Abstract:
A method for preventing interference between electromagnetic (EM) signals, consisting of toggling a first transceiver, adapted to transmit in a first EM frequency band, and a second transceiver, adapted to transmit in a second EM frequency band, different from the first EM frequency band, between an on-period when the first transceiver is able to transmit and the second transceiver is prevented from transmitting, and an off-period when the first transceiver is prevented from transmitting and the second transceiver is able to transmit. The method further includes inhibiting a third transceiver, adapted to transmit in the first EM band, from transmitting during the off-period, and inhibiting a fourth transceiver, adapted to transmit in the second EM band, from transmitting during the on-period.

Description:
RELATED APPLICATIONS  
       [0001]    This applications claims priority to U.S. Provisional Application No. 60/355,741, filed on Feb. 5, 2002. 
     
    
     
       FIELD OF THE INVENTION  
         [0002]    The present invention relates generally to communication systems, and specifically to communication systems which may interfere with each other.  
         BACKGROUND OF THE INVENTION  
         [0003]    Electromagnetic (EM) transmission frequency bands are allocated so that systems operating on different bands do not interfere with each other. In practice, however, interference may occur between transceivers operating in different bands because, inter alia, a transceiver nominally operating within one band generates EM frequencies outside the band. Furthermore, if the transceivers are physically close to each other, interference may occur regardless of the frequencies at which the transceivers operate.  
           [0004]    [0004]FIG. 1 is a schematic diagram of the frequency spectrum of two frequency bands which are relatively close to each other, as is known in the art. A first band  12  is an upper Industrial, Scientific and Medical (ISM) band, operating at frequencies of approximately 2.4 GHz. A second band  14  is a Multichannel Multipoint Distribution Service (MMDS) band, operating in an approximate range 2.596-2.680 GHz. Both bands are used for implementation of broadband wireless access systems, which are intended to provide digital services such as Internet access, wide-area networks (WANs), and Voice-over Internet Protocol (VoIP).  
           [0005]    Chapter 15 of a protocol ANSI/IEEE 802.11 (1999) published by The Institute of Electrical and Electronics Engineers, Inc. New York, N.Y., specifies 2.400-2.497 GHz as frequencies within the ISM band to be used in wireless local area networks (WLANs). The specified frequencies are shown schematically as region  16 . The Institute of Electrical and Electronics Engineers, Inc. publish supplements 802.11a, 802.11b, . . . to the ANSI/IEEE 802.11 protocol. Hereinafter, the term ANSI/IEEE 802.11 protocol is assumed to comprise the supplements.  
           [0006]    Chapters 9 and 11 of the ANSI/IEEE 802.11 protocol, describe functions of a medium access control (MAC) sub-layer of stations operating according to the protocol. Operations of the stations are timed, and stations maintain local clocks which are periodically synchronized by a “beacon” frame transmitted by an “Access Point” (AP) station, acting as a timing master for all the stations. The beacon frame is typically followed by one or more management frames which are also referred to hereinbelow. As described in chapter 9, the protocol defines a contention period and a contention-free period for operation of the stations. The contention period is implemented by a distributed coordination function (DCF) in each station. The DCF operates a carrier sense multiple access with collision avoidance (CSMA/CA) system, wherein a station wanting to transmit first “senses” the medium to determine if another station is transmitting. If the medium is determined to be available, the transmission may proceed.  
           [0007]    The contention-free period is implemented by a point coordination function (PCF), which is optionally implemented in the AP station. When implemented, a PCF AP station transmits a management frame which gives control of the medium to the AP station, and enables the AP station to poll the other stations. The management frame prevents non-PCF stations from transmitting by setting a network allocation vector (NAV) of each station. The NAV is an indicator, maintained by each station, of time periods when transmission will not be initiated by the station.  
           [0008]    A protocol TIA/EIA/IS-856, published by the Technical Specification Group C of the Third Generation Partnership Project 2, and which is available from the Telecommunications Industry Association, Arlington, Va., gives specifications for code division multiple access (CDMA) transceivers supporting the protocol. Although not in the specification, it is known in the art for CDMA transceivers to operate in two sections of the MMDS band in the 2.6-2.7 GHz range. In each section, shown as regions  18  and  20 , the CDMA transceivers may transmit and receive.  
