Patent Publication Number: US-8976808-B2

Title: Methods for transmitting closely-spaced packets in WLAN devices and systems

Description:
CROSS-REFERENCE TO RELATED APPLICATION 
     The present application is a divisional of U.S. patent application Ser. No. 13/038,626 and filed Mar. 2, 2011, which is in turn a divisional of U.S. patent application Ser. No. 12/454,006 and filed May 11, 2009, which is in turn a divisional of U.S. patent application Ser. No. 10/676,139 (now issued as U.S. Pat. No. 7,551,581) and filed Sep. 30, 2003, and claims priority to the filing date of Sep. 30, 2003. 
    
    
     TECHNICAL FIELD 
     The inventive subject matter pertains to wireless local area networks (WLANs) and, more particularly, to transmission of data packets between a transmitter and a receiver in a WLAN. 
     BACKGROUND 
     Any Wireless Local Area Network (WLAN) device that supports an Institute of Electrical and Electronics Engineers (IEEE) 802.11 Standard (e.g., IEEE Std 802.11-1997, 802.11a, 802.11e, etc.) includes two main parts: 1) a physical (PHY) layer signaling control device; and 2) a medium access control (MAC) device. The function of the PHY device is to transfer data packets over the air interface. Among other things, the function of the MAC device is to fairly control access to the shared air interface. 
     The minimal MAC protocol consists of two frames: 1) a frame sent from a transmitter to a receiver; and 2) an acknowledgment (ACK) from the receiver that the frame was received correctly. If a transmitter has multiple packets to send to the receiver, some versions of the 802.11 Standard require the transmitter to wait for an ACK after transmission of each packet. In addition, the transmitter must wait for a particular time interval, referred to as the Interframe Space (IFS), after receiving the ACK and before transmitting the next packet. 
     Other versions of the 802.11 Standard (e.g., IEEE Std 802.11e) support transmission of packets with selective acknowledgement. This feature is referred to as “Block ACK.” The Block ACK feature enables the transmitter to send the next packet to the same receiver without necessarily waiting for an ACK. Instead, after negotiating for access to the air interface, the transmitter sends the first packet, waits an IFS after the end of the first packet, and sends the next packet. After the transmitter has sent all of its packets to the receiver, the transmitter asks the receiver for a response, which indicates an ACK for all of the previously transmitted packets. 
     Although the Block ACK feature has provided some throughput improvements, developers continue to strive for ways of further increasing throughput. Accordingly, what are needed are methods and apparatus for further improving throughput using burst-mode transmissions. 
    
    
     
       BRIEF DESCRIPTION OF THE DRAWINGS 
       The appended claims point out different embodiments of the inventive subject matter with particularity. However, the detailed description presents a more complete understanding of the inventive subject matter when considered in connection with the figures, wherein like-reference numbers refer to similar items throughout the figures and: 
         FIG. 1  is a simplified diagram of example WLANs, in accordance with an embodiment of the inventive subject matter; 
         FIG. 2  is a simplified block diagram of a WLAN station, in accordance with an embodiment of the inventive subject matter; 
         FIG. 3  illustrates an example of a timing diagram for transmitting multiple PHY protocol data unit (PPDU) frames, each with a single service data unit (SDU); 
         FIG. 4  illustrates an example of a timing diagram for transmitting a PPDU that may contain multiple SDUs and delimiters, in accordance with an embodiment of the inventive subject matter; 
         FIG. 5  is a flowchart of a procedure for a transmitter to assemble and transmit a PPDU, such as that illustrated in  FIG. 4 , in accordance with an embodiment of the inventive subject matter; 
         FIG. 6  is a flowchart of a procedure for a receiver to receive and divide a PPDU, such as that illustrated in  FIG. 4 , in accordance with an embodiment of the inventive subject matter; 
         FIG. 7  illustrates an example of a timing diagram for transmitting a PPDU with multiple SDUs without intervening data in accordance with an embodiment of the inventive subject matter; 
         FIG. 8  is a flowchart of a procedure for a transmitter to assemble and transmit a PPDU, such as that illustrated in  FIG. 7 , in accordance with an embodiment of the inventive subject matter; 
         FIG. 9  is a flowchart of a procedure for a receiver to receive and divide a PPDU, such as that illustrated in  FIG. 7 , in accordance with an embodiment of the inventive subject matter; 
         FIG. 10  illustrates an example of a timing diagram for transmitting a burst of multiple PPDUs in accordance with an embodiment of the inventive subject matter; 
         FIG. 11  is a flowchart of a procedure for a transmitter to transmit a burst of multiple PPDUs, such as those illustrated in  FIG. 10 , in accordance with an embodiment of the inventive subject matter; 
         FIG. 12  is a flowchart of a procedure for a receiver to receive a burst of multiple PPDUs, such as those illustrated in  FIG. 10 , in accordance with an embodiment of the inventive subject matter; 
         FIG. 13  illustrates an example of a timing diagram for transmitting a burst of multiple PPDUs with shortened intervening preambles in accordance with an embodiment of the inventive subject matter; 
         FIG. 14  is a flowchart of a procedure for a transmitter to transmit a burst of multiple PPDUs, such as those illustrated in  FIG. 13 , in accordance with an embodiment of the inventive subject matter; 
         FIG. 15  is a flowchart of a procedure for a receiver to receive a burst of multiple PPDUs, such as those illustrated in  FIG. 13 , in accordance with an embodiment of the inventive subject matter; 
         FIG. 16  illustrates an example of a timing diagram for transmitting a burst of multiple PPDUs without intervening preambles, in accordance with an embodiment of the inventive subject matter; 
         FIG. 17  is a flowchart of a procedure for a transmitter to transmit a burst of multiple PPDUs, such as those illustrated in  FIG. 16 , in accordance with an embodiment of the inventive subject matter; and 
         FIG. 18  is a flowchart of a procedure for a receiver to receive a burst of multiple PPDUs, such as those illustrated in  FIG. 16 , in accordance with an embodiment of the inventive subject matter. 
     
    
    
     DETAILED DESCRIPTION 
     In the following description of various embodiments, reference is made to the accompanying drawings, which form a part hereof and show, by way of illustration, specific embodiments in which the inventive subject matter may be practiced. Various embodiments are described in sufficient detail to enable those skilled in the art to practice the inventive subject matter, and it is to be understood that other embodiments may be utilized, and that process or mechanical changes may be made, without departing from the scope of the inventive subject matter. Such embodiments of the inventive subject matter may be referred to, individually and/or collectively, herein by the term “invention” merely for convenience and without intending to voluntarily limit the scope of this application to any single invention or inventive concept if more than one is in fact disclosed. It will be recognized that the methods of various embodiments can be combined in practice, either concurrently or in succession. Various permutations and combinations will be readily apparent to those skilled in the art. 
     Embodiments of the inventive subject matter include ways of transmitting multiple packets in a burst mode (i.e., in succession). Various embodiments will be described in detail below, after a description of a wireless local area network (WLAN) system and a WLAN device, in conjunction with  FIGS. 1 and 2 . Various embodiments can be implemented in systems and devices such as the system and device described in conjunction with  FIGS. 1 and 2 . Various embodiments also can be implemented in other systems and devices, which have different configurations. 
       FIG. 1  is a simplified diagram of example WLANs in accordance with an embodiment of the inventive subject matter. A WLAN may include multiple network stations  102  and zero or more access points (APs)  104 . 
     In a WLAN, network stations  102  communicate over the medium of free space, commonly referred to as the “air interface.” Generally, a station  102  may be referred to as a network adapter or network interface card (NIC). A station  102  may be mobile, portable or stationary. For example, a station  102  may be a laptop computer, a handheld radio, a desktop computer, or virtually any other one-way or two-way device with the capability of communicating with other devices  102  or APs  104  over a wireless medium. 
     A set of stations  102  may communicate directly with each other, as is the case in a Basic Service Set (BSS). An Independent BSS (IBSS)  110  is a BSS in which there is no connection to a wired network. 
