WO2003007552A2 - Method for dynamically electing the master among computers or stations of a mobile wireless local area network - Google Patents

Method for dynamically electing the master among computers or stations of a mobile wireless local area network Download PDF

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Publication number
WO2003007552A2
WO2003007552A2 PCT/IT2002/000456 IT0200456W WO03007552A2 WO 2003007552 A2 WO2003007552 A2 WO 2003007552A2 IT 0200456 W IT0200456 W IT 0200456W WO 03007552 A2 WO03007552 A2 WO 03007552A2
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WIPO (PCT)
Prior art keywords
station
master
stations
cluster
network
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PCT/IT2002/000456
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French (fr)
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WO2003007552A3 (en
Inventor
Gaetano Ferrari
Massimiliano Torregiani
Francesco Delli Priscoli
Roberto Cusani
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Universita' Degli Studi Di Roma 'la Sapienza'
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Priority to AU2002354599A priority Critical patent/AU2002354599A1/en
Publication of WO2003007552A2 publication Critical patent/WO2003007552A2/en
Publication of WO2003007552A3 publication Critical patent/WO2003007552A3/en

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    • H—ELECTRICITY
    • H04—ELECTRIC COMMUNICATION TECHNIQUE
    • H04W—WIRELESS COMMUNICATION NETWORKS
    • H04W84/00—Network topologies
    • H04W84/18—Self-organising networks, e.g. ad-hoc networks or sensor networks
    • H04W84/20—Master-slave selection or change arrangements

Definitions

  • This invention relates to a method for dynamically electing the controller or Master among an assembly of computers or stations of a wireless local area network or WLAN, namely a radio connection network, which enables to select, among those connected to the network, the computer having the best radio-visibility properties, in order to perform the role of Master unit of the network, while minimising the information transfer from the old Master to the new Master, so as to make such transfer transparent (or "seamless") to the other stations , thereby making the concerned network robust, flexible, configurable and adaptable to the movements of the stations.
  • the present invention also relates to the instruments needed to perform the method and to the apparatuses performing the method.
  • WLAN wireless local area networks
  • one or more, preferably mobile computers or stations are connected among each other in order to form an assembly, so-called “cluster", in which they are in condition of mutual electromagnetic visibility, namely they are adapted to directly exchange data between each other without passing through intermediate stations, by utilising a radio communication channel.
  • Each station is provided with a radio transceiver apparatus.
  • the radio channel is shared among the various stations and, therefore, it is necessary to regulate the access thereto in order to enable ordered and collisionless communications among them.
  • a station in the network is intended to perform this medium access control (or MAC) function and it is named controller or Master, while the other stations of the network are named Slave or servoed stations.
  • controller or Master medium access control
  • the station operating as Master carries out a particular assembly of functionalities that enable to decide and control the access to the radio channel from all stations, inclusive of the Master one itself.
  • the Slave units are servoed to the decisions of the Master unit and provide it with all necessary information to take the control decisions.
  • PCT Patent Application No. WO9429986 discloses a network of stations, one of which performs the Master role and establishes the radio communication frequency which the other stations must lock on.
  • the selection of the Master unit is carried out by following a designation method according to which two different possible selection procedures are provided.
  • the network always designates a pre-established station as initial Master station, usually defined by a network administrator when the network is installed.
  • any station can perform the Master role.
  • any station on connecting to the network, carries out a check to ascertain that a Master unit is present and, should it ascertain that no Master unit is present, then it itself takes on the Master role, preferably in compliance to a selection criterium in which some rules are to be fulfilled, such as minimising the signal power and the multipath propagation, as well as the stability achieved in the previous connections to the network.
  • a selection criterium in which some rules are to be fulfilled, such as minimising the signal power and the multipath propagation, as well as the stability achieved in the previous connections to the network.
  • WO9429986 has certain drawbacks mainly connected with the fact that the selection of the Master unit among the stations of the network is based upon constancy of the electromagnetic visibility conditions between the stations, both in respect of the previous connections and in respect of the current connection.
  • the electromagnetic configuration be modified , for instance due to modifications in the spatial configuration, the initially designated Master station will continue operating as Master even if the connection quality is degraded. This severely limits or even inhibits the station mobility.