           [0009]    Chapter 9 of the protocol, which is incorporated herein by reference, describes a data rate control (DRC) channel that indicates a rate at which an access terminal can receive traffic in a specific channel. The rate may be set to any of a pre-determined set of values including zero. Setting the rate to zero effectively prevents the channel which is “pointed to” by the DRC channel from receiving.  
           [0010]    Particularly in cases where the transceivers are relatively close physically, a WLAN transceiver operating in region  16  may generate interference in a CDMA transceiver operating in region  18 , and vice versa. There are four main reasons for the interference:  
           [0011]    Low receiver selectivity, causing the receiver to be unable to distinguish signals directed to the receiver in the presence of signals of different frequencies.  
           [0012]    Insufficient receiver blocking handling, where receiver operation is degraded due to strong signals, different from the tuned frequency of the receiver, input to the receiver.  
           [0013]    Transmitter out-of-band emissions, where significant power is emitted from the transmitter due to insufficient filtering.  
           [0014]    Transmitter wide band noise.  
           [0015]    Methods for reducing interference between a WLAN transceiver and a CDMA transceiver which are physically close typically comprise using high quality filters (radio-frequency (RF), intermediate frequency (IF) and baseband) and careful RF design. Both methods lead to increased transceiver costs. Furthermore, RF solutions are not able to solve the problems generated by the close physical proximity of transceivers. Thus, an alternative system for reducing interference which avoids these costs and which overcomes the problems caused by the proximity of the transceivers would be advantageous.  
         SUMMARY OF THE INVENTION  
         [0016]    It is an object of some aspects of the present invention to provide a method and apparatus for reducing interference between two collocated transceiving subsystems.  
           [0017]    In preferred embodiments of the present invention, two collocated transceivers operate in respective different frequency bands. If allowed to operate unrestrictedly, the transmissions of each of the transceivers cause interference between the transceivers, because a frequency separation between edges of the frequency bands is small, and there is some power “leakage” between the bands. There is also interference because of the physical proximity of the collocated transceivers. To prevent interference, the transceivers are toggled between respective active and quiescent periods in a time domain multiplexed (TDM) manner.  
           [0018]    Each collocated transceiver communicates with one or more respective distant transceivers within its frequency band. At the end of its active period, each collocated transceiver either transmits one or more blocking signals, or does not transmit signals, to the one or more distant transceivers operating on its frequency band. Both methods prevent the respective distant transceivers transmitting during the quiescent period of each of the collocated transceivers. Operating the collocated transceivers in a TDM manner, and preventing transmissions from respective distant transceivers during the quiescent period of each of the collocated transceivers, substantially prevents interference at the collocated transceivers without incurring costs of interference prevention systems known in the art.  
           [0019]    Each collocated transceiver (and its respective distant transceivers) most preferably operates according to a different industry-standard protocol. The one or more blocking signals transmitted according to a first of the protocols preferably comprise clear-to-send (CTS) signals to a fictitious transceiver enabling it to send, or request-to-send (RTS) signals from the fictitious transceiver. Consequently, other actual transceivers operating according to the protocol are prevented from sending. Alternatively, the one or more blocking signals are CTS/RTS signals not directed to a specific (actual or fictitious) transceiver, but are generalized CTS/RTS signals, supported by the first protocol. Transceivers operating according to a second protocol are effectively prevented from transmitting by setting a data rate channel (DRC), supported by the second protocol, equal to zero.  
           [0020]    There is therefore provided, according to a preferred embodiment of the present invention, a method for preventing interference between electromagnetic (EM) signals, including:  
           [0021]    toggling a first transceiver, adapted to transmit in a first EM frequency band, and a second transceiver, adapted to transmit in a second EM frequency band, different from the first EM frequency band, between an on-period when the first transceiver is able to transmit and the second transceiver is prevented from transmitting, and an off-period when the first transceiver is prevented from transmitting and the second transceiver is able to transmit;  
           [0022]    inhibiting a third transceiver, adapted to transmit in the first EM band, from transmitting during the off-period; and  
           [0023]    inhibiting a fourth transceiver, adapted to transmit in the second EM band, from transmitting during the on-period.  
