PATENT ABSTRACT
A communication network includes a local area network (LAN) and a wireless access point coupled to the LAN. In one embodiment, each access point includes a medium access control (MAC) stage, and a radio frequency (RF) transmitter/receiver for communicating unsecure message data via RF links with users of associated wireless devices. An optical transmitter/receiver in the access point enables the users to communicate secure message data over the LAN via free space optical (FSO) links with the users. The MAC stage operates (i) to direct unsecure data from the LAN to the wireless device users and to direct unsecure data from the users to the LAN, via the RF transmitter/receiver; and (ii) to direct secure data from the LAN to the wireless device users and to direct secure data from the users to the LAN, via the optical transmitter/receiver. An integrated VoIP/FSO portable handset is also disclosed.

PATENT DESCRIPTION
BACKGROUND OF THE INVENTION 
       [0001]    1. Field of the Invention 
         [0002]    This invention relates to communication networks, and particularly to a network that provides portable users with secure access when exchanging information with other users on the network. 
         [0003]    2. Discussion of the Known Art 
         [0004]    As military conflicts are being resolved more through the use of a network-centric rather than a platform-centric paradigm, vital communications over the established networks must be secure, reliable, interoperative, survivable, and timely. The implementation of a high capacity, multimedia network is also desirable. 
         [0005]    Free space optical (FSO) or photonic communication links have been deployed in fixed, point-to-point links for commercial and military applications. Such links may be preferred over microwave or millimeter wavelength radio frequency (RF) links for short range communications, especially when other communication infrastructure is unavailable, unreliable, or untrustworthy. FSO links have the following advantages: 
         [0006]    1. The links are highly directional, and therefore quite immune to interception, interference or jamming. 
         [0007]    2. Secure communications during periods of radio silence. 
         [0008]    3. Elimination of any detectable RF signature. 
         [0009]    4. FSO terminals can be made small, lightweight, and are easily portable. Optical antennas including light emitters (e.g., laser LEDs) and detectors (e.g., photodiodes) have typical gains on the order of one million times those of isotropic RF antennas. 
         [0010]    5. Low power consumption. 
         [0011]    6. The availability of a wide frequency spectrum with no governmental regulatory restrictions. 
         [0012]    7. Large data bandwidth capacity. 
         [0013]    8. Direct baseband signaling, thus simplifying modulation and demodulation processes. 
         [0014]    9. Ease of multiplexing, de-multiplexing, and switching of optical channels. 
         [0015]    10. Tactically useful range. 
         [0016]    Projects are being pursued that would enable laser communication on the move between platforms ground to ground, ground to air, air to air, air to satellite, and satellite to air. Infrared (IR) light sources and detectors suitable for use in high data rate FSO transmitters and receivers are commercially available at low cost. 
         [0017]    IR light penetrates clear glass but will not propagate through walls or other opaque building structures. FSO links are therefore confined to rooms or other areas inside buildings where the links are established. Such confinement enhances the security of FSO transmissions against interception or casual eavesdropping, and avoids interference between optical links operating in physically separate regions, thus making possible a high degree of spectrum reuse. Also, while multipath fading may cause signals to fluctuate in strength and phase over RF links, FSO links are immune to fading if intensity modulation and direct detection (IM/DD) techniques are applied. See, J. M. Kahn et al., “Wireless Infrared Communications”, 85 (2) Proceedings of the IEEE (Feb. 1997), at 265-98, which is incorporated by reference. 
       Portable Infrared (IR) Devices 
       [0018]    For short range (up to a few meters) applications, consumer devices are available that allow data to be transferred between the devices via infrared light. The Infrared Data Association (IrDA) defines specifications for point-to-point communication using directional half duplex serial IR links through space, at data rates up to and including 115.2 kbit/s; 0.576 Mbps, 1.152 Mbps, 4.0 Mbps and 16 Mbps. Cell phones are available with IR ports that follow these standards for enabling the phones to dump data into stationary printers, PDAs, or PCs equipped with IR ports. See “Motorola i930/i920”, at &lt;www.phonescoop.com/phones/phone.php?p=627&gt;. IR ports of typical cell phones do not carry active voice communications and, as mentioned, are limited in range to 1 to 2 meters. 
       VoIP Telephony and Wireless Local Area Networks 
       [0019]    The use of voice-over-Internet protocol (VoIP) telephony, both wired and RF wireless, is expanding. In a conventional circuit switched telephone system, a dedicated physical connection is established between a calling and a called party over the duration of the call. The continuous connection assures that voice signals carried between end points of the system are not interrupted. With a VoIP system, however, there is no dedicated connection. Instead, analog voice signals from a microphone transducer in a user&#39;s handset or headset are digitized, and corresponding digital data is transmitted over a system network in separate groups of data called “packets”. Each packet contains the sender&#39;s and the recipient&#39;s IP addresses, and a piece of digitized voice information (“payload” data). The packets may be routed through the network over different paths, and eventually arrive with some delay at a common destination to be recombined in the proper sequence. Further, each packet may arrive with a different delay. Variations in arrival time are defined as “jitter”. Some packets may never reach the destination, resulting in “packet loss”. Most vendors adhere to strict limits on tolerable packet loss, delay, and jitter. For example, Cisco Systems adopted the following guidelines for VoIP network operation: 
         [0000]    
       