  • the information amount handled by the Master station in controlling the network is extremely large, thereby making the transfer of the Master functionality from a station to an other station particularly cumbersome: this suggests to perform such transfer only in cases of operative failure of the current Master station. In such case, however, the selection of a new Master station entails an interruption of the communications currently taking place between the stations of the network, which, consequently, are affected by said transfer of the Master functionality.
  • a second object of the present invention is to minimise the transfer of the information needed to control the network from the old Master to the new Master station, so as to make such transfer reliable and transparent (or "seamless") for the other stations.
  • said at least one physical layer measurement can comprise:
  • bit decision error (BER) frequency of the signals each station receives from any other station BER
  • noise both the thermal noise and the possible interferences of various kinds are to be meant.
  • said one or more quality figures can comprise, in respect of any actively connected station,
  • said one or more rules can comprise:
  • the best average value is always to be referred to the measured physical parameter and it can be the maximum average value, for instance in connection with the power of the received signals or with the sig ⁇ al-to-noise ratio, or the minimum average value, for instance in connection with BER.
  • said one or more quality figures can comprise, in respect of any actively connected station,
  • said one or more rules can comprise: - the selection of the station having the best sum value.
  • the best sum is always to be referred to the measured physical parameter and it can be the maximum sum (for instance in connection with the power of the received signals or with the signal-to-noise ratio), or the minimum sum (for instance in connection with BER).
  • said one or more quality figures can comprise, in respect of any actively connected station,
  • said one or more rules can comprise: - the selection of the station having the best one among said worst values.
  • said election procedure can be periodically carried out by the station operating as Master, preferably upon expiring of a manually or automatically adjustable time interval.
  • said election procedure can be carried out by the station operating as Master every time a station is actively connected to the network.
  • said election procedure can be carried out by the station operating as Master before switching off.
  • said election procedure can be carried out by any station operating as Slave upon detecting the absence of a station operating as Master.
  • the method can comprise, subsequently to said election procedure when a station operating as Slave is elected to operate as new Master by the station currently operating as Master, a procedure aimed at transferring information needed for operation of the network, such information including only those information items that do not vary during the current connections, preferably the connection identification codes of the actively connected stations and/or the identification codes of the sender and addressee stations and/or traffic characteristic information and/or information relating to the quality requirements of the communications.
  • a computer or station comprising a radio transceiver apparatus, characterised in that it is adapted to perform the method for dynamic election of the controller, or Master, among the stations of a cluster of a WLAN as above described.
  • the above said station is a mobile station. It is further subject-matter of this invention a mobile radio connection wireless local area network or WLAN, characterised in that it comprises at least one computer or station as above illustrated.
  • a mobile radio connection wireless local area network or WLAN characterised in that it comprises at least one computer or station as above illustrated.
  • the above said mobile network can comprise one only cluster, the actively connected stations directly communicating with one another. Such kind of communication being known to those skilled in the art under the term "single-hop”.
  • the above said mobile network can comprise at least two clusters, any two actively connected stations belonging to two different clusters communicating with one another by means of at least one intermediate station. Such kind of communication being known to those skilled in the art under the term "multi-hop”.
  • FIG. 1 shows a diagram of a WLAN network comprising stations adapted to perform the method for dynamic election of the Master according to this invention
  • Figure 2 shows a flow chart diagrammatically illustrating the steps of the method according to this invention, as performed by a computer or station firstly accessing the network of figure 1 ,
  • Figure 3 shows a flow chart diagrammatically illustrating a first transfer procedure for the information needed to control the network according to the method for dynamic election of the Master of this invention
  • Figure 4 shows a flow chart diagrammatically illustrating a second transfer procedure for the information needed to control the network according to the method for dynamic election of the Master of this invention.
  • FIG. 1 By referring to Figure 1 , one can observe an illustrative but not limitative diagram of a wireless local area network or WLAN comprising a cluster of four computers or stations.
  • the cluster of figure 1 comprises a first stationary computer 1 , a first and a second mobile computers 2 and 3 and a hand-held computer 4.
  • the stations of the cluster are provided with a transceiver radio apparatus and directly communicate with one another, without interposition of intermediate stations, by utilising a radio communication channel.
  • the above said stations are all potentially adapted to operate as Master stations.
  • the Master role is not decided a priori , but it can be dynamically allotted during the time, when the network conditions vary, either in respect of the topology or in respect of the involved radio channel.
  • one only station operates as Master, while all remaining ones operate as Slave stations.
  • the functionality's relating to the control of the stations accessing the network, both from the Master side and from the Slave side, are resident in the MAC (Medium Access Control) of the ISO-OSI protocol stack of a station.
