Abstract:
A wireless local area network adapted for use by users traveling on a mobile platform such as an aircraft. The network includes a network server located on the mobile platform, and at least one network access point connected to the server and accessible wirelessly by at least one user portable electronic device over one of a plurality of non-overlapping network frequency channels. The RF characteristics of this wireless network are specifically tailored to meet applicable standards for electromagnetic compatibility with aircraft systems and RF exposure levels for passengers and flight crews.

Description:
FIELD OF THE INVENTION  
         [0001]    The present invention relates generally to communication systems on board mobile platforms such as aircraft and, more particularly, to an on-board wireless local area network (WLAN) accessible by passengers&#39; portable electronic devices such as laptop computers.  
         BACKGROUND OF THE INVENTION  
         [0002]    Mobile network systems have traditionally been limited in bandwidth and link capacity, making it prohibitively expensive and/or unacceptably slow to distribute broadband data and video services to all passengers on a mobile platform such as an aircraft, boat or train. There is great interest in making such services available to users on mobile platforms. A system for supplying television and data services to mobile platforms is described in co-pending U.S. patent application Ser. No. 09/639,912, the entire disclosure of which is incorporated herein.  
           [0003]    The system described in application Ser. No. 09/639,912 provides bi-directional data transfer via satellite communications link between a ground-based control segment and a mobile RF transceiver system carried on each mobile platform. Each user on each mobile platform is able to interface with an on-board server by using a laptop, personal digital assistant (PDA) seat-back-mounted computer/display or other computing device. Each user can independently request and obtain Internet access, company intranet access, stored video and audio programming and live television programming.  
           [0004]    It would be desirable to provide passengers with wireless connections to network services available on mobile platforms such as aircraft. There are concerns, however, about the possibility of interference to aircraft systems from portable electronic devices (PEDs) that might be used by passengers to make wireless connections to an on-board network. Of particular concern is the possibility of PED interference during critical phases of flight, for example, during takeoff and landing. There also are concerns that such networks might expose passengers and flight crews to radiated RF fields exceeding recommended health and safety limits for RF exposure.  
           [0005]    Generally there are two types of PEDs: (1) intentional transmitters, which must transmit a signal in order to accomplish their function (e.g. cell phones, two-way radios, pagers and remote-control devices), and (2) non-intentional transmitters, which do not need to transmit a signal to accomplish their function, but nevertheless emit some level of radiation (e.g. laptop computers, compact disk players, tape recorders and electronic hand-held games). The Federal Aviation Administration (FAA) has not issued certification regulations for PEDs. The FAA does, however, restrict the use of PEDs on commercial airlines. FAA advisory circular AC91.21-1 paragraph 6.a (7) states that, unless otherwise authorized, use of PEDs classified as intentional transmitters should be prohibited during aircraft operation. General Operating and Flight Rules, 14 CFR 91.21(b)(5) (“Portable Electronic Devices”) prohibits the operation of a PED on an aircraft, unless the aircraft operator has determined that the device will not cause interference with the navigation or communication systems on board the aircraft. Thus it is desirable to provide a wireless network that can be determined to be accessible by passenger-operated PEDs without causing such interference and thus could be authorized for on-board use. It also is desirable to provide an on-board wireless network that produces RF emission levels within recommended health and safety limits.  
         SUMMARY OF THE INVENTION  
         [0006]    In one preferred form, the present invention provides a wireless local area network adapted for use by users traveling on a mobile platform such as an aircraft. The network includes a network server located on the mobile platform, and at least one network access point connected to the server and accessible wirelessly by at least one user portable electronic device over one of a plurality of non-overlapping network frequency channels. This wireless local area network can provide two-way communication, data and entertainment for aircraft passengers, cabin crews and flight crews. Such information may be obtained via e-mail, internet, company intranet access, and/or from data stored on board or off board the aircraft. The RF characteristics of this wireless network are specifically tailored to meet applicable standards for electromagnetic compatibility with aircraft systems and RF exposure levels for passengers and flight crews.  
           [0007]    Further areas of applicability of the present invention will become apparent from the detailed description provided hereinafter. It should be understood that the detailed description and specific examples, while indicating the preferred embodiment of the invention, are intended for purposes of illustration only and are not intended to limit the scope of the invention. 
