Abstract:
A wireless communications method and system includes an industrial sensor network that comprises a plurality of industrial sensors, which communicates with the industrial sensor network at a particular operating frequency band through one or more infrastructure nodes associated with the industrial sensor network. A wireless communications network is established, which includes a plurality of wireless communications stations that communicates with one or more of the infrastructure nodes. The infrastructure node(s) can be modified to additionally function as a wireless access point for the wireless communications network utilizing Time Division Multiplexing Algorithm (TDMA), thereby permitting the industrial sensor network to coexist and accommodate the wireless communications network.

Description:
TECHNICAL FIELD 
     Embodiments are generally related to data communication methods and systems. Embodiments are additionally related to wireless communication methods and systems. Embodiments also relate to Wi-Fi networks and other types of wireless communications networks. 
     BACKGROUND OF THE INVENTION 
     Many processes and devices have been used for wireless communication for the past few years. A number of wireless integrated systems have been introduced for the monitoring and maintenance of industrial process systems. Two of the major products in this area are, for example, the IntelaTrac mobile PKS and the XYR 5000 wireless sensing devices. 
     The IntelaTrac system, for example, provides a mobile control room for operators in the field in an industrial environment. All process data including current and historical trends, plant layouts, piping and instrumentation diagrams, which are available for the operator in the control room, can now be accessed by the mobile wireless worker in the field. This can be achieved by laying out a wireless Wi-Fi infrastructure in the field and using a web pad that connects to the infrastructure for access to the data much like that found in an office setting. These communications are referred to as “Level 3” (i.e., an indication of the security of the data) communications. 
     Referring to  FIG. 1 , a diagram of a prior art system  10  is illustrated indicative of a communication architecture, which can be implemented in an industrial control system. System  10  can be used to manage workflow and field workforce activities  11 . As indicated in  FIG. 1 , an intela trac work process system  12  can be implemented for mobile communication in association with an intela trac mobile manager  13 . A wireless LAN (802.11b) provides data synchronization “on demand” for a mobile communication device  15 , which provides level 3 communications. A reliability CM system  16  is also provided in association with a maintenance management system  17  and an ERP system (i.e. SAP)  18  with the assistance of an application network (TCP/IP). An operator console module  19  can be configured over the network by using a local control network and an application network (TCP/IP) for operational functionality of the entire system. An alert management system  20  can also be provided, which generates system alerts. Security can be provided by using a firewall  21 , whose settings and historical trends are processed by a process historian  22  by enabling configuration with a local control network. A central loop  23  provides a functionality for image processing  25  and for video camera related conferencing. 
     The XYR5000 and next generation WNSIA based sensor networks also use wireless technologies. Such systems, however, are used to transmit periodic data from sensing elements in the field to controllers that monitor and/or control low level loops. Such a configuration can be referred to as a “Level 1” communication and may be provided through the use of a wireless infrastructure. 
     The two types of communications (i.e., Level 1 and Level 3) possess different data rates and range requirements. Hence, two wireless networks with different characteristics need to be laid out and implemented. Moreover since these solutions need to be globally applicable, these two networks should preferably operate in the same ISM band (2.4 GHz). This means that for both wireless networks to be operational in a plant, two different infrastructure nodes must be installed in the same plant or location. Also, the security requirements for Level 1 and Level 3 communications are different. As far as possible, these two levels of communication should be kept disjointed from one another. Any device that serves both communication levels should be certified as compliant with different security protocols and should guarantee the Quality of Service (QoS) limits of both networks. 
     The majority of industrial customers prefer a common integrated infrastructure that is maintainable in the field. Maintenance of the two different infrastructures would be cost prohibitive or may wipe out the cost benefits of a wireless network. One of the problems with prior art systems is that such configurations do not provide for the definition and design of a common infrastructure for a Local Sensor Network and an Application Level Network in the field. Such a system should preferably manage different security protocols depending on the level of the communications that it is serving. 
       FIG. 2  illustrates a graphical representation  20  with reference to a “backoff time”. In general, the backoff time=Random ( )×a SlotTime, where Random ( ) is equivalent to a pseudorandom integer drawn from a uniform distribution over an interval [0,CW], where CW represents an integer within the range of values of the PHY characteristics aCWmin and a CWmax, wherein aCWmin≦CW≦aCWmax. It is important that designers recognize the need for statistical independence among the random number streams among one or more Slot Times (ST&#39;s) and that the SlotTime is equal to the value of the correspondingly named PHY characteristic. 