     An infrastructure BSS  112  is a BSS in which a BSS includes an AP  104 . In an infrastructure BSS, all stations  102  communicate with an AP  104 . The AP  104  provides the connection to the wired LAN, if any, and the local relay function for the BSS. Accordingly, if a first station  102  wants to communicate with a second station  102 , the first station  102  sends the communication to the AP  104 , and the AP  104  relays the communication to the second station  102 . 
     An Extended Service Set (ESS)  114  is a set of infrastructure BSSs  112 , where the APs  104  communicate among themselves to forward traffic from one BSS  112  to another, and to facilitate the movement of stations  102  from one BSS to another. The Distribution System (DS) is a mechanism by which one AP  104  communicates with another to exchange frames from stations  102  in their BSSs  112 , forward frames to follow mobile stations  102  from one BSS  112  to another, and exchange frames with wired networks, if any. 
     Embodiments of the invention will now be described in more detail. Although various embodiments are described in detail, below, using terms that are similar to terms used in the context of an IEEE 802.11 Standard (e.g., IEEE Std 802.11-1997, 802.11a, 802.11e, etc.), the invention is not meant to be limited to use within a system that uses an IEEE 802.11 Standard. Instead, embodiments of the invention could be used in conjunction with other WLAN standards, as well. 
       FIG. 2  is a simplified block diagram of a WLAN station  200  (e.g., stations  102 ,  104 ,  FIG. 1 ) in accordance with an embodiment of the inventive subject matter. Any WLAN station  200  that supports an IEEE 802.11 Standard includes a physical (PHY) layer signaling control device  202  (PHY device), a medium access control (MAC) device  204 , and a MAC client  206 . WLAN station  200  supports station services, which are provided by PHY device  202  and MAC device  204 , and used by MAC client  206 . These services may include authentication, deauthentication, privacy, and delivery of data. 
     The MAC client  206  creates and processes data, among other things. The purpose of the PHY and MAC devices  202 ,  204  is to ensure that two network stations are communicating with the correct frame format and protocol. An IEEE Std 802.11 defines the communication protocol between the PHY and MAC devices  202 ,  204 . 
     The function of the PHY device  202  is threefold: 1) to provide a frame exchange between the MAC  204  and PHY  202  under the control of a physical layer convergence procedure (PLCP) sublayer; 2) to transmit data frames over the air interface under the control of the physical medium dependent (PMD) sublayer; and 3) to provide a carrier sense indication back to the MAC  204  so the MAC  204  is able to verify activity on the air interface. 
     The PHY device  202  implements one of several physical layer specifications, such as infrared (IR) baseband, frequency hopping spread spectrum (FHSS), direct sequence spread spectrum (DSSS), or orthogonal frequency domain multiplexing (OFDM). Other specifications can be implemented in other embodiments. 
     In general, the PHY device  202  includes PLCP apparatus  210 , and transmit and receive PMD apparatuses  212 ,  214 . Each of these may or may not use some or all of the same physical circuitry (e.g., processors, busses, clocks, storage, etc.). In addition, one or more antennae  216  may be interconnected with PMD apparatus  212 ,  214 . When an IR baseband specification is implemented, a light-emitting diode (LED) (not shown) or other optical transmission device may be used instead of the antennae  216 . 
     As mentioned above, a function of PLCP apparatus  210  is to control the frame exchange between the MAC device  204  and the PHY device  202 . The function of PMD apparatuses  212 ,  214  is to control signal carrier and spread spectrum modulation and demodulation for transmitting and receiving data frames over the air interface. 
     The structures of PMD apparatuses  212 ,  214  depend on the particular physical layer specification (e.g., the modulation type) implemented in the station. For example, if DSSS is used, the transmit PMD apparatus  212  may include a scrambler, an adder, a mask filter, and a DBPSK DQPSK modulator, and the receive PMD apparatus  214  may include a de-spread correlator, a DBPSK DQPSK de-modulator, a de-scrambler, and a timing clock recovery device. If FHSS is used, the transmit PMD apparatus  212  may include a data whitener, a symbol mapper, a Gaussian shaping filter, and a modulator, and the receive PMD apparatus  214  may include a de-modulator, a data de-whitener, and a hop timing recovery device. If IR is used, the transmit PMD apparatus  212  may include a symbol mapper, a modulator, and an LED driver, and the receive PMD apparatus  214  may include a diode detector, a de-modulator, and a symbol mapper. If OFDM is used, the transmit PMD apparatus  212  may include a convolutional encoder, a bit interleaving and mapping device, an inverse fast Fourier transform (FFT), a symbol shaper, and a quadrature amplitude modulation (QAM) modulator, and the receive PMD apparatus  214  may include a PSK QAM de-modulator, an FFT, a bit de-interleaving and de-mapping device, a convolutional decoder, and a clock recovery device. 
     Among other things, the function of the MAC device  204  is to control access to the shared air interface. The MAC device  204  provides an interface between the MAC client  206  and the PHY device  202 . In addition, the MAC device  204  may or may not perform encryption and decryption. In on embodiment, the MAC device supports the MAC sublayer according to an IEEE Std 802.11. In other embodiments, the MAC device supports the MAC sublayer according to another standard. 
     Because the air interface is often very noisy and unreliable, an IEEE Std 802.11 MAC device  204  implements a frame exchange protocol to allow the source of a data frame to determine whether the frame has been successfully received at the destination. The minimal MAC protocol consists of two frames: 1) a sent frame that includes a frame sent from the transmitter to the receiver; and 2) a response frame that includes an acknowledgment (ACK) from the receiver that the sent frame was received correctly. In addition, a sent frame might be one of the following: an acknowledgement (ACK), a request to send (RTS), a clear to send (CTS), or a PS-Poll. The corresponding response frames would be, respectively: a fragment; a CTS; a data frame; and an ACK. 
       FIG. 3  illustrates an example of a timing diagram for transmitting multiple PHY protocol data unit (PPDU) frames, each carrying a single service data unit (SDU). The PPDU frames  300 ,  310  represent the format of a frame as it is transmitted over the air interface. A PPDU frame includes a preamble  302 , a PHY header  304 , and a SDU  306 , in an embodiment. 
     The preamble  302 , the PHY header  304 , and the SDU  306  each are transmitted at the beginning of a symbol boundary, as indicated by the tic marks on the time axis  320  of  FIG. 3 . Each symbol may have a pre-determined duration, or a duration that changes within different parts of the packet. For example, a symbol duration may be 4 microseconds, although it can be longer or shorter, as well. 
     The preamble  302  includes a pattern of bits, which the receiver uses to synchronize itself. Specifically, the receiver may use the preamble  302  to perform the following tasks: 1) packet start acquisition; 2) channel estimation; 3) antenna diversity and training; 4) receiver automatic gain control (AGC); 5) carrier offset; and 6) symbol timing. 
     Within the PHY header  304  are a rate field and a length/size field, in one embodiment. The rate field indicates which type of modulation must be used to receive the incoming SDU  306 . In an alternate embodiment, the rate of the incoming SDU  306  is determined in advance between a transmitting and receiving station, and thus the rate information may not be included in the PHY header  304 . 
     The length/size field indicates the length of the SDU  306 . In various embodiments, the length/size field can include a number of microseconds necessary to transmit the SDU  306 , a number of bytes in the SDU  306 , or some other value indicating the length of the SDU  306 . The PHY header  304  may also include a checksum or other field, which enables its contents to be validated. The PHY header  304  could have a fixed or variable length. 
     The SDU  306  is a series of fields that is assembled by the MAC  204  ( FIG. 2 ) and passed to the PHY  202  via PLCP  210 . As far as the PHY  202  is concerned, the SDU  306  includes “opaque data,” meaning that the PHY  202  does not know or care what data is included in the SDU  306 . 
     The SDU  306  can be of a variable length. An SDU frame may be used by the MAC device  204  to transport its MAC protocol data unit (MPDU), which may include an MPDU header, a frame body field, and a frame check sequence (FCS) field. The frame body field is of a variable length, and its contents may or may not be encrypted. This field may contain all or part of a MAC service data unit (MSDU) or protocol service data unit (PSDU) from higher layer protocols. 