           [0024]    Preferably, the first and the second transceivers are positioned sufficiently close to each other so that a transmission in the first EM frequency band interferes with operation of the second transceiver.  
           [0025]    Preferably, the method includes detecting at the first transceiver one or more first EM band interference parameters, detecting at the second transceiver one or more second EM band interference parameters, and computing the off-period and the on-period responsive to the one or more first EM band interference parameters and the one or more second EM band interference parameters.  
           [0026]    Further preferably, the method includes measuring statistics of the EM signals responsive thereto, and computing the off-period and the on-period responsive to the statistics.  
           [0027]    Preferably, inhibiting the third transceiver includes transmitting one or more inhibiting signals from the first transceiver, and receiving the one or more inhibiting signals at the third transceiver.  
           [0028]    Further preferably, the one or more inhibiting signals include one or more signals directed to a fictitious fifth transceiver in the first EM band, wherein the one or more inhibiting signals enable the fictitious fifth transceiver to transmit.  
           [0029]    Alternatively or additionally, the one or more inhibiting signals include one or more signals directed from a fictitious fifth transceiver in the first EM band, wherein the one or more inhibiting signals request permission for the fictitious fifth transceiver to transmit.  
           [0030]    Preferably, the one or more inhibiting signals include a data rate channel (DRC) signal, wherein the DRC signal is set to have a value of zero so as to inhibit the third transceiver from transmitting.  
           [0031]    Preferably, a frequency separation between the first EM frequency band and the second EM frequency band is sufficiently small so that a transmission in the first EM frequency band interferes with operation of the second transceiver.  
           [0032]    Preferably, the first and the third transceiver are adapted to communicate via a first communication protocol, and the second and the fourth transceivers are adapted to communicate via a second communication protocol, different from the first communication protocol.  
           [0033]    Further preferably, the first communication protocol is operative as a wireless local area network (WLAN) protocol and the second communication protocol is operative as a code division multiple access (CDMA) protocol.  
           [0034]    Preferably, the CDMA protocol includes an IS-856 protocol of the Telecommunications Industry Association/Electronic Industries Association (TIA/EIA).  
           [0035]    Preferably, the first transceiver acts as a master transceiver controlling the second transceiver acting as a slave transceiver.  
           [0036]    There is further provided, according to a preferred embodiment of the present invention, apparatus for preventing interference between electromagnetic (EM) signals, including:  
           [0037]    a first transceiver which is adapted to transmit in a first EM frequency band;  
           [0038]    a second transceiver which is adapted to transmit in a second EM frequency band, different from the first EM frequency band;  
           [0039]    a controller which is adapted to permit the first transceiver to transmit and to prevent the second transceiver from transmitting during an on-period, and to prevent the first transceiver from transmitting and to permit the second transceiver to transmit during an off-period, and wherein the first transceiver substantially prevents a third transceiver, adapted to transmit in the first EM band, from transmitting during the off-period, and wherein the second transceiver substantially prevents a fourth transceiver, adapted to transmit in the second EM band, from transmitting during the on-period.  
           [0040]    Preferably, the first and the second transceivers are positioned sufficiently close to each other so that a transmission in the first EM frequency band interferes with operation of the second transceiver.  
           [0041]    Preferably, the first transceiver is adapted to detect one or more first EM band interference parameters, the second transceiver is adapted to detect one or more second EM band interference parameters, and the controller is adapted to compute the off-period and the on-period responsive to the one or more first EM band interference parameters and the one or more second EM band interference parameters.  
           [0042]    Further preferably, the first and the second transceivers are adapted to measure statistics of the EM signals responsive thereto, and the controller is adapted to compute the off-period and the on-period responsive to the statistics.  
