Abstract:
A wireless local area network system includes a plurality of base stations connected in a wired local area network. A mobile wireless station can roam through communication cells defined by the base stations. The base stations transmit beacon messages at regular intervals. The mobile station determines the communications quality of the beacon message for the cell in which the mobile station is currently located and if this quality becomes unacceptable, switches to a search mode wherein beacon messages from any base station are received and their communications quality is determined. The mobile station switches to communicate with a base station providing a beacon message with an acceptable communications quality.

Description:
BACKGROUND OF THE INVENTION 
     The present invention relates to a method of operating a wireless local area network. 
     With a view toward obviating the need for wired cabling connections between stations in local area networks (LANs), wireless local area networks have been developed and are now commercially available. However, the coverage area of wireless LANs is restricted especially in an indoor environment, due to the presence of structural features such as walls and floors in buildings, for example. Also it may be desirable for stations in a wireless LAN to communicate with remote facilities or resources such as servers. Therefore, it has been proposed to connect the wireless LAN via interface devices to a backbone LAN such as a wired LAN, which can be connected to the remote facilities and/or other wireless LANs. 
     Furthermore, where a mobile wireless station, such as a portable data processing unit provided with a wireless transceiver, for example, moves around it may move in and out of the range of various interface devices connected to the backbone LAN. It is therefore necessary for the mobile station to hand over communication from one interface device to another as it moves around. 
     European Patent Application No. 0 483 544 discloses a wireless communications network including a plurality of mobile data stations in bidirectional communication with header stations which are connected into a wired LAN and which define communication cells. If a mobile data station crosses over from one cell to another, the mobile station determines a new header station for communications access to the wired LAN. The header stations periodically transmit broadcast messages identifying the header station and the loading factors at the header station. Using these messages a mobile station will determine all header stations that are potential owners of the mobile station and maintain in memory a list of potential owner stations. If the current owner station remains a potential owner, and the signal strength of the current owner station is acceptable, then the current owner is retained. Otherwise, the mobile station determines a new header station based on the loading factors of the potential owner stations, and, if the loading factors are equal, the signal strengths of the signals received at the mobile station. This procedure has the disadvantages of using memory space in the mobile station to maintain in memory the list of potential owner stations, and the need to constantly monitor the messages periodically transmitted by the base stations. 
     International Patent Application No. WO 92/19059 discloses a system which includes a cable-based network to which are attached controllers which maintain a portable device in communication with the network for data transfer. The portable unit transmits polling packets at regular intervals. Normally, a response packet is received from the current controller. If no response packet is received for a predetermined number of attempts the portable unit initiates a procedure for registering with a new controller. This procedure involves transmitting a packet from the portable unit requesting a response from any controllers, transmitting a response packet from each controller receiving the request, selecting one of the responding controllers at the portable unit according to criteria that enhance data transfer, and transmitting from the portable unit a packet identifying the selected controller to complete registration. This procedure has the disadvantage of the need for the portable unit to transmit a request packet to initiate the handover procedure. 
     SUMMARY OF THE INVENTION 
     In accordance with one embodiment of the present invention, there is provided a method of operating a local area network system which includes a plurality of base stations and a mobile station which includes the steps of (1) transmitting beacon messages from the base station at regular intervals, each beacon message including an identification of the base station transmitting the message, (2) operating the mobile station in a normal mode wherein beacon messages from a first base station only are monitored, (3) determining a communications quality value for the beacon messages from the first base station, (4) determining if the communications quality value becomes unacceptable, and if so, changing the operating mode of the mobile station to a search operating mode wherein beacon messages from any of the base stations may be monitored, (5) selecting a base station providing an acceptable communications quality value for monitored beacon messages, and (6) changing the operating mode of the mobile station to the normal station operating mode wherein beacon messages from the selected base station only are monitored. 
     In accordance with another embodiment of the present invention, there is provided a method of operating a wireless local area network system having a plurality of base stations which transmit beacon messages to a mobile station. The method includes the steps of (1) operating the mobile station in a normal mode wherein the mobile station monitors beacon messages from a first base station only, and (2) changing the operating mode of the mobile station to a search operating mode wherein beacon messages from any of the base stations may be monitored if a communications quality value of the beacon messages from the first base station is unacceptable. 
     It is therefore an object of the present invention to provide a new and useful method of operating a wireless local area network system. 
     It is another object of the present invention to provide an improved method of operating a wireless local area network system. 
     It is yet another object of the present invention to provide a method of operating a wireless local area network system wherein mobile station handover may be achieved in a simple manner. 