       
    
    
     BRIEF DESCRIPTION OF THE DRAWINGS  
       [0008]    The present invention will become more fully understood from the detailed description and the accompanying drawings, wherein:  
         [0009]    [0009]FIG. 1 is a view of a wireless LAN (“WLAN”) adapted for use in a mobile platform such as an aircraft;  
         [0010]    [0010]FIG. 2 is a plan view of WLAN cells in an aircraft passenger cabin, shown from above the overhead area;  
         [0011]    [0011]FIG. 3 is a graph of E-field strength of emissions versus transmitter-to-victim distance for a WLAN;  
         [0012]    [0012]FIG. 4 is a view of a portion of a passenger cabin, shown from above the overhead area, in which more than one user PED is in use; and  
         [0013]    [0013]FIG. 5 is a graph of margins of compliance with FCC OET Bulletin 65 for the effect of adjacent laptops on RF exposure versus distance from transmitter. 
     
    
     DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENTS  
       [0014]    The following description of the preferred embodiments is merely exemplary in nature and is in no way intended to limit the invention, its application, or uses. As described below, the present invention in one embodiment is directed to a wireless LAN (“WLAN”) for use in a mobile platform. The mobile platform could include an aircraft, cruise ship or any other mobile vehicle. Thus the reference to the mobile platform as an aircraft throughout the following description should not be construed as limiting the applicability of the WLAN  10  and/or the present invention to only aircraft.  
         [0015]    A preferred embodiment of a wireless LAN for use in a mobile platform such as an aircraft is indicated generally by the reference numeral  10  in FIG. 1. The WLAN  10  includes an Ethernet router/server  14  (hereinafter “server”) wired to a plurality of access points  18  via at least one switching device such as an Ethernet switch  22 . In the embodiment shown in FIG. 1, the server  14  is connected to a transmit antenna, in this example, a transmit phased array antenna system  26 , and to a receive antenna, which in this example comprises a receive phased array antenna system  30 . The antenna systems  26  and  30  provide for two-way communication via satellite link between the WLAN  10  and a ground based network segment, as described in co-pending U.S. patent application Ser. No. 09/639,912. The server  14  can interface with other systems, for example, with in-flight entertainment and/or telephone service systems. In another embodiment the WLAN  10  operates standalone in the mobile platform.  
         [0016]    Each access point  18  has an antenna  34  located, for example, in the passenger cabin overhead. Each access point  18  is configured to transmit RF signals to, and receive RF signals from, one or more PEDs  38  carried on board by passengers. Such PEDs are fabricated for wireless use or have a wireless adapter antenna (not shown) and can include laptops, PDAs or the like. The access point antenna  34  may be, for example, an omni-directional or patch antenna. The number and location of access points  18 , and the number of PEDs  38  associated with an access point  18 , can vary as further described below.  
         [0017]    An exemplary arrangement of access point antennas  34  relative to PEDs  38  is shown in FIG. 2, which is a plan view of a portion  76  of an aircraft passenger cabin. Two access points  18  (not shown in FIG. 2) and associated antennas  34  are located in the overhead. Although an access point  18  could be located outside the cabin overhead, locating it close to its antenna  34  in the overhead reduces the length of a cable connection between them. Each access point  18  broadcasts over a cell  80  that includes eighteen seats  84 . Other cell sizes and numbers of associated seats can be used, as further described below. Factors influencing the sizes and numbers of cells  80  include seat width  92 , seat pitch  96 , distance  100  between antennas  34 , interior width  104  of the cabin, and the width  108  of each of the rows of seats  84 .  
         [0018]    The WLAN  10  operates in the 2.40 to 2.483 GHz ISM band, which is designated for unlicensed commercial or public use. Other licensed or unlicensed bands above 2.4 GHz, for example, the ISM 5.725 to 5.875 GHz band, could also be used. The WLAN  10  is configured in conformance with the IEEE 802.11b (High Rate) standard. The invention is not so limited, and other bands, standards, and protocols can be used. Each access point  18  communicates with the server  14  through the Ethernet switch  22  at full available bandwidth. The WLAN  10  utilizes Direct Sequence Spread Spectrum (DSSS) transmission between each access point  18  and its associated user PEDs  38 . That is, the spectrum is divided into three non-overlapping frequency channels of approximately 22 MHz each. It is contemplated that other spread-spectrum modulation methods also could be used.  