         
               
               
               
             
           
               
                   
                   
               
               
                   
                 Network Performance 
                 Value 
               
               
                   
                   
               
             
             
               
                   
                 Delay 
                 &lt;=150 milliseconds (ms) one-way 
               
               
                   
                 Jitter 
                 &lt;=30 ms 
               
               
                   
                 Packet loss 
                 &lt;=1% 
               
               
                   
                   
               
             
          
         
       
     
         [0020]    VoIP may offer many features above and beyond those afforded by traditional telephony systems, whether wired or remote. See, e.g., A. Noser, “Combining VoIP and Wireless Services”, at &lt;www.ncstate.net/wireless/presentations/wirelessvoip/wirelessvoip.html&gt;, which is incorporated by reference. Manufacturers claim their wireless VoIP products allow mobile users to engage in conversations anywhere in an IP network with reliability and voice quality equivalent to that of a desktop office phone. Internet gateways and RF access points are positioned to ensure that user conversations do not drop out or experience gaps, regardless of a user&#39;s location within a defined area. As voice quality, reliability and security improve, IP wireless communication including the use of convenient portable VoIP handsets is likely to increase. 
         [0021]    A typical VoIP local area network (LAN)  10  is illustrated in  FIG. 1  including commercial off-the-shelf (COTS) products. To connect with a legacy public telephone switching exchange (PBX)  12 , a telephony gateway  14  is configured to convert analog voice signals received over the PBX  12  into IP voice data packets. The packets are routed through an Ethernet cable  15  that connects with RF wireless access points  16 . Voice data packets arriving at the gateway  14  over the cable  15  are converted to analog voice signals for transmission into the PBX  12 . The gateway  14  may be omitted if the PBX  12  is a so-called telephony server. 
         [0022]    The access points  16  may comprise RF wireless routers each of which operates according to, e.g., known IEEE 802.11x signaling protocols. A voice priority server  20  available, for example, from Spectralink SVP® may be provided to ensure that the voice data packets have priority over other kinds of data carried over the network  10 . The access points  16  may join or bridge various wireless clients such as, for example, a number of portable VoIP telephone sets  26 , a notebook computer  27  and a PDA  28 , with fixed users and devices connected by wire to the network  10 . 
         [0023]      FIG. 2  shows a typical high level architecture for a wireless access point  16 . Access point  16  may operate, for example, under one or more defined RF signaling protocols per the IEEE Standards 802.xxx. Because voice data transmitted by a user of a RF device may be received by users of like RF devices within range, some security measures are available to ensure that a user&#39;s data is not captured or manipulated by unauthorized intruders. When classified or other highly sensitive voice messages are involved, however, commercial security (COMSEC) is insufficient for the task. For example, adding improved Type I security can significantly increase cost and management complexity, since such security must be controlled and crypto keys must be managed. 
       Wireless VoIP Phone Sets 
       [0024]    Several vendors provide RF wireless VoIP telephone sets that can access a LAN using IEEE 802.11x or other newly emerging IEEE 802.xxx RF signaling protocols. See, e.g., “Linksys WIP330 Wireless-G IP Phone”, at &lt;www.tomsnetworking.com/Sections-article147-page1.php&gt;. A block diagram of a typical wireless VoIP telephone  26  is shown in  FIG. 3 . Core subsystems include: 
         [0025]    A RF transceiver/power amplifier  30  that performs frequency translation between the RF and the baseband (voice) signals, and amplifies RF signals to be radiated from the phone from an antenna  31 . 