           [0043]    Preferably, the first transceiver is adapted to transmit one or more inhibiting signals which are received by the third transceiver, and which prevent the third transceiver from transmitting.  
           [0044]    Preferably, the one or more inhibiting signals include one or more signals directed to a fictitious fifth transceiver in the first EM band, wherein the one or more inhibiting signals enable the fictitious fifth transceiver to transmit.  
           [0045]    Alternatively or additionally, the one or more inhibiting signals include one or more signals directed from a fictitious fifth transceiver in the first EM band, wherein the one or more inhibiting signals request permission for the fictitious fifth transceiver to transmit.  
           [0046]    Preferably, the one or more inhibiting signals include a data rate channel (DRC) signal, and the DRC signal is set to have a value of zero.  
           [0047]    Preferably, a frequency separation between the first EM frequency band and the second EM frequency band is sufficiently small so that a transmission in the first EM frequency band interferes with operation of the second transceiver.  
           [0048]    Preferably, the first and the third transceiver are adapted to communicate via a first communication protocol, and the second and the fourth transceivers are adapted to communicate via a second communication protocol, different from the first communication protocol.  
           [0049]    Preferably, the first communication protocol is operative as a wireless local area network (WLAN) protocol and the second communication protocol is operative as a code division multiple access (CDMA) protocol.  
           [0050]    Further preferably, the CDMA protocol includes an IS-856 protocol of the Telecommunications Industry Association/Electronic Industries Association (TIA/EIA).  
           [0051]    Preferably, the first transceiver acts as a master transceiver controlling the second transceiver acting as a slave transceiver.  
       
    
    
     BRIEF DESCRIPTION OF THE DRAWINGS  
       [0052]    The present invention will be more fully understood from the following detailed description of the preferred embodiments thereof, taken together with the drawings, in which:  
         [0053]    [0053]FIG. 1 is a schematic diagram of the frequency spectrum of two frequency bands which are relatively close to each other, as is known in the art;  
         [0054]    [0054]FIG. 2 is a schematic diagram of a Broadband Wireless Access (BWA) system, according to a preferred embodiment of the present invention;  
         [0055]    [0055]FIG. 3 is a schematic block diagram of Consumer Premises Equipment operating in the BWA system of FIG. 2, according to a preferred embodiment of the present invention;  
         [0056]    [0056]FIG. 4 shows schematic graphs of the operation of the BWA system of FIG. 2, according to a preferred embodiment of the present invention; and  
         [0057]    [0057]FIG. 5 is a flowchart showing steps involved in implementation of a toggling system used in the BWA system of FIG. 2, according to a preferred embodiment of the present invention. 
     
    
     DETAILED DESCRIPTION OF PREFERRED EMBODIMENTS  
       [0058]    Reference is now made to FIG. 2, which is a schematic diagram of a Broadband Wireless Access (BWA) system  30 , according to a preferred embodiment of the present invention. Typically, system  30  is installed in a small office/home office (SOHO) environment to give stations in the environment access to a distributed network such as the Internet. However, it will be appreciated that preferred embodiments of the present invention may be implemented in environments other than a SOHO environment. BWA system  30  comprises Consumer Premises Equipment (CPE)  32 , which acts as a gateway transferring information between a wide area network (WAN)  36  and a wireless local area network (WLAN)  34 . The WLAN comprises one or more generally similar stations  38 , and operates in an upper Industrial, Scientific and Medical (ISM) band, using frequencies in a range of approximately 2.400-2.497 GHz, as described with reference to FIG. 1. Each station  38  comprises an ISM transceiver  40 , and CPE  32  comprises an ISM transceiver  42 , enabling stations  38  to communicate with CPE  32 . Communications in WLAN  34  are preferably implemented according to a protocol ANSI/IEEE 802.11, described in more detail in the Background of the Invention.  
