Patent Publication Number: US-7586948-B2

Title: Packet sub-frame structure for selective acknowledgment

Description:
BACKGROUND OF THE INVENTION 
     1. Field of the Invention 
     The present invention relates to communication networks, and, more particularly, to a frame format for selective acknowledgment in a wireless communication network. 
     2. Description of the Related Art 
     Wireless local area networks (WLANs) include one or more non-fixed stations (or mobile terminals) such as cell phones, notebook (laptop) computers, and hand-held computers, equipped with generally available, WLAN PC cards that enable them to communicate among themselves as well as through a network server. The network server provides support for communication between mobile terminals in different service sets (service areas) which are associated with different access points (APs). An AP is a terminal or other device that provides connectivity to other networks or service areas, and may be either fixed or mobile. Such WLAN networks allow mobile terminals to be moved within a particular service area without regard to physical connections among the mobile terminals within that service area. An example of a WLAN network is a network that conforms to standards developed and proposed by the Institute of Electrical and Electronic Engineers (IEEE) 802.11 Committee (termed herein as a network operating in accordance with one or more editions of the IEEE 802.11 standard). Typically, all messages transmitted among the mobile terminals of the same cell (i.e., those terminals associated with the same AP) in such WLAN networks are transmitted to the access point (AP) rather than being directly transmitted between the mobile terminals. Such centralized wireless communication provides significant advantages in terms of simplicity of the communication link as well as in power savings. 
     Most networks are organized as a series of layers, each one built upon its predecessor. The purpose of each layer is to offer services to the higher layers, shielding those layers from implementation details of lower layers. Between each pair of adjacent layers is an interface that defines those services. The International Standards Organization has developed a layered network architecture called the Open Systems Interconnection (OSI) Reference model that has seven protocol layers: application, presentation, session, transport, network, data link, and physical. The function of the lowest level, the physical layer, is to transfer bits over a communication medium. The function of the data link layer is to partition input data into data frames and transmit the frames over the physical layer sequentially. Each data frame includes a header that contains control and sequence information for the frames. 
     The interface between the data link layer and the physical layer includes a medium access control (MAC) device and a physical layer signaling control device, called a PHY device. The purpose of the MAC device and the PHY device is to ensure two network stations are communicating with the correct frame format and protocol. Not all communication networks require all layers of the OSI model. For example, in a wireless communication networks, physical, network, and application layers are typically sufficient to enable operation. 
     In WLANs, the physical device is a radio and the physical communication medium is free space. The IEEE 802.11 standard for WLANs defines the communication protocol between a MAC device and a PHY device. According to the WLAN data communication protocol, each data frame transferred between the MAC device and the PHY device has a PHY header, a MAC header, MAC data, and error checking fields. The PHY header includes a preamble that is used to indicate the presence of a signal, unique words, frame length, etc. The MAC header includes frame control, duration, source (i.e., MAC) and destination address, and data sequence number.  FIG. 1  shows a typical frame format  100  for 802.11 wireless LAN systems. Frame format  100  comprises packet header  101 , payload data  102  and frame check sequence (FCS)  103 . 
     The maximum achievable throughput of 802.11 wireless LAN systems largely depends on the length of the frames that carry the data information (payload). With relatively good channel quality, the throughput efficiency increases when a larger frame size is used. This increase in throughput efficiency is mainly due to the large, fixed-size of the MAC overhead, such as: Inter Frame Space (IFS), preamble and Physical Layer Convergence Protocol (PLCP) header, MAC header, and FCS. 
     Section 7.6 of the 802.11 version “e” draft D3.0 specifies MAC-Level Forward Error Correction (FEC) and FEC frame formats in which frames are also divided in blocks. The MAC header and each block have their own FEC fields enabling detection and correction of errors in the separate blocks. However, the frame is dropped if one or more of the blocks are corrupted. 
     SUMMARY OF THE INVENTION 
     The present invention relates to transmission of data packets between endpoints in which packet frames are divided into sub-frames. On reception of a frame, the integrity of each individual sub-frame is checked. Sub-frames that pass the integrity check are acknowledged and retransmission is requested only for sub-frames that failed the integrity check. Consequently, only the affected sub-frames require retransmission instead of the whole frame. 