         [0026]    A medium access control (MAC)/baseband processor  32  which implements the applicable IEEE 802.xxx protocols and provides modem functionality to control wireless signaling and communication between the telephone  26  and the wireless access points of the LAN. 
         [0027]    A DSP/microcontroller/OMAP  34  that executes VoIP call controls and voice processing, and provides a user interface. 
         [0028]    Various memories including flash, ROM and RAM stages for storing programming code, voice and other data. 
         [0029]    A voice coder-decoder (CODEC)  36  which interfaces with a user headset  37  having a microphone  38  and a speaker or earpiece  39 . The CODEC  36  operates to convert a user&#39;s analog voice signals as produced by the microphone  38 , into corresponding digital voice data to be processed by the OMAP  34 . 
         [0030]    The RF bandwidth required for each voice call depends on (i) the type of CODEC  36 , (ii) the number of CODEC samples per data packet, and (iii) the packet header compression. The number of CODEC samples per packet affects the delay of a VoIP call. As the size of the sample data increases, the required bandwidth decreases but the overall delay increases. 
         [0031]    As mentioned, if a wireless VoIP telephone set user desires to discuss classified subject matter, COMSEC items must be provided thereby increasing equipment cost and management complexity. Accordingly, there is a need for a robust multi-user local area wireless network that is not only capable of interfacing with current VoIP telephone sets, but which also provides security for portable users who want to convey sensitive information without having to invoke costly COMSEC measures. 
       SUMMARY OF THE INVENTION 
       [0032]    The inventive network allows a portable user to engage in wireless communications wherein normal messaging is routed over a RF link with the user, and classified or other highly sensitive messages are contained over a secure FSO link that can be established by or with the user when desired. 
         [0033]    According to the invention, a communication network includes a local area network (LAN) and a wireless access point coupled to the LAN. In one embodiment, each access point includes a medium access control (MAC) stage, and a radio frequency (RF) transmitter/receiver for communicating unsecure message data via RF links with users of associated wireless devices. An optical transmitter/receiver in the access point enables the users to communicate secure message data over the LAN via free space optical (FSO) links with the users. 
         [0034]    The MAC stage operates (i) to direct unsecure data from the LAN to the wireless device users and to direct unsecure data from the users to the LAN, via the RF transmitter/receiver; and (ii) to direct secure data from the LAN to the wireless device users and to direct secure data from the users to the LAN, via the optical transmitter/receiver. 
         [0035]    According to another aspect of the invention, a wireless handset includes a message data source, and a radio frequency (RF) transceiver for transmitting RF signals corresponding to unsecure message data to a network access point, and for receiving RF signals corresponding to unsecure message data radiated from the access point. An optical transceiver operates to transmit free space optical (FSO) signals corresponding to secure message data to an optical access antenna system associated with the access point, and to receive FSO signals corresponding to secure message data emitted from the optical antenna system. A switching stage has a first port coupled to the message data source, a second port coupled to the RF transceiver, and a third port coupled to the optical transceiver. The switching stage is configured to couple the message data source to the RF transceiver for unsecure message data, and to the optical transceiver for secure message data. 
         [0036]    For a better understanding of the invention, reference is made to the following description taken in conjunction with the accompanying drawing and the appended claims. 
     