     Various types of wireless networks are often co-located or operate in a similar or the same frequency band. A typical wireless public network, for example, may be composed of a plurality of base stations that communicate with mobile stations. Each base station can communicate with mobile stations up to several miles away. Wireless local area network access points can be utilized for communicating with selected mobile stations located in the wireless local area network. An access point can be configured as a first transceiver that communicates with selected mobile stations within a short distance of the access point, a second transceiver that communicates with a first base station of the wireless public network. In such a configuration, interconnection circuitry can also be provided for transferring data traffic between the first transceiver and the second transceiver, such that the access point enables the selected mobile stations to communicate with the first base station via the access point. 
     A drawback with the use of different wireless networks and/or wireless communications protocols is that interference typically results between two or more wireless networks operating in the same frequency band. A need thus exists for the ability to overcome such interference and allow for the seamless coexistence of two or more networks as disclosed in greater detail herein. 
     BRIEF SUMMARY 
     The following summary is provided to facilitate an understanding of some of the innovative features unique to the embodiments disclosed and is not intended to be a full description. A full appreciation of the various aspects of the embodiments can be gained by taking the entire specification, claims, drawings, and abstract as a whole. 
     It is, therefore, one aspect of the present invention to provide for an improved wireless communication. 
     It is another aspect of the present invention to provide for a time division multiple access (TDMA) based infrastructure system that schedules the communications of level 1 and level 3 networks, and which incorporates all security protocols defined for these levels. 
     It is a further aspect of the present invention to provide for a system in which communications networks share information concerning the time frequency and possible codes indicative of each other&#39;s coexistence. 
     The aforementioned aspects and other objectives and advantages can now be achieved as described herein. A wireless communications method and system are disclosed. In general, wireless network can be provided, which may be composed of a group of sensors that communicates with the wireless network at a particular operating frequency band through one or more infrastructure nodes associated with the wireless network. An auxiliary wireless communications network can be established which includes a plurality of wireless communications stations that communicates with one or more of the infrastructure nodes at the same frequency band or at another operating frequency band. The infrastructure node(s) can be modified to additionally function as a wireless access point for the wireless communications network, thereby permitting the wireless network to coexist and accommodate the auxiliary wireless communications network. 
     The wireless network can be implemented, for example, as an industrial sensor network. A TDMA data structure can be implemented to support coexistence of the industrial sensor network and the wireless communications network. The wireless communications network can be provided as, for example, a Wi-Fi network, while the wireless access point can be configured as a Wi-Fi access point. The industrial sensor network communicates with one or more of the infrastructure nodes, for example, utilizing, but not limited to, a Frequency Hopping Spectrum (FHSS) mechanism. Additionally, the wireless access point can communicate with the wireless communications network utilizing, for example, but not limited to, an IEEE 802.11 communications protocol. 
     The method and system disclosed herein involves an approach based on a design and infrastructure that serves different types of networks utilizing, for example, a TDMA type scheduling. In such a design, time scheduling aspects of the architecture can be implemented while minimizing modifications and optimizations to protocol stacks of, for example, an 802.11 network and, for example, an industrial wireless sensor network. The system architecture is generally transparent to end nodes of wireless networks (e.g., Webpads in case of Level 3 networks). Additionally, sensor nodes in an industrial sensor network can be modified to transmit packets only at FHSS slots, depending upon design considerations. This means that the end devices can seamlessly integrate and communicate with the networks. These modifications and time scheduling aspects are restricted only to the infrastructure. A major functionality of the infrastructure is to orchestrate and communicate various end devices during their particular time slots. 
    
    
     
       BRIEF DESCRIPTION OF THE DRAWINGS 
       The accompanying figures, in which like reference numerals refer to identical or functionally-similar elements throughout the separate views and which are incorporated in and form a part of the specification, further illustrate the embodiments and, together with the detailed description, serve to explain the embodiments disclosed herein. 
