17, which is its home address, and the base station  402  has care-of address 10.17.6.100, which is one of its free care-of addresses. The mobile device sends packet  1  to the correspondent node via the base station and via the network server as follows. The base station receives  405  packet  1  and replaces  406  the sender address of packet  1  with, for example, care-of address 10.17.6.100. Thus, the sender address 194.89.126.17 is replaced  406  by 10.17.6.100. The base station transmits  407  packet  1  to the network server, which replaces  408 , the sender address of packet  1  with the mobile device&#39;s home address, i.e. 10.17.6.100 is replaced with 194.89.126.17. Then the network server transmits  409  packet  1  to the correspondent node  404 , which receives packet  1  and sends  410  packet  2  to the network server as response to packet  1 . The network server replaces  411  the receiver address of packet  2  with the care-of address, i.e. 194.89.126.17 is replaced with 10.17.6.100. The network server transmits  412  packet  2  to the base station, which replaces  413  care-of address 10.17.6.100 with 194.89.126.17 and transmits the packet to the mobile device.  
         [0037]    [0037]FIG. 4B shows a second use example of the method. The second use example concerns the same mobile device  401 , network server  403 , and correspondent node  404  as the first use example, but this time the mobile device  401  moves out of the area of the base station  402  during the time when the mobile device  401  communicates with the correspondent node  404 . We may suppose, for example, that the mobile device is located in a moving train and therefore the mobile device is moved from the sub-network area of the base station  402  to the sub-network area of another base station  415  during the time period between phases  408  and  411 . Then the other base station  415  starts to serve the mobile device  401 . Phases  405 - 409  are the same as in the first use example, thus phases  405 - 409  are omitted from FIG. 4B. Also phase  410  is the same as in the first use example, but phase  410  is included in FIG. 4B to point out that the correspondent node does not need information about the location of the mobile device  401 . The correspondent node sends packet  2  to the network server. The network server receives  410  packet  2  and replaces  416  the receiver address of packet  2  with a current care-of address of the mobile device  401 . The current care-of address could be, for example, 12.17.6.103 belonging to the base station  415 . Thus, the receiver address 194.89.126.17 is replaced with 12.17.6.103. The network server transmits  417  packet  2  to the base station  415 . The base station  415  replaces  418  the receiver address with the mobile device&#39;s home address, i.e. 12.17.6.103 is replaced with 194.89.126.17.  
         [0038]    As described above in phases  411  and  416 , a network server places the mobile device&#39;s current care-of address to the receiver address of a packet that a correspondent node has sent to the said mobile device. Thus, mobile device&#39;s care-of address may change during a communication. Because the care-of addresses of numerous mobile devices may change during their communications, a network server at least needs data structures to manage mobile devices&#39; IP addresses and care-of addresses.  
         [0039]    [0039]FIG. 5 shows an example of a mobile device list. The said list is composed of records, which include at least two fields: one field for the mobile device&#39;s home address and another field for the mobile device&#39;s current care-of address. The first field is termed “home address” and the latter field is termed “care-of address”. Of course, a record could include more than these two fields. In record  501  in the home address field the value 194.89.126.17 is stored and in the care-of address field the value 10.17.6.100 is stored. In record  502  in the home address field the value 194.89.126.21 is stored and in the care-of address field the value 12.17.6.103 is stored. Thus, the records of the mobile device list are mappings between the mobile devices&#39; home addresses and the mobile devices&#39; current care-of addresses. In addition, a second list could be composed of records that are mappings between the mobile devices&#39; home addresses and the mobile devices&#39; identifiers. For example, medium access control number (MAC number) can be used as a mobile device&#39;s identifier (ID). The second list could include all mobile devices belonging to a certain network. Instead, the mobile device list shown in FIG. 5 could include only those mobile devices that are switched on and/or logged into the certain network.  
         [0040]    The mobile device list and the second list can be used as follows. When the home address of a mobile device is known, the home address can be used as a search key, i.e. the home address identifies a certain record of the mobile device list. Correspondingly, when the mobile device ID is known, the mobile device ID can be used as a search key because it identifies a certain record in the second list. The said record includes the home address, which can be used as a search key when finding a corresponding record in the mobile device list.  
         [0041]    The base stations of a network may also have mobile device lists, but each base station has its own list including only those mobile devices that are located in the transmission area of the base station. Then each base station has its own IP address space from which it allocates a free care-of address to a mobile device.  
         [0042]    We may consider that a care-of database includes at least one mobile device list. In addition, we may consider that the care-of database is up to date so that a network server and a base station obtain a mapping between the home address and the care-of address of a mobile device from the care-of database when the network server and the base station replaces IP-addresses as described in FIG. 4. The care-of database can be implemented differently by using various data structures and database models.  
         [0043]    [0043]FIG. 6 shows four embodiments of updating a care-of database. In each embodiment a base station  601  and a network server  602  communicate with each other, and the base station performs a handoff  603  for the mobile device. The handoff is performed because the mobile device has moved from the area of a second base station to the area of the base station  601 .  