     In accordance with a first exemplary embodiment of the present invention, a packet frame for a data payload is generated by (a) dividing the data payload into one or more sub-frame payloads; (b) appending, to each of the sub-frame payloads, a corresponding sub-frame sequence number and a corresponding sub-frame integrity check value to generate a sub-frame; (c) arranging the sub-frames into a sub-frame sequence; (d) generating a header integrity check value for a header; and (e) appending the header integrity check value to the sub-frame sequence to form a frame. 
     In accordance with a second exemplary embodiment of the present invention, a transmitter sends data by (a) forming a packet frame from payload data, the packet frame including one or more sub-frames and each sub-frame comprising sub-frame payload data and a sub-frame sequence number; (b) transmitting the packet frame to a receiver; (c) receiving an acknowledgment message from the receiver, the acknowledgment message identifying each sub-frame of the packet frame received by the receiver; (d) determining each sub-frame of the packet frame not received by the receiver; and (e) re-transmitting each sub-frame not received by the receiver to the receiver. 
     In accordance with a third exemplary embodiment of the present invention, a receiver processes a packet frame into payload data by (a) receiving the packet frame, wherein 1) the packet frame comprises a header, a header integrity check value, a packet frame integrity check value, and one or more sub-frames, and 2) each sub-frame comprises a sub-frame integrity check value and a sub-frame sequence number; (b) verifying the integrity of the received packet frame based on the packet frame integrity check value; (c) verifying, if the integrity of the received packet frame is not verified, the integrity of the header based on the header integrity check value; and (d) verifying, if the integrity of the header of the received packet frame is verified, the integrity of each sub-frame based on the corresponding sub-frame integrity check value. 
    
    
     
       BRIEF DESCRIPTION OF THE DRAWINGS 
       Other aspects, features, and advantages of the present invention will become more fully apparent from the following detailed description, the appended claims, and the accompanying drawings in which: 
         FIG. 1  shows a typical prior art frame format for 802.11 wireless LAN systems; 
         FIG. 2  is a graph of throughput efficiency versus payload data length for a system under ideal channel conditions; 
         FIG. 3  is a graph of packet error rate versus payload data length with various bit error rate (BER) values; 
         FIG. 4  shows a graph of efficiency versus payload data length where both MAC overhead and BER are taken into account; 
         FIG. 5  shows a frame format in accordance with an exemplary embodiment of the present invention in which a frame is divided into sub-frames; 
         FIG. 6  shows a graph of efficiency versus payload data length where both MAC overhead and BER are taken into account and employing the frame format of  FIG. 5 ; 
         FIG. 7  shows an exemplary method employed by a sender using the exemplary format of  FIG. 5 ; and 
         FIG. 8  shows an exemplary method employed by a receiver using the exemplary format of  FIG. 5 . 
     
    
    
     DETAILED DESCRIPTION 
     As an aid to understanding the present invention,  FIG. 2  is a graph of throughput efficiency versus payload data length, where the points of the graph are generated by simulation for an 802.11 wireless LAN system using orthogonal frequency division multiplex (OFDM) modulation and a transmit rate of 24 Mbps with ideal channel conditions. The efficiency indicates which percentage of the active medium time is actually used to transmit payload data. The simulation employed for generating the graph of  FIG. 2  accounts for the fixed-size MAC overhead, but does not include contention, polling, and acknowledgment overhead. 
     In practice, the channel conditions are not ideal and channel errors occur. When a bit in a frame (packet) is corrupted, the FCS check at the receiver fails and the frame is discarded. The Packet Error Rate (PER) should be low to achieve a high net throughput, so, for a low PER at a given Bit Error Rate (BER), frames should include a relatively short payload data length. Assuming bit errors are randomly distributed,  FIG. 3  is a graph of PER versus payload data length for various BER values. 
     To overcome inefficiency from the fixed-size MAC overhead, frame payload data length should be large, but to maintain a low PER at a given BER, frame payload data length should be small.  FIG. 4  shows the resulting graph of efficiency versus payload data length where both MAC overhead and BER are taken into account.  FIG. 4  shows that the efficiency for, for example, a BER of 10 −4  increases as expected when the payload data length is increased up to about 300 bytes. Above this length, the PER dominates and the efficiency drops. 
     In accordance with embodiments of the present invention, a frame format allows for variation of frame payload data length, and allows for selective acknowledgment of portions of the data that are received. Such frame format allows for increased efficiency due to fixed size MAC overhead and maintains a low PER for a given set of channel conditions. 