         FIG. 1  illustrates a diagram of a prior art system indicative of a communication architecture, which can be implemented in an industrial control system; 
         FIG. 2  illustrates a graphical representation with reference to a backoff time; 
         FIG. 3  illustrates a block diagram of a wireless application network system that is utilized support an integrated infrastructure for the coexistence of Wi-Fi networks with other networks in accordance with a preferred embodiment; 
         FIG. 4  illustrates a block diagram of a super frame TDMA data structure that can be utilized to support an integrated infrastructure for the coexistence of Wi-Fi networks with other networks in accordance with a preferred embodiment; 
         FIG. 5  illustrates an event flow diagram of IEEE 802.11 DCF to support integrated infrastructure for coexistence of Wi-Fi networks with other networks in accordance with a preferred embodiment; 
         FIG. 6  illustrates an event flow diagram of backoff mechanism to support integrated infrastructure for coexistence of Wi-Fi networks with other networks in accordance with a preferred embodiment; 
         FIG. 7  illustrates a block diagram of different power levels of FHSS to support integrated infrastructure for coexistence of Wi-Fi networks with other networks in accordance with a preferred embodiment; 
         FIG. 8  illustrates a high-level flow chart of operations illustrating logical operational steps for providing a TDMA scheme to support an integrated infrastructure for the coexistence of Wi-Fi networks with other networks in accordance with a preferred embodiment; 
         FIG. 9  illustrates a high-level flow chart of operations illustrating logical operational steps for providing Wi-Fi data transmission to support an integrated infrastructure for the coexistence of Wi-Fi networks with other networks in accordance with a preferred embodiment; 
         FIG. 10  illustrates a high-level flow chart of operations illustrating logical operational steps for Wi-Fi access point modification to support an integrated infrastructure for the coexistence of Wi-Fi networks with other networks in accordance with a preferred embodiment; 
         FIG. 11  illustrates a high-level flow chart of operations illustrating logical operational steps with reference to iNode for Wi-Fi access point modification to support an integrated infrastructure for the coexistence of Wi-Fi networks with other networks in accordance with a preferred embodiment; 
         FIG. 12  illustrates a high-level flow chart of operations illustrating logical operational steps with reference to industrial sensors for Wi-Fi access point modification to support an integrated infrastructure for the coexistence of Wi-Fi networks with other networks in accordance with a preferred embodiment; and 
         FIGS. 13A to 13D  illustrate packet formats in association with an integrated infrastructure that supports the coexistence of Wi-Fi networks with other networks, in accordance with a preferred embodiment. 
     
    
    
     DETAILED DESCRIPTION 
     The particular values and configurations discussed in these non-limiting examples can be varied and are cited merely to illustrate at least one embodiment and are not intended to limit the scope thereof. 
     The embodiments disclosed herein may be executed in a variety of systems, including a variety of computers running under a number of different operating systems. The computer may be, for example, a personal computer, a network computer, a mid-range computer or a mainframe computer. In the preferred embodiment, the computer is utilized as a control point of network processor services architecture within a local-area network (LAN) or a wide-area network (WAN) based on wireless technology. 
     Referring now to the drawings and in particular to  FIG. 3 , a block diagram of a wireless application network system  30  is depicted, which can be utilized to support an integrated infrastructure for the coexistence of wireless networks (e.g., Wi-Fi networks) with other networks in accordance with a preferred. System  30  generally includes a plurality of Wi-Fi nodes  331 ,  333 ,  335 ,  337 , and  339  that incorporates wireless communication for Wi-Fi end users. System  30  also includes a plurality of industrial infrastructure nodes  345 ,  347 ,  349 ,  351  and one or more industrial sensors  36 . In the network of system  30 , industrial sensors  36  and Wi-Fi nodes  331 ,  333 ,  335 ,  337 , and  339  can communicate with an infrastructure  32  that defines one or more wireless access points. 
     System  30  further includes one or more advanced applications  34  that function as the software-related portion of system  30 . Network monitoring can be accomplished through the use of the advanced applications  34  coupled with a control system  33  in order to control the entire system  30 . Each industrial infrastructure nodes  345 ,  347 ,  349 ,  351  can also behave as an access point for a Wi-Fi station. Note that the term “industrial infrastructure node” can be referred to by the term “INode”. To support two different communication paradigms, wherein the first communication type includes data transfer from one or more industrial sensors  36  to one or more of INodes  345 ,  347 ,  349 ,  351 , and one or more INodes  345 ,  347 ,  349 ,  351  to another INode, a Frequency Hopping Spread Spectrum (FHSS) mechanism can be utilized. Second, a Wi-Fi station can transmit data to an access point and an access point to another access point using, for example, but not limited to an IEEE 802.11 communications protocol. 