    
     
       BRIEF DESCRIPTION OF THE DRAWING 
         [0037]    In the drawing: 
           [0038]      FIG. 1  is a block diagram of a typical local area network (LAN) with wireless access points; 
           [0039]      FIG. 2  is a block diagram of a typical wireless access point in the network of  FIG. 1 ; 
           [0040]      FIG. 3  is a block diagram of a typical voice over Internet protocol (VoIP) wireless telephone; 
           [0041]      FIG. 4  is a block diagram of a communication network according to the invention; 
           [0042]      FIG. 5  is a block diagram of an integrated radio frequency (RF) and free space optical (FSO) wireless access point, according to the invention; 
           [0043]      FIG. 6  is a block diagram of an integrated RF and FSO wireless handset, according to the invention; 
           [0044]      FIG. 7(   a ) illustrates a first embodiment of an optical access antenna system, including a number of optical antennas associated with the access point of  FIG. 5 ; 
           [0045]      FIG. 7(   b ) illustrates a second embodiment of the optical access antenna system; 
           [0046]      FIG. 8  shows optical transmitting and receiving elements mounted on the handset of  FIG. 6 ; and 
           [0047]      FIGS. 9(   a ) to  9 ( d ) illustrate arrays of light receiving elements that may form part of each optical antenna in the system of  FIG. 7(   a ) or  FIG. 7(   b ). 
       
    
    
