Abstract:
A method is provided for a local device in a network to determine media qualities for the transmission paths between it and all of the remote devices in a wireless network. Each of the devices in the network will be assigned at least one of a plurality of management time slots in a superframe rotation. Each device will always transmit a frame during this assigned time slot, whether it is a management frame or a null frame. Individual devices can listen during these frames, determine quality information about the transmission medium between the receiving device and the transmitting device, and based on this quality information set the transmission and reception parameters that the receiving device will use when later communicating with that particular transmitting device. The criteria for determining transmitting parameters and reception parameters can be different to make certain that compatible transmission and reception parameters are chosen among devices.

Description:
CROSS-REFERENCE TO RELATED PATENT DOCUMENTS  
       [0001]    This application relies for priority on U.S. provisional application serial No. 60/349,358, by Knut T. Odman and William M. Shvodian, filed Jan. 22, 2002, entitled “MEDIA QUALITY FEEDBACK,” the contents of which is hereby incorporated by reference in its entirety. 
     
    
     
       BACKGROUND OF THE INVENTION  
         [0002]    The present invention relates to wireless personal area networks and wireless local area networks. More particularly, the present invention relates to a method for improving the ability of devices in a network to determine the current quality of the transmission media.  
           [0003]    The International Standards Organization&#39;s (ISO) Open Systems Interconnection (OSI) standard provides a seven-layered hierarchy between an end user and a physical device through which different systems can communicate. Each layer is responsible for different tasks, and the OSI standard specifies the interaction between layers, as well as between devices complying with the standard.  
           [0004]    [0004]FIG. 1 shows the hierarchy of the seven-layered OSI standard. As seen in FIG. 1, the OSI standard  100  includes a physical layer  110 , a data link layer  120 , a network layer  130 , a transport layer  140 , a session layer  150 , a presentation layer  160 , and an application layer  170 .  
           [0005]    The physical (PHY) layer  110  conveys the bit stream through the network at the electrical, mechanical, functional, and procedural level. It provides the hardware means of sending and receiving data on a carrier. The data link layer  120  describes the representation of bits on the physical medium and the format of messages on the medium, sending blocks of data (such as frames) with proper synchronization. The networking layer  130  handles the routing and forwarding of the data to proper destinations, maintaining and terminating connections. The transport layer  140  manages the end-to-end control and error checking to ensure complete data transfer. The session layer  150  sets up, coordinates, and terminates conversations, exchanges, and dialogs between the applications at each end. The presentation layer  160  converts incoming and outgoing data from one presentation format to another. The application layer  170  is where communication partners are identified, quality of service is identified, user authentication and privacy are considered, and any constraints on data syntax are identified.  
           [0006]    The IEEE 802 Committee has developed a three-layer architecture for local networks that roughly corresponds to the physical layer  110  and the data link layer  120  of the OSI standard  100 . FIG. 2 shows the IEEE 802 standard  200 .  
           [0007]    As shown in FIG. 2, the IEEE 802 standard  200  includes a physical (PHY) layer  210 , a media access control (MAC) layer  220 , and a logical link control (LLC) layer  225 . The PHY layer  210  operates essentially as the PHY layer  110  in the OSI standard  100 . The MAC and LLC layers  220  and  225  share the functions of the data link layer  120  in the OSI standard  100 . The LLC layer  225  places data into frames that can be communicated at the PHY layer  210 ; and the MAC layer  220  manages communication over the data link, sending data frames and receiving acknowledgement (ACK) frames. Together the MAC and LLC layers  220  and  225  are responsible for error checking as well as retransmission of frames that are not received and acknowledged.  
           [0008]    [0008]FIG. 3 is a block diagram of a wireless network  300  that could use the IEEE 802 standard  200 . In a preferred embodiment the network  300  is a wireless personal area network (WPAN), or piconet. However, it should be understood that the present invention also applies to other settings where bandwidth is to be shared among several users, such as, for example, wireless local area networks (WLAN), or any other appropriate wireless network.  
           [0009]    When the term piconet is used, it refers to a network of devices connected in an ad hoc fashion, having one device act as a coordinator (i.e., it functions as a server) while the other devices (sometimes called stations or nodes) follow the time allocation instructions of the coordinator (i.e., they function as clients). Although the term “device” will be used throughout this disclosure for the sake of clarity, the terms “station,” “node,” and “client” can be freely used in its place.  
