Patent Publication Number: US-9432962-B2

Title: Wireless link monitoring and active troubleshooting

Description:
BENEFIT CLAIM; RELATED APPLICATIONS; INCORPORATION BY REFERENCE 
     This application claims priority as a Continuation of application Ser. No. 11/495,084 filed on Jul. 28, 2006, now U.S. Pat. No. 8,675,547, the content of which is hereby incorporated by reference. 
    
    
     FIELD 
     Embodiments of the invention relate to the field of communications, and in particular, to a network and method for monitoring wireless links through the transmission of one or more Layer 2 (L2) frames from any wireless device to any wireless device. 
     GENERAL BACKGROUND 
     Over the last decade or so, many commercial and governmental entities have installed communication networks in order to allow their employees access to various network resources. To improve efficiency and to support mobility, many wireless access enhancements have been added to local, personal, and wide area networks. Based on these enhancements, Wireless Local Area Networks (WLANs), Personal Area Networks (PANs) and Wide Area Networks (WLANs) have been and continue to be utilized by more and more users. 
     For instance, a WLAN supports communications between a number of wireless devices without any required line of sight for such communications. In current network configurations, multiple Access Points (APs), base stations or a central coordinator are connected to a wired network, such as an Ethernet network for example, and each AP operates as a relay station by supporting communications between resources of the wired network and wireless stations (STAs). STAs are fixed or mobile wireless devices, where the mobile devices enable users to alter their physical locations, but still communicate over the network. 
     In accordance with many WLAN transmission schemes, including transmission schemes operating in accordance with any Institute of Electrical and Electronics Engineers (IEEE) 802.11 standard, each wireless device must acknowledge any directed frames within a prescribed period of time commonly referred to as a “Short Interframe Space” or “SIFS”. This acknowledgement signaling occurs without verification of the content or source of the frame, and is merely used to confirm that the transmitted frame was successfully received so as to avoid unnecessary retransmissions. 
     One disadvantage associated with current WLANs is that the efficiency of these networks degrades as the number of wireless devices increase. Moreover, a substantial number of wireless devices may be incapable of initiating directed L2 unicast communications and only capable of responding to a communication initiated from another device or forwarding a communication from one device to another. However, L2 communications are valuable for gathering information concerning a particular wireless device and the quality of the wireless channel or link. Information gathered from L2 communications can be used to improve network security as well as overall performance. For instance, many conventional APs produced by other manufacturers are adapted to initiate broadcast transmissions, commonly referred to as “beacons,” but are incapable of initiating L2 unicast communications to a STA. 
     Hence, it would be advantageous to develop a communication scheme that enables any wireless device to communicate with another wireless device, and to make use of the L2 acknowledgement signaling requirements described above. 
    
    
     
       BRIEF DESCRIPTION OF THE DRAWINGS 
       The invention may best be understood by referring to the following description and accompanying drawings that are used to illustrate embodiments of the invention. 
         FIG. 1  is an exemplary embodiment of a wireless local area network in accordance with an embodiment of the invention. 
         FIG. 2  is an exemplary embodiment of a Layer 2 (L2) frame transmitted by one wireless device to another in order to perform link monitoring. 
         FIG. 3A  is an exemplary embodiment of address and control information loaded into the data or management frame of  FIG. 2  for link monitoring. 
         FIG. 3B  is an exemplary embodiment of a L2 Return frame received in response to the L2 frame of  FIG. 2 . 
         FIG. 4A  is an exemplary embodiment of a L2 frame being a Return-to-Send (RTS) Control frame for link monitoring. 
         FIG. 4B  is an exemplary embodiment of a L2 Return frame being a Clear-to-Send (CTS) Control frame for link monitoring. 
         FIG. 4C  is an exemplary embodiment of a L2 frame being a Power-Save (PS) Poll Control frame for link monitoring. 
         FIG. 5  is an exemplary flowchart illustrating the operations performed by the wireless device transmitting the L2 frame for location estimation. 