     DETAILED DESCRIPTION OF THE INVENTION 
       [0048]      FIG. 4  is a schematic block diagram of a communication network  40  according to the invention. The network  40  has one or more associated wireless access points (WAP)  50 , described below, which enable the network to be accessed by users of one or more portable handsets or headsets  70 . In addition to signaling via RF links with the access points  50 , the handsets  70  are capable of establishing FSO links when necessary to exchange secure (e.g., classified) voice data over the network  40 . Details of the handsets  70  are set out below in connection with  FIG. 6 . 
         [0049]      FIG. 5  is a schematic block diagram of a first embodiment of an integrated RF and FSO network access point  50 , according to the invention. In addition to the RF components of the typical wireless access point  16  in  FIG. 2 , the inventive access point  50  includes an optical transceiver. The optical transceiver comprises an optical receiver  52 , an optical transmitter  54 , and an optical access antenna system  56  that is coupled to an input of the receiver  52  and to an output of the transmitter  54 . 
         [0050]    The optical access antenna system  56  may be coupled to the optical receiver  52  and the optical transmitter  54  through a passive optical network (PON)  57 , as shown in  FIG. 7(   a ). The individual optical antennas  100  may be mounted, for example, in a grid array on the ceiling of one or more secure rooms access to which is restricted to authorized personnel. 
         [0051]    In the access point  50 , a baseband output of the optical receiver  52  is coupled to an input of a medium access controller (MAC)  58  through a desired crypto device  60 . The crypto device  60  operates to encrypt voice data detected by the optical receiver  52 , and to supply the encrypted voice data to the MAC  58 . Further, the optical transmitter  54  has an input coupled to a baseband output of the MAC  58  through a corresponding decrypto processor  62 . The decrypto processor  62  is configured to decode encrypted voice data received over the LAN  10  and output from the MAC  58 , and to supply the decoded data to the optical transmitter  54 . 
         [0052]    Depending on the nature of voice data originating from the LAN  10  and destined to a particular handset user, the MAC  58  routes the data through only one of the optical transceiver ( 52 ,  54 ), or the RF transceiver  64 . For encrypted secure data to be delivered from the LAN  10  to an authorized handset user, the decrypto baseband processor  62  decrypts the data before it is modulated onto a light signal by the optical transmitter  54 . Voice data originating from a handset user over his/her established FSO link, is detected by the optical receiver  52  and input to the crypto device  60 , as shown in  FIG. 5 . 
         [0053]      FIG. 6  is a schematic representation of an integrated wireless RF and FSO handset (or headset)  70 , according to the invention. In addition to the components of the wireless VoIP telephone  26  in  FIG. 3 , the handset  70  includes an optical transceiver  72 , an optical antenna  74  which is coupled to the transceiver  72 , and a switching stage  76 . In the illustrated embodiment, the switching stage  76  has a first port  78  coupled to the MAC/baseband processor  32  of the handset  70 , a second port  80  coupled to the RF transceiver  30 , and a third port  82  coupled to the optical transceiver  72 . The handset  70  may also feature a ringer unit  84  that is coupled to an output of the optical transceiver  72 . The ringer unit  84  is constructed and arranged to produce, for example, a distinct alert sound and a blinking red LED display when the transceiver  72  detects a light signal having message data that is addressed to a user of the handset  70 . 
         [0054]    The optical antenna  74  and the transceiver  72  may be housed together in an optical module  86  that is constructed and configured to connect with the MAC/baseband processor  32  inside the handset  70  via, for example, an RJ-45 or other common wire connector interface that has been mounted onto the handset housing. The optical module  86  may be powered, e.g., by an existing voltage source (not shown) disposed in the handset  70 . If desired, the switching stage  76  and the ringer  84  may be mounted and arranged inside an existing VoIP handset. 
         [0055]    When a user of the handset  70  wants to communicate classified or other sensitive information to an authorized person on the network  40 , the user operates the switching stage  76  to establish a FSO link between the handset antenna  74  and one or more of the optical antennas  100  in line of sight of the user. As mentioned, the FSO link provides communication security since the user&#39;s light signals will not propagate beyond the room or other area in which the user and the optical antennas  100  are located. A password may be entered by the user before the switching stage  76  can be operated to establish the FSO link. It is also preferable to configure the switching stage  76  so that only one of an RF or an FSO link can be established by the user at any given time. Thus, once an FSO link has been selected, there is no possibility of an inadvertent leakage of the user&#39;s secure information onto an RF link with one of the network access points  50 . 