           [0010]    The coordinator can be a designated device, or simply one of the devices chosen to function as a coordinator. One primary difference between the coordinator and non-coordinator devices is that the coordinator must be able to communicate with all of the devices in the network, while the various non-coordinator devices need not be able to communicate with all of the other non-coordinator devices.  
           [0011]    As shown in FIG. 3, the network  300  includes a coordinator  310  and a plurality of non-coordinator devices  320 . The coordinator  310  serves to control the operation of the network  300 . As noted above, the system of coordinator  310  and non-coordinator devices  320  may be called a piconet, in which case the coordinator  310  may be referred to as a piconet coordinator (PNC). Each of the non-coordinator devices  320  must be connected to the coordinator  310  via primary wireless links  330 , and may also be connected to one or more other non-coordinator devices  320  via secondary wireless links  340 , also called peer-to-peer links.  
           [0012]    In addition, although FIG. 3 shows bi-directional links between devices, they could also be unidirectional. In this case, each bi-directional link  330 ,  340  could be shown as two unidirectional links, the first going in one direction and the second going in the opposite direction.  
           [0013]    In some embodiments the coordinator  310  may be the same sort of device as any of the non-coordinator devices  320 , except with the additional functionality for coordinating the system, and the requirement that it communicate with every device  320  in the network  300 . In other embodiments the coordinator  310  may be a separate designated control unit that does not function as one of the devices  320 .  
           [0014]    Through the course if the following disclosure the coordinator  310  will be considered to be a device just like the non-coordinator devices  320 . However, alternate embodiments could use a dedicated coordinator  310 . Furthermore, individual non-coordinator devices  320  could include the functional elements of a coordinator  310 , but not use them, functioning as non-coordinator devices. This could be the case where any device is a potential coordinator  310 , but only one actually serves that function in a given network.  
           [0015]    Each device of the network  300  may be a different wireless device, for example, a digital still camera, a digital video camera, a personal data assistant (PDA), a digital music player, or other personal wireless device.  
           [0016]    The various non-coordinator devices  320  are confined to a usable physical area  350 , which is set based on the extent to which the coordinator  310  can successfully communicate with each of the non-coordinator devices  320 . Any non-coordinator device  320  that is able to communicate with the coordinator  310  (and vice versa) is within the usable area  350  of the network  300 . As noted, however, it is not necessary for every non-coordinator device  320  in the network  300  to communicate with every other non-coordinator device  320 .  
           [0017]    [0017]FIG. 4 is a block diagram of a device  310 ,  320  from the network  300  of FIG. 3. As shown in FIG. 4, each device (i.e., each coordinator  310  or non-coordinator device  320 ) includes a physical (PHY) layer  410 , a media access control (MAC) layer  420 , a set of upper layers  430 , and a management entity  440 .  
           [0018]    The PHY layer  410  communicates with the rest of the network  300  via a primary or secondary wireless link  330  or  340 . It generates and receives data in a transmittable data format and converts it to and from a format usable through the MAC layer  420 .  
           [0019]    The MAC layer  420  serves as an interface between the data formats required by the PHY layer  410  and those required by the upper layers  430 .  
           [0020]    The upper layers  430  include the functionality of the device  310 ,  320 . These upper layers  430  may include TCP/IP, TCP, UDP, RTP, IP, LLC, or the like.  
           [0021]    The management entity  440  provides monitoring and control functions to the MAC layer  420  and the PHY layer  410 , and facilitates communication between the upper layers and the MAC layer  420 . The management entity  440  may include a device management entity (DME) for controlling the operation of the device and a MAC layer management entity (MLME) for managing operation of the MAC layer  420 . In alternate embodiments the DME can be called a station management entity (SME).  
           [0022]    Typically, the coordinator  310  and the non-coordinator devices  320  in a WPAN share the same bandwidth. Accordingly, the coordinator  310  coordinates the sharing of that bandwidth. Standards have been developed to establish protocols for sharing bandwidth in a wireless personal area network (WPAN) setting. For example, the IEEE standard 802.15.3 provides a specification for the PHY layer  410  and the MAC layer  420  in such a setting where bandwidth is shared using a form of time division multiple access (TDMA). Using this standard, the MAC layer  420  defines frames and superframes through which the sharing of the bandwidth by the devices  310 ,  320  is managed by the coordinator  310  and/or the non-coordinator devices  320 .  