  • the station operating in the role of Master performs an assembly of functionalities that are not performed when it operates in the role of Slave and that enable it to co-ordinate itself and the remaining stations of the network, the Slave stations, in accessing the communication channel.
  • Such functionalities comprise:
  • FIG. 2 it illustrates the procedure for initialise a station, such procedure being the same for all stations, which is performed on switching-on the concerned station and, consequently, at its first accessing the network.
  • the Master functionality is performed by the first station to be switched-on.
  • Such a variation can be due, for instance, to switching-on other stations.
  • the station currently operating as Master could be in a no more suitable position from a connection quality point of view, thereby making it necessary to elect the station that at this time is placed in the best position with respect to the other ones as Master, in order to improve the operation of the network itself.
  • a similar variation in the topologic configuration of the network could be caused by one or more displacements of the involved stations, so that the spatial mutual positions of the stations are changed thereby modifying the quality of the radio connections established between them.
  • An election of a new Master station could turn out to be necessary also in this circumstance.
  • a particular case occurs when the current Master unit is moved away from the other stations, up to causing a loss of the magnetic visibility thereof or up to getting out of the cluster. In this case, it is necessary that the transfer of the Master role be effected before the current Master unit completely loses its capability to communicate with the other stations.
  • the method according to this invention also provides for transfer of the Master role when the current Master unit is switched off or fails, because otherwise the network could not go on operating due to lack of co-ordination in accessing the tra ⁇ smissive means.
  • the left hand portion of the chart illustrates the steps carried out by the current Master unit that, when one of the above mentioned events occurs, cause a new Master unit to be elected.
  • the right hand portion of the chart illustrates the steps carried out by the Slave elected as new Master unit, such steps being carried out after the related actions performed by the current Master unit (such chronological succession is designated by a dashed arrow in Figure 3).
  • Figure 4 shows the flow chart illustrating the Soft Master Handover protocol in the particular case in which the current Master unit is switched off.
  • the selection of the Master takes place by means of an election procedure, as designated in the above mentioned flow charts of Figures 3 and 4 by reference numeral 5, in which all currently active stations participate.
  • the Master election procedure indicates the station that has the best visibility level within the network and that, therefore, appears to be the most convenient in performing the Master role.
  • the protocol is carried out in the current Master station and it is entered in asynchronous manner or in synchronous manner: in the first case, the election protocol is carried out every time an event causing a change in the network topology occurs, such as, for instance, when the Master station is switched off or a new station is associated to the network; in the second case, the election protocol is carried out when a periodical time interval expires (for instance, calculated a priori based upon the mobility grade of the stations), as provided by a timer, in order to check whether the topology of the network can be changed in such a way as to make the transfer of the Master functionality convenient, in view of the fact that, for instance, a Slave unit is in better radio-visibility conditions than the current Master unit.
  • a periodical time interval expires (for instance, calculated a priori based upon the mobility grade of the stations), as provided by a timer, in order to check whether the topology of the network can be changed in such a way as to make the transfer of the Master functionality convenient, in view
  • the election method selects as Master the station that is placed in the position believed as the best one based upon physical layer measurements in respect of the quality of the radio channels established between the various stations.
  • the best station is the one that, as a function of one or more rules based upon one or more quality figures depending on at least one physical layer measurement in respect of the quality of all radio channels established between the actively connected stations of the cluster, offers the best connection quality.
  • such physical layer measurements can comprise: the power of the signals each station receives from any other station, and/or the signal/noise ratio of the signals each station receives from any other station (where the noise can be due either to thermal noise or to interference) and/or the bit decision error frequency of the signals each station receives from any other station.
  • Said one or more quality figures can comprise, in respect of any actively connected station, the average values of said at least one physic layer measurements in respect of the quality of all radio connection channels existing with the other actively connected stations in the cluster, and/or the total sums of said at least one physic layer measurements in respect of the quality of all radio connection channels existing with the other actively connected stations in the cluster, and/or the worst values of said at least one physic layer measurements in respect of the quality of all radio connection channels existing with the other actively connected stations in the cluster.
  • the procedure terminates and the Master unit is not changed. If, on the contrary, the station in the best position is one of the Slave stations, the procedure is prosecuted by transferring the Master functionality to the latter, under application of a suitably designed protocol. Obviously, when the current Master unit is going to be switched off, it is not considered as possible new Master.