         [0056]    As mentioned, the optical access antenna system  56  may include a grid of the individual optical antennas  100  mounted, for example, on the ceiling of a restricted occupancy room or other limited access area in a building. In the embodiment of  FIG. 7(   a ), the passive optical network (PON)  57  may include one or more large core (e.g., &gt;100μ) multimode optical fibers to couple the optical transceiver in the access point  50  with each of the optical antennas  100  forming the grid. Light reflectors or diffusers (not shown) may, if necessary, be provided in a given room to obtain 100% FSO connectivity for authorized users at various locations in the room. A variety of common building materials may also act as efficient diffuse infrared reflectors. For example, in the 800 to 900 nm range, plaster walls and acoustical ceiling tiles have diffuse reflectivities typically in a range between 0.6 and 0.9. 
         [0057]    The PON  57  in  FIG. 7(   a ) may, for example, implement a known coarse wavelength division multiplexing (CWDM) scheme. The CWDM scheme maintains large spectrum separation between the transmitting and the receiving light signals, so that available optical filters with high isolation can be used to separate the transmitted and the received light signals from one another at both ends. For example, to permit the use of low cost, large area silicon diode based detectors, a high power 950 nm laser may be used as a light source at the access point transmitter  54  for downstream (access point to user) transmissions, and an 880 nm GaAs laser/LED may be used as a light source for the transceiver  72  in the handsets  70  for upstream transmissions. The 950 nm laser can deliver up to 1 watt (W) of power which is sufficient to feed the multiple optical antennas  100 , thus eliminating the need for an optical amplifier. Because eye safety is of paramount importance, however, a 950 nm wavelength may not be suitable for all applications. In such case, a 1550 nm laser may be used together with an optical amplifier to increase power level. Alternatively, an element such as a diffuser may be employed to destroy special coherence of the laser beam and spread the radiation over a sufficiently extended aperture and angle. 
         [0058]    A second embodiment of the optical access antenna system  56  is shown in  FIG. 7(   b ). In the embodiment of  FIG. 7(   b ), the access point  50  is preferably located in the same room or other restricted area as the array of optical antennas  100  forming the optical access antenna system  56 . An electrical wire or cable distribution system  67  is arranged to couple the input of the crypto device  60  and the output of the decrypto baseband processor  62  in the access point  50 , with pairs of electrical to optical (E/O) media converters  102 ,  104 . Each pair of E/O converters is associated with a given one of the antennas  100 . The E/O converter  102  is configured to convert electrical signals from the decrypto baseband processor  62 , into corresponding light signals to be emitted from the associated optical antenna  100  on an FSO link. The E/O converter  104  is configured to convert light signals received by the antenna  100  on the FSO link, into corresponding electrical signals for input to the crypto device  60 . The E/O converters  102 ,  104  may incorporate suitable LEDs in the 880 to 1550 nm wavelength range for the uplink (E/O converter  104 ) and the downlink (E/O converter  102 ) message data flows. Because decrypted electrical data signals may be present on the cable distribution system  67  in  FIG. 7(   b ), it is important that appropriate measures are taken to prevent unauthorized access or detection of any signals on the distribution system  67 . 
         [0059]    If the handsets  70  include the mentioned type G.729 codecs with compressed data headers and 100 simultaneous system users are assumed, less than 1.2 Mbit/s total bandwidth is needed in each direction for voice traffic. The light source in each antenna  100  may then take the form of a LED, a Fabry Perot (FP) broad area laser, or a GaAs VCSEL based transmitter, all of which can support the mentioned data rate. 
         [0060]    In  FIGS. 7(   a ) and  7 ( b ), the optical antennas  100  are arrayed so as to enable a handset user to have a clear LOS to at least one of the antennas from any location in a given secure area. Because the PON architecture of  FIG. 7(   a ) requires no active optical components between the access point  50  and the optical antennas  100 , micro-cells  102  each of radius R less than, e.g., ten feet, may be defined. All cells may be in the same building, or spread over different buildings/rooms. For example, if the size of the building room in  FIG. 7(   a ) is about 80′×40′, it may be divided into 15 micro cells each with a radius R of eight feet. If a downstream laser from the access point transmitter  54  produces 100 mW of power and is split through a series of 1×4 splitters as shown in the drawing, at least 5 mW of power will be available at each optical antenna  100 . The available antenna power may then be split further to feed four or five light transmitting elements that define each antenna  100  for covering all directions. Each antenna element will then radiate about 1 mW power for downstream optical signals after discounting any losses in the PON  57 . About 1 mW of power may also be satisfactory for upstream optical signals transmitted from an antenna element on the handset  70  (see  FIG. 8) . 
         [0061]    In the arrangement of  FIG. 7(   b ), LEDs can be used as transmitting elements for both uplink and downlink, with each LED emitting more than 1 mw power. The antenna grid in  FIG. 7(   b ) will not, however, be passive since the pairs of E/O converters  102 ,  104  associated with each antenna  100  will require electrical power supplied, e.g., from the access point  50  in order to operate. 