           [0023]    Device IDs and MAC Addresses  
           [0024]    One important aspect of working with devices  310 ,  320  in a network  300  is uniquely identifying each of the devices  310 ,  320 . There are several ways in which this can be accomplished.  
           [0025]    Independent of any network it is in, each device  310 ,  320  has a unique MAC address that can be used to identify it. This MAC address is generally assigned to the device by the manufacturer such that no two devices  310 ,  320  have the same MAC address. One set of standards that is used in preferred embodiments of the present invention to govern MAC addresses can be found in IEEE Std. 802-1990, “IEEE Standards for Local and Metropolitan Area Networks: Overview and Architecture.” 
           [0026]    For ease of operation, the network  300  can also assign a device ID to each device  310 ,  320  in the network  300  to use in addition its unique MAC address. In the preferred embodiments the MAC  420  uses ad hoc device IDs to identify devices  310 ,  320 . These device IDs can be used, for example, to route packets within the network  300  based on the ad hoc device ID of the destination of the packet. The device IDs are generally much smaller than the MAC addresses for each device  310 ,  320 . In the preferred embodiments the device IDs are 4-bits and the MAC addresses are 48-bits.  
           [0027]    Each device  310 ,  320  should maintain mapping table that maps the correspondence between device IDs and MAC addresses. The table is filled in based on the device ID and MAC address information provided to the non-coordinator devices  320  by the coordinator  310 . This allows each device  310 ,  320  to reference themselves and the other devices in the network  300  by either device ID or MAC address.  
           [0028]    The present invention can be used with the IEEE 803.15.3 standard for high-rate WPANs, which is currently under development by the IEEE 802.15 WPAN™ Task Group 3 (TG3). The details of the current draft 802.15.3 standard, including archives of the 802.15.3 working group can be found at: http://www.ieee802.org/15/pub/TG3.html. Nothing in this disclosure should be considered to be incompatible with the draft 802.15.3 standard, as set forth on the IEEE 802 LAN/MAN Standards Committee web page.  
           [0029]    Superframes  
           [0030]    The available bandwidth in a given network  300  is split up in time by the coordinator  310  into a series of repeated superframes. These superframes define how the available transmission time is split up among various tasks. Individual frames of data are then transferred within these superframes in accordance with the timing set forth in the superframe.  
           [0031]    [0031]FIG. 5 is a block diagram of a superframe according to preferred embodiments of the present invention. As shown in FIG. 5, each superframe  500  may include a beacon period  510 , a contention access period (CAP)  520 , and a contention free period (CFP)  530 .  
           [0032]    The beacon period  510  is set aside for the coordinator  310  to send a beacon frame out to the non-coordinator devices  320  in the network  300 . Such a beacon frame will include information for organizing the operation of devices within the superframe. Each non-coordinator device  320  knows how to recognize a beacon  510  prior to joining the network  300 , and uses the beacon  510  both to identify an existing network  300  and to coordinate communication within the network  300 .  
           [0033]    The CAP  520  is used to transmit commands or asynchronous data across the network. The CAP  520  may be eliminated in many embodiments and the system would then pass commands solely during the CFP  530 .  
           [0034]    The CFP  530  includes a plurality of time slots  540 . These time slots  540  are assigned by the coordinator  310  to a single transmitting device  310 ,  320  and one or more receiving devices  310 ,  320  for transmission of information between them. Generally each time slot  540  is assigned to a specific transmitter-receiver pair, though in some cases a single transmitter will transmit to multiple receivers at the same time. Exemplary types of time slots are: management time slots (MTS) and guaranteed time slots (GTS).  
           [0035]    An MTS is a time slot that is used for transmitting administrative information between the coordinator  310  and one of the non-coordinator devices  320 . As such it must have the coordinator  310  be one member of the transmission pair. An MTS may be further defined as an uplink MTS (UMTS) if the coordinator  310  is the receiving device, or a downlink MTS (DMTS) if the coordinator  310  is the transmitting device.  