         [0059]    CPE  32  also communicates in a Multichannel Multipoint Distribution Service (MMDS) band, comprising frequencies in an approximate range 2.5-2.7 GHz, with an access unit  44 , which gives CPE  32  access to WAN  36 . Access unit  44  comprises a code division multiple access (CDMA) transceiver  46 , which communicates with a CDMA transceiver  50  comprised in CPE  32 . Communications between the transceivers in unit  44  and CPE  32  are on a carrier in the MMDS band, corresponding to region  18  or  20  of FIG. 1, according to a TIA/EIA/IS-856 protocol, described in more detail in the Background of the Invention. It will be appreciated that CPE  32  comprises two transceivers  42  and  50  which are substantially collocated. A more detailed description of CPE  32  is given below. It will also be appreciated that ISM transceiver  42  and CDMA transceiver  50  may interfere with each other, if measures to prevent interference are not taken, both because of the physical positioning of the two types of transceivers close to each other, and because of the small frequency separation between edges of the bands within which the two types of transceivers operate.  
         [0060]    [0060]FIG. 3 is a schematic block diagram of CPE  32 , according to a preferred embodiment of the present invention. A host central processing unit (CPU)  60  acts as an overall controller of ISM transceiver  42  and CDMA transceiver  50 . CPU  60  uses a memory  62  to implement a time division multiplexing (TDM) process which toggles ISM transceiver  42  and CDMA transceiver  50  between two states. In a first state, ISM transceiver  42  is active and CDMA transceiver  50  is quiescent. In a second state, ISM transceiver  42  is quiescent and CDMA transceiver  50  is active. CPU  60  also implements a short guard period, between the second and the first states, during which neither of transceivers  42  or  50  operate fully. The TDM process, described in more detail with reference to FIG. 4 and FIG. 5 below, varies the time periods for the first, second, and guard states according to status and statistics parameters received from transceivers  42  and  50 . Preferably, ISM transceiver  42  is implemented as a local master transceiver which controls CDMA transceiver  50  as a local slave, i.e., controlling which of the first and second states the CDMA transceiver is in, via a TDM synchronization line  64 .  
         [0061]    [0061]FIG. 4 shows schematic graphs of the operation of system  30 , according to a preferred embodiment of the present invention. A graph  70  shows activity of ISM transceiver  42  vs. time, and a graph  72  shows activity of CDMA transceiver  50  vs. time. At beacon times  74  a beacon frame (described in more detail in the Background of the Invention) is transmitted from ISM transceiver  42 , initiating a first period  86  during which the ISM transceiver is in the first state and is active. A period t B  between beacon times  74  is configured by CPU  60 , and is preferably set to be approximately between 20 ms and 1,000 ms, although other periods may be used. Period t B  corresponds to an overall time of repetition of the toggling system applied to transceivers  42  and  50 .  
         [0062]    Preferably, the beacon frame initiates a contention-free period  76 , during which transceiver  42  coordinates traffic between the transceiver and stations  38 . Stations  38  which implement a point coordination function (PCF) are polled during this period, using one or more management frames transmitted after the beacon frame, and are prevented from initiating unsolicited transmissions during period  76 . Stations  38  which do not implement a PCF are prevented from transmitting by incorporating control functions of a network activity vector (NAV) for the specific stations in the beacon frame. The PCF and NAV are described in more detail in the Background of the Invention. Transceiver  42  terminates contention-free period  76  by transmitting a contention-free end (CFE) frame to stations  38 . The CFE frame is transmitted at a time t 1  from the end of the beacon frame, a maximum value of t 1  being set by CPU  60 , as described with reference to FIG. 5 below. When the CFE frame has been transmitted, a contention period  78  begins.  
         [0063]    Alternatively, for example when none of stations  38  implement a PCF function, the beacon frame does not initiate contention-free period  76 , i.e., t 1 =0, and contention period  78  begins immediately after the beacon frame.  
         [0064]    A management frame transmitted immediately after the beacon frame transmits information which determines when “sleeping” stations  38 , described in more detail below, should “wake up.” 