     In general, FHSS can be described as a transmission technology used in particular transmissions, wherein the data signal is modulated with a narrowband carrier signal that “hops” in a random but predictable sequence from frequency to frequency as a function of time over a wide band of frequencies. The signal energy is spread in time domain rather than chopping each bit into small pieces in the frequency domain. This technique reduces interference because a signal from a narrowband system will only affect the spread spectrum signal if both are transmitting at the same frequency at the same time. If synchronized properly, a single logical channel is maintained. The transmission frequencies are determined by a spreading, or hopping, code. The receiver must be set to the same hopping code and must listen to the incoming signal at the right time and correct frequency in order to properly receive the signal. 
     Note that although the use of a FHSS is discussed herein, it can be appreciated that other communications protocols can be utilized in place of or in addition to a FHSS. For example, the embodiments can be configured to permit the described industrial sensor network to communicate with one or more infrastructure nodes utilizing an IEEE 802.15.1 Bluetooth communications protocol and/or an IEEE 802.15.4 ZigBee communications protocol in addition to or in place of FHSS, depending upon design considerations. 
       FIG. 4  illustrates a block diagram of a super frame TDMA data structure  40  that can be utilized to implement a TDMA scheme that supports an integrated infrastructure for the coexistence of Wi-Fi networks with other networks in accordance with a preferred embodiment. As depicted in  FIG. 4 , the TDMA data structure  40  can be provided as a super frame TDMA (Time-Division Multiple Access) data structure that supports two types of data transfers First, the TDMA data structure  40  constitutes a communications type that includes data transfer from an industrial sensor to an INode and an INode to another INode using a Frequency Hopping Spread Spectrum mechanism, as illustrated above with respect to  FIG. 3 . Second, a Wi-Fi station can transmit data to an access point and an access point to another access point using an IEEE 802.11 protocol. Note that a Frequency Hopping Spread Spectrum (FHSS) MAC  43  and an IEEE 802.11 (802.11) MAC  44  can share a 2.4 GHz ISM band. 
     To use a TDMA scheme, industrial sensors, such as sensors  36  depicted in  FIG. 3  and INode/AP nodes can be synchronized. Since FHSS MAC requires global time synchronization, one can assume the use of a globally synchronized clock among these devices. With a synchronized clock as indicated by a TDMA super frame  41 , the total time can be divided into equally-sized frames wherein each frame is composed of a guard time  42 , an FHSS slot  43 , a guard time  42 , and an 802.11 slot  44 . In an FHSS slot, an INode/AP behaves as an INode and becomes an AP in an 802.11 slot. The industrial sensors  36  depicted in  FIG. 3  can attempt to transmit data to an INode only in the FHSS slot  43  and can then go to “sleep” in the 802.11 slot  44 . The period of guard time is very small (e.g., 40 us). The duration of the FHSS  43  slot and 802.11 slot  44  can be decided based on two components: (1) minimizing the packet drops from Wi-Fi stations which are not aware of FHSS slots; and (2) allowing enough time to FHSS slot to satisfy bandwidth requirement of industrial sensors. 
       FIG. 5  illustrates an event flow diagram  50  of IEEE 802.11 DCF (Distributed Coordination Function) mode. In an 802.11 DCF mode, each node can access a shared medium by utilizing a Carrier Sensing Medium Access/Collision Avoidance (CSMA/CA) mechanism. Since DCF mode is mostly used in Wi-Fi communication, in DCF mode, each node attempts to transmit a packet if it senses idle medium more than DIFS time. With a packet whose size is larger than RTS_Threshold, the sender and receiver will handshake via RTS/CTS/DATA/ACK procedure. Note that as utilized herein the acronym “RTS” generally refers to “Request to Send” and “CTS” refers to “Clear to Send”. All other nodes who hear RTS, CTS or DATA will set NAV (Network Allocation Vector) and keep quite for NAV duration. Once the transmission ends and the channel become idle, each node having the packet to transmit sets a random backoff timer to avoid simultaneous channel access. Once the backoff timer expires, it will attempt to transmit a packet. Note that the embodiments disclosed herein result in the transmission via a transmission mechanism of a “dummy” or unsolicited CTS signal at an end of a 802.11 time slot (e.g., predefined time period)_from one or more infrastructure nodes, where the NAV duration of this unsolicited CTS is equal to the size of guard+FHSS slot size+guard. Upon receiving a dummy CTS, 802.11 stations will keep quite for the NAV duration. Thus 802.11 stations will not interfere transmissions with industrial sensors in an FHSS slot. 
