Abstract:
There is disclosed a method of helping mobile stations such as voice over IP devices to roam between wireless access points, by each access point transmitting the MAC address of a spanning tree algorithm root switch of the local network domain. This MAC address is used by mobile stations to detect if two access points are in a common network domain.

Description:
CROSS-REFERENCE AND PRIORITY CLAIM 
       [0001]    This application is a filing under 35 U.S.C. 371 from PCT/GB2005/004386 filed Nov. 14, 2005, which claims the benefit of European (GB) Application No. 04257037.4 filed Nov. 12, 2004, the disclosures of which are hereby incorporated by reference. 
     
    
     FIELD OF THE INVENTION 
       [0002]    The present invention relates to network stations moving between network access points, and in particular, but not exclusively, to the roaming of stations, such as voice over IP devices or laptop computers, communicating over wireless connections in an IP network environment. 
       DISCUSSION OF THE PRIOR ART 
       [0003]      FIG. 1  illustrates elements of data network, and in particular of a single network domain. Four network segments are indicated by broken lines. The network segments are interconnected by bridges  10  and  12 . A router  14  provides a connection between bridge  12  and other networks such as the wider Internet  16 . The solid connections of the network are provided by Ethernet, while the broken lines are provided by wireless connections, for example, operating under an IEEE 802.11 protocol. 
         [0004]    The term “network domain” is intended to refer to a collection of network elements communicating directly, without intervening routers, at the second “data link” layer of the OSI model or similar protocol layer models, typically using a flat addressing scheme. A network domain may therefore also be referred to as a layer 2 network, in contrast to a layer 3 network which uses a hierarchical addressing scheme such as IP. Each subnet of a layer 3 network is generally implemented using a separate underlying layer 2 network. 
         [0005]    The bridges  10  and  12  divide the illustrated network into network segments so as to reduce the total amount of network traffic at any one point and thereby reduce the load on the network components. Typical bridges are designed to learn automatically which network messages need to be passed between segments and which messages do not. 
         [0006]    Two of the network segments include static or fixed stations  18 , such as desktop computers. Three of the network segments include wireless access points  20  which enable mobile stations  22  to connect with the rest of the network. The router, the bridges and any other devices providing message switching functions within the network may be referred to as network switches. The wireless access points may operate as switches to control the movement of messages into and out of the wireless environment. 
         [0007]    Typically, all of the network components illustrated in  FIG. 1  will reside on the same IP subnet connected to other subnets via the router  14 , and each mobile station  22  will operate using a static or dynamically allocated IP address belonging to that subnet. As a mobile station  22  moves it may wish or need to disconnect from one access point and connect to another. This may be handled smoothly if both access points are in the same network domain. This is because the IP address of the mobile station will remain valid because the mobile station  22  can still access the other subnets via the same router. However, should the new access point belong to a new network domain then the IP address of the mobile station will not be valid because the previous router  14  cannot be accessed, and any established network session will be interrupted. 
         [0008]    Sometimes a network domain such as that illustrated in  FIG. 1  may contain two IP subnets running concurrently, for example, by means of two different routers providing connections to the external network  16 . 
         [0009]    It would be desirable to enable a mobile station to recognize whether or not an access point is on the same network domain as the one to which it is currently connected, so that the mobile station can preferably select an access point on the same network domain and minimize service disruption. Wireless access points already transmit, in probe response and broadcast beacon messages, a service set identifier (SSID) identifying the local wireless network. However, an organization with multiple wireless networks will in general use the same SSID for all networks, so it is not useful for distinguishing between them. 
         [0010]    There is a need to address these and other problems of the related prior art. 
       SUMMARY 
       [0011]    An embodiment provides a method of enabling stations to roam between network access points, in which each access point belongs to a network domain. A separate spanning tree algorithm is operated within each network domain, and switches in a particular network domain are aware of the spanning tree algorithm root switch for that network domain. Each access point then receives the MAC address of the root switch, or another unique physical network identifier for the network domain, and transmits this information for reception by said stations. 
         [0012]    Since the MAC address of the root switch is globally unique, yet the same for the whole network domain on which a particular instance of a spanning tree algorithm is being used, a mobile station roaming between access points can use the received root switch MAC address to identify whether a second access point is on the same network domain as a first access point, and use this information in controlling its own roaming process. 
         [0013]    The access points may be wireless access points communicating with the mobile stations using a wireless protocol such as an 802.11 protocol, but may also be physical plug and socket access points or similar. The switches in the network may be connected using Ethernet protocols or similar. 
         [0014]    Suitable unique network domain identifiers other than the spanning tree root switch MAC address may be used. For example, a different algorithm which identifies the MAC address of a unique element of a network domain may be used to derive an identifier. 
         [0015]    An embodiment further provides a network, an access point and a mobile station to put into effect the above method. 
     