  • the above Master functionality hand-over protocol provides for transferring all information needed for controlling the network to the station just elected as new Master unit. The information transfer is minimised in view of the possibility to utilise the radio communicated information circulating in the network or "broadcast" information. Furthermore, this transfer is carried out without interrupting the operation of the network itself, thereby making it transparent to the other Slave stations (“seamless hand-over") that have no need to interrupt any possibly established connections.
  • the information items the Master unit needs to carry out its role are related to the established connections and are of two kinds: a) "static” information items, namely information items that do not vary during an established connection, such as, for instance, the connection identification codes, sender and addressee stations, traffic characteristic information, service quality requirements; and b) "dynamic” information items, namely information items that can vary during an established connection, such as information items relating to the buffer status, for instance the number of packets in queue.

Abstract

This invention relates to a method for dynamically electing the controller or Master among an assembly of computers or stations in condition of reciprocal electromagnetic visibility, or 'cluster', said cluster belonging to a radio communication wireless local area network or WLAN, said cluster comprising no more than one station operating as Master at each time at least one station of the cluster is actively connected to the network,the other stations operating as dependant or Slave, such method comprising a procedure for electing the Master among all stations of the cluster actively connected to the network, characterised in that said election procedure designates as Master the station having the best connection quality, as a function of one or more rules based upon one or more quality figures depending on at least one physical layer measurement related to the quality of all radio channels existing between the actively connected stations of the cluster. The invention also relates to the instruments needed to perform the method and to the apparatuses performing the method.

Description

, METHOD FOR DYNAMICALLY ELECTING THE MASTER AMONG COMPUTERS OR STATIONS OF A MOBILE WIRELESS LOCAL AREA NETWORK OR WLAN
This invention relates to a method for dynamically electing the controller or Master among an assembly of computers or stations of a wireless local area network or WLAN, namely a radio connection network, which enables to select, among those connected to the network, the computer having the best radio-visibility properties, in order to perform the role of Master unit of the network, while minimising the information transfer from the old Master to the new Master, so as to make such transfer transparent (or "seamless") to the other stations , thereby making the concerned network robust, flexible, configurable and adaptable to the movements of the stations. The present invention also relates to the instruments needed to perform the method and to the apparatuses performing the method.
It is known that at present there is an increasing diffusion of wireless local area networks or WLAN, in which one or more, preferably mobile computers or stations are connected among each other in order to form an assembly, so-called "cluster", in which they are in condition of mutual electromagnetic visibility, namely they are adapted to directly exchange data between each other without passing through intermediate stations, by utilising a radio communication channel. Each station is provided with a radio transceiver apparatus. The radio channel is shared among the various stations and, therefore, it is necessary to regulate the access thereto in order to enable ordered and collisionless communications among them. A station in the network is intended to perform this medium access control (or MAC) function and it is named controller or Master, while the other stations of the network are named Slave or servoed stations. The station operating as Master carries out a particular assembly of functionalities that enable to decide and control the access to the radio channel from all stations, inclusive of the Master one itself. The Slave units are servoed to the decisions of the Master unit and provide it with all necessary information to take the control decisions.
PCT Patent Application No. WO9429986 discloses a network of stations, one of which performs the Master role and establishes the radio communication frequency which the other stations must lock on. The selection of the Master unit is carried out by following a designation method according to which two different possible selection procedures are provided.
According to the first procedure, the network always designates a pre-established station as initial Master station, usually defined by a network administrator when the network is installed.
According to the second procedure, any station can perform the Master role. In particular, any station, on connecting to the network, carries out a check to ascertain that a Master unit is present and, should it ascertain that no Master unit is present, then it itself takes on the Master role, preferably in compliance to a selection criterium in which some rules are to be fulfilled, such as minimising the signal power and the multipath propagation, as well as the stability achieved in the previous connections to the network. The system as described in said PCT Patent Application No.
WO9429986 has certain drawbacks mainly connected with the fact that the selection of the Master unit among the stations of the network is based upon constancy of the electromagnetic visibility conditions between the stations, both in respect of the previous connections and in respect of the current connection. In view of the above, should the electromagnetic configuration be modified , for instance due to modifications in the spatial configuration, the initially designated Master station will continue operating as Master even if the connection quality is degraded. This severely limits or even inhibits the station mobility. Furthermore, the information amount handled by the Master station in controlling the network is extremely large, thereby making the transfer of the Master functionality from a station to an other station particularly cumbersome: this suggests to perform such transfer only in cases of operative failure of the current Master station. In such case, however, the selection of a new Master station entails an interruption of the communications currently taking place between the stations of the network, which, consequently, are affected by said transfer of the Master functionality.