       FIG. 6  illustrates an event flow diagram  60  of a backoff mechanism. The backoff timer will be decreased only when the medium is idle and suspended when any channel activity is observed (such as busy channel by carrier sensing or NAV setting). Once the channel becomes idle again for DIFS or EIFS time, it will resume decrease. Thus, a Wi-Fi station will not decrease backoff timer if industrial sensors or INodes use the same frequency to the Wi-Fi station in an FHSS slot, such as, for example, the FHSS slot  43  depicted in  FIG. 4 . 
       FIG. 7  illustrates a block diagram of a system  70  of different power levels of FHSS to support an integrated infrastructure for the coexistence of Wi-Fi networks with other networks in accordance with a preferred embodiment. As depicted in  FIG. 7  that shows the cellular division in wireless networks. The FHSS range is configured using the cell portions  72 ,  74  with the nearby cell i.e.  73 . IEEE 802.11 range is gauged that shows  75 ,  71 . The interference area range is configured shown in  77 ,  73 . The cell  78  and its integration related with FHSS range and 802.11 range. The larger power of FHSS is based on potential interfering 802.11 stations that can sense channel busy thereby to prevent hidden terminals TX attempt. The same power of FHSS is based on potential interfering of IEEE 802.11 stations that may cause collision. The embodiment depicted in  FIG. 7  generally describes a system  70  or mechanism for varying the power level of transmission from one or more infrastructure nodes. Such a technique can be utilized in association with the previously described feature of transmitting a dummy or unsolicited CTS signal at the end of a predefined time period from one or more of the infrastructure node. 
       FIG. 8  illustrates a high-level flow chart  80  of operations illustrating logical operational steps for implementing a TDMA scheme to support an integrated infrastructure for the coexistence of Wi-Fi networks with other networks in accordance with a preferred embodiment. As depicted at block  81 , initialization can occur. Next, as indicated at block  82 , the TDMA scheduling can be established. Data transfer can be accomplished by using INode to another INode and by using an access point (AP) to another access point (AP) as depicted at block  83  and by using the TDMA frame the data transfer is done as described at block  84 . 
     As described at block  85 , a globally synchronized clock can be used among devices. Thereafter, as indicated at block  86 , a frame can be transferred such that the frame is composed of a guard time, an FHSS slot, a guard time and/or an IEEE 802.11. The transmission is based on the duration of an FHSS slot and an IEEE 802.11 slot as depicted at block  87 . Next, as described at block  88 , minimizing packet drops from Wi-Fi stations, which are not aware of FHSS slots, and allowing enough time to FHSS slot can be utilized to satisfy bandwidth requirement of industrial sensors, such as the industrial sensors  36  depicted in  FIG. 3 . The process can then terminate as indicated at block  89 . 
       FIG. 9  illustrates a high-level flow chart  90  of operations illustrating logical operational steps for providing Wi-Fi data transmission to support an integrated infrastructure for the coexistence of Wi-Fi networks with other networks in accordance with a preferred embodiment. As depicted at block  91  a Wi-Fi initialization can occur. Next, as depicted at block  92  a Wi-Fi station can transmit packets during non-802.11 slots. Wi-Fi stations communicate with access points that freeze transmission during non-802.11 slots as indicated at block  93 . Thereafter as described at block  94 , the Wi-Fi stations transmission may not be successful. Thus, it may be necessary to repeat an exponential backoff and provide for re-transmissions for a limited number of retrial times. If the Wi-Fi stations fail during the limited number of transmission retrials, the Wi-Fi stations can drop the packet as indicated at block  95 . 
     Next, as indicated at block  96 , in order to minimize such a packet drop, a very small FHSS slot size should be used so that the total retrials will not be completed during one FHSS slot thereby permitting successive retrials and packet drops to occur as indicated thereafter at block  97 . Following processing of a non-fragmented packet transmission whose size is larger than RTS_Threshold from a Wi-Fi station (STA) to AP at a non-802.11 slot as indicated at block  98 , the operation described at block  99  can be processed, in which fragmented packet transmissions are permitted whose size is larger than an RTS_Threshold from STA to AP. Finally, as described at block  100 , the packet transmission whose size is less than RTS_threshold from STA to AP and beacon interval can be considered such that an FHSS slot≦1 (i.e., a beacon period of IEEE 802.11). The process can then terminate as indicated at block  101 . 