         [0019]    Each access point  18  is configured to communicate with PEDs  38  over one of the three channels. For example, as shown in FIG. 1, three access points  18  communicate using channels 1, 6 and 11 respectively. Adjacent access points  18  broadcast over different channels. For example, referring to FIG. 2, a user sitting in a cell  80  in which the associated access point  18  broadcasts over channel 1 could communicate with the WLAN  10  via channel 1. Another passenger sitting in an adjacent cell  80  would communicate with the WLAN  10  over channel 6 or channel 11.  
         [0020]    Where the number of access points  18  exceeds three, each channel can be re-assigned to another access point  18  that is not adjacent to an access point to which the channel is already assigned. For example, seven access points  18  located sequentially along the aircraft aisle overhead could use channels 1, 6, 11, 1, 6, 11 and 1 respectively. Thus use of each of the three channels can be distributed spatially over the aggregate of cells  80 , for example, to users distributed over the entire passenger cabin. Of course, the channels can be distributed over a plurality of cells in many different ways. Additionally, a connected user PED  38  can roam, e.g. as supported by the IEEE 802.11b protocol. That is, a WLAN  10  connection established with a user PED  38  in one cell  80  over one channel can be maintained over another channel if the user PED  38  roams to other cells. For example, a user carrying a PED  38  can walk, from one cell  80  in which the PED is connected to the WLAN  10  via channel 1, into an adjacent cell  80  in which, for example, channel 6 is being used, and maintain the connection to the WLAN  10 .  
         [0021]    Communication between the PEDs  38  and the access points  18  is half-duplex. That is, in each frequency channel, at any one time either the access point  18  or one user PED  38  can transmit. PEDs communicate via CSMA/CA (Carrier Sense Multiple Access/Collision Avoidance). That is, a PED  38  checks for a quiet channel before transmitting to its associated access point  18 . If the channel is busy, the PED waits a random amount of time and then retransmits. Several PEDs  38  could transmit simultaneously when contending for channel use. If a collision of their signals is detected, each of the transmitting PEDs “backs off” and waits a random time period before retransmitting. Eventually one PED gains control of the channel and transmits.  
         [0022]    The WLAN  10  is configured such that only access points  18  and PEDs  38  that meet applicable interference, health and safety requirements are allowed to operate within the network. PEDs that do not comply with such standards are excluded from connecting to the WLAN  10 . More specifically, and for example, according to IEEE 802.11b protocol, each type of PED  38  that has passed testing for compliance with applicable interference, health and safety standards is identified in the MAC (Media Access Control) layer of the WLAN  10 . Thus it can be determined at each access point  18  whether a remote PED  38  has been predetermined to be suitable for connection to the WLAN  10 . If the PED is one that has been approved for connection, it is allowed to connect to the network; if not, the PED request for network access is ignored.  
         [0023]    Configuring a WLAN for use in aircraft entails consideration of a variety of factors, including those related, for example, to aircraft and passenger safety. Not all of such factors, however, are unique to aircraft. Thus many of the considerations for configuring an aircraft WLAN also pertain to configuring a WLAN for use in other types of mobile platforms. Embodiments of a mobile WLAN as described above can be configured in accordance with the following assumptions, determinations and considerations.  
         [0024]    Distance Assumptions and Far Field Calculations  
         [0025]    Emissions by 802.11b wireless LANs can be treated as a far field problem. The wavelength, λ, at 2.4 GHz is 0.125 meters. The far field limit is approximated by 2*d 2 /λ where “d” is the largest dimension of the transmitting antenna. For a typical omni-directional or patch antenna utilized at a wireless access point mounted, for example, in the overhead in an aircraft passenger cabin, the largest dimension is assumed to be approximately 9 inches or 0.23 meters. The far field limit for such an antenna  34 , then, is approximately 0.85 meters.  