A similar system is described in PCT Patent Application No. WO0062485 in which a dynamical transfer procedure for the network Master functionality in a wireless station network is disclosed. In particular, such system provides for defining a pre-established network Master station as well as possible alternative Master stations, which perform the Master role if and when the network Master station has an operative failure.
Also this system has the drawbacks already mentioned in respect of the system according to PCT Patent Application No. WO9429986.
It is a first object of this invention, therefore, to provide a method for selecting the Master station among the stations connected to a WLAN network, in such a way as to be adaptable to the changes in the radio visibility conditions among the stations, particularly allowing all movements of the stations, so as to make the network robust, flexible and configurable.
A second object of the present invention is to minimise the transfer of the information needed to control the network from the old Master to the new Master station, so as to make such transfer reliable and transparent (or "seamless") for the other stations.
It is a further object of the present invention to provide all necessary instruments for performing the method as well as the apparatuses performing the method itself. It is specific subject-matter of this invention a method for dynamically electing the controller or Master among an assembly of computers or stations in condition of reciprocal electromagnetic visibility, or "cluster", said cluster belonging to a radio communication wireless local area network or WLAN, said cluster comprising no more than one station operating as Master at each time at least one station of the cluster is actively connected to the network, the other stations operating as dependant or Slave, such method comprising a procedure for electing the Master among all stations of the cluster actively connected to the network, characterised in that said election procedure designates as Master the station having the best connection quality, as a function of one or more rules based upon one or more quality figures depending on at least one physical layer measurement related to the quality of all radio channels existing between the actively connected stations of the cluster.
Further according to this invention, said at least one physical layer measurement can comprise:
- the power of the signals each station receives from any other station, and/or - the signal/noise ratio of the signals each station receives from any other station,
- the bit decision error (BER) frequency of the signals each station receives from any other station. In particular, under the term noise, both the thermal noise and the possible interferences of various kinds are to be meant.
Still according to this invention, said one or more quality figures can comprise, in respect of any actively connected station,
- the average values of said at least one physic layer measurements in respect of the quality of all radio connection channels existing with the other actively connected stations in the cluster, and said one or more rules can comprise:
- the selection of the station having the best average value.
In particular, the best average value is always to be referred to the measured physical parameter and it can be the maximum average value, for instance in connection with the power of the received signals or with the sigπal-to-noise ratio, or the minimum average value, for instance in connection with BER.
Further according to this invention, said one or more quality figures can comprise, in respect of any actively connected station,
- the total sums of said at least one physic layer measurement in respect of the quality of all radio connection channels existing with the other actively connected stations in the cluster, and said one or more rules can comprise: - the selection of the station having the best sum value.
Similarly to the above set forth statement, the best sum is always to be referred to the measured physical parameter and it can be the maximum sum (for instance in connection with the power of the received signals or with the signal-to-noise ratio), or the minimum sum (for instance in connection with BER).
Again according to this invention, said one or more quality figures can comprise, in respect of any actively connected station,
- the worst values of said at least one physic layer measurements in respect of the quality of all radio connection channels existing with the other actively connected stations in the cluster, and said one or more rules can comprise: - the selection of the station having the best one among said worst values.
Still according to this invention, said election procedure can be periodically carried out by the station operating as Master, preferably upon expiring of a manually or automatically adjustable time interval.
Again according to this invention, said election procedure can be carried out by the station operating as Master every time a station is actively connected to the network.
Further according to this invention, said election procedure can be carried out by the station operating as Master before switching off.
Still according to this invention, said election procedure can be carried out by any station operating as Slave upon detecting the absence of a station operating as Master.
Further according to this invention, the method can comprise, subsequently to said election procedure when a station operating as Slave is elected to operate as new Master by the station currently operating as Master, a procedure aimed at transferring information needed for operation of the network, such information including only those information items that do not vary during the current connections, preferably the connection identification codes of the actively connected stations and/or the identification codes of the sender and addressee stations and/or traffic characteristic information and/or information relating to the quality requirements of the communications.