     Different modulation rates may be used to transmit the preamble  302 , PHY header  304 , and SDU  306 . The preamble  302  and PHY header  304  are transmitted at a first rate, referred to herein as a “robust modulation rate.” The robust modulation rate may be in a range of approximately 6 megabits per second (Mbps) to 12 Mbps, in an embodiment, although rates higher or lower can be used in other embodiments. The robust modulation rate does not change, in an embodiment. When the robust modulation rate does not change, a receiver knows to look for a preamble  302  and PHY header  304  at the known robust modulation rate. In another embodiment, the robust modulation rate may change. In still another embodiment, the preamble  302  and the PHY header  304  are transmitted at different modulation rates. 
     In contrast, the SDU  306  may be transmitted at a second rate, referred to herein as a “data modulation rate.” For illustration purposes, the SDU  306  is cross-hatched, indicating that it is transmitted at the data modulation rate, as opposed to the robust modulation rate. 
     The data modulation rate can vary from frame to frame. In an embodiment, the rate varies in a range of between approximately 6 to 240 Mbps. In one embodiment, a receiver determines the data modulation rate for a particular SDU by evaluating the rate field of the PHY header  304 , as described above. 
     The lower modulation rates may be more robust, meaning that the data can tolerate worse channel conditions. The preamble  302  and PHY header  304  are sent at the lower modulation rate, so that the data within the PHY header  304  is less likely to be corrupted, even though corruption is possible given enough interference signal power. If the data within the rate field of the PHY header  304  were corrupted, for example, the receiver would be unable to demodulate the SDU  306 . If the data within the size field of the PHY header  304  were corrupted, the receiver would either truncate the SDU  306  or would extend the SDU  306 , causing the receiver to demodulate invalid data after the end of the SDU  306 . 
     The data modulation rate for the SDU  306  can be chosen based on an estimate of channel conditions. If the channel is excellent, then a high rate (e.g., approaching 240 Mbps) may be selected, thus increasing the throughput of the system. If the channel is very noisy, then a relatively low rate (e.g., approaching 6 Mbps) may be selected, so that the data integrity can be maintained as best possible. 
     In various embodiments, each packet is fully or partially “self-describing,” meaning that the receiver does not need a priori information about the structure (i.e., the data rate and/or size) of the upcoming packet. In one embodiment, each packet is fully self-describing, meaning that each packet includes both the data modulation rate, in the PHY header  304 , and also the length/size information. The length/size information is included in the PHY header  304 , in one embodiment, and in the SDU itself, in another embodiment. 
     In another embodiment, each packet is partially self-describing, meaning that each packet includes the length/size information, but the data modulation rate may be defined between the transmitting and receiving station in a prior training exchange. Accordingly, the data modulation rate is not necessarily included in the PHY header  304 . 
     This “self-describing” feature differentiates embodiments of the invention from other protocols, such as the Hiperlan 2 protocol, for example. Using the Hiperlan 2 protocol, the transmitter emits a known block of data every 2 milliseconds. That block of data includes a complete map of everything that the transmitter will transmit for the remainder of the upcoming 2 millisecond time period. This means that all receivers have a priori information about the modulation rates and lengths of the packets that the transmitter will send. The modulation and length information is not included in each packet, using Hiperlan 2, and thus the packets are not “self-describing.” 
     As discussed previously, prior art systems support “burst-mode” transmission of PPDU frames with selective acknowledgement using the “Block ACK” feature. The Block ACK feature enables the transmitter to send the next PPDU frame to the same receiver without necessarily waiting for an ACK. Instead, after negotiating for access to the air interface, the transmitter sends the first PPDU frame, waits an Interframe Space (IFS) after the end of the first packet, and sends the next PPDU frame. 
     The term “IFS,” as used herein, is meant to include various related time periods, including but not limited to the IFS, a Short IFS (SIFS), a Priority IFS (PIFS), a Distributed IFS (DIFS), and an Extended IFS (EIFS), as defined in an IEEE 802.11 Standard, although the term IFS is not meant to be limited to time periods defined only in such a standard. The IFS may consume multiple symbol boundaries.  FIG. 3  illustrates a second PPDU  310  being transmitted after a symbol gap  312  of four or more symbols, which can represent an IFS. An IFS can be an integer or non-integer number of symbol widths. In addition, the duration of the IFS can be longer or shorter than four symbols. 
     Using the Block ACK feature, after the transmitter has sent all of its PPDU frames to the receiver, the transmitter asks the receiver for a response, which indicates an ACK for all of the previously transmitted frames. Using prior art methods, each PPDU frame includes a single SDU, and each PPDU frame is formatted essentially as described in conjunction with  FIG. 3 . 
     In accordance with various embodiments of the inventive subject matter, a single PPDU frame includes one or more opaque, concatenated SDUs, where the one or more SDUs are referred to herein as the “payload.” In an embodiment, each SDU includes a “delimiter,” which indicates the size of the SDU, and the PHY header may include a length field that includes the entire length of the concatenated SDUs. In another embodiment, the PHY header contains length information for each SDU, enabling the receiver to assemble and re-divide the payload into the distinct SDUs. 
     In still another embodiment, each PPDU frame includes a single SDU. However, during burst mode, multiple PPDU frames are concatenated together, rather than waiting the IFS between each frame. In still another embodiment, multiple PPDU frames are concatenated together, but a shortened preamble is included with each PPDU frame after the first frame. In still another embodiment, multiple PPDU frames are concatenated together, but the preamble is eliminated for each PPDU frame after the first frame. Various embodiments will now be described in conjunction with  FIGS. 4-18 . 
       FIG. 4  illustrates an example of a timing diagram for transmitting a PPDU that may contain multiple SDUs and delimiters, in accordance with an embodiment of the inventive subject matter. The PPDU  400  includes a preamble  402 , PHY header  404 , and a consolidated payload  406  with at least one SDU  420 ,  422 ,  424 . In the illustrated example, the consolidated payload  406  includes three SDUs. More or fewer SDUs can be included in a single payload. 
     The preamble  402  includes a pattern of bits, which the receiver uses to synchronize itself, as described above. The PHY header  404  includes a rate field, in one embodiment, which indicates which data modulation rate is used for the consolidated payload  406 . In an embodiment, the PHY header  404  also includes a length/size field, which defines the total length of the consolidated payload  406 . In various embodiments, the length/size field can include a number of microseconds necessary to transmit the consolidated payload  406 , a number of bytes in the consolidated payload  406 , or some other value indicating the length of the consolidated payload  406 . In another embodiment, the PHY header  404  does not include the total length information. The PHY header  404  could have a fixed or variable length. 
     Each SDU  420 ,  422 ,  424  must be separated and delivered intact by the PHY in the receiver. To facilitate decomposition of the consolidated payload  406  into individual SDUs, the consolidated payload also includes information indicating the lengths of each of the multiple SDUs. In an embodiment, the information includes multiple “delimiters”  408 ,  410 ,  412 , where each SDU is preceded by a delimiter, in an embodiment. 
     Each delimiter includes a length field, which indicates the variable length  430 ,  432 ,  434  of the SDU  420 ,  422 ,  424 , respectively, that follows. If the following SDU is not the last SDU in the consolidated payload  406 , the delimiter information also enables a receiver to determine where the next SDU&#39;s delimiter should be located. 
     In an embodiment, each delimiter  408 ,  410 ,  412  also includes a length validation field, which enables a receiver to determine whether or not the length field has been corrupted, as described in more detail later. In an embodiment, the length validation field includes a checksum or CRC, although other validation information can be used in other embodiments. The length validation field enables robust error detection, as will be described in detail in conjunction with  FIGS. 5 and 6 . 
     In addition, in an embodiment, each delimiter  408 ,  410 ,  412  may also include a sequence field, which indicates that the SDU  420 ,  422 ,  424 , respectively, that follows is either the last SDU or is not the last SDU. In other embodiments, the delimiter may not include either or both the length validation field or the sequence field. 