         [0026]    A typical user PED  38  PCMCIA adapter antenna is assumed to have a largest dimension of 2 inches or 0.05 meters. The far field limit for such an antenna, then, is approximately 0.04 meters. Based on the foregoing assumptions and determinations, all WLAN  10  emissions more than one meter from an access point antenna  34  or more than four centimeters from a user PED  38  antenna can be treated as being in the far field.  
         [0027]    Non-coaxial aircraft system cables can be lossy at the frequencies contemplated for use in the airborne WLAN  10 . Therefore, possible effects of WLAN-radiated field levels at line replaceable units (LRUs) of an aircraft system are considered. An access point antenna  34  transmitting to users in an aircraft passenger compartment would be prevented by its ground plane (not shown) from radiating at significant levels into the overhead compartment. Access point antenna  34  emissions, then, are investigated primarily for their effect on equipment in avionics bays under the floor or in the sidewalls of the aircraft. The user PED  38  antennas could radiate into both the overhead and underfloor areas of the aircraft. System LRUs can be installed in equipment bays and/or in the overhead throughout the aircraft. Therefore the minimum distance from an operating access point antenna  34  or a user PED  38  adapter to an airborne system LRU is assumed to be one meter.  
         [0028]    Field Strength Levels  
         [0029]    The following methodology is used to evaluate field strength levels for both aircraft system RF susceptibility and for RF exposure compliance. For the following analysis of field strength levels, it is assumed that a transmit antenna on either an access point or user adapter has a maximum gain value of 2.2 dBi (numerical value 1.66), and that transmit cable losses are zero dB. The far field radiated power density is given by:  
           P   d =( P   t   *G )/(4*π *D   2 )  (1)  
         [0030]    where “P t ” is transmitter power at antenna input in watts, “G” is numerical gain of the transmit antenna relative to an isotropic source, and “D” is distance from center of transmit antenna to measuring point in meters.  
         [0031]    The E-field in free space is given by:  
           E ( v/m )= SQRT (P d *377), or  (2)  
           E ( dBuv/m )=20 LOG   10 ( E* 10 6 )  (3)  
         [0032]    where “E” is the E-field strength in volts per meter and “dBuv/m” is field strength in dB above 1 microvolt per meter. Referring to FIG. 3, test data indicate that, for a single transmitter, transmitted power levels of both 1 and 3 milliwatts with a nominal unity gain (0 dBi) transmit antenna, the field strength is at or below 110 dBuv/m (0.3 volts per meter) for all distances greater than one meter. For multiple transmitters operating simultaneously using 802.11b protocol, field strength levels are analyzed as further described below.  
         [0033]    Maximum Permissible Exposure (MPE) Levels  
         [0034]    A 802.11b network operates in the 2.4 to 2.483 GHz ISM band. The IEEE C.95.1-1999 standard for human exposure to RF electromagnetic fields specifies a maximum permissible whole body exposure (MPE) level for this frequency region in an uncontrolled environment of f/1500 mw/cm 2  averaged over 30 minutes, where f is frequency expressed in MHz. The worst case or minimum value is at the lower end of the frequency band where MPE=2400/1500=1.6 mw/cm 2  or 16 w/m 2 . The FCC requirement as specified in OET Bulletin 65 for this frequency range is 1.0 mw/cm 2  or 10 w/m 2  averaged over 30 minutes. Although the European CENELEC ES59005 maximum allowable RF exposure levels are less stringent than the FCC limits, the more conservative FCC requirements for compliance are used herein.  
         [0035]    Maximum 802.11b Radiated Field Strengths  
         [0036]    It is assumed that over any 30-minute interval the separation distance from an individual to an access point antenna  34  in the overhead is 1.0 meters. Table 1 below describes 2.4 GHz WLAN radiated emissions at transmit powers from 1 to 100 milliwatts and at a transmitter-to-victim distance of 1 meter.  