         FIG. 6  is an exemplary flowchart illustrating the operations performed by the wireless device transmitting the L2 frame in order to determine antenna connectivity. 
         FIG. 7  is an exemplary flowchart illustrating the operations performed by the wireless device transmitting the L2 frame in order to determine link quality. 
         FIG. 8  is an exemplary flowchart illustrating the operations performed by the wireless device transmitting the L2 frame in order to perform interference mitigation. 
         FIG. 9  is an exemplary flowchart illustrating the operations performed by the wireless device transmitting the L2 frame to operate as a L2 ping mechanism. 
         FIG. 10  is an exemplary flowchart illustrating the operations performed by the wireless device transmitting the L2 frame in order to perform age-out and roaming operations to determine the status of a device. 
         FIG. 11  is an exemplary flowchart illustrating the operations performed by the wireless device transmitting the L2 frame in order to orient directional antennas. 
     
    
    
     DETAILED DESCRIPTION 
     Embodiments of the invention relate to a network and method for monitoring wireless links through the transmission of one or more Layer 2 (L2) frames from any wireless device to any wireless device. Such monitoring may be performed by the transmitting device, the receiving device or a third non-participating wireless device. No pre-established communications (e.g., association) is required before transmission of the L2 frame. As a result, any wireless device can transmit a L2 frame to any other wireless device within its range and may use information gathered from a return frame produced by the wireless device to better understand the state and location of that wireless device in efforts to optimize network performance and security. 
     According to one embodiment of the invention, a wireless link may be monitored by a first wireless device (e.g., access point “AP”) transmitting a L2 frame to a second wireless device (e.g., wireless station “STA”). It is contemplated that a L2 frame may be a NULL-Data frame, although other types of L2, including data, control and management frames may be used. Furthermore, the second wireless device does not need to be associated with the first wireless device. According to another embodiment of the invention, the wireless link may be monitored by transmitting a NULL-Data frame from a first STA to a second STA in order to gather information concerning the second STA. 
     In summary, the wireless link may be monitored for communications between any wireless devices, including AP-to-STA and STA-to-STA as described above as well as AP-to-AP and STA-to-AP communications. Moreover, communications between an intermediary wireless device (e.g., a mesh or relay node) and either a STA, AP or another intermediary node may be monitored in accordance with the scope and spirit of the invention. 
     For these and other embodiments of the invention, a L2 frame is transmitted and a L2 Return frame from the receiving wireless device is received upon successful receipt of the L2 frame. In many cases, the L2 Return frame may be an Acknowledgement (ACK) frame sent within a prescribed time period after transmission of the L2 frame. This signaling protocol can be used in a variety of applications as described below. 
     Certain details are set forth below in order to provide a thorough understanding of various embodiments of the invention, albeit the invention may be practiced through many embodiments other than those illustrated. Well-known logic and operations are not set forth in detail in order to avoid unnecessarily obscuring this description. 
     Herein, a “wireless device” may be broadly construed as any device that supports wireless communications. For instance, one type of wireless device may operate as an AP while another type of wireless device may operate as a STA. Specific examples of wireless devices operating as APs or STAs include, but are not limited or restricted to servers, portable and desktop computers, VoIP phones, personal digital assistants “PDAs”, customer premises equipment, or the like. 
     A “wireless link” may be broadly construed as a pathway that supports wireless communications between two wireless devices. The wireless communications are in accordance with a WLAN communication standard such as High Performance Radio LAN (HiperLan) or IEEE 802.11. Examples of different types of IEEE 802.11 standards include, but are not limited or restricted to (i) an IEEE 802.11b standard entitled “Wireless LAN Medium Access Control (MAC) and Physical Layer (PHY) specifications: Higher-Speed Physical Layer Extension in the 2.4 GHz Band” (IEEE 802.11b, 1999), (ii) an IEEE 802.11a standard entitled “Wireless LAN Medium Access Control (MAC) and Physical Layer (PHY) specifications: High-Speed Physical Layer in the 5 GHz Band” (IEEE 802.11a, 1999), (iii) a revised IEEE 802.11 standard “Wireless LAN Medium Access Control (MAC) and Physical Layer (PHY) specifications” (IEEE 802.11, 2003), or the like. 