         [0062]    In some applications it may also be desirable to employ an optical concentrator or lens to increase the effective area of each optical antenna  100 . An angle-diversity receiving array using multiple receiving elements  120  oriented in different directions together with a light concentrator, may be used advantageously in place of a single receiving element as shown in  FIG. 9(   a ). This scheme allows the receiving elements  120  to achieve high optical gain and a wide field of view (FOV) simultaneously, and may also reduce the impact of any ambient light noise and multi-path distortion. Multiple signals may be summed with equal weights, or the signal having the best signal to noise ratio (SNR) may be selected by operation of a selector/combiner stage  122 . 
         [0063]      FIG. 9(   b ) shows an alternative arrangement to implement angle-diversity reception, using an array of photo detector elements  130  disposed at a focal plane of an optical concentrator  132 . Each detector element has an associated preamplifier  134 , and the elements  130  can be fabricated in large number monolithically. Only one concentrator  132  may be needed regardless of the number of detector elements  130 . The  FIG. 9(   b ) arrangement results in a narrower FOV as shown in  FIG. 9(   d ), when compared to the FOV in  FIG. 9(   c ) obtained when using the receiving elements  120  in  FIG. 9(   a ). 
         [0064]    For upstream light signals to be beamed from the handsets  70  to one or more of the optical antennas  100 , any of the mentioned devices capable of emitting light at wavelengths of 850 nm to 1550 nm may be used for the handset transmitting element  112 . Typical packaged LEDs emit light into semi-angles (at half power) ranging from about 10 to 30 degrees, making them suitable for directed transmissions. A disadvantage of LEDs is their broad spectral width (typically 25 to 100 nm) which would require a wide passband for the light detectors that define the optical antennas  100  in  FIG. 7(   a ), resulting in poor rejection of the ambient light. An array of available, low cost 850 nm VCSELs may therefore be useful to form directive light beams to carry the upstream signals from the handsets  70  in place of the single transmitting element  112 . For ease of implementation and to prevent inter symbol interference due to different times of arrival of voice data from a handset user, it may be desirable to use short pulse (RZ type) on-off key modulation, NRZ, or 4-PPM. 
         [0065]    Experimental results reported in the literature suggest that the above mentioned power levels for the light sources in the access point  50  and the handset  70 , will provide adequate margins to support a data rate of about 5 Mbps using a 10 mm aperture for the handset receiving elements  110  in  FIG. 8 . Ultimate system performance will, of course, be limited by ambient noise and noise suppression methods. 
         [0066]    Intense ambient IR noise in the environment of a handset user may be reduced through optical filtering and/or the use of a directional light receiving array on the handset  70  to discern a desired signal from the noise.  FIG. 8  shows a quadrant array of light receiving elements  110  for high collection efficiency, and a central light transmitting element  112 . The elements  110 ,  112  may be mounted together, for example, on an outside surface of the handset housing or on an associated headset. 
         [0067]    Multi-megabit capacity FSO links may therefore be established by portable users on the network  40 , and known time-division multiple-access (TDMA) techniques may be applied to share available bandwidth so that a number of independent voice streams will be supported simultaneously. Some level of security may also be obtained for RF links carrying unclassified voice communications between the handsets  70  and the access point  50 , by using VoIP phones that incorporate known secure socket layer (SSL) technology. As mentioned, the switching stage  76  is preferably configured so as to make it impossible for the handset  70  to establish an RF link once an FSO link has been selected for secure communication. 
         [0068]    It will be understood that final configurations of the handset optical antenna  74 , and the optical access antenna system  56 , will depend on the physical size and nature of the building in which the antenna system  56  is installed and the number of handset users, among other parameters. Because the voice data is preferably IP in nature and the FSO links allow a large data carrying capacity, the same architecture will support multimedia services (voice, image, and other kinds of data) seamlessly, if needed. 
         [0069]    The inventive communication network  40  integrates optical communication techniques with emerging commercial VoIP handset technology. The network features secure photonic voice links including, if desired, a TDMA access scheme for classified audio transport within restricted areas. The network may therefore support any service (voice, data or image) now supported by existing RF wireless VoIP phone sets. 
         [0070]    While the foregoing description represents preferred embodiments of the invention, it will be obvious to those skilled in the art that various changes and modifications may be made without departing from the spirit and scope of the invention as defined by the following claims. For example, the network  40  may extend and enhance any existing military (e.g., JTRS) or homeland security infrastructure for which a secure access feature is desired for portable or mobile users. Also, the PON  57  in the embodiment of  FIG. 7(   a ) may implement optical wavelength division multiplexing using two wavelengths in each direction, one wavelength being used for classified and the other for unclassified voice signals.