       FIG. 10  illustrates a high-level flow chart  300  of operations illustrating logical operational steps for Wi-Fi access point modification to support an integrated infrastructure for the coexistence of Wi-Fi networks with other networks in accordance with a preferred embodiment. As depicted at block  301 , a Wi-Fi access point can be modified, thereby freezing transmission at non-802.11 slots as indicated thereafter at block  302 . Next, as indicated at block  303 , an RTS request may be rejected if successive data transmissions are not accomplished before an 802.11 slot ends according to the following formulation: current time+ duration of data transmission&gt; end of current 802.11 slot→ reject RTS and does not reply with a CTS (Clear to Send). An RTS or DATA packet may not be transmitted if the transmission is not accomplished before the 802.11 slot ends as described at block  304 . Next, as indicated at block  305 , the timer for the end of the current 802.11 slot and the start of the next 802.11 slot can be set. A counting beacon timer can send a beacon in 802.11 slots as described thereafter at block  306 . Next, as indicated at block  308  the modification of an FHSS MAC at an iNode and industrial sensors should freeze transmission at non-FHSS slots. The process can then terminate as indicated at block  310 . 
       FIG. 11  illustrates a high-level flow chart  400  of operations illustrating logical operational steps with reference to INode for Wi-Fi access point modification to support an integrated infrastructure for the coexistence of Wi-Fi networks with other networks in accordance with a preferred embodiment. As depicted at block  402 , initialization can occur. Data transfer can be accomplished by using INode to another INode and by using an access point (AP) to another access point (AP) as depicted at block  404  and by using the TDMA scheme for data transfer for INode is done as described at block  406 .Thereafter, as described at block  408  the TDMA slots are divided into INode and Wi-Fi AP roles. 
     Next as depicted at block  410 . The FHSS MAC requires global synchronization, assuming the use of a globally synchronized clock among the devices as described at block  412 . In the FHSS slot, the node behaves as an INode and can function as an AP in an 802.11 slot as indicated at block  414 . The MAC protocol of an INode and an AP may be modified based on the modification of the Wi-Fi access point as depicted at block  416 . Next, as indicated at block  418 , the modification of the MAC protocol for the AP can be minimized. The modification of the INode and the interference of the Wi-Fi stations at the FHSS slot and packet drops at Wi-Fi stations can be finally modified due to the time-division behavior of the AP as indicated at block  420 . The process can then terminate as indicated at block  422 . 
       FIG. 12  illustrates a high-level flow chart  500  of operations illustrating logical operational steps with reference to industrial sensors for Wi-Fi access point modification to support an integrated infrastructure for the coexistence of Wi-Fi networks with other networks in accordance with a preferred embodiment. As depicted at block  502 , initialization can occur. By using the TDMA scheme for data transfer for industrial sensors is done as described at block  504  by which the industrial sensors are synchronized as indicated at block  506 . The FHSS MAC requires global time synchronization so assuming globally synchronized clock among the devices as described at block  508 . Thereafter as described at block  510 , the industrial sensors can attempt to transmit data to INode only in an FHSS slot and then move into a “sleep mode” within an 802.11 slot. The duration of the FHSS slot and the 802.11 slots can be determined by allowing sufficient time for an FHSS slot to satisfy the bandwidth requirement of industrial sensors as indicated at block  512 . Next, as depicted at block  514  modifying the Mac protocol of industrial sensors. Finally, the modification of industrial sensor&#39;s MAC can be minimized as indicated at block  516 . The process can then terminate as indicated at block  518 . 
       FIGS. 13A-13D  illustrate the block diagrams of respective packet formats  302 ,  304 ,  306  and  308  in association with an integrated infrastructure to support the coexistence of Wi-Fi networks with other networks in accordance with a preferred embodiment.  FIG. 13A  illustrates a format  302  in which a MAC frame format (i.e., data frame) is composed of frame control, duration ID, address1, address2, address3, sequence control, address, frame body and frame check sequence (FCS).  FIG. 13B  illustrates a format  304  in which a frame control field is composed of a protocol version, type, subtype, to destination source (DS), from destination source (DS), more flag, retry, power management, more data, WEP and order data.  FIG. 13C  illustrates a format  406  that includes an RTS frame with 20 bytes consisting of frame control, duration, RA, TA and frame check sequence (FCS).  FIG. 13D  illustrates a format  308  in which a CTS frame is provided with 14 bytes including frame control, duration, RA and frame check sequence (FCS). 
     It will be appreciated that variations of the above-disclosed and other features and functions, or alternatives thereof, may be desirably combined into many other different systems or applications. Also that various presently unforeseen or unanticipated alternatives, modifications, variations or improvements therein may be subsequently made by those skilled in the art which are also intended to be encompassed by the following claims.