                                                                         TABLE 1                           2.4 GHz WLAN Radiated Emissions            Victim to Transmitter Distance = 1 m       Assume Tx antenna gain (dBi) = 2.2 = numeric 1.659587       lambda = 0.125 m       Eff Area = 0.001875 m{circumflex over ( )}2 short dipole            Transmit   Tx Power   Tx Field   Tx Field   Received   Received       Power   Density   Strength   Strength   Power   Power       (mw)   w/m{circumflex over ( )}2   v/m   dBuv/m   w   dBm                    1   0.000132066   0.223134   106.9713   2.48 E-07   −36.06209       3   0.000396197   0.386479   111.7425   7.43 E-07   −31.29087       5   0.000660329   0.498943   113.961   1.24 E-06   −29.07239       10   0.001320657   0.705612   116.9713   2.48 E-06   −26.06209       20   0.002641316   0.997886   119.9816   4.95 E-06   −23.05179       30   0.003961972   1.222155   121.7425   7.43 E-06   −21.29087       50   0.006603286   1.577796   123.961   1.24 E-05   −19.07239       100   0.013206573   2.23134   126.9713   2.48 E-05   −16.06209                  
 
         [0037]    Referring to Table 1, test data indicate that an 802.11b system radiating at 3 mw maximum output power will generate a radiated power density of 4×10 −4  w/m 2  at the distance of 1 meter from the access point antenna  34 . This power density is 4.0×10 −5  times the maximum allowed FCC level, which is equal to a margin of 44 dB.  
         [0038]    It is possible for tall individuals to be within 0.25 meters of an overhead access point antenna  34  in a single-aisle aircraft or for a user to be within 0.05 meters of his/her PED  38  antenna. Table 2 below describes 2.4 GHz WLAN radiated emissions at transmit powers from 1 to 100 milliwatts and at a transmitter-to-victim distance of 0.05 meter.  
                                                                         TABLE 2                           2.4 GHz WLAN Radiated Emissions            Victim to Transmitter Distance = 0.05 m       Assume Tx antenna gain (dBi) = 2.2 = numeric 1.659587       lambda = 0.125 m       Eff Area = 0.001875 m{circumflex over ( )}2 short dipole            Transmit   Tx Power   Tx Field   Tx Field   Received   Received       Power   Density   Strength   Strength   Power   Power       (mw)   w/m{circumflex over ( )}2   v/m   dBuv/m   w   dBm                    1   0.052826292   4.46268   132.9919   9.9 E-05   −10.0415       3   0.158478876   7.729588   137.7631   0.000297   −5.27027       5   0.26413146   9.978856   139.9816   0.000495   −3.05179       10   0.52826292   14.11223   142.9919   0.00099   −0.04149       20   1.056525839   19.95771   146.0022   0.001981   2.968814       30   1.584788759   24.4431   147.7631   0.002971   4.729727       50   2.641314598   31.55591   149.9816   0.004952   6.948214       100   5.282629196   44.6268   152.9919   0.009905   9.958514                  
 
         [0039]    Table 3 below describes margins of compliance with FCC OET Bulletin 65 for worst-case exposure with access points separated by 3 meters and with multiple transmitters.  
                                                                                                                                                                                                               TABLE 3                           Worst Case Exposure with Multiple Transmitters       uz,9/32 Compliance Margins for FCC OET Bulletin 65 Reqmt            Seat spacing (row) = 0.8 m   in = 31.496   Self Dist = 0.05 m            Seat spacing (side) = 0.5 m   in = 19.685 FCC Rqmt   10w/m 2                 Access Pt Spacing = 3 m            Assume Tx antenna gain (dBi) = 2.2 = numeric 1.659587                    Single Emitter       Tx Pwr = 3                Distance   Pwr Dens               m   w/m 2                              0.75   0.000704               0.7   0.000809               0.6   0.001101               0.5   0.001585               0.4   0.002476               0.3   0.004402               0.25   0.006339               0.2   0.009905               0.1   0.03962               0.05   0.158479                        Two &amp; Four Adjacent Emitters + Own @ 0.05 meter       Tx Pwr = 3 mw                        Two + own   Single       Four + own       Dist-TX#1   Dist-TX#2   Two + own   Margin   Margin   Four + own   Margin       m   m   w/m 2     dB   dB   