           [0036]    A GTS is a time slot that is used for transmitting isochronous non-administrative data between devices  310 ,  320  in the network  300 . This can include data transmitted between two non-coordinator devices  320 , or non-administrative data transmitted between the coordinator  310  and a non-coordinator device  320 .  
           [0037]    As used in this application, a stream is a communication between a source device and one or more destination devices. The source and destination devices can be any devices  310 ,  320  in the network  300 . For streams to multiple destinations, the destination devices can be all or some of the devices  310 ,  320  in the network  300 .  
           [0038]    In some embodiments the uplink MTS may be positioned at the front of the CFP  530  and the downlink MTS positioned at the end of the CFP  530  to give the coordinator  310  a chance to respond to an uplink MTS in the in the downlink MTS of the same superframe  500 . However, it is not required that the coordinator  310  respond to a request in the same superframe  500 . The coordinator  310  may instead respond in another downlink MTS assigned to that non-coordinator device  320  in a later superframe  500 .  
           [0039]    The superframe  500  is a fixed time construct that is repeated in time. The specific duration of the superframe  500  is described in the beacon  510 . In fact, the beacon  510  generally includes information regarding how often the beacon  510  is repeated, which effectively corresponds to the duration of the superframe  500 . The beacon  510  also contains information regarding the network  300 , such as the identity of the transmitter and receiver of each time slot  540 , and the identity of the coordinator  310 .  
           [0040]    The system clock for the network  300  is preferably synchronized through the generation and reception of the beacons  510 . Each non-coordinator device  320  will store a synchronization point time upon successful reception of a valid beacon  510 , and will then use this synchronization point time to adjust its own timing.  
           [0041]    Although not shown in FIG. 5, there are preferably guard times interspersed between time slots  540  in a CFP  530 . Guard times are used in TDMA systems to prevent two transmissions from overlapping in time because of inevitable errors in clock accuracies and differences in propagation times based on spatial positions.  
           [0042]    In a WPAN, the propagation time will generally be insignificant compared to the clock accuracy. Thus the amount of guard time required is preferably based primarily on the clock accuracy and the duration since the previous synchronization event. Such a synchronizing event will generally occur when a non-coordinator device  320  successfully receives a beacon frame from the coordinator  310 .  
           [0043]    For simplicity, a single guard time value may be used for the entire superframe. The guard time will preferably be placed at the end of each beacon frame, GTS, ATS, and MTS.  
           [0044]    The exact design of a superframe  500  can vary according to implementation. FIG. 6 shows an example of a specific superframe design. As shown in FIG. 6, the transmission scheme  600  involves dividing the available transmission time into a plurality of superframes  610 . Each individual superframe  610  includes a beacon frame  620 , an uplink MTS  630 , a plurality of GTS  640 , and a downlink MTS  660 . This exemplary superframe includes no contention access period.  
           [0045]    The beacon frame  620  indicates by association ID (known as a device ID in the IEEE 802.15.3 draft standard) a non-coordinator device  320  that is assigned to the current superframe  610 . It also indicates via a receive-transmit table the transmitter/receiver assignments for the individual GTS  640 .  
           [0046]    In the exemplary superframe structure shown in FIG. 6, the uplink MTS  630  is set aside for the non-coordinator device  320  assigned to the current superframe  610  to upload signals to the coordinator  310 . All other non-coordinator devices  320  remain silent on the current channel during this time slot. In alternate embodiments that use multiple channels, all other non-coordinator devices  320  on that channel must remain silent during an uplink MTS  630 , though they may still transmit on alternate channels.  
           [0047]    The plurality of GTS  640  are the time slots set aside for each of the devices  310 ,  320  to allow communication between devices. They do so in accordance with the information set forth in the receive-transmit table in the beacon  620 . Each GTS  640  is preferably large enough to transmit one or more data frames. When a transmitter-receiver set is assigned multiple GTS  640 , they are preferably contiguous.  
           [0048]    The downlink MTS  660  is set aside for the coordinator  310  to download signals to the non-coordinator device  320  assigned to the current superframe  610 . All other non-coordinator devices  320  may ignore all transmissions during this time slot.  