         [0044]    In the first embodiment (FIG. 6A) the base station  601  allocates a free care-of address  604  from its care-of address set. Of course, the care-of addresses, which are already included in the base station&#39;s mobile device list, are not free and cannot be allocated. Then the base station updates the care-of database  605 . In more detail, the base station creates a new record having the mobile device&#39;s home address and the care-of address and adds  605  the said record to its mobile device list. Finally, the base station transmits the mobile device&#39;s home address and care-of address  606  to the network server  602 . The network server updates the care-of database  607 . In more detail, the network server updates its own mobile device list and, in addition, the network server informs the second base station in which area the mobile device was previously located, when the second base station can remove a record related to the said mobile device from its mobile device list.  
         [0045]    In the second embodiment (FIG. 6B) the base station  601  transmits  608  the home address of a mobile device to the network server  602 . The network server allocates  609  a care-of address to the mobile device and updates  610  the care-of database. Also in this (second) embodiment, as well as in the third and fourth embodiment, the network server informs a second base station in which area the mobile device was previously located, when the second base station removes a record related to the said mobile device from its mobile device list. Finally, the network server sends  611  the care-of address to the base station that updates  612  the care-of database.  
         [0046]    In the following embodiments the base station  601  does not obtain or does not accept the mobile device&#39;s home address when performing the handoff  603 . Instead, the base station obtains the mobile device&#39;s ID, such as a MAC number. Then the base station transmits the mobile device&#39;s ID to the network server and as response to the ID transmitted, the network server transmits the mobile device&#39;s home address to the base station. In one case, the network server uses an ID as a search key to obtain the corresponding home address. In more detail, the network server uses a list composed of mappings between the mobile devices&#39; home addresses and the mobile devices&#39; identifiers to find the corresponding home address. In another case, the network server allocates a home address from a set of home addresses. However, to keep the patent application text simple, we have used the phrase “a network server obtains a home address” throughout the patent application.  
         [0047]    In the third embodiment (FIG. 6C) The base station allocates  613  a care-of address and transmits  614  the mobile device ID and the care-of address to the network server  602 . The network server obtains  615  the home address of the mobile device and updates  616  the care-of database. In more detail, the network server uses the mobile device ID as a search key and places the care-of address in a record identified by the search key. Then the network server sends  617  the home address to the base station that updates  618  the care-of database.  
         [0048]    In the fourth embodiment (FIG. 6D) the base station  601  transmits  619  the mobile device ID to the network server  602 . The network server obtains  620  the home address of the mobile device, allocates  621  a care-of address to the mobile device, and updates  622  the care-of database. Then the network server sends  623  the home address and the care-of address to the base station that updates  624  the care-of database.  
         [0049]    In all the above-described embodiments the base station  601  and the network server may handle several mobile devices at once. This reduces the transmission need between the network server and the base station. Therefore, in FIG. 6, the network server could send fifty home address and care-of address pairs to the base station.  
         [0050]    When a mobile device logins a network the operation is the same as in a handoff. Therefore the above-described embodiments also concern the login of a mobile device.  
         [0051]    The method further enables that a mobile device can move from its home network to a foreign network, if the foreign network supports the method. The method can be considered as a protocol for wireless networks. The protocol, which is termed MagelP, is an alternative to MobilelP.  
         [0052]    [0052]FIG. 7 depicts communication when a mobile device locates in a foreign network that supports MagelP. The mobile device  701  having home address 194.89.126.17 communicates with a correspondent node  702 . The mobile device is located in a sub-network area belonging to the foreign network  703 . The sub-network&#39;s care-of addresses are in this example form  33 .* . * . * . A foreign network server  704  uses and updates the foreign network&#39;s care-of database. In addition, the foreign network server communicates with the mobile device&#39;s home network server  705  in order that the home network server can update a care-of database. The foreign network and home network  706  have their own care-of databases. From the point of view of the home network server  705 , the foreign network server is similar to a base station belonging to a home network  706 . When the foreign network server and the home network server have finished the login of the mobile device  701 , the mobile device can communicate with the correspondent node  702 , so that packets are transmitted, in both ways, via a base station belonging to the foreign network, via the foreign network server  704 , and via the home network server  705 . Some IP-tunneling can be used when transmitting the packets between the foreign network server and the home network server.  
         [0053]    When a mobile device logs into a foreign network, the owner of the mobile device should declare the home network server&#39;s IP address to the foreign network server so that the foreign network server can communicate with the home network server. Alternatively, the foreign network server may obtain the home network server&#39;s IP address from a certain service.  
         [0054]    A new field termed “visitor address” can be added to the network server&#39;s mobile device list to support visiting. Visiting means that a mobile device is currently located in a foreign network.  