It is further subject-matter of this invention a computer or station comprising a radio transceiver apparatus, characterised in that it is adapted to perform the method for dynamic election of the controller, or Master, among the stations of a cluster of a WLAN as above described.
Preferably, according to this invention, the above said station is a mobile station. It is further subject-matter of this invention a mobile radio connection wireless local area network or WLAN, characterised in that it comprises at least one computer or station as above illustrated.
Still according to this invention, the above said mobile network can comprise one only cluster, the actively connected stations directly communicating with one another. Such kind of communication being known to those skilled in the art under the term "single-hop". Alternatively, according to this invention, the above said mobile network can comprise at least two clusters, any two actively connected stations belonging to two different clusters communicating with one another by means of at least one intermediate station. Such kind of communication being known to those skilled in the art under the term "multi-hop".
It is further specific subject-matter of this invention a computer program characterised in that it comprises code means adapted to perform, when they operate on a computer, the method for dynamic election of the controller, or Master, among the stations of a cluster of a
WLAN as above described.
It is still further subject-matter of this invention a computer readable memory support having a program stored therein, characterised in that said program is a program as above described. This invention will be now described by way of illustration, not by way of limitation, according to its preferred embodiments, by particularly referring to the Figures of the annexed drawings, in which:
Figure 1 shows a diagram of a WLAN network comprising stations adapted to perform the method for dynamic election of the Master according to this invention,
Figure 2 shows a flow chart diagrammatically illustrating the steps of the method according to this invention, as performed by a computer or station firstly accessing the network of figure 1 ,
Figure 3 shows a flow chart diagrammatically illustrating a first transfer procedure for the information needed to control the network according to the method for dynamic election of the Master of this invention,
Figure 4 shows a flow chart diagrammatically illustrating a second transfer procedure for the information needed to control the network according to the method for dynamic election of the Master of this invention.
By referring to Figure 1 , one can observe an illustrative but not limitative diagram of a wireless local area network or WLAN comprising a cluster of four computers or stations. In particular the cluster of figure 1 comprises a first stationary computer 1 , a first and a second mobile computers 2 and 3 and a hand-held computer 4. The stations of the cluster are provided with a transceiver radio apparatus and directly communicate with one another, without interposition of intermediate stations, by utilising a radio communication channel.
The above said stations are all potentially adapted to operate as Master stations. In the WLAN network according to this invention, the Master role is not decided a priori , but it can be dynamically allotted during the time, when the network conditions vary, either in respect of the topology or in respect of the involved radio channel. At any time, one only station operates as Master, while all remaining ones operate as Slave stations. The functionality's relating to the control of the stations accessing the network, both from the Master side and from the Slave side, are resident in the MAC (Medium Access Control) of the ISO-OSI protocol stack of a station.
The station operating in the role of Master performs an assembly of functionalities that are not performed when it operates in the role of Slave and that enable it to co-ordinate itself and the remaining stations of the network, the Slave stations, in accessing the communication channel. Such functionalities comprise:
- the transmission of the information relating to synchronisation in the transmit-receive function in the involved radio channel;
- the management of the association procedure of new stations to the network;
- the management of the set-up (and set-off) procedure for connections between the concerned stations; and - the decision in respect of sharing the radio channel among the various connections established between the present stations and the communication of this decision to the Slave stations.
By referring to Figure 2, it illustrates the procedure for initialise a station, such procedure being the same for all stations, which is performed on switching-on the concerned station and, consequently, at its first accessing the network. In the situation in which all of the stations of the network are switched-off, the Master functionality is performed by the first station to be switched-on.
During the operation of the network, other stations can be connected to the above said first station and, therefore, it could turn out to be suitable or even necessary to transfer the Master functionality from a station to an other one, in order to guarantee the best possible performances of the network.
As above mentioned, such transfer can take place not only when the current Master is switched-off or fails, but also as a consequence of a variation in the topological configuration of the network that affects the radio-visibility properties among the stations.
Such a variation can be due, for instance, to switching-on other stations. In this case, the station currently operating as Master could be in a no more suitable position from a connection quality point of view, thereby making it necessary to elect the station that at this time is placed in the best position with respect to the other ones as Master, in order to improve the operation of the network itself.
A similar variation in the topologic configuration of the network could be caused by one or more displacements of the involved stations, so that the spatial mutual positions of the stations are changed thereby modifying the quality of the radio connections established between them. An election of a new Master station could turn out to be necessary also in this circumstance. A particular case occurs when the current Master unit is moved away from the other stations, up to causing a loss of the magnetic visibility thereof or up to getting out of the cluster. In this case, it is necessary that the transfer of the Master role be effected before the current Master unit completely loses its capability to communicate with the other stations.