     In an embodiment described above, a one-to-one correlation exists between delimiters and SDUs. In another embodiment, a one-to-one correlation may not exist between delimiters and SDUs. Instead, a fewer number of delimiters than the number of SDUs can be transmitted. For example, a single delimiter can be transmitted, which indicates the lengths of all of the SDUs. 
       FIG. 5  is a flowchart of a procedure for a transmitter to assemble and transmit a PPDU, such as that illustrated in  FIG. 4 , in accordance with an embodiment of the inventive subject matter. The method begins, in block  502  when the PHY device obtains at least one SDU. In one embodiment, the SDUs are intermediately or finally destined for the same receiver, although it is possible for the SDUs to have different destinations. 
     In block  504 , the lengths of each SDU and length validation data are determined. For example, in an embodiment, the SDU length is represented by two bytes, and the length validation field includes a checksum or CRC for the two-byte length field. Accordingly, the length validation field also can be two bytes. In other embodiments the length and/or length validation fields can be larger or smaller. 
     The delimiters for each SDU are assembled, in an embodiment, in block  506 . Each delimiter includes the length field, the length validation field, and a sequence field, which indicates whether or not the SDU is the last. In other embodiments, either or both the length validation field or the sequence field can be excluded from the delimiter. 
     In an embodiment, the total length of the consolidated payload is determined, in block  508 , for inclusion in the PHY header. The total length includes the lengths of each of the delimiters, plus the lengths of each of the SDUs. The total length enables the receiver to determine when the end of the consolidated payload should occur. In another embodiment, the total length is not included in the PHY header. For example, in another embodiment, the receiver can rely instead on the sequence field of the delimiter to predict the end of the consolidated payload. As will be explained in more detail later, if a delimiter is corrupted, the receiver may measure the symbol energy to determine whether the end of the consolidated payload has been reached. 
     After negotiating access to the air interface, the transmitter transmits the preamble and the PHY header over the air at the robust modulation rate, in block  510 . In an embodiment, the transmitter begins transmitting each of the preamble and the PHY header at the beginning of a symbol boundary. In an embodiment, the preamble is transmitted for two symbols, and the PHY header is transmitted for one symbol. In other embodiments, either the preamble or the PHY header can be transmitted for longer or shorter time durations. 
     When transmission of the PHY header is complete, the transmitter switches to the data modulation rate, in block  512 , which will be used to transmit the consolidated payload. The transmitter begins transmitting the first delimiter, in block  514 . In an embodiment, the transmitter begins transmitting the first delimiter at the beginning of the next symbol boundary after completion of the PHY header. Alternatively, the first delimiter can begin at a time other than a symbol boundary. In other words, transmission can begin before or after a symbol boundary. In an embodiment, the transmitter begins transmitting the consolidated payload within one symbol width of the end of the PHY header. Internal block padding may be included at the end of each SDU. 
     In an embodiment, the delimiter may take less than one symbol to complete, and the transmitter may begin to transmit the SDU within the latter part of the same symbol as the delimiter. In another embodiment, the transmitter begins transmitting the SDU at the next symbol boundary after completion of the delimiter transmission. Internal block padding may be included at the end of each SDU. 
     In block  516 , a determination is made whether more delimiters and SDUs remain to be transmitted. In another embodiment, this determination can be excluded. If more delimiters and SDUs remain to be transmitted, the transmitter begins transmitting the next delimiter and its associated SDU, in block  514 . 
     In an embodiment, the transmitter begins transmitting the next delimiter promptly upon completion of transmitting the previous SDU, whether or not that time occurs on a symbol boundary. In addition, the transmitter begins transmitting the associated SDU promptly upon completion of transmitting the delimiter. Accordingly, in this embodiment, all of the data within the consolidated payload is efficiently concatenated together. In other embodiments, gaps or filler data can exist between subsequent delimiters and/or SDUs. After transmitting the last SDU, the method ends. 
       FIG. 6  is a flowchart of a procedure for a receiver to receive and divide a PPDU, such as that illustrated in  FIG. 4 , in accordance with an embodiment of the inventive subject matter. The method begins, in block  602 , when the receiver detects an incoming preamble at the robust modulation rate. The receiver uses the preamble to become synchronized with the incoming PPDU frame, in block  604   
     In one embodiment, the receiver determines the modulation rate of the PPDU&#39;s consolidated payload from the PHY header, in block  606 . In an alternate embodiment, the data modulation rate may be determined during a prior training exchange. 
     In an embodiment, the receiver also determines the entire length of the consolidated payload from the PHY header. This enables the receiver to know how long it should demodulate incoming data at the data modulation rate. In another embodiment, the receiver uses the length and sequence fields in the delimiters to make this determination, and the total length is not necessarily provided in the PHY header. Once receipt of the PHY header has completed, the receiver switches to demodulating at the data modulation rate, in block  608 , in order to receive and demodulate the consolidated payload. 
     The first thing that occurs in the consolidated payload is a delimiter, in an embodiment. Therefore, in block  610 , the receiver receives and attempts to validate a segment of data having the size of a delimiter. In an embodiment, the delimiter size is the size of the length field (e.g., two bytes), plus the size of the length validation field (e.g., two bytes), plus the size of the sequence field (e.g., one byte), if it is included. In other embodiments, the absolute or relative sizes of the various delimiter fields can be different. 
     Validation is performed by determining if the length validation field correlates with the data in the length field. In an embodiment, the length validation field includes a checksum or CRC, which enables the receiver to determine whether the length data is corrupted or uncorrupted. 
     A determination is made, in block  612 , whether the delimiter-sized data segment includes what appears to be a valid delimiter. If so, then in block  614 , the receiver receives and stores an amount of SDU data with a length as indicated in the delimiter&#39;s length field, and the method proceeds to block  622 , which will be described later. 
     If the delimiter-sized segment does not include what appears to be a valid delimiter, then the receiver transitions to a delimiter search mode, indicated by blocks  616 ,  618 , and  620 . In this mode, the receiver determines whether the end of the payload may have been reached, in block  616 . In various embodiments, the end of the payload can be determined if no delimiter is detected within an amount of time, or if a known end point has been reached, or if the symbol energy drops below a threshold. If the end of the payload has been reached, the method ends. 
     If the end of the payload has not been reached, then the receiver receives and evaluates each subsequent delimiter-sized segment of data, in block  618 . Subsequent segments can be overlapping or sequential. 
     A determination is made, in block  620 , whether the next delimiter-sized segment of data appears to be a possible delimiter by validating what can be the length field with what can be the length validation field. If the delimiter-sized segment of data does not appear to be a possible delimiter, then the procedure iterates, all the while storing the received data as a potential SDU. When a possible delimiter is detected, the receiver discontinues the delimiter search mode. 
     A determination is made, in block  622 , whether the end of the consolidated payload has been reached. In an embodiment, the receiver knows that it has reached the end of the consolidated payload if it has received an amount of data that corresponds to the total length field provided in the PHY header. In another embodiment, the receiver knows that it has reached the end of the consolidated payload if it has received an amount of data indicated in the last delimiter as the length of the last SDU. In an embodiment, the receiver knows whether or not an SDU is the last SDU of the consolidated payload by evaluating the sequence field of the last SDU&#39;s delimiter. In other embodiments, either or both the total length field in the PHY header or the sequence field in the delimiter can be excluded, and another way of determining the end of the consolidated payload can be used. For example, the receiver may measure the symbol energy to determine whether the end of the consolidated payload has been reached. 
     If the end of the consolidated payload has not yet been reached, the procedure iterates as shown. Specifically, the receiver evaluates the next delimiter-sized data segment, in block  610 , and the procedure repeats. 
     If the end of the consolidated payload has been reached, the receiver delivers the various SDUs that it parsed from the consolidated payload, in block  624 , and the method ends. In another embodiment, the receiver can deliver each SDU while the SDU is being received, or in parallel with receiving other SDUs. 
     Embodiments described above in conjunction with  FIGS. 4-6  provide a high throughput method of burst-mode transmission with robust error detection and recovery. Throughput is improved from prior art methods by eliminating the IFS between SDUs, as well as by eliminating intervening preambles and PHY headers associated with SDUs that occur after the first SDU. 