     The wireless communications between two wireless devices are generally controlled by hardware or software such as an application, an applet, a routine or even one or more executable instructions stored in a storage medium. The hardware may include any data processing unit including an Application Specific Integrated Circuit (ASIC), a Field Programmable Gate Array (FPGA), or the like. Firmware is considered merely one type of software. The “storage medium” may include, but is not limited or restricted to a programmable electronic circuit, a semiconductor memory device, a volatile memory (e.g., random access memory, etc.), a non-volatile memory (e.g., read-only memory, flash memory, a hard drive, etc.), a portable memory device (e.g., floppy diskette, a compact disk “CD”, digital versatile disc “DVD”, a digital tape, a Universal Serial Bus “USB” flash drive), or the like. 
     A “frame” is information arranged in a selected format that is transmitted over a link. Examples of different frame types may include, but are not limited or restricted to Data, Management, or Control frames. 
     I. General Architecture 
     Referring to  FIG. 1 , an exemplary embodiment of a network  100  is shown. In accordance with one embodiment of the invention, wireless network  100  is a wireless local area network (WLAN) that comprises a backbone (wired or wireless) network  110  in communication with a wireless network  120 . Alternatively, network  100  may be deployed as a personal area network (PAN) or a wide area network (WAN). 
     Wireless network  120  comprises a plurality of wireless devices  130   1 - 130   N  that are adapted to receive and/or transmit data. For this embodiment of the invention, wireless devices  130   1 - 130   N  may include an access point (AP)  130   1  and multiple wireless stations (STAs)  130   2 - 130   4 , where AP  130   1  provides STAs  130   2 - 130   4  access to resources on backbone network  110 . 
     As shown in detail, according to one embodiment of the invention, wireless device  130   1  transmits an L2 frame  150  to wireless device  130   2  for link monitoring or active troubleshooting such as obtaining information concerning one of the devices. As shown in  FIG. 2 , consistent with the IEEE 802.11 standard, L2 frame  150  comprises a media access control (MAC) header  200 , a frame body  250  and a frame check sequence (FCS)  260 . FCS  260  is used to determine whether L2 frame  150  has been correctly received. Frame body  250  is a variable length field that contains information specific to individual frame types and subtypes in the form of information elements or data. Of course, where L2 frame  150  is a NULL-Data frame or a Control frame, frame body  250  is zero octets in size. 
     According to one embodiment of the invention, MAC header  200  comprises a frame control field  210  and a plurality of address fields  230 . Frame control field  210  includes a “To DS” field  220  and a “From DS” field  225 , which are used to identify an intended direction of L2 frame  150 . “To DS” field  220  is a single bit that is set to a first logical value (e.g., logic “1”) for data types transmitted from a STA to an AP. Otherwise, “To DS” field  220  is set to a second logical value (e.g., logic “0”). Similarly, “From DS” field  225  is a single bit that is set to the first logical value (e.g., logic “1”) for data types transmitted from the AP. Otherwise, “From DS” field  225  is set to the second logical value (e.g., logic “0”). 
     Therefore, as illustrated in  FIG. 3A , for a first entry  300 , transmissions between STAs, both “To DS” field  220  and “From DS” field  225  are set to the second logical value. For transmissions from an AP to STA, “To DS” field  220  is set to the second logical value and “From DS” field  225  is set to the first logical value as set forth in entry  310 . For transmissions from a STA to an AP as illustrated in entry  320 , “To DS” field  220  is set to the first logical value and “From DS” field  225  is set to the second logical value. Finally, as shown in entry  330 , transmissions to an intermediary wireless device such as a mesh or relay node that receives and forwards an incoming frame, both “To DS” field  220  and “From DS” field  225  are set to the first logical value. 