         [0065]    During contention period  78 , assumed to continue for a time interval t 2 , stations  38  are able to transmit and receive, operating according to the carrier sense multiple access with collision avoidance (CSMA/CA) system of protocol ANSI/IEEE 802.11. Contention period  78  enables stations  38  to transmit to transceiver  42  and to transmit data between themselves, as well as to perform other network functions such as registration of a new station with the network. Contention period  78  also enables transceiver  42  to coordinate traffic between the transceiver and stations  38  which do not implement a PCF. Contention period  78  is concluded by transceiver  42 , acting on instructions from CPU  60 , transmitting a predetermined clear-to-send (CTS) signal  80 , described in more detail below. CTS signal  80  concludes first period  86 , during which system  30  is in the first state, and initiates a second period  84 , during which the system is in the second state.  
         [0066]    CTS signal  80  comprises a duration/identity field which defines a period during which a station operating according to the 802.11 protocol is able to transmit. As also defined in the protocol, a CTS signal is directed to a requesting station operating in WLAN  34  which has transmitted a request-to-send (RTS) signal. The CTS signal incorporates an identity of the requesting station, taken from the RTS signal, so that the requesting station knows to whom the CTS signal is directed. Other stations in WLAN  34  receive the signal, and from the information therein refrain from transmitting during the period. In some preferred embodiments of the present invention, a fictitious station, having a fictitious identity, is implemented in WLAN  34  by CPU  60 . Transceiver  42  transmits CTS signal  80  to the fictitious station, so that actual stations  38  refrain from transmitting during the period defined by CTS signal  80 . Alternatively, CTS signal  80  comprises a generalized CTS signal, not to a specific station within WLAN  34 , as supported by the 802.11 protocol. After transmitting CTS signal  80 , transceiver  42  is instructed by CPU  80  to refrain from transmission, except for transmitting CTS signals  82  described below, until it transmits its next beacon frame.  
         [0067]    CPU  60  instructs transceiver  42  to transmit further CTS signals  82 , substantially similar to CTS signal  80 , on a periodic basis. The period between the signals is set to be less than the period defined in the duration/identity field, so that stations  38  are substantially blocked from transmitting during period  84 , i.e., as long as signals  82  are transmitted. Near the end of period  84  CPU  60  instructs transceiver  42  to transmit a final signal  82  at a time before the next beacon frame, so that period  84  does not overlap the beacon.  
         [0068]    Protocol 802.11 allows one or more of stations  38  to go into a “sleep” state, during which they are not able to receive CTS signal  80  or CTS signals  82 . A particular station  38  may emerge from a sleep state and begin transmission of a data-frame during period  84 . In this case, transceiver  42  does not acknowledge the data-frame, so that the station  38  initiates a back-off mechanism, and is able to detect subsequent CTS signals  82 . The station  38  may re-transmit before receiving any CTS signal  82 , but preferably the NAV of the station is set so that back-off times between such re-transmissions are increased each time no acknowledgement is received, thus reducing the likelihood of re-transmission before receiving a CTS signal  82 .  
         [0069]    It will be appreciated that during time period  84  transmission in the WLAN band is substantially reduced by transceiver  42 :  
         [0070]    Transmitting CTS signal  80 ;  
         [0071]    Transmitting CTS signals  82 ;  
         [0072]    Refraining from transmission of other data-frames; and  
         [0073]    Not acknowledging any received data-frames.  
         [0074]    Those skilled in the art will appreciate that request-to-send (RTS) signals may be transmitted from transceiver  42  in place of, and/or as well as, at least some of the CTS signals described above, for the purposes of blocking transmission from stations  38 . The RTS signals may comprise RTS signals from the fictitious transceiver, or a generalized RTS signal. Thus, preferred embodiments of the present invention comprise using CTS and/or RTS signals for reducing transmission.  
         [0075]    As shown in graph  72  for transceiver  50 , during first period  86 , CPU  60  configures the transceiver to refrain from transmitting up-link signals to access unit  44 . Most preferably, during a guard time  88 , described in more detail below, the transceiver is also configured to transmit a data rate control (DRC) channel setting of zero (DRC=0) to access unit  44 , indicating that a rate at which transceiver  46  can transmit down-link data is effectively zero. The DRC channel is described in the Background of the Invention.  