     The above and other objects, features, and advantages of the present invention will become apparent from the following description and the attached drawings. 
    
    
     BRIEF DESCRIPTION OF THE DRAWINGS 
     FIG. 1 shows a block diagram of a wireless LAN system in which the present invention is embodied; 
     FIG. 2 shows a block diagram of a mobile wireless station included in the system of FIG. 1; 
     FIG. 3 shows a block diagram of a base station included in the system of FIG. 1; 
     FIG. 4 shows the format of a beacon message; and 
     FIGS. 5A and 5B are a flowchart illustrating the operation of a mobile wireless station during a handover procedure. 
    
    
     DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENT 
     While the invention is susceptible to various modifications and alternative forms, a specific embodiment thereof has been shown by way of example in the drawings and will herein be described in detail. It should be understood, however, that it is not intended to limit the invention to the particular form disclosed, but on the contrary, the intention is to cover all modifications, equivalents, and alternatives falling within the spirit and scope of the invention as defined by the appended claims. 
     Referring now to FIG. 1, there is shown a block diagram of a wireless LAN (local area network) system  10 , embodying the present invention. The LAN system  10  includes a backbone LAN  12  which is a wired cable-based LAN, and which includes a cable  14  connecting a plurality of base stations  16 , referenced individually as base stations  16 - 1 ,  16 - 2 , . . . ,  16 -N. The base stations  16  have antennas  18 , referenced individually as  18 - 1 ,  18 - 2 , . . . ,  18 -N. A server  19  is connected to the cable  14  to provide a server function for devices communicating with the LAN  12 . Also included in the system  10  is a mobile station  20  having an antenna  22 . Of course there may be more than one mobile station operating in the system. The base stations have coverage areas, referred to as cells  24 , referenced individually as cell  24 - 1 ,  24 - 2 , . . . ,  24 -N. 
     Referring now to FIG. 2, there is shown a block diagram of the mobile station  20 . The mobile station includes a wireless transceiver  30  coupled to the antenna  22  and to a bus  32 . The mobile station  20  also includes, connected to the bus  32 , a processor  34 , a memory  36 , and (optionally) a keyboard  38  and display  40 . Other devices such as a printer (not shown) may be connected to the bus  32 . A communications quality measurement circuit  42  is connected to the antenna  22  and to the bus  32 , and is adapted to measure the communications quality of signals received on the antenna  22 , as will be explained in more detail hereinafter. 
     Referring now to FIG. 3, there is shown a block diagram of a base station  16 . The base station  16  includes a wireless transceiver  50  coupled to the antenna  18  and to a bus  52 . A further transceiver  54 , connected to the bus  52 , connects the base station  16  to the cable  14  of the backbone LAN  12  (FIG.  1 ). Also connected to the bus  52  are a processor  56 , a memory  58  and a filtering database  60  which stores data identifying the location of devices in the LAN system  10 , in relation to the base station  16 . An interference level measurement circuit  62  is connected to the antenna  18  and to the bus  52  and is adapted to measure the interference level experienced by the base station  16 . 
     It should be understood that in operation of the LAN system  10 , FIG. 1, information messages are transmitted between the mobile station  20  and the backbone LAN  12  via one of the base stations  16 . It should be understood that such messages contain a network ID portion which identifies the particular cell  24  in which the message is transmitted. It will be appreciated that as the mobile station  20  moves around it may leave the coverage area of one cell  24  and enter the coverage area of a new cell  24 . The present invention is concerned with a handover procedure for handing over communication from one base station  16  to another base station  16  when the mobile station  20  moves in such manner. 
     In this connection, it is provided that the base stations  16  transmit beacon messages at regular intervals, for example at intervals of one second. Referring to FIG. 4, there is shown the format of such a beacon message  70 . The beacon message  70  includes a first preamble portion (PR- 1 )  72 , a network ID (NWID) portion  74 , a second preamble portion (PR- 2 )  76 , a start delimiter (SD) portion  78 , a destination address (DA) portion  80 , which is a broadcast address ensuring reception by any mobile station in the cell  24  covered by the base station  16  such as the mobile station  20 , a source address (SA) portion  82 , which is the base station address for the base station&#39;s cell  24 , a length portion  84 , a message identification (MID) portion  86 , which identifies the message  70  as a beacon message, a wired address (WA) portion  88 , which is the base station address for the wired LAN  12 , an interference level (IL) portion  90 , which represents the background interference (noise) level as measured by the interference level measurement circuit  62  (FIG.  3 ), a sequence number (SEQ) portion  92 , a pad (PAD) portion  94  and a frame check (FCS) portion  96 . 