     The length validation field enables robust error detection and recovery. First, the length validation field enables the receiver to determine whether the length field is corrupted in the delimiter. If the receiver determines that the length field is corrupted, the receiver can look at every byte that follows, to try to find a segment of data that appears to be a delimiter. If the receiver finds a segment of data that appears to be a delimiter, the receiver assumes that the data represents a delimiter, and the receiver re-synchronizes itself for receipt of the next SDU. 
     In an embodiment, the chance is very slim of the receiver finding a data segment that appears to be a delimiter, but is not. In an embodiment that includes a 2-byte CRC, the chance of incorrectly detecting a delimiter is approximately 1 in 65,000. Even if this occurs, the receiver will again detect an error when it does not find a valid delimiter at the end of the supposed SDU. And again, the receiver will search for a data segment that appears to be a delimiter. Therefore, even if a delimiter is corrupted, and another data segment coincidentally looks like a delimiter, the receiver eventually will recover when it finds a valid delimiter. Accordingly, this embodiment provides a robust method of error detection and recovery. 
     In another embodiment, the delimiter includes only a length field, and the length validation field is excluded. This embodiment works well when the channel is robust, and the delimiter&#39;s length field is highly unlikely to be corrupted. If the data in the length field is more likely to be corrupted, then the lack of a length validation field may make it more difficult for the receiver to recover from an error in the length validation field. The receiver may look for a next delimiter based on the corrupted length, and it would likely find only random data there, which would cause it to be even more difficult for the receiver to recover from the erroneous data. 
     In an embodiment described in conjunction with  FIGS. 6-8 , the delimiter is transmitted at the data modulation rate. Although this may increase the chance that the delimiter&#39;s length field might become corrupted, the delimiter&#39;s length validation field enables robust error detection and recovery. 
     In another embodiment, described in conjunction with  FIGS. 7-9 , the PPDU frame may include multiple SDUs, but the length of each SDU is included in the PHY header, and thus it is transmitted at the robust modulation rate. In this embodiment, the chance that the SDU length fields will be corrupted is less than it would be if the lengths were transmitted at the data modulation rate. 
       FIG. 7  illustrates an example of a timing diagram for transmitting a PPDU with multiple SDUs without intervening data in accordance with an embodiment of the inventive subject matter. The PPDU  700  includes a preamble  702 , PHY header  704 , and a consolidated payload with at least one SDU  706 ,  716 ,  726 . In the illustrated example, the consolidated payload includes three SDUs. More or fewer SDUs can be included in a single consolidated payload. 
     The preamble  702  includes a pattern of bits, which the receiver uses to synchronize itself, as described above. The PHY header  704  includes a rate field, which indicates which data modulation rate is used for the consolidated payload. The PHY header  704  could have a fixed or variable length. 
     Each SDU  706 ,  716 ,  726  must be separated and delivered intact by the PHY in the receiver. To facilitate decomposition of the consolidated payload into individual SDUs, the PHY header  704  also includes a length/size field associated with each SDU  706 ,  716 ,  726  included in the consolidated payload, in an embodiment. 
     Each length/size field indicates the length of its associated SDU, in an embodiment. In another embodiment, the length/size field defines the aggregate length of the associated SDU and any preceding SDUs within the consolidated payload. Thus, the length of SDU  706  would be represented as the length  730  of SDU  706 . The length of SDU  716  would be represented as the aggregate length  732  of SDU  706  and  716 . Finally, the length of SDU  726  would be represented as the aggregate length  734  of SDUs  706 ,  716 , and  726 . In various embodiments, the length/size field can include a number of microseconds, a number of bytes, or some other value indicating length. 
     The values in the length/size fields enable the receiver to determine where one SDU ends, and another begins. Accordingly, in yet another embodiment, the length/size field can instead include an “offset” value, which indicates the magnitude of an offset into the consolidated payload where the beginning of the next SDU occurs (or where the end of a previous SDU occurs). 
       FIG. 8  is a flowchart of a procedure for a transmitter to assemble and transmit a PPDU, such as that illustrated in  FIG. 7 , in accordance with an embodiment of the inventive subject matter. The method begins, in block  802  when the PHY device obtains at least one SDU. In one embodiment, the SDUs are intermediately or finally destined for the same receiver, although it is possible for the SDUs to have different destinations. 
     In block  804 , the lengths (or offsets) associated with each SDU are determined. The lengths can be the individual length of each SDU, or the aggregate length of each SDU within the consolidated payload. For example, in an embodiment, the SDU length is represented by two bytes. In other embodiments the length field can be larger or smaller. In still other embodiments, an offset value can be used to enable a determination of the end of one SDU and the beginning of a next SDU, rather than using a length value. 
     Each of the lengths or offsets is included in the PHY header. Accordingly, if the consolidated payload includes three SDUs, the PHY header would include at least three length fields. In one embodiment, the PHY header is fixed in size, which limits the number of SDUs that the PHY header may describe. In another embodiment, the PHY header has a variable size. In such an embodiment, the PHY header may include information that enables a determination of how many SDUs the PHY header describes and/or the length of the PHY header. 
     After negotiating access to the air interface, the transmitter transmits the preamble and the PHY header over the air at the robust modulation rate, in block  806 . In an embodiment, the transmitter begins transmitting each of the preamble and the PHY header at the beginning of a symbol boundary. In an embodiment, the preamble is transmitted for two symbols, and the PHY header is transmitted for one symbol. In other embodiments, either the preamble or the PHY header can be transmitted for longer or shorter time durations. 
     When transmission of the PHY header is complete, the transmitter switches to the data modulation rate, in block  808 . The transmitter begins transmitting the first SDU, in block  810 . In an embodiment, the transmitter begins transmitting the first SDU at the beginning of the next symbol boundary after completion of the PHY header. Alternatively, the first SDU can begin at a time other than a symbol boundary. In other words, transmission can begin before or after a symbol boundary. In an embodiment, the transmitter begins transmitting the consolidated payload within one symbol width of the end of the PHY header. Internal block padding may be included at the end of each SDU. 
     In block  812 , a determination is made whether more SDUs remain to be transmitted. If more SDUs remain to be transmitted, the transmitter begins transmitting the next SDU, in block  810 . In an embodiment, the transmitter begins transmitting the next SDU immediately upon completion of transmitting the previous SDU, whether or not that position within the payload occurs on a symbol boundary. Accordingly, in this embodiment, all of the data within the consolidated payload is efficiently concatenated together. In other embodiments, gaps or filler data can exist between subsequent SDUs. After transmitting the last SDU, the method ends. 
       FIG. 9  is a flowchart of a procedure for a receiver to receive and divide a PPDU, such as that illustrated in  FIG. 7 , in accordance with an embodiment of the inventive subject matter. The method begins, in block  902 , when the receiver detects an incoming preamble at the robust modulation rate. The receiver uses the preamble to become synchronized with the incoming PPDU frame, in block  904 . 
     In one embodiment, the receiver determines the modulation rate of the PPDU&#39;s consolidated payload from the PHY header, in block  906 . In an alternate embodiment, the data modulation rate may be determined in a prior training exchange. 
     In an embodiment, the receiver also determines the lengths or offsets associated with each SDU in the consolidated payload from the PHY header. This enables the receiver to know where the SDU boundaries occur, and how long it should demodulate incoming data at the data modulation rate. Once receipt of the PHY header has completed, the receiver switches to demodulating at the data modulation rate, in block  908 . 
     In block  910 , the receiver receives and stores an amount of SDU data with a length as indicated in the associated length field for the SDU in the PHY header. A determination is made, in block  912 , whether the end of the consolidated payload has been reached. In an embodiment, the receiver knows that it has reached the end of the consolidated payload if it has received an amount of data that corresponds to the length field for the last SDU provided in the PHY header, whether that length field indicates the length of the last SDU separately, or whether that length field indicates the aggregate length. In an alternate embodiment, the receiver may determine that the end of the payload has been reached using a measurement of symbol energy. 