     Address fields  230  comprise four (4) address fields, namely a first address field  240 , a second address field  242 , a third address field  244 , and a fourth address field  246 . The contents of address fields  230  are shown in detail in  FIG. 3A , where such content is also dependent on the intended direction of L2 frame  150 . Normally, the receiving wireless device utilizes the address within second address field  242  as the destination address for an acknowledgement (ACK) frame if an ACK frame is necessary. 
     For instance, where L2 frame  150  is exchanged between STAs (“To DS” field  220  and “From DS” field  225 =“0”), fourth address field  246  is not utilized. However, address fields  240 ,  242  and  244  are configured to include a destination address (DA), a source address (SA) and a Basic Service Set Identifier (BSSID). In general, the address “DA” is the destination of the L2 frame while the address “SA” is the address of the wireless device transmitting the L2 frame. “BSSID” may be (i) an address currently in use by the wireless device when an AP or associated with an AP, or (ii) an identifier of an Independent Basic Service Set (IBSS) that the wireless device belongs to, or (iii) a specific identifier for a particular frame type. 
     As another exemplary embodiment, where L2 frame  150  is sent to an intermediary device for forwarding to another device (“To DS” field  220  and “From DS” field  225 =“1”), address fields  240 ,  242 ,  244  and  246  are configured to include a receiver address (RA), a transmitter address (TA), a destination address (DA) and a source address (SA), respectively. In general, “RA” is an address of the wireless device that is the next immediate intended recipient of L2 frame  150  of  FIG. 1 . “TA” is the address of the wireless device that is transmitting L2 frame  150 . Also, “DA” is the intended destination of L2 frame  150  while the address “SA” is the address of the wireless device initiating transmission of L2 frame  150 . 
     Returning back to  FIG. 1 , in response to receiving L2 frame  150 , wireless device  130   2  transmits an L2 Return frame  160  to wireless device  130   1 . According to one embodiment of the invention and as further shown in  FIG. 3B , L2 Return frame  160  is an ACK frame that includes a MAC header  350  and a FCS  360 . The ACK frame has no frame body for data transfer. MAC header  350  includes a receiving address (RA)  370  that is adapted to include the contents of address field  242  of L2 frame  150  (see  FIG. 2 ) and is used as a destination address for ACK frame  160 . 
     It is contemplated that other wireless devices, such as wireless device  130   3 , may be within range to detect L2 frame  150  and L2 Return frame  160  during transmission. Wireless device  130   3  may monitor wireless link  140  created by these unicast communications to obtain information concerning both wireless devices  130   1  and  130   2 . For instance, based on the signal strength of L2 frame  150  and L2 Return frame  160 , wireless device  130   3  may be able to triangulate locations of wireless devices  130   1  and  130   2 , respectively. 
     According to another embodiment of the invention, when a control frame is used as L2 frame  150 , MAC header  200  comprises frame control field  210  and one or more address fields  250 . Frame control field  210  includes a “To DS” field  220  and a “From DS” field  225  as described above, but the values of these fields  220  and  225  is set to second logical value (e.g., logic “0”). 
     As shown in  FIG. 4A , with respect to L2 frame  150  being a Return To Send (RTS) frame, MAC header  200  comprises frame control field  210 , a receiver address (RA)  400 , and a transmitter address (TA)  405 . In general, RA  400  includes the address of the wireless device intended to receive L2 frame  150  and TA  405  is the address of the wireless device that is transmitting L2 frame  150 . 
     In response to receiving L2 frame  150 , as shown in  FIG. 1 , wireless device  130   2  transmits L2 Return frame  160  to wireless device  130   1 . According to this embodiment of the invention and as further shown in  FIG. 4B , L2 Return frame  160  is a Clear To Send (CTS) frame that includes a receiver address (RA)  410  that includes the contents of TA field  405  of L2 frame  150  (see  FIG. 4A ) and is used as a destination address for the CTS frame. 