         [0076]    Preferably, during second period  84 , transceiver  50  transmits a DRC channel having settings which enable transceiver  46  to transmit at a rate which compensates for its suspension of transmission in period  86 .  
         [0077]    In order to allow completion of operations at the end of second period  84 , CPU  60  sets guard time  88 , of duration t G , between a conclusion  73  of second period  84  and start time  74  of first period  86 . During guard time  88 , transceiver  50  is able to transmit DRC=0 data packets, in order to notify access unit  44  to avoid scheduling transmission of data packets during the upcoming first period. During guard time  88 , apart from transmitting DRC=0 data packets and acknowledging down-link frames transmitted from unit  44 , transceiver  50  operates substantially as described above for first period  86 .  
         [0078]    CPU  60  is able to configure the lengths of guard time  88 , first period  86 , a maximum value for period  76 , and second period  84 , according to statistics of data transfer measured by transceivers  42  and  50 . It will be appreciated that, depending on the statistics, CPU  60  may also decide not to implement the toggling system described with reference to FIG. 3 and FIG. 4.  
         [0079]    [0079]FIG. 5 is a flowchart showing steps comprised in a process for implementation of the toggling system, according to a preferred embodiment of the present invention. In an initial step CPU  60  determines a reference frame error rate (FER) in communications in WLAN  34  and in WAN  36  when the toggling system is not implemented. The CPU also determines a reference level of RF interference that is present when the toggling system is not implemented.  
         [0080]    In a second step, CPU  60  receives continuous statistics on traffic flow and system state from transceivers  42  and  50  (FIG. 3). The statistics preferably include parameters measuring up-link payload, down-link payload, and payload transferred between stations  38 , during a predefined time period prior to the third step being performed. Other statistics used by CPU  60  include data rates of the up- and down-link in WAN  36 , data rates used in WLAN  34 , whether a PCF function is implemented in stations  38 , a number of stations  38  in WLAN  34  and their state, i.e., if they are in an active state, an inactive state, or a sleep mode. The statistics are used to update values of the reference FER and RF reference level measured in the first step.  
         [0081]    In a third step, CPU  60  measures an operating value of the FER. If the operating value is greater than the reference FER, the toggling system is implemented by continuing to a fourth step, described below. Otherwise the process returns to the second step.  
         [0082]    In the fourth step, CPU  60  sets values of first period  86 , a maximum value of contention free period  76 , contention period  78 , second period  84 , and guard period  88 . The values are set according to statistics evaluated in the third step.  
         [0083]    Guard time  88  is preferably set so that there is enough time to complete a frame which is being transmitted in WAN  36 . Typically, data packets in WAN  36  are transmitted at a rate of at least one slot per packet. To derive the actual guard time  88 , CPU  60  adds a time for transceiver  50  to respond to its instruction, transmitted via line  64 , to toggle off. When one slot per packet is used, guard time  88  is preferably set to 4 slots plus the response time.  
         [0084]    First period  86 , contention free period  76 , contention period  78 , and second period  84  are all set by CPU  60  so as to optimize performance of WLAN  34  and WAN  36 . It will be appreciated that for the optimization, CPU  60  may also vary guard time  88  and an overall period of repetition  92 .  
         [0085]    During operation of system  30 , the second, third, and fourth steps are implemented on a repeating basis, so allowing for changing conditions in the WLAN and in the WAN.  
         [0086]    CPE  32  in system  30  acts as a gateway between WLAN  34  and WAN  36 , wherein information may be transferred between the WLAN and the WAN. It will be appreciated, however, that the scope of the present invention is not limited to systems transferring information in such a manner, but includes dual frequency systems where information may be transferred in other ways known in the art, such as between the WLAN and a third network and between the WAN and the third network.  
         [0087]    It will thus be appreciated that the preferred embodiments described above are cited by way of example, and that the present invention is not limited to what has been particularly shown and described hereinabove. Rather, the scope of the present invention includes both combinations and subcombinations of the various features described hereinabove, as well as variations and modifications thereof which would occur to persons skilled in the art upon reading the foregoing description and which are not disclosed in the prior art.