     If the end of the consolidated payload has not yet been reached, the procedure iterates as shown. Specifically, the receiver receives and stores the next SDU, in block  910 , and the procedure repeats. 
     If the end of the consolidated payload has been reached, in block  914 , the receiver delivers the various SDUs that it parsed from the consolidated payload, and the method ends. In another embodiment, the receiver can deliver each SDU while the SDU is being received, or in parallel with receiving other SDUs. 
     In embodiments described in conjunction with  FIGS. 7-9 , no preambles or PHY headers are transmitted between SDUs. Accordingly, it is not necessary for the receiver to switch back and forth between the data modulation rate and the robust modulation rate while the receiver is receiving the consolidated payload. In another embodiment, illustrated in conjunction with  FIGS. 10-12 , a preamble and PHY header are transmitted for each SDU. However, the transmitter does not wait for the IFS before transmitting subsequent PPDU frames that are intermediately or finally destined for the same receiver. Instead, the transmitter begins transmitting the next PPDU frame at the next symbol boundary after completion of a previous frame. 
       FIG. 10  illustrates an example of a timing diagram for transmitting multiple PPDUs in accordance with an embodiment of the inventive subject matter. Each PPDU  1000 ,  1010 ,  1020  includes a preamble  1002 ,  1012 ,  1022 , PHY header  1004 ,  1014 ,  1024 , and an SDU  1006 ,  1016 ,  1026 . In the illustrated example, three concatenated PPDUs are shown. More or fewer PPDUs can be sent in accordance with embodiments described in conjunction with  FIGS. 10-12 . 
     Each preamble  1002 ,  1012 ,  1022  includes a pattern of bits, which the receiver uses to synchronize itself, as described above. Each PHY header  1004 ,  1014 ,  1024  includes a rate field, which indicates which data modulation rate is used for the payload. The data modulation rate may or may not be the same for each payload. In addition, each PHY header  1004 ,  1014 ,  1024  includes a length/size field for the SDU  1006 ,  1016 ,  1026  that follows it. The lengths of the payloads may or may not be the same. Each length/size field indicates the length of its associated SDU, in an embodiment. In various embodiments, the length/size field can include a number of microseconds, a number of bytes, or some other value indicating length. The PHY header  1004  could have a fixed or variable length. 
       FIG. 11  is a flowchart of a procedure for a transmitter to transmit a burst of multiple PPDUs, such as those illustrated in  FIG. 10 , in accordance with an embodiment of the inventive subject matter. The method begins, in block  1102  when the PHY device obtains at least one SDU. In one embodiment, the SDUs are intermediately or finally destined for the same receiver, although it is possible for the SDUs to have different destinations. 
     In block  1104 , the length associated with the next SDU to be transmitted is determined. For example, in an embodiment, the SDU length is represented by two bytes. In other embodiments the length field can be larger or smaller. The length is included in the PHY header for that SDU. 
     After negotiating access to the air interface, the transmitter transmits the preamble and the PHY header for the SDU over the air at the robust modulation rate, in block  1106 . In an embodiment, the transmitter begins transmitting each of the preamble and the PHY header at the beginning of a symbol boundary. In an embodiment, the preamble is transmitted for two symbols, and the PHY header is transmitted for one symbol. In other embodiments, either the preamble or the PHY header can be transmitted for longer or shorter time durations. 
     When transmission of the PHY header is complete, the transmitter switches to the data modulation rate, in block  1108 . The transmitter begins transmitting the first SDU, in block  1110 . In an embodiment, the transmitter begins transmitting the first SDU at the beginning of the next symbol boundary after completion of the PHY header. Alternatively, the first SDU can begin at a time other than a symbol boundary. In other words, transmission can begin before or after a symbol boundary. Although not illustrated in  FIG. 11 , the last symbol in which an SDU is transmitted can be only partially used. In such a case, a gap may exist between the end of the SDU and the beginning of the next symbol boundary. In addition, internal block padding may be included at the end of each SDU. 
     In block  1112 , a determination is made whether more SDUs remain to be transmitted. If more SDUs remain to be transmitted, then the procedure iterates as shown. Specifically, the transmitter prepares and transmits the next preamble, PHY header, and SDU. 
     In an embodiment, the transmitter begins transmitting the preamble for the next PPDU at the beginning of the next symbol boundary after completion of the previous SDU. Alternatively, the next PPDU can begin at a time other than a symbol boundary. In other words, transmission can begin before or after a symbol boundary. After transmitting the last SDU, the method ends. 
       FIG. 12  is a flowchart of a procedure for a receiver to receive a burst of multiple PPDUs, such as those illustrated in  FIG. 10 , in accordance with an embodiment of the inventive subject matter. The method begins, in block  1202 , when the receiver detects an incoming preamble at the robust modulation rate. The receiver uses the preamble to become synchronized with the incoming PPDU frame, in block  1204 . 
     In one embodiment, the receiver determines the modulation rate of the PPDU&#39;s payload from the PHY header, in block  1206 . In another embodiment, the data modulation rate may be determined during a prior training exchange. 
     The receiver also determines the length of the associated SDU from the PHY header, in one embodiment. Once receipt of the PHY header has completed, the receiver switches to demodulating at the data modulation rate, in block  1208 . 
     In block  1210 , the receiver receives and stores an amount of SDU data with a length as indicated in the associated length field for the SDU in the PHY header. In an alternate embodiment, the receiver may determine that the end of the payload has been reached using a measurement of symbol energy. 
     When the end of the SDU has been reached, in block  1212 , the receiver delivers the SDU. In another embodiment, the receiver can begin delivery of the SDU while the SDU is being received. The procedure then iterates as shown, by the receiver attempting to detect a preamble at the robust modulation rate, in block  1202 . 
     In embodiments described in conjunction with  FIGS. 10-12 , a full-length preamble and PHY header is transmitted between SDUs. In another embodiment, illustrated in conjunction with  FIGS. 13-15 , a full-length preamble is transmitted for the first PPDU frame, and partial preambles are transmitted for subsequent PPDU frames. 
       FIG. 13  illustrates an example of a timing diagram for transmitting a burst of multiple PPDUs with shortened intervening preambles in accordance with an embodiment of the inventive subject matter. The first PPDU  1300  includes a full-length preamble  1302 , and subsequent PPDUs  1310 ,  1320  in the burst include partial preambles  1312 ,  1322 , in an embodiment. 
     The full-length preamble  1302  includes a pattern of bits, which the receiver uses to synchronize itself. Specifically, the receiver may use the preamble  1302  to perform the following tasks: 1) packet start acquisition; 2) channel estimation; 3) antenna diversity and training; 4) receiver automatic gain control (AGC); 5) carrier offset; and 6) symbol timing. In an embodiment, all of these tasks, except for packet start acquisition, can be performed once at the start of the burst. For subsequent PPDUs after the first PPDU of the burst, a partial preamble  1312 ,  1322  is transmitted. The partial preamble  1312 ,  1322  may be used by the receiver to perform a packet start acquisition task. 
     In an embodiment, each PPDU  1300 ,  1310 ,  1320  also includes a PHY header  1304 ,  1314 ,  1324 , and an SDU  1306 ,  1316 ,  1326 . Each PHY header  1304 ,  1314 ,  1324  includes a rate field, in one embodiment, which indicates which data modulation rate is used for the payload. In another embodiment, the data modulation rate could be determined during a training exchange. 
     In addition, each PHY header  1304 ,  1314 ,  1324  includes a length/size field for the SDU  1306 ,  1316 ,  1326  that follows it. Each length/size field indicates the length of its associated SDU, in an embodiment. In various embodiments, the length/size field can include a number of microseconds, a number of bytes, or some other value indicating length. In the illustrated example, three concatenated PPDUs are shown. More or fewer PPDUs can be sent in accordance with embodiments described in conjunction with  FIGS. 13-15 . The PHY header  1304  could have a fixed or variable length. 