           [0049]    The lengths of the uplink and downlink MTS  630  and  660  must be chosen to handle the largest possible management frame, an immediate acknowledgement (ACK) frame, and the receiver-transmitter turnaround time. For the GTS  640 , the length and number must be chosen to accommodate the specific requirements of frames to be transmitted, e.g., short MPEG frames, large frames of the maximum allowable length, and streaming vs. immediate ACK operation.  
           [0050]    Although the disclosed embodiment uses a plurality of GTS  640 , one uplink MTS  630  placed before the GTS  640 , and one downlink MTS  660  placed after the GTS  640 , the number, distribution, and placement of GTS  640  and MTS  630 ,  660  may be varied in alternate embodiments. Preferred embodiments of the present invention will be described below. And while the embodiments described herein will be in the context of a WPAN (or piconet), it should be understood that the present invention also applies to other settings where bandwidth is to be shared among several users, such as, for example, wireless local area networks (WLAN), or any other appropriate wireless network.  
           [0051]    However, conventional network designs can operate at higher power levels or lower transmission rates than they might otherwise because the non-coordinator devices  320  have no way of telling whether they are transmitting at too high a power level. The present invention provides a system and method for achieving such feedback.  
         SUMMARY OF THE INVENTION  
         [0052]    Consistent with the title of this section, only a brief description of selected features of the present invention is now presented. A more complete description of the present invention is the subject of this entire document.  
           [0053]    An object of the present invention is to provide a method by which each device in a network can continually monitor the quality of the media between it and each of the other devices in the network.  
           [0054]    Another object of the present invention is to provide a method of media quality that will reduce the possibility of a transmitting device using a transmission format that is at odds with the reception format used by a receiving device.  
           [0055]    Another feature of the present invention is to ensure that a signal is being transmitted by each device during a management time slot assigned to it so that all other devices in the network will be guaranteed to be able to hear a signal from that device with a known frequency.  
           [0056]    These and other objects are accomplished by way of a method for a local device to determine media qualities of a plurality of transmission paths between the local device and a plurality of remote devices in a wireless network, each of the local device and the remote devices being assigned at least one of a plurality of management time slots in a superframe rotation, comprising: transmitting a local frame from the local device to a network coordinator during each of the management time slots assigned to the local device, the local frame being transmitted according to a set of transmission criteria; receiving a remote frame during each of the management time slots assigned to one of the remote devices, the remote frame being received according to a set of reception criteria; determining quality information about a transmission medium between the local device and the remote device that transmitted the remote frame; determining a modified set of transmission criteria in the local device for transmissions to the remote device based on the quality information; determining a modified set of reception criteria in the local device for transmissions from the remote device based on the quality information; and storing in the local device the modified set of transmission criteria and the modified set of reception criteria as new transmission criteria and new reception criteria, respectively. The local frame is preferably a management frame if the local device has management data to send to a network coordinator, or a null frame if the local device has no management data to send the coordinator.  
           [0057]    The quality information may be signal-to-noise ratio values. The local device preferably stores the signal-to-noise ratio values.  
           [0058]    The method may further comprise sending data from the local device to one of the remote devices during a time slot assigned to the local unit using the new transmission criteria.  
           [0059]    The transmission criteria preferably comprises at least one of: preamble size, transmission power, transmission rate, amount of forward error correction used, acknowledgement policy, and fragment size. The reception criteria preferably comprises at least one of: preamble size, transmission power, transmission rate, amount of forward error correction used, acknowledgement policy, and fragment size.  
           [0060]    The modified set of transmission criteria is preferably determined using a different set of criteria than the modified set of reception criteria is determined using. 
       
    
    
     BRIEF DESCRIPTION OF THE DRAWINGS  
       [0061]    A more complete appreciation of the invention and its many attendant advantages will be readily obtained as it becomes better understood with reference to the following detailed description when considered in connection with the accompanying drawings, in which:  
         [0062]    [0062]FIG. 1 is a diagram showing the hierarchy of the seven-layered OSI standard;  
         [0063]    [0063]FIG. 2 is a diagram showing the IEEE 802 standard;  
         [0064]    [0064]FIG. 3 is a block diagram of a wireless network according to a preferred embodiment of the present invention;  
         [0065]    [0065]FIG. 4 is a block diagram of a device from the network of FIG. 3;  
         [0066]    [0066]FIG. 5 is a block diagram of a superframe according to preferred embodiments of the present invention; and  
         [0067]    [0067]FIG. 6 is a block diagram of a specific superframe design according to a preferred embodiment of the present invention.  