       FIG. 5  shows a frame format  500  in accordance with an exemplary embodiment of the present invention comprising header  501 , header integrity check (HIC)  502 , frame body  503  having sub-frames  504 ( i ) (i=1,2,3), and frame check sequence (FCS)  505 . Header  501  is shown as a MAC layer header and might conform to a typical MAC layer protocol for an 802.11 standard. FCS  505  is a checksum value computed for the entire frame format  500 , and HIC  502  is a checksum value computed for header  501 . 
     In  FIG. 5 , frame body  503  is divided into sub-frames  504 ( 1 ),  504 ( 2 ), and  504 ( 3 ), but the present invention is not so limited and the number of sub-frames may vary depending on specific design and channel conditions. To enable an acknowledgment and retransmission method in accordance with the present invention, each sub-frame  504 ( i ) has its own sub-frame sequence number (SFSN)  506 ( i ) and sub-frame check sequence (SFCS)  507 ( i ). SFSN  506 ( i ) is a sequence number pre-pended to and associated with corresponding sub-frame payload data block  508 ( i ) (e.g., user data), while SFCS  507 ( i ) is a checksum value computed for sub-frame payload  508 ( i ). Sub-frames are preferably numbered independently of the sequence field in header  501 , since frames might include some re-transmitted sub-frames as well as new sub-frames. SFCS  507 ( i ) might be computed including SFSN  506 ( i ). 
     When a receiver examines a packet having frame format  500 , the checksum for the packet is computed and compared to FCS  505 . If the computed packet checksum and FCS  505  match, the packet contains no errors and may be processed. If the computed packet checksum and FCS  505  do not match, the packet contains at least one error. To enable the verification of the integrity of (e.g., MAC) header  501  in case of a failed comparison, a checksum for header  501  is computed and compared to HIC  502 . If the comparison of the checksum for header  501  and HIC  502  indicates that header  501  is corrupted, the receiver will drop the entire packet. 
     If the comparison of the checksum for header  501  and HC  502  indicates that header  501  is valid, at least one of the sub-frames has an error. In addition, the FCS may be corrupted, but in this case the header information and sub-frame information will be verified. For each sub-frame  504 ( i ), a checksum is calculated and compared with corresponding SFCS  507 ( i ). For those sub-frames in which the comparison passes, the sub-frame is valid and the corresponding payload data block is passed on for processing. For those sub-frames in which the comparison fails, the sub-frame is corrupted, the receiver will not acknowledge receipt of a correct sub-frame, and the receiver causes the sender to re-transmit each sub-frame having a failed comparison. 
     Frame format  500  enables a receiver to selectively acknowledge reception of sub-frames, thus selectively informing the sender of correct sub-frames that have been received. Instead of re-transmitting the whole frame, the sender might then retransmit only the missing sub-frames.  FIG. 6  shows a graph of efficiency versus payload data length where both MAC overhead and BER are taken into account and employing the frame format of  FIG. 5 . The graph of  FIG. 6  might be generated using an HIC field length of 2 bytes, an SFSN field length of 2 bytes, an SFCS field length of 2 bytes, and a sub-frame payload data length of 100 bytes. When comparing the graph of  FIG. 6  with the graph of  FIG. 4 , employing a frame format in accordance with an exemplary embodiment of the present invention significantly increases the efficiency at bit error rates (BERs) higher than or equal to 10 5 . 
     An exemplary implementation as might be employed with wireless LANs operating in accordance with an 802.11 standard is now described. The 802.11 standard specifies the MAC layer protocol as carrier sense multiple access with collision avoidance (CSMA/CA) with random backoff. Optionally, request-to-send (RTS) and clear-to-send (CTS) messages exchanged between senders and receivers might also be employed. The exemplary implementation exhibits relatively small changes to MAC layer assembly/disassembly of frames, since selective acknowledgment/retransmission and re-ordering of sub-frames is accomplished at the application layer running on the host above the MAC layer. 
       FIG. 7  shows an exemplary method employed by a sender using the exemplary format of  FIG. 5 . At step  701 , on availability of data to transmit, the application layer of the sender generates sub-frame payload data. If needed, the application layer divides and/or concatenates payload data to fit the sub-frame size. For preferred embodiments of the present invention, both the sender and the receiver agree on sub-frame size, which might be equal to a multimedia format&#39;s packet size. 