     Furthermore, according to another embodiment of the invention, as shown in  FIG. 4C , with respect to L2 frame  150  being a Power-Save Poll (PS-Poll) frame, MAC header  200  comprises frame control field  210 , AID field  420 , a BSSID field  425 , and a transmitter address (TA) field  430 . Similar in operation as describe above, upon receipt of L2 frame  150 , wireless device  130   2  transmits L2 Return frame  160  to wireless device  130   1 . According to this embodiment of the invention and as further shown in  FIG. 3B , L2 Return frame  160  is an ACK frame that includes the contents of TA field  430  of L2 frame with RA field  370 . 
     It is contemplated that L2 frame  150  and L2 Return frame  160  may be an exchange of any type data or control frame, provided the transmission of L2 frame  150  causes a required transmission of L2 Return frame  160 . 
     II. General Applications of Link Monitoring 
     As described above, link monitoring enables any wireless device, namely APs or STAs or a mesh or relay node, to transmit a L2 frame to any other wireless device in order to receive information in response to the L2 frame. This information may be an ACK frame in accordance with the IEEE 802.11 standard or another type of response signal in accordance with HiperLan or other communication schemes. There is no requirement that the wireless device needs to be associated to the transmitting device. Moreover, there is no requirement that this link monitoring needs to be conducted using the current operating channel utilized by the transmitting wireless device, but rather, can be conducted over other wireless channels. 
     A. Location Tracking 
     Link monitoring can be used to track the location of a wireless device. As shown in  FIG. 5 , an exemplary flowchart of the location tracking operation is shown. To commence link monitoring, a first wireless device (WD) transmits a L2 frame to a second wireless device (block  500 ). The purpose of the L2 frame is to cause the second wireless device to transmit a frame in response to receiving the L2 frame. For instance, the L2 frame may be a NULL-Data frame. 
     Upon receiving the L2 frame, within a prescribed time period without error (indicated by the successful FCS validation), the second wireless device transmits a (L2) Return frame to the first wireless device (blocks  510  and  520 ). Otherwise, second wireless device does not provide a return frame, and thus, the first wireless device now is aware that the second wireless device either (i) failed to receive the L2 frame or (ii) received the L2 frame with errors (block  530 ). It is also possible that the ACK frame was transmitted by the second wireless device, but did not reach the first wireless device. 
     If the Return frame is received, the first wireless device may be adapted to measure the signal strength (blocks  540 ,  545  and  550 ). The signal strength of the Return frame can be used to determine the proximity of the second wireless device from the first wireless device. Alternatively, if the return frame is received, the first wireless device may be adapted to measure a time difference between transmission of the L2 frame and receipt of the Return frame (blocks  545  and  560 ). This signal propagation delay can be used to determine the proximity of the second wireless device from the first wireless device. Despite the technique used, the measurements conducted for the Return frame are used to determine the location of the first wireless device from the second wireless device (block  570 ). 
     B. Antenna Connectivity 
     Link monitoring can be used to determine antenna connectivity by transmitting frames from all antennas associated with a first wireless device and comparing signal strength of return frames if received. As shown in  FIG. 6 , an exemplary flowchart for determining antenna connectivity is shown. To commence the antenna connectivity analysis, a first wireless device transmits a L2 frame to a second wireless device via each antenna (block  600 ). This transmission may involve the first wireless device repeatedly transmitting a L2 frame over each antenna of the first wireless device concurrently or in succession. 
     In response to receiving the L2 frame transmitted via a first antenna, within a prescribed time period and upon successful receipt of the L2 frame, the second wireless device sends a L2 Return frame to the first wireless device (blocks  610  and  620 ). Otherwise, the second wireless device does not send the Return frame, and thus, the first wireless device now is aware that the second wireless device may not have received the L2 frame (block  630 ). If the first wireless device fails to receive the Return frame associated with one of the remaining antennas or the signal strengths of the Return frames for any of these antenna is significantly different, this information could denote that antenna connectivity problems (blocks  640  and  650 ). As a result, a notification signal may be generated to request the user to investigate the antenna connections (block  660 ). 