     As mentioned hereinabove, the base stations  16  transmit beacon messages  70  at regular intervals. By arranging that the interval between successive beacon messages  70  is slightly different for each base station  16 , the incidence of collisions between beacon messages is reduced. Thus station  16 - 1  may transmit beacon messages  70  at intervals of one second, whereas station  16 - 2  may transmit beacon messages at intervals of one second plus one microsecond, for example, and station  16 - 3  may transmit beacon messages at intervals of one second plus two microseconds, for example. With this procedure, the loss of beacon messages due to collisions is rendered very small. 
     Referring now to FIGS. 5A and 5B, there is shown a flowchart  100  illustrating the operation of the mobile station  20  during a cell handover procedure. Initially, as shown in block  102 , the station is operating in its normal mode, communicating with a current base station, assumed to be base station  16 - 1  (FIG.  1 ). When a beacon message is received from the current base station (block  104 ), the flowchart proceeds to block  106 , where it is seen that the communications quality of this beacon message is determined. This measurement involves first determining the maximum of the background noise (interference) level measured at the mobile station  20  and the background noise (interference) level measured at the base station  16  and included as the IL portion  90  of the beacon message  70  received from the base station  16 . The measured signal strength of the beacon message  70  at the mobile station  20  is then divided by the determined maximum noise level to provide a communications quality value based on a signal-to-noise ratio. It is then determined whether or not this communications quality value is above or below a first predetermined threshold value TH 1 . If above, the flowchart returns to block  104  via arrow  110 . If below, the flowchart moves to block  112 , which shows that the mobile station  20  changes to a search mode of operation. In the search mode of operation, the receiver portion of the transceiver (FIG. 2) switches to a so-called cell-promiscuous mode of operation, in which beacon messages from any of the base stations  16  may be received and processed. This may be achieved by ignoring the network ID portion  74  (FIG. 4) of beacon messages. 
     Assume, as shown in block  116 , that the first beacon message  70  received by the mobile station  20  in search mode is from base station X. As shown in block  118 , the communications quality of this beacon message is measured by the communications quality measurement circuit  42  (FIG. 2) as described hereinabove, by first determining the maximum of the background noise (interference) levels at the mobile station  20  and base station X, using the IL portion  90  of the beacon message  70 , and then dividing the signal level value of the beacon message  70  by the thus determined maximum noise level. A determination is then made as to whether or not this communications quality value is above a second predetermined threshold value TH 2  (block  120 ). If below, then the flowchart proceeds as shown via line  122  to block  116 , and the next beacon message from a base station  16  is awaited. If above, then the flowchart proceeds to block  123  where it is determined whether the base station X is or is not the current base station. If the base station X is not the current base station, then the mobile station  20  switches to the new base station, i.e. switches to a state wherein messages having the network ID of the new base station are processed (block  124 ), and the receiver is returned from the cell-promiscuous mode to the normal mode (block  126 ). The flowchart then returns to block  104  as shown by line  128 . If base station X is the current base station, the flowchart proceeds directly via line  130  to block  126 , and the receiver returns to normal cell mode operation, in communication with the current base station  16 . 
     In connection with the above-described handover operation, it should be understood that the value of TH 2  is greater than the value of TH 1 , so that the stop cell search threshold TH 2  indicates a better communications quality than the start cell search threshold TH 1 . This arrangement is effective as a hysteresis feature, and avoids oscillation between base stations  16  where coverage areas  24  overlap. 
     Many modifications to the described embodiment are possible. Thus, in a simplified arrangement, measurement of the noise level at the base stations  16  could be omitted and the beacon messages  70  (FIG. 4) transmitted without the IL portion  90 . Communications quality at the mobile station  20  would then be determined on the basis of a local signal strength to local noise value ratio only. In another modification, when the mobile station is in the cell-promiscuous search mode of operation, searching for a new base station  16 , a further comparison is made to determine whether the communication quality value of the current base station  16  is less than the threshold value TH 1 , and there is no base station  16  with a communications quality value above the stop cell search threshold value TH 2 . If so, then the mobile station  20  will switch over to any base station which has a communications quality value above the lower threshold value TH 3 . This fast cell search procedure assists in enabling the mobile station  20  to maintain communication with a base station  16  when the communications quality becomes so low that a new base station  16  has to be identified within as short a time as possible. 
     While the invention has been illustrated and described in detail in the drawings and foregoing description, such illustration and description is to be considered as exemplary and not restrictive in character, it being understood that only the preferred embodiment has been shown and described and that all changes and modifications that come within the spirit of the invention are desired to be protected.