       FIG. 14  is a flowchart of a procedure for a transmitter to transmit a burst of multiple PPDUs, such as those illustrated in  FIG. 13 , in accordance with an embodiment of the inventive subject matter. The method begins, in block  1402  when the PHY device obtains at least one SDU. In one embodiment, the SDUs are intermediately or finally destined for the same receiver, although it is possible for the SDUs to have different destinations. 
     In block  1404 , the length associated with the next SDU to be transmitted is determined. For example, in an embodiment, the SDU length is represented by two bytes. In other embodiments the length field can be larger or smaller. The length is included in the PHY header for that SDU. 
     After negotiating access to the air interface, the transmitter transmits a full-length preamble and the PHY header for the SDU over the air at the robust modulation rate, in block  1406 . In an embodiment, the transmitter begins transmitting each of the full-length preamble and the PHY header at the beginning of a symbol boundary. In an embodiment, the full-length preamble is transmitted for two symbols, and the PHY header is transmitted for one symbol. In other embodiments, either the full-length preamble or the PHY header can be transmitted for longer or shorter time durations. 
     When transmission of the PHY header is complete, the transmitter switches to the data modulation rate, in block  1408 . The transmitter begins transmitting the first SDU, in block  1410 . In an embodiment, the transmitter begins transmitting the first SDU at the beginning of the next symbol boundary after completion of the PHY header. Alternatively, the first SDU can begin at a time other than a symbol boundary. In other words, transmission can begin before or after a symbol boundary. Although not illustrated in  FIG. 14 , the last symbol in which an SDU is transmitted can be only partially used. In such a case, a gap may exist between the end of the SDU and the beginning of the next symbol boundary. In addition, internal block padding may be included at the end of each SDU. 
     In block  1412 , the transmitter switches back to the robust modulation rate, so that it will be ready to transmit the next preamble. In block  1414 , a determination is made whether more SDUs remain to be transmitted. In another embodiment, this determination can be excluded. If more SDUs remain to be transmitted, the transmitter determines the length of the next SDU to be transmitted, in block  1416 . 
     The transmitter begins transmitting a partial preamble and the PHY header for the SDU over the air at the robust modulation rate, in block  1418 . In an embodiment, the partial preamble is transmitted for one symbol, and the PHY header is transmitted for one symbol. In other embodiments, either the partial preamble or the PHY header can be transmitted for longer or shorter time durations. In an embodiment, the transmitter begins transmitting the partial preamble for the next PPDU at the beginning of the next symbol boundary after completion of the previous SDU. Alternatively, the next PPDU can begin at a time other than a symbol boundary. In other words, transmission can begin before or after a symbol boundary. 
     After transmitting the partial preamble and PHY header, the method iterates as shown. Specifically, the transmitter switches back to the data modulation rate, and transmits the next SDU. After all SDUs in the burst have been transmitted, the method ends. 
       FIG. 15  is a flowchart of a procedure for a receiver to receive a burst of multiple PPDUs, such as those illustrated in  FIG. 13 , in accordance with an embodiment of the inventive subject matter. The method begins, in block  1502 , when the receiver detects an incoming, full-length preamble at the robust modulation rate. The receiver uses the preamble to become fully synchronized with the incoming PPDU frame, in block  1504 . As described previously, full synchronization includes the tasks of: 1) packet start acquisition; 2) channel estimation; 3) antenna diversity and training; 4) receiver automatic gain control (AGC); 5) carrier offset; and 6) symbol timing. In other embodiments, more, fewer or different tasks can be performed during a full synchronization. 
     In one embodiment, the receiver determines the modulation rate of the PPDU&#39;s payload from the PHY header, in block  1506 . In another embodiment, the data modulation rate may be determined during a prior training exchange. 
     The receiver also determines the length of the associated SDU from the PHY header, in one embodiment. Once receipt of the PHY header has completed, the receiver switches to demodulating at the data modulation rate, in block  1508 . 
     In block  1510 , the receiver receives and stores an amount of SDU data with a length as indicated in the associated length field for the SDU in the PHY header. In an alternate embodiment, the receiver may determine that the end of the payload has been reached using a measurement of symbol energy. 
     When the end of the SDU has been reached, the receiver switches back to the robust modulation rate, in block  1512 , so that it will be ready to receive the next incoming preamble. In addition, in block  1514 , the receiver delivers the SDU. In another embodiment, the receiver can begin delivery of the SDU while the SDU is being received. 
     After receipt of the first SDU is completed, the receiver determines whether a partial preamble is detected, in block  1516 . If no partial preamble is detected within an amount of time, the receiver can assume that the burst is complete, and the procedure ends. 
     If a partial preamble is detected, the receiver uses the partial preamble to perform a partial synchronization process, in block  1518 . In an embodiment, this involves performing at least the task of packet start acquisition. Because at least one of the other previously-performed synchronization tasks need not be repeated, the receiver will take significantly less time to synchronize with the upcoming PPDU, and the partial preamble can be significantly shorter than the full-length preamble. In other embodiments, the partial preamble can be used by the receiver to perform more or other tasks. After partially synchronizing itself, in block  1518 , the procedure iterates as shown. Specifically, the receiver receives and processes the PHY header and SDU for the PPDU associated with the partial preamble. 
     In embodiments described in conjunction with  FIGS. 13-15 , a partial preamble and a PHY header are transmitted between SDUs. In another embodiment, illustrated in conjunction with  FIGS. 16-18 , only a PHY header is transmitted between SDUs, and the intervening preambles are excluded. Although a preamble is helpful in performing the task of packet start acquisition, it is possible to acquire the start of a packet even after loss of structural information by entering a PHY header search mode, which will be described in more detail in conjunction with  FIG. 18 , in an embodiment. Accordingly, acceptable synchronization, error detection, and error recovery may be possible even without intervening preambles between SDUs. 
       FIG. 16  illustrates an example of a timing diagram for transmitting a burst of multiple PPDUs without intervening preambles in accordance with an embodiment of the inventive subject matter. The first PPDU  1600  includes a preamble  1602 . The preamble  1602  includes a pattern of bits, which the receiver uses to synchronize itself. Subsequent PPDUs  1610 ,  1620  in the burst do not include preambles, in an embodiment. 
     In an embodiment, each PPDU  1600 ,  1610 ,  1620  includes a PHY header  1604 ,  1614 ,  1624 , and an SDU  1606 ,  1616 ,  1626 . Each PHY header  1604 ,  1614 ,  1624  includes a rate field, in one embodiment, which indicates which data modulation rate is used for the payload. In another embodiment, the data modulation rate can be determined during a training exchange. 
     In addition, each PHY header  1604 ,  1614 ,  1624  includes a length/size field for the SDU  1606 ,  1616 ,  1626  that follows it. Each length/size field indicates the length of its associated SDU, in an embodiment. Accordingly, the length/size field enables the receiver to determine when the end of the SDU will occur, and to predict when the beginning of the next PHY header in the burst should occur. In various embodiments, the length/size field can include a number of microseconds, a number of bytes, or some other value indicating length. In the illustrated example, three concatenated PPDUs are shown. More or fewer PPDUs can be sent in accordance with embodiments described in conjunction with  FIGS. 16-18 . The PHY header  1604  could have a fixed or variable length. 
       FIG. 17  is a flowchart of a procedure for a transmitter to transmit a burst of multiple PPDUs, such as those illustrated in  FIG. 16 , in accordance with an embodiment of the inventive subject matter. The method begins, in block  1702  when the PHY device obtains at least one SDU. In one embodiment, the SDUs are intermediately or finally destined for the same receiver, although it is possible for the SDUs to have different destinations. 
     In block  1704 , the length associated with the first SDU to be transmitted is determined. For example, in an embodiment, the SDU length is represented by two bytes. In other embodiments the length field can be larger or smaller. The length is included in the PHY header for that SDU. 
     After negotiating access to the air interface, the transmitter transmits a preamble and the PHY header for the SDU over the air at the robust modulation rate, in block  1706 . In an embodiment, the transmitter begins transmitting each of the preamble and the PHY header at the beginning of a symbol boundary. In an embodiment, the preamble is transmitted for two symbols, and the PHY header is transmitted for one symbol. In other embodiments, either the preamble or the PHY header can be transmitted for longer or shorter time durations. 