     C. Link Quality 
     Link monitoring can be used to determine link quality by transmitting frames at different transmission rates and determining whether such frames are properly received. For instance, the IEEE 802.11g standard supports twelve transmission rates. Normally, the transmission rate over a wireless link between two wireless devices is selected based on the amount of data lost during transmissions. For instance, if the percentage of data loss is above a predetermined threshold, the current transmission (TX) rate supported by the wireless link is proactively reduced. If there is no data loss, the current TX rate may be increased and monitored. An application of the invention is desired to determine an optimal transmission rate without loss of valuable data. 
     As shown in  FIG. 7 , an exemplary flowchart for determining link quality is shown. Herein, a first wireless device (WD) transmits a L2 frame to a second wireless device (WD) at a first transmission rate (block  700 ). The first transmission rate (rate 1) may be greater than the current TX rate supported by a wireless link between the first and second wireless devices. 
     If the first wireless device fails to receive a frame in response to the L2 frame (e.g., L2 Return frame such as an ACK frame), the first wireless device may set the transmission rate to another rate (block  720 ). For instance, the TX rate may be set to the current TX rate or a rate that falls between the current TX rate and the first transmission rate identified above. 
     In response to receiving the L2 Return frame, which indicates that the second wireless device has properly received the L2 frame, the first wireless device transmits a L2 frame at a second transmission rate “rate 2” (blocks  710  and  730 ). The second transmission rate (rate 2) would be greater than the first transmission rate (rate 1). 
     If the first wireless device receives a L2 Return frame, the first wireless device now transmits a L2 frame at a third transmission rate (rate 3) that is greater than the second transmission rate (blocks  740  and  760 ). This process is continued as necessary with incremental rate increases and the determination whether communications can be supported at this transmission rate. 
     However, if the L2 Return frame is not received within a prescribed time period, the first wireless device determines that second transmission rate cannot be currently supported by the wireless link, and thus, either selects the first transmission rate (rate 1) or conducts further link quality measurements for one or more transmission rates between the first and second transmission rates (blocks  740  and  750 ). 
     Similarly, if the first wireless device determines that the third transmission rate cannot be currently supported, the second transmission rate (rate 2) is selected or further link quality measurements are conducted between the second and third transmission rates (blocks  770  and  780 ). This process continues through the transmission of L2 frames to gather information until a TX rate is selected. 
     D. Interference Mitigation 
     Link monitoring can be used to mitigate interface by transmitting L2 frames at different transmit (TX) powers and determining whether L2 Return frames are received. If L2 Return frames are received for transmissions at a reduced TX power level, this power level is selected because transmissions at this reduced TX power level mitigate the chances of signal interference with other wireless devices. 
     As shown in  FIG. 8 , an exemplary flowchart by using directed L2 frames for mitigating interference is shown. For instance, the first wireless device is currently transmitting frames at a first power level such as 20 decibels (dB) for example. In order to mitigate interference, the first wireless device reduces its TX power level to a second power level (e.g., 17 dB) that is less than the first power level. Thereafter, the first wireless device transmits a L2 frame to the second wireless device over a wireless link (block  800 ). 
     In response to receiving a frame in response to the L2 frame (e.g., a L2 Return frame such as an ACK frame), which indicates that the second wireless device properly received the L2 frame, the first wireless device has now be identified that current communications with the second wireless device can be accomplished at the second power level (block  810 ). Otherwise, the power level is increased to a level closer the first power level (block  820 ). 
     Thereafter, the first wireless device can further reduce the TX power level to a third power level (e.g., 14 dB) that is less than the second power level and transmit another L2 frame (blocks  830 ). If an L2 Return frame is received (block  840 ), the same reduction in TX power may be performed as set forth at  860 . However, if the L2 Return frame is not received within a prescribed time period, the first wireless device determines that third power level is insufficient to support communications over the wireless link, and thus, the first wireless device either selects the second power level or conducts further TX power measurements to select a power level between the second and third power levels (block  850 ). 