     
    
     DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENTS  
       [0068]    Preferred embodiments of the present invention will now be described with reference to the drawings. Throughout the several views, like reference numerals designate identical or corresponding parts.  
         [0069]    In normal operation devices can often tell that they are transmitting at too high a transmission rate by the simple fact that the device  310 ,  320  that they are trying to reach cannot hear their transmission. However, there has been provided no useful way for them to determine whether they are operating at too low a transmission rate, since the destination device  310 ,  320  for a given packet of data successfully receives the packet whether it is at a good transmission rate or a transmission rate that is too low. Previously no feedback existed that enabled a transmitting device  310 ,  320  to select the most appropriate settings for the transmission.  
         [0070]    The preferred embodiments described below allow a device  310 ,  320  to select appropriate transmission rate and acknowledgement policies based on the current status of the transmission media, without any additions to the operation protocol.  
         [0071]    The preferred embodiments employ a null frame, used as a “ping” signal, that is sent from the non-coordinator devices  320  to the coordinator  310  during any MTS assigned to a given non-coordinator device  320  in which no other traffic exists. By using these null frames, the system ensures that each non-coordinator device  320  will transmit something in the MTS assigned to it. In this way, the other non-coordinator devices  320  can obtain media information, i.e., information about the quality of the transmission media, about all of the other non-coordinator devices  320  without having to use explicit feedback signals or feedback information fields in any other signals.  
         [0072]    By forcing the coordinator  310  to acknowledge these null frames, the non-coordinator devices  320  can obtain media information, i.e., information about the quality of the transmission media, from the coordinator  310  without having to use explicit feedback signals or feedback information fields in any other signals.  
         [0073]    The disclosed system and method can be used with any wireless network, e.g. a network based on the IEEE 802.15.3 standard, or any other protocol in which the quality of the media needs to be known. However, the null frame transmissions make it particularly suitable for ultrawide bandwidth (UWB) signals, since there is no significant overhead for the non-coordinator devices  320  to transmit.  
         [0074]    The length of each superframe (i.e., the interval between the beacons) needs to be short enough to keep the information updated. Preferably this would be below 50 ms for an eight station network. However, any appropriate superframe length (beacon interval) can be chosen that will keep the media information sufficiently up-to-date. This can change according to the particular embodiment.  
         [0075]    In alternate embodiments networks could use reception quality information fields in the acknowledgement (ACK) frame to pass media information. For example, each ACK frame could include not only an indication that a frame was received, but also an indication of the received power, signal strength, etc. of the received frame. However, this will require that: (1) devices  310 ,  320  always receive the ACK frame; (2) devices may have to send an extra signal to get the ACK frame; and (3) a given device may waste its first frame if it&#39;s sent using inappropriate parameters, since the ACK frame is required to get the latest status.  
         [0076]    Introduction  
         [0077]    The preferred embodiments of the present invention relate to a TDMA method of accessing a wireless medium using a network topology consisting of several devices  320  including a coordinator  310 . (See FIG. 3)  
         [0078]    The preferred embodiment disclosed below meets three important requirements. It can support at least three device  310 ,  320  at an isochronous rate of 25 Mbps per device  310 ,  320 . It can support devices associating with and disassociating from the network  300 . And it support two transmission rates: one transmission rate for the preamble, and one transmission rate for data.  
         [0079]    Preambles  
         [0080]    A preamble is used at the beginning of each frame transmitted between two devices  310 ,  320  for receiver acquisition. The preamble allows the receiving device to lock onto and synchronize with the transmitting device, and to train itself so that it knows how to extract the modulated payload out of the frame. Depending upon the media quality and the transmission parameters, this preamble could be varied in length. For example, if the media conditions were such that signal quality was poor, a longer preamble might be needed to allow more time to prepare the receiver to process the incoming frame. If, however, the media conditions were good such that signal quality was poor, the frame could afford a shorter preamble.  
         [0081]    A feature of the present invention is that the length of the preambles in PMD may be changed as needed.  