Obviously, the method according to this invention also provides for transfer of the Master role when the current Master unit is switched off or fails, because otherwise the network could not go on operating due to lack of co-ordination in accessing the traπsmissive means.
By referring to Figure 3, it can be observed that the transfer of the Master functionality from a station to an other one takes place in gradual mode which is hereinafter designated as "Soft Master Hand-over". In particular, the flow chart shown in Figure 3 illustrates the Soft Master Hand-over protocol in respect of both the current Master unit and the Slave unit elected as new Master
The left hand portion of the chart illustrates the steps carried out by the current Master unit that, when one of the above mentioned events occurs, cause a new Master unit to be elected. The right hand portion of the chart illustrates the steps carried out by the Slave elected as new Master unit, such steps being carried out after the related actions performed by the current Master unit (such chronological succession is designated by a dashed arrow in Figure 3). Figure 4 shows the flow chart illustrating the Soft Master Handover protocol in the particular case in which the current Master unit is switched off.
The selection of the Master takes place by means of an election procedure, as designated in the above mentioned flow charts of Figures 3 and 4 by reference numeral 5, in which all currently active stations participate. The Master election procedure indicates the station that has the best visibility level within the network and that, therefore, appears to be the most convenient in performing the Master role. The protocol is carried out in the current Master station and it is entered in asynchronous manner or in synchronous manner: in the first case, the election protocol is carried out every time an event causing a change in the network topology occurs, such as, for instance, when the Master station is switched off or a new station is associated to the network; in the second case, the election protocol is carried out when a periodical time interval expires (for instance, calculated a priori based upon the mobility grade of the stations), as provided by a timer, in order to check whether the topology of the network can be changed in such a way as to make the transfer of the Master functionality convenient, in view of the fact that, for instance, a Slave unit is in better radio-visibility conditions than the current Master unit.
The election method according to this invention selects as Master the station that is placed in the position believed as the best one based upon physical layer measurements in respect of the quality of the radio channels established between the various stations. In particular, the best station is the one that, as a function of one or more rules based upon one or more quality figures depending on at least one physical layer measurement in respect of the quality of all radio channels established between the actively connected stations of the cluster, offers the best connection quality. By way of exemplification and not by way of limitation, such physical layer measurements can comprise: the power of the signals each station receives from any other station, and/or the signal/noise ratio of the signals each station receives from any other station (where the noise can be due either to thermal noise or to interference) and/or the bit decision error frequency of the signals each station receives from any other station. Said one or more quality figures can comprise, in respect of any actively connected station, the average values of said at least one physic layer measurements in respect of the quality of all radio connection channels existing with the other actively connected stations in the cluster, and/or the total sums of said at least one physic layer measurements in respect of the quality of all radio connection channels existing with the other actively connected stations in the cluster, and/or the worst values of said at least one physic layer measurements in respect of the quality of all radio connection channels existing with the other actively connected stations in the cluster.
When the station in the best position is the current Master station, then the procedure terminates and the Master unit is not changed. If, on the contrary, the station in the best position is one of the Slave stations, the procedure is prosecuted by transferring the Master functionality to the latter, under application of a suitably designed protocol. Obviously, when the current Master unit is going to be switched off, it is not considered as possible new Master. The above Master functionality hand-over protocol provides for transferring all information needed for controlling the network to the station just elected as new Master unit. The information transfer is minimised in view of the possibility to utilise the radio communicated information circulating in the network or "broadcast" information. Furthermore, this transfer is carried out without interrupting the operation of the network itself, thereby making it transparent to the other Slave stations ("seamless hand-over") that have no need to interrupt any possibly established connections.
The information items the Master unit needs to carry out its role are related to the established connections and are of two kinds: a) "static" information items, namely information items that do not vary during an established connection, such as, for instance, the connection identification codes, sender and addressee stations, traffic characteristic information, service quality requirements; and b) "dynamic" information items, namely information items that can vary during an established connection, such as information items relating to the buffer status, for instance the number of packets in queue. The information items transferred from the old to the new
Master are only the static ones, while the dynamic information items are reconstructed by the new Master in a short time without an explicit transfer, because they are transmitted in broadcast mode by all of the stations of the cluster.