    
     
       BRIEF DESCRIPTION OF THE DRAWINGS 
         [0016]    Embodiments of the invention will now be described by way of example only, with reference to the drawings, of which: 
           [0017]      FIG. 1  shows an example prior art network combining Ethernet and wireless connections; 
           [0018]      FIG. 2  illustrates the need for a spanning tree protocol in a network domain; 
           [0019]      FIG. 3  shows the flow of a root switch identity in the network of  FIG. 2 ; and 
           [0020]      FIG. 4  illustrates aspects of a wireless access point and a mobile station. 
       
    
    
     DETAILED DESCRIPTION OF PREFERRED EMBODIMENTS 
       [0021]    Referring now to  FIG. 2  there is shown a network which is built using a router, bridges, fixed stations, access points and mobile stations in the same way as the network of  FIG. 1 . However, this network contains a network link  30  that joins two bridges in such a way that a closed loop in the network is formed. A further bridge link  32  duplicates a link provided by an adjacent bridge, for example in case the duplicated bridge fails. 
         [0022]    Multiple paths linking one network segment to another can cause various problems such as infinite message looping. To avoid these problems, while retaining the benefits of increased network connectivity and robustness, the bridges and other switches communicate with each other to implement a spanning tree algorithm or protocol, for example as defined in IEEE Standard 802.1D. This protocol is used to avoid transmission on selected links, such as links  30  and  32  illustrated in  FIG. 2 , until such time as a change in the network allows or requires them to be reactivated. 
         [0023]    The switches in a sub network operating under a spanning tree protocol gather information on other switches in the sub network through an exchange of data messages in the form of bridge protocol data units. This exchange of messages results in the election of a root switch for a spanning tree network topology. In  FIG. 2 , bridge  34  has been selected as the root switch. The identity of the root switch may be changed from time to time by the spanning tree protocol, for example to adapt to physical changes in the network, but in practice such changes are rare. 
         [0024]      FIG. 3  illustrates an embodiment set in the context of the network of  FIG. 2 . For clarity, only the active connections between the bridges and wireless access points are shown. An additional access point  42  is located on a different network domain to the rest of the illustrated components. Each active element in the network, and in particular each bridge, has a physical network address, or MAC (media access control) address. For example, for an Ethernet connection this is an Ethernet address. The MAC address  36  of the bridge selected as root switch  34  is transmitted, under the spanning tree protocol, to each of the other bridges, as illustrated by the arrows on the network connections, and can be read by the wireless access points, even if they are not configured to join in the spanning tree protocol, as long as they can extract the root switch MAC from the appropriate network messages. 
         [0025]    The access points then include the root switch MAC address  36  in their wireless transmissions, including for example in probe responses and beacon broadcasts. Each MAC address is globally unique, so there is no chance of two different root switches having the same MAC address. 
         [0026]    The mobile stations are configured to read the root switch MAC address from the access point transmissions and use this information in roaming functions to determine if a second access point is in the same network domain as a first access point. For example, when mobile station  38  may wish to disconnect from a present access point  40  and connect to a new access point, it checks whether the root switch MAC address is the same for both the present and new access points, and if so, concludes that the two access points are on the same network domain. This information may be used by the mobile station to decide whether or not to roam from the present to the new access point, or to decide which of two or more new access points  42 ,  44  it should connect to. For example, the information may be used to avoid roaming to an access point  42  which resides on a different network domain. 
         [0027]      FIG. 4  illustrates in more detail an access point  50  and a mobile station  60 . A control process  52  in the access point reads spanning tree protocol messages from the network, extracts the MAC address of the spanning tree root switch and stores this in a memory  54 . The control process causes this root MAC address to be transmitted to mobile station  60 . 
         [0028]    A control process  62  in mobile station  60  reads the root MAC address from the transmissions made by the access point  50  and stores it in a memory  64 . The MAC IDs received from other access points to which the mobile station could connect are also stored in the memory  64 . From time to time a roaming function  66  reads the MAC IDs from the memory  64  for use in determining to which new access point a roaming transfer may be made. 
         [0029]    As mentioned above, a spanning tree protocol may sometimes cause the identity of the root switch to change, for example to adapt to changes in the state or topology of the network. In practice, this happens only rarely, but to allow for this possibility, the mobile stations are programmed such that there is no firm assumption that a change in root switch MAC address is caused by a change in network domain. In fact, when the apparent root switch MAC address changes, there is no disadvantage over prior art systems in a mobile station going through a conventional roaming procedure, for example by pinging a router to establish whether it is still accessible. 
         [0030]    If a spanning tree protocol is not being used in a particular network domain then an access point according to the embodiment will not receive any spanning tree protocol messages, so will not know the MAC address of the root switch. Thus, this information will not be broadcast to the mobile stations which will then use a conventional roaming procedure. 
         [0031]    Although embodiments have been described in which the MAC address of a root switch node is used as a network domain identification tag, the network elements may provide a facility for setting a different, for example a user selected network domain identification tag. Other network domain identification tags may also be used, for example other MAC level or level 2 network identifiers. 
         [0032]    While the present invention has been described with reference to one or more particular embodiments, those skilled in the art will recognize that many changes may be made thereto without departing from the scope of the invention. Accordingly, each of the foregoing embodiments and obvious variations thereof is contemplated as falling within the scope of the claimed invention, as is set forth in the following claims.