w/m 2     dB                    0.25   0.75   0.165522   17.81143   41.52211   0.166931   17.77463       0.3   0.7   0.16369   17.85979   40.92285   0.165098   17.82257       0.4   0.6   0.162056   17.90336   39.58391   0.163464   17.86577       0.5   0.5   0.161648   17.91428   38.00029   0.163057   17.8766       0.6   0.4   0.162056   17.90336   36.06209   0.163464   17.86577       0.7   0.3   0.16369   17.85979   33.56331   0.165098   17.82257       0.75   0.25   0.165522   17.81143   31.97969   0.166931   17.77463       0.8   0.2           30.04149           0.9   0.1           24.02089           0.95   0.05           18.00029                    Exposure from Two Adjacent Access Pts       Tx Pwr = 3 mw                Dist-TX#1   Dist-TX#2   PwrDens   Margin           m   m   w/m 2     dB                            0.25   2.75   0.006392   31.94394           0.3   2.7   0.004457   33.51002           0.4   2.6   0.002535   35.96049           0.5   2.5   0.001648   37.82995           0.6   2.4   0.001169   39.32062           0.7   2.3   0.000883   40.53813           0.8   2.2   0.000701   41.54333           0.9   2.1   0.000579   42.37342           1   2   0.000495   43.05179           1.1   1.9   0.000437   43.59334           1.2   1.8   0.000397   44.0075           1.3   1.7   0.000372   44.30008           1.4   1.6   0.000357   44.47446           1.5   1.5   0.000352   44.53241                      
 
         [0040]    Referring to Tables 2 and 3, test data indicate that an 802.11b system radiating at 3 mw maximum output power will generate a radiated power density of 6.3×10 −3  w/m 2  at the worst-case minimum distance of 0.25 meters from the access point antenna  34  and 1.6×10 −1  w/m 2  at the worst-case minimum distance of 0.05 meters from the user PED  38  antenna. For the access point antenna  34 , this power density is 6.3×10 −4  of the maximum allowed FCC level, which is equal to a margin of 32 dB. For the user PED  38  antenna, this is 1.6×10 −2  of the maximum allowed FCC level, which is equal to a margin of 18 dB.  
         [0041]    Contribution from Multiple WLAN Sources  
         [0042]    The contribution of multiple WLAN RF emission sources simultaneously transmitting is addressed next. Referring to FIG. 2, the width  92  of each seat  84  is assumed to be 0.5 meters. The seat pitch  96  is assumed to be 0.8 meters (32 inches) and the distance  100  between access point antennas  34  is assumed to be a minimum of 2.5 to 3 meters. Thus it is assumed that the worst-case RF levels are generated by multiple users transmitting via PEDs  38  while sitting in the seats  84  or otherwise closely spaced in the cell areas  80 . It is assumed that the user PEDs  38  transmit simultaneously when they contend for the RF medium as previously described. Such simultaneous transmissions occur only for short periods of time (before one PED is granted access to transmit), compared to the 30-minute exposure time described above in connection with the FCC maximum allowed level of power density. The possibility nevertheless is considered, however, that such transmissions might generate RF signal levels that might interfere with airframe systems. It also is assumed that these asynchronous sources are in phase and that their transmitted signals will add constructively, even though this is unlikely.  
         [0043]    A layout of a plurality of PEDs  38  in adjacent seats  84  is shown in FIG. 4. The predominant source of EMI is likely to be a user&#39;s own laptop  38  antenna, which was assumed above to be at the worst-case distance of 0.05 meters from the user. FIG. 5 shows margins of compliance to FCC emission requirements for a single laptop and for a laptop adjacent to other laptops. At the assumed seat width of 0.5 meters, the effect of one adjacent emissions source diminishes as the user approaches (e.g. leans toward) the other source. The seat pitch is assumed to be 0.8 meters (32 inches). Therefore the contributions from sources in seat rows in front of or behind the subject will not significantly affect the margin of compliance. As shown in Table 3, including two more sources at 0.75 meters (directly in front and in back of the subject laptop and transmitting at 3 mw) to the two sources in the same seat group plus the subject&#39;s laptop will only change the margin for RF exposure compliance from 17.81 to 17.77 dB.  