     When transmission of the PHY header is complete, the transmitter switches to the data modulation rate, in block  1708 . The transmitter begins transmitting the first SDU, in block  1710 . In an embodiment, the transmitter begins transmitting the first SDU at the beginning of the next symbol boundary after completion of the PHY header. Alternatively, the first SDU can begin at a time other than a symbol boundary. In other words, transmission can begin before or after a symbol boundary. Although not illustrated in  FIG. 17 , the last symbol in which an SDU is transmitted can be only partially used. In such a case, a gap may exist between the end of the SDU and the beginning of the next symbol boundary. In addition, internal block padding may be included at the end of each SDU. 
     In block  1712 , the transmitter switches back to the robust modulation rate, so that it will be ready to transmit the next PHY header. In block  1714 , a determination is made whether more SDUs remain to be transmitted. In another embodiment, this determination can be excluded. If more SDUs remain to be transmitted, the transmitter determines the length of the next SDU to be transmitted, in block  1716 . 
     The transmitter begins transmitting the PHY header for the SDU over the air at the robust modulation rate, in block  1718 . In an embodiment, the PHY header is transmitted for one symbol. In other embodiments, the PHY header can be transmitted for longer or shorter time durations. In an embodiment, the transmitter begins transmitting the PHY header for the next PPDU at the beginning of the next symbol boundary after completion of the previous SDU. Alternatively, the next PPDU can begin at a time other than a symbol boundary. In other words, transmission can begin before or after a symbol boundary. 
     After transmitting the PHY header, the method iterates as shown. Specifically, the transmitter switches back to the data modulation rate, and transmits the next SDU. After all SDUs in the burst have been transmitted, the method ends. 
       FIG. 18  is a flowchart of a procedure for a receiver to receive a burst of multiple PPDUs, such as those illustrated in  FIG. 16 , in accordance with an embodiment of the inventive subject matter. The method begins, in block  1802 , when the receiver detects an incoming preamble at the robust modulation rate. The receiver uses the preamble to become synchronized with the incoming PPDU frame, in block  1804 . 
     At the next symbol boundary after the end of the preamble, the receiver should begin receiving a PHY header. Therefore, the receiver receives and attempts to validate a segment of data having the size of a PHY header. In an embodiment, the PHY header size is one symbol width. 
     Validation is performed by determining if a data integrity field within the PHY header correlates with the data in the PHY header. In an embodiment, the data integrity field includes a checksum or CRC, which enables the receiver to determine whether the data is corrupted or uncorrupted. 
     A determination is made, in block  1806 , whether the PHY header-sized data segment includes what appears to be a valid PHY header. If not, then the receiver transitions to a PHY header search mode, indicated by blocks  1808 ,  1810 , and  1812 . In this mode, the receiver determines whether the end of the burst may have been reached, in block  1808 . In various embodiments, the end of the burst can be determined if no PHY header is detected within an amount of time, or if a known end point has been reached, or if the symbol energy drops below a threshold. If the end of the burst has been reached, the method ends. 
     If the end of the burst has not been reached, then the receiver receives and evaluates each subsequent PHY header-sized segment of data, in block  1810 . Subsequent segments can be overlapping or sequential. 
     A determination is made, in block  1812 , whether the next PHY header-sized segment of data appears to be a possible PHY header by validating what can be the header data with what can be the header integrity field. If the PHY header-sized segment of data does not appear to be a possible PHY header, then the procedure iterates. When a possible PHY header is detected, the receiver discontinues the PHY header search mode. 
     When the PHY header search mode is exited, or when the next PHY header has been validated, the receiver determines the modulation rate of the PPDU&#39;s payload from the PHY header, in block  1814 , in one embodiment. In another embodiment, the data modulation rate may be determined during a prior training exchange. 
     The receiver also determines the length of the associated SDU from the PHY header, in one embodiment. Once receipt of the PHY header has completed, the receiver switches to demodulating at the data modulation rate, in block  1816 . 
     In block  1818 , the receiver receives and stores an amount of SDU data with a length as indicated in the associated length field for the SDU in the PHY header. In an alternate embodiment, the receiver may determine that the end of the payload has been reached using a measurement of symbol energy. 
     When the end of the SDU has been reached, the receiver switches back to the robust modulation rate, in block  1820 , so that it will be ready to receive the next incoming PHY header. In addition, in block  1822 , the receiver delivers the SDU. In another embodiment, the receiver can begin delivery of the SDU while the SDU is being received. 
     After receipt of the first SDU is completed, the receiver determines whether the end of the burst has occurred, in block  1824 . If no PHY header is detected within an amount of time, or if a known end point has been reached, or if the symbol energy drops below a threshold, the receiver can assume that the burst is complete, and the procedure ends. If a PHY header is detected, the procedure iterates as shown. Specifically, the receiver receives and processes the PHY header and SDU for the next PPDU. 
     An embodiment described above in conjunction with  FIGS. 16-18  provides a high throughput method of burst-mode transmission with robust error detection and recovery. Throughput is improved from prior art methods by eliminating the IFS and the preambles between SDUs. 
     The header integrity field of the PHY header enables robust error detection and recovery. If the receiver determines that PHY data is corrupted, which can indicate an out of sync condition, the receiver can look at every byte that follows, to try to find a segment of data that appears to be a PHY header. If the receiver finds a segment of data that appears to be a PHY header, the receiver assumes that the data represents a PHY header, and the receiver re-synchronizes itself for receipt of the next SDU. 
     Thus, various embodiments of a method, apparatus, and system have been described which enable higher throughput data burst transmissions. The foregoing description of specific embodiments reveals the general nature of the inventive subject matter sufficiently that others can, by applying current knowledge, readily modify and/or adapt it for various applications without departing from the generic concept. Therefore such adaptations and modifications are within the meaning and range of equivalents of the disclosed embodiments. The phraseology or terminology employed herein is for the purpose of description and not of limitation. Accordingly, the inventive subject matter embraces all such alternatives, modifications, equivalents and variations as fall within the spirit and broad scope of the appended claims. 
     The operations described above, with respect to the methods illustrated and described herein, can be performed in a different order from that disclosed. Also, it will be understood that, although some methods are described as having an “end,” they may be continuously performed. 
     Although embodiments, above, have been described in conjunction with an 802.11 Standard, embodiments can be implemented in conjunction with other standards that have fully or partially “self-describing” frames. In other words, embodiments are not meant to be limited to methods, systems, and devices that implement an 802.11 Standard. 
     The various procedures described herein can be implemented in hardware, firmware or software. A software implementation can use microcode, assembly language code, or a higher-level language code. The code may be stored on one or more volatile or non-volatile computer-readable media during execution or at other times. These computer-readable media may include hard disks, removable magnetic disks, removable optical disks, magnetic cassettes, flash memory cards, digital video disks, Bernoulli cartridges, random access memories (RAMs), read only memories (ROMs), and the like. 
     Embodiments of the inventive subject matter may pertain to any of a variety of types of PHY layers that support an IEEE Std 802.11 and other WLAN standards, including but not limited to, infrared (IR) baseband PHY, frequency hopping spread spectrum (FHSS) radios (e.g., in the 2.4 GHz band), direct sequence spread spectrum (DSSS) radios (e.g., in the 2.4 GHz band), orthogonal frequency domain multiplexing (OFDM) radios (e.g., in the UNIT bands), and other types of PHY layers for which IEEE Std 802.11 and other WLAN standards are being extended to now and in the future. Further, embodiments of the inventive subject matter can be used in conjunction with any IEEE Std 802.11, including IEEE Std 802.11-1997, 802.11a, 802.11b, 802.11e, other variants of the IEEE Std 802.11 existing or being developed now or in the future, and other WLAN standards besides IEEE Std 802.11. 
     In the claims, the terms “first modulation rate,” “second modulation rate,” and “third modulation rate” are used. It is to be understood that any of these modulation rates can be the same or different from one another.