         [0055]    [0055]FIG. 8 shows two mobile device lists in which records include the visitor address field. When the visitor address field includes the value 0, it means the record relates to the network&#39;s own mobile device. When the visitor address field includes another value than 0, it means the record relates to a foreign mobile device. We may suppose that the home network server shown in FIG. 7 uses a mobile device list  801  and the foreign network server shown in FIG. 7 uses a mobile device list  802 . The upper mobile device list  801  includes records  803  and  804 . The home network server can determine that home network&#39;s IP-addresses are form  194  . * . * . * and it can also determine all the forms of the home network&#39;s care-of addresses. Thus, the record  803  indicates that a mobile device having home address 194.89.126.17 is located in a foreign network because the mobile device&#39;s care-of address starts with “201”. The said IP address is termed “visitor address”. Correspondingly, the record  804  indicates that a mobile device having home address 201.12.133.2 is from a certain foreign network because the said IP address starts with “ 201 ”. The lower mobile device list  802  includes records  805  and  806 . The record  805  indicates that the mobile device having home address 194.89.126.17 is located in a sub-network area whose care-of addresses are in the form  33 .* . * . * . The visitor address field of the record  805  includes visitor address 201.90.44.13 allocated by the foreign network server. The same IP-address, 201.90.44.13, is written in the record  803  belonging to the mobile device list updated by the home network server, but the IP-address is placed in the care-of field of the record. The visiting is possible in both ways. For example, a record  806  indicates that a mobile device having ID 201.12.133.2 is on visit in the network of which IP-addresses are in the form  194 .* . * . * .  
         [0056]    When a mobile device is visiting in a foreign network, the mobile devices&#39; home network server has the mobile device&#39;s visitor address, but not its care-of address. Only the foreign network server has the mobile device&#39;s care-of address indicating the mobile device&#39;s current sub-network area. When the mobile device moves in the foreign network from a sub-network area to another sub-network area, the base station of the other sub-network area performs a handoff. After the handoff, the said base station and the foreign network server updates the foreign network&#39;s care-of database. The four embodiments shown in FIGS. 6A, 6B,  6 C, and  6 D also concern the updating of the foreign network&#39;s care-of database.  
         [0057]    [0057]FIG. 9 shows two embodiments for updating care-of databases when a mobile device logs into a foreign network. In both embodiments a foreign network server  901  and a home network server  902  communicate with each other in order to update the care-of databases.  
         [0058]    In the first embodiment (FIG. 9A) it is supposed that the foreign network server  901  obtains the mobile device&#39;s home address when the mobile device logs into the foreign network. Then the foreign network server allocates  903  a visitor address to the mobile device and updates  904  the care-of database. In more detail, the foreign network server places the visitor address in the visitor address field of a certain record belonging to a mobile device list. The record  805  in FIG. 8 is an example of the said record. Finally, the foreign network server transmits the mobile device&#39;s home address and visitor address to the home network server  902 . The home network server updates the home network&#39;s care-of database. In more detail, the home network server places the visitor address transmitted in the care-of field of a certain record of a mobile device list. The record  803  in FIG. 8 is an example of the said record.  
         [0059]    In the second embodiment (FIG. 9B) it is supposed that the foreign network server  901  does not obtain the mobile device&#39;s home address when the mobile device logs into the foreign network. Instead, it obtains the mobile device&#39;s ID, such as a MAC number. The foreign network server allocates  903  a visitor address to the mobile device, and transmits  907  the mobile device&#39;s ID and visitor address to the home network server  902 . The home network server obtains  908  a home address of the mobile device. Then the home network server updates  909  the home network&#39;s care-of database as in the first embodiment. Finally, the home network server sends  910  the mobile device&#39;s home address to the foreign network server that updates  911  the care-of database as in the first embodiment.  
         [0060]    In the both above-described embodiments the network servers may handle several mobile devices at once, for example, fifty mobile devices at once. This reduces the transmission need between the network servers.  
         [0061]    As shown in FIG. 7, communication packets can be transmitted, in both ways, via the base station belonging to the foreign network, via the foreign network server  704 , and via the home network server  705 . However, it is possible to support the mobility of a mobile device which either don&#39;t have a home network or which don&#39;t have a reason to use a home address belonging its home network. We would like to point out that a home address does not have to be a fixed IP address, which is intended to a certain mobile device. We would also like to point out that a home address does not have to be intended only to the mobile devices of a home network. For example, we may suppose that the mobile device  701  have no reason to use a home address belonging to its home network  706 . Then the mobile device  701  obtains a home address from the foreign network server  704  in order to communicate with the correspondent node  702 .  
         [0062]    As described above, the method can be considered as a protocol for wireless networks. Wireless networks may be e.g. WLANs or mobile networks. Of course, a network must support the protocol, i.e. MagelP. The network should also have some authentication procedure to prohibit communication hijacking. Communication between a mobile device and a base station may be IP-based, but it also may be, for example, Bluetooth-based. Communication between the base station and a network server is IP-based when the communication concerns the IP-packets to be transmitted. In addition, the base station and the network server may have communication that is not IP-based.  
         [0063]    The method can be especially utilized when implementing a network system composed of a network server and at least one base station.