         [0044]    Radiated Cell Dimensions  
         [0045]    Cell size is determined based on the contemplated power level for the WLAN, the aggregate bandwidth contemplated to be available, and the number of users contemplated to share the bandwidth. For example, in the embodiment shown in FIG. 2, a cell population of 3 rows includes 18 seats per access point. Such could be the case for a narrow body aircraft, e.g. a Boeing 737 or 757. A cell population of three rows on a wide body, e.g. a Boeing 767 or 200, could include 21 seats. A worst-case demand for bandwidth is likely to be for users requesting streaming video services. While systems using 802.11b protocol have been demonstrated to provide up to 8 Mbps per access point, a bandwidth of 6 Mbps is assumed to be achievable on a repeatable basis using standard hardware components. Thus it is assumed that a maximum aggregate bandwidth of 6 Mbps is available per access point  18  using short transmission preambles, and that typically 30 percent, i.e. 6 or 7 user PEDs  38 , in a cell  80  are active and sharing the 6 Mbps bandwidth. Less bandwidth-demanding services such as e-mail or Internet access can support more users per access point  18 . It is contemplated that power radiated by components of the WLAN  10  is kept in the 1- to 5-mw range in order to meet interference, health and safety requirements.  
         [0046]    Received Signal Strength  
         [0047]    Table 4 below describes 2.4 GHz WLAN radiated emissions at transmit powers from 1 to 100 milliwatts and at a transmitter-to-victim distance of 3 meters. Assuming a maximum distance of 3 meters between an access point and its cell boundary, as shown in Table 4, a user PED  38  at the maximum distance from an access point antenna  34  broadcasting at 1 milliwatt receives a signal in the range of −45 to −50 dBm. This signal exceeds the 802.11b-specified value of −76 dBm required to support 11 Mbps communication. Such margin protects against signal fading due to mulltipath within the aircraft cabin.  
                                                                         TABLE 4                           2.4 GHz WLAN Radiated Emissions            Victim to Transmitter Distance = 3 m       Assume Tx antenna gain (dBi) = 2.2 = numeric 1.659587       lambda = 0.125 m       Eff Area = 0.001875 m{circumflex over ( )}2 short dipole            Trans-                           mit   Tx Power   Tx Field   Tx Field   Received   Received       Power   Density   Strength   Strength   Power   Power       (mw)   w/m{circumflex over ( )}2   v/m   dBuv/m   w   dBm                    1    1.4674 E-05   0.074378   97.42889   2.75 E-08   −45.60451       3   4.40219 E-05   0.128826   102.2001   8.25 E-08   −40.8333       5   7.33698 E-05   0.166314   104.4186   1.38 E-07   −38.61481       10   0.00014674   0.235204   107.4289   2.75 E-07   −35.60451       20   0.000293479   0.332629   110.4392    5.5 E-07   −32.59421       30   0.000440219   0.407385   112.2001   8.25 E-07   −30.8333       50   0.000733698   0.525932   114.4186   1.38 E-06   −28.61481       100   0.001467397   0.74378   117.4289   2.75 E-06   −25.60451                  
 
         [0048]    RF Susceptibility Test Levels for Aircraft Equipment  
         [0049]    Aircraft systems have been qualified to varying RF susceptibility test levels and frequency ranges. Those systems that have been determined to be flight-critical and essential are required to demonstrate immunity to the effects of High Intensity Radiated Fields (HIRF) and have been tested to field strengths that are many orders of magnitude above the RF field strength generated by an 802.11b WLAN system. Other systems qualified to levels below the HIRF levels also have demonstrated RF immunity in the 2.4 to 2.483 GHz frequency range. For any aircraft system for which there is no radiated susceptibility test data in the 802.11b operating band of 2.4 to 2.483 GHz, it is proposed that aircraft level susceptibility testing be performed to demonstrate that there will be no interference from the worst case operation of an 802.11b wireless LAN configured in accordance with the embodiments described herein.  
         [0050]    The above-described WLAN  10  includes multiple intentional RF transmitters that operate at very low levels of RF field strength. These low levels provide significant margins of compliance for both electromagnetic interference and RF exposure limit regulations for operators, airframe manufacturers, and the traveling public. This makes it possible to safely operate the above-described WLAN  10  on board commercial aircraft in flight.  
         [0051]    The description of the invention is merely exemplary in nature and, thus, variations that do not depart from the gist of the invention are intended to be within the scope of the invention. Such variations are not to be regarded as a departure from the spirit and scope of the invention.