         [0082]    In one preferred embodiment, the network  310  will start with a default short preamble and change to a long preamble during bad media quality transmissions. In alternate embodiments, however, the network  300  could start with a long preamble and switch to a short preamble during good media quality transmissions to make sure initial packets are safely transmitted. In other embodiments some portions of the superframe  500 ,  710 , e.g., the beacon and MTS, could always use long preambles to ensure their successful transmission in all circumstances.  
         [0083]    For the purposes of the examples below, it is assumed that the media quality is the same in both directions between two devices  310 ,  320 . Therefore a received signal can indicate the reception quality in the other end. It is also assumed that the PHY layer  410  in a given device  310 ,  320  can deliver a signal-to-noise ratio (SNR) indicator to the corresponding MAC layer  410 .  
         [0084]    Network Operation  
         [0085]    As noted above, periodically the superframes may contain one or more MTS. In particular, it is preferable that in each superframe one non-coordinator device  320  in the network be assigned at least one MTS. The available MTS in consecutive superframes are preferably distributed among the non-coordinator devices  320  using a fair algorithm such that each non-coordinator device  320  is periodically assigned at least one MTS. (See FIG. 7 and related disclosure.)  
         [0086]    According to preferred embodiments of the present invention, all non-coordinator devices  320  stay awake and listen to all traffic during every MTS. While they may enter a low power sleep mode during other portions of the superframe  500 ,  710 , they will always listen during each MTS, regardless of whom it assigned to. Because the MTS are short in comparison to the remainder of the superframe, this will not cause a great increase in power consumption.  
         [0087]    During each uplink MTS assigned to it, a given non-coordinator device  320  will send any necessary network maintenance frames to the coordinator  310 . Similarly, during each downlink MTS assigned to it, a given non-coordinator device  320  will receive any necessary network maintenance frames from the coordinator  310 . However, unlike in conventional networks, when a non-coordinator device  320  has nothing to send/receive during one of its assigned MTS, instead of remaining silent it will send a null frame directed to the coordinator  310 . According to this preferred embodiment the coordinator  310  must always acknowledge these null frames (i.e., by sending the non-coordinator device  320  an ACK frame), regardless of the general acknowledgement policy in the network.  
         [0088]    Then, during each MTS, all of the other non-coordinator devices  320  listen for both the frame transmitted by the device  320  assigned to the MTS, and any ACK frame from the coordinator  310 . Each non-coordinator device  320  will always be transmitting something, whether it be a management frame or a null frame; and the coordinator  310  will more often than not be responding with an ACK frame.  
         [0089]    Based on an analysis of one or more incoming signals from each device over the course of one or more MTS, the each other device can determine certain signal quality parameters, e.g., signal-to-noise ratios (SNR). These values are preferably stored in a table in each device that indicates the relative SNR of each other device in the network  300 .  
         [0090]    Preferably, the PHY layer  410  in each non-coordinator device  320  determines the SNR for the non-coordinator device  320  assigned to the current MTS based on the quality of the received null/management frame, and determines the SNR for the coordinator  310  based on the SNR of the ACK frame sent by the coordinator  310  to the non-coordinator device  320  assigned to the current MTS.  
         [0091]    Thus, through this process, every non-coordinator device  320  will periodically be able to calculate a SNR reading for every other device  310 ,  320  in the network. Each non-coordinator device  320  must be assigned an MTS with some reasonable frequency, and will always transmit something in that MTS, even if it&#39;s just a null frame. Furthermore, the coordinator  310  replies to most MTS frames and all null frames, so it will transmit even more frequently.  
         [0092]    Once it calculates the SNR for a device  310 ,  320 , a non-coordinator device  320  compares the SNR value a set threshold value set in a network information base. Then, based on this comparison the transmissions to that device will be assigned certain transmission parameters. These parameters could include fragment size, PHY rate, preamble length, use of FEC, ACK policy, transmit power, and the like. As an example for this disclosure, the device will choose between a long or a short preamble based on the SNR comparison. In alternate embodiments other parameters could also be changed. In addition, multiple thresholds could be provided to allow more than two choices of parameters.  