     E. L2 Ping Functionality 
     Link monitoring can be used as a L2 “ping” mechanism so that a first wireless device can determine whether a specific MAC address for a second wireless device is accessible. The second wireless device does not need to be associated with the first wireless device. In contrast with conventional ping mechanisms that are designed to troubleshoot problems with Internet connections, embodiments of the invention may be directed toward troubleshooting MAC layer connectivity problems. 
     As shown in  FIG. 9 , an exemplary flowchart of a L2 ping mechanism being applied as an embodiment of the invention is shown. Herein, a first wireless device transmits a L2 frame to a second wireless device (block  900 ). In other words, if the first wireless device receives a frame from the second wireless device in response to the L2 frame (e.g., ACK frame) as shown in blocks  910  and  920 , the first wireless device has effectively identified that a wireless link between the first and second wireless devices remains intact or can be established. Otherwise, no wireless link is currently in place or may not be established (block  930 ). 
     F. Assisting System Functionality 
     Link monitoring can be used as to assist in the control of system functions such as age-out and roaming by providing a mechanism where a first wireless device can quickly determine the status of a second wireless device, where the second wireless device may not be actively transmitting data. 
     For instance, WLANs may be adapted with low-power devices that transmit very infrequently. This poses a problem for many wireless devices within the WLAN concerning “age-out,” which is a destructive process where information concerning one or more wireless devices that have not been transmitting for a prolonged period of time is deleted. In accordance with WLAN communication standards, when used correctly, “Age-out” prevents wireless devices from indefinitely storing information, and thus, a lesser amount of internal memory is needed by the wireless device. 
     Similarly, “Roaming” is a condition where the second wireless device discontinues communications with the first wireless device. In some situations, the second wireless device disassociates with the first wireless device and re-associates with a different wireless device. However, there are many conditions where the second wireless device merely stops transmission without initiating any disassociation requests. For these conditions, the first wireless device is unsure whether the second wireless device is still part of the WLAN or not. 
     As shown in  FIG. 10 , an exemplary flowchart of an embodiment of the invention being used for Age-out and roaming control is shown. Herein, upon detection that a second wireless device has not sent any transmissions detected by a first wireless device for a prolonged period of time, first wireless device transmits a L2 frame to a second wireless device (blocks  1000  and  1010 ). If the first wireless device receives a frame from the second wireless device in response to the L2 frame (e.g., ACK frame), the first wireless device has effectively identified that a wireless link between the first and second wireless devices remains intact or can be established (blocks  1020  and  1030 ). Otherwise, the first wireless device may initiate an “age-out” or disassociation process in order to free up resources (block  1040 ). 
     G. Antenna Orientation 
     Link monitoring can be used to orient a directional antenna of a wireless device. As shown in  FIG. 11 , an exemplary flowchart for determining antenna orientation of a wireless device is shown. Initiating, the wireless device transmits a L2 frame to a second wireless device that is known to be operational (block  1100 ). For this transmission, the directional antenna of the wireless device is positioned in a first orientation. If the wireless device fails to receive an L2 Return frame, the directional antenna is repositioned in a second orientation that differs from the first orientation (blocks  1110  and  1120 ). 
     This process is performed iteratively, until a L2 Return frame is received with sufficient signal strength or the directional antenna has been repositioned in orientations that span the entire coverage area such as 360° orientation (blocks  1130  and  1150 ). If no L2 Return frame is received or the signal strength is insufficient, a warning is provided to the user regarding the inability of the first wireless device to reliably communicate with the second wireless device (block  1160 ). 
     If the L2 Return frame is received at any orientation of the directional antenna with sufficient signal strength, the orientation is saved along with the signal strength and used for subsequent communications with the second wireless device (block  1140 ). 
     While the invention has been described in terms of several embodiments, the invention should not limited to only those embodiments described, but can be practiced with modification and alteration within the spirit and scope of the appended claims. The description is thus to be regarded as illustrative instead of limiting.