The preferred embodiments of this invention have been described and a number of variations have been suggested hereinbefore, but it should expressly be understood that those skilled in the art can make other variations and changes, without so departing from the scope thereof, as defined by the following claims.

Claims

1.- A method for dynamically electing the controller or Master among an assembly of computers or stations in condition of reciprocal electromagnetic visibility, or "cluster", said cluster belonging to a radio communication wireless local area network or WLAN, said cluster comprising no more than one station operating as Master at each time at least one station of the cluster is actively connected to the network, the other stations operating as dependant or Slave, such method comprising a procedure for electing the Master among all stations of the cluster actively connected to the network, characterised in that said election procedure designates as Master the station having the best connection quality, as a function of one or more rules based upon one or more quality figures depending on at least one physical layer measurement related to the quality of all radio channels existing between the actively connected stations of the cluster.
2.- A method according to claim 1 , characterised in that said at least one physical layer measurement comprises: - the power of the signals each station receives from any other station, and/or
- the signal/noise ratio of the signals each station receives from any other station,
- the bit decision error frequency of the signals each station receives from any other station.
3.- A method according to claim 1 or 2, characterised in that said one or more quality figures comprise, in respect of any actively connected station,
- the average values of said at least one physic layer measurements in respect of the quality of all radio connection channels existing with the other actively connected stations in the cluster, and in that said one or more rules comprise:
- the selection of the station having the best average value.
4.- A method according to any one of the preceding claims, characterised in that said one or more quality figures comprise, in respect of any actively connected station, - the total sums of said at least one physic layer measurements in respect of the quality of all radio connection channels existing with the other actively connected stations in the cluster, and in that said one or more rules comprise: - the selection of the station having the best sum value.
5.- A method according to any one of the preceding claims, characterised in that said one or more quality figures comprise, in respect of any actively connected station,
- the worst values of said at least one physic layer measurements in respect of the quality of all radio connection channels existing with the other actively connected stations in the cluster, and in that said one or more rules comprise:
- the selection of the station having the best one among said worst values.
6.- A method according to any one of the preceding claims, characterised in that said election procedure is periodically carried out by the station operating as Master.
7.- A method according to claim 6, characterised in that said election procedure is carried out by the station operating as Master upon expiring of a manually or automatically adjustable time interval.
8.- A method according to any one of the preceding claims, characterised in that said election procedure is carried out by the station operating as Master every time a station is actively connected to the network.
9.- A method according to any one of the preceding claims, characterised in that said election procedure is carried out by the station operating as Master before switching off.
10.- A method according to any one of the preceding claims, characterised in that said election procedure is carried out by any station operating as Slave upon detecting the absence of a station operating as
Master.
11.- A method according to any one of the preceding claims, characterised in that it further comprises, subsequently to said election procedure, when a station operating as Slave is elected to operate as new Master by the station currently operating as Master, a procedure aimed at transferring information needed for operation of the network, such information including only those information items that do not vary during the current connections.
12.- A method according to claim 11 , characterised in that said network communication static information items include the connection identification codes of the actively connected stations and/or the identification codes of the sender and addressee stations and/or traffic characteristic information and/or information relating to the quality requirements of the communications.
13.- A computer or station comprising a radio transceiver apparatus, characterised in that it is adapted to perform the method for dynamic election of the controller, or Master, among the stations of a cluster of a WLAN according to any one of preceding claims 1 to 12.
14.- A computer or station according to claim 13, characterised in that it is a mobile station.
15.- A mobile radio connection wireless local area network or
WLAN, characterised in that it comprises at least one computer or station according to claim 13 or 14.
16.- A mobile network according to claim 15, characterised in that it comprises one only cluster, the actively connected stations directly communicating with one another.
17.- A mobile network according to claim 15, characterised in that it comprises at least two clusters, any two actively connected stations belonging to two different clusters communicating with one another by means of at least one intermediate station.
18.- A computer program characterised in that it comprises code means adapted to perform, when they operate on a computer, the method for dynamic election of the controller, or Master, among the stations of a cluster of a WLAN according to any one of preceding claims 1 to 12.
19.- A computer readable memory support having a program stored therein, characterised in that said program is a program according to claim 18.
PCT/IT2002/000456 2001-07-13 2002-07-11 Method for dynamically electing the master among computers or stations of a mobile wireless local area network WO2003007552A2 (en)

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