         [0093]    The MAC layer  420  of each non-coordinator device  320  preferably maintains a table including the current preferred preamble length for each other device  310 ,  320  in the network  300 , determined based on the comparison above. As noted above, in alternate embodiments this table entry could comprise additional parameters, such as the preferred transmission ACK-policy, the preferred transmission rate, etc. that should be used when transmitting to a particular device. The devices  310 ,  320  fill in these fields based on the SNR comparisons made during MTS assigned to other devices  310 ,  320  in the network  300  and any corresponding ACK from the coordinator  310 .  
         [0094]    Preferably each non-coordinator device  320  uses the SNR value before it is processed by a decision feedback equalizer (DFE), if any. The only reason to change the preamble rate and leave the PHY payload rate the same is because the DFE is improving the signal sufficiently to compensate for a multipath channel).  
         [0095]    Then, when a given non-coordinator device  320  sends a frame to any given device  310 ,  320 , it can read the current preamble mode (or whatever other parameters are determined) for transmissions made to that device  310 ,  320  from the table and indicate that preamble length (or other parameters) to the PHY layer  410 . The PHY layer  410  can then prepare the frames accordingly.  
         [0096]    Likewise, when receiving a frame from any given device  310 ,  320 , the current preamble mode (or other parameters) for messages received from that device  310 ,  320  can be read from the table and indicated to the PHY layer  410 . The PHY layer  410  can then process the incoming frames accordingly.  
         [0097]    Although a single preamble table can be maintained, it is preferable to keep separate transmitter and receiver preamble tables, filled using slightly different SNR thresholds. In particular, a transmit SNR threshold for using a long preamble should be lower than a receive SNR threshold for using a long preamble. Thus, when operating as a transmitter, a non-coordinator device  320  will be more likely to use a long preamble than when it was operating as a receiver.  
         [0098]    This address the problem of what happens when the SNR is close to the threshold. Without two separate thresholds, one device  310 ,  320  might choose to send a long preamble while the other device  310 ,  320  chose to receive a short preamble. This would cause the data to be incorrectly received. However, by adjusting the transmit and receive thresholds accordingly, the protocol can make certain that if an error is made, it will be for the transmitter to send a long preamble when it only needed to send a short preamble. This is because a device  310 ,  320  expecting a short preamble can receive a long preamble, while a device  310 ,  320  expecting a long preamble will not be able to receive a frame with a short preamble.  
         [0099]    As shown above, the network  300  can support both a best-case and a worst-case length of preamble. When transmission conditions are poor, a worst-case (long) preamble length can be used. And when transmission conditions are good, a best-case (short) preamble length can be used. And since the preambles are determined on a device-by-device basis, different preamble lengths can be used within the network  300  as needed based on conditions between various devices  310 ,  320 .  
         [0100]    In some embodiments a transmitting device  310 ,  320  could also fall back to more conservative parameters (e.g., a long preamble) if a transmission using less conservative parameters (e.g., a short preamble) does not pass successfully. In other words, if a device  310 ,  320  sends a frame using parameters from its database and the transmission is unsuccessful, e.g., it doesn&#39;t receive a response or an ACK frame, then the transmitting device  320  could transmit using more conservative parameters, despite what the relevant entry in the database says.  
         [0101]    Likewise, a receiving device  310 ,  320  could also fall back to more conservative parameters (e.g., a long preamble) if a transmission using less conservative parameters (e.g., a short preamble) does not pass successfully. In other words, if a receiving device  310 ,  320  cannot successfully receive a frame using parameters from its database, then the receiving device  320  could change to use more conservative parameters, despite what the relevant entry in the database says.  
         [0102]    The worst case scenario for using the wrong preamble mode is the maximum time between two transmissions from a device  310 ,  320  which would be super frame length*MTS cycle length, or in current implementation 4*16 ms. In other words, if a device is wrong about the parameters to use, it suffers a loss in time equal to the time it can transmit again using parameters more likely to succeed.  
         [0103]    Although this disclosed embodiment uses only two preamble lengths, alternate embodiments could use multiple preamble lengths using the same mechanism. Such embodiments would have to provide multiple thresholds for determining which preambles to assigned to each device.  
         [0104]    Obviously, numerous modifications and variations of the present invention are possible in light of the above teachings. It is therefore to be understood that within the scope of the appended claims, the invention may be practiced otherwise than as specifically described herein.