     At step  702 , the application layer generates each sub-frame by calculating and pre-pending/appending each corresponding SFSN and SFCS to the sub-frame payload. At step  703 , each sub-frame is stored in a buffer until its receipt is acknowledged or a (per sub-frame) retry limit is exceeded. 
     At step  704 , the application layer calculates the HIC based on MAC header information known by the application. Some of the fields of the MAC header (such as Sequence Number) are based on the local state of the MAC layer and are not known by the application. Consequently, some embodiments of the present invention might generate or re-generate the HIC at the MAC layer prior to transmission. At step  705 , the frame body is constructed from the sub-frames and passed to the MAC for transmission. 
     At step  706 , the MAC layer calculates the FCS of the whole frame and pre-pends/appends the FCS to the frame. At step  707 , the frame is sent using the “No Acknowledgment” policy as defined in, for example, the 802.11e draft 4.0. 
     The 802.11e draft 4.0 specifies a quality of service (QoS) data frame format that includes a QoS control field. The QoS control field includes a subfield that signals the acknowledgment (Ack) policy. One Ack policy is the “No Acknowledgment” (No Ack) policy. When a transmitter sends a QoS data frame using the No Ack policy, the addressed recipient will not respond with an ACK frame upon receipt of the frame. The transmitter assumes that the frame has been received successfully without regard of the actual result. 
     Returning to  FIG. 7 , at step  708 , the sender receives an acknowledgment from the receiver of those sub-frames that were received correctly. The acknowledgment may comprise a list of sub-frame sequence numbers correctly received, or may comprise a start sub-frame sequence number followed by a bitmap in which each bit corresponds to a sub-frame relative to the start number (e.g., this acknowledgment format may be employed by the Block Ack mechanism described in a 802.11e draft 6.0). 
     At step  709 , the sender retransmits, if necessary, those sub-frames that were not received by the receiver. In practice, steps  708  and  709  may occur repeatedly until all sub-frame data is acknowledged as being received by the receiver. Then the transmitter clears the sub-frame(s) from the buffers. Many re-transmission schemes are known in the art, and for some re-transmission schemes dedicated packets containing re-transmitted sub-frames might be employed. However, preferred embodiments of the present invention employ a frame format in which both new and re-transmitted sub-frames are included. Thus, the receipt of re-transmitted frames along with new frames might be similar to that described for steps  701 - 708 . 
     To enable a receiver&#39;s application layer to receive data frames that have an invalid FCS comparison, but might have useful sub-frames, the MAC layer of the receiver allows for the following in its receive decision. The frame is not discarded when i) a quality of service (QoS) data frame using “No Ack” policy is received, ii) the Address 1 field matches the receiver&#39;s own address, and iii) the FCS is incorrect. Instead, the frame is passed from the MAC layer to the host with an indication that there is an FCS error. 
       FIG. 8  shows an exemplary method employed by a receiver using the exemplary format of  FIG. 5 . At step  801 , on reception of a frame, a test of the receiver&#39;s application layer verifies the integrity of the MAC header of the frame using the HIC field. If the test of step  801  fails, indicating a corrupted MAC header, at step  802 , the receiver discards the whole frame. 
     If the test of step  801  passes, indicating a valid MAC header, at step  803  the receiver verifies the integrity of each sub-frame using the corresponding SFCS field. At step  804 , each sub-frame that fails its integrity check is discarded. At step  805 , the SFSN and payload data block of each correct sub-frame is stored in a re-ordering buffer. At step  806 , the receiver sends an acknowledgment to the sender indicating those sub-frames that are correctly received. At step  807 , the receiver receives those sub-frames from the sender in supplemental packets that were previously corrupted. Such supplemental packets might also conform to frame format  500  ( FIG. 5 ), and such supplemental packets might be generated and received as described herein with respect to  FIGS. 7 and 8 . 
     For steps  806  and  807 , acknowledgment and re-transmission might be repeated.(shown by the connecting arrow) and might be accomplished using various methods known in the art. For example, in response to every received frame, the receiver sends a normal directed data frame to the sender containing the sequence numbers of the sub-frames that were received correctly. Alternatively, methods providing selective acknowledgment/retransmission of whole frames, such as Block Acknowledgment as defined in the 802.11e draft version 4.0 or TCP Selective Acknowledgment, might be employed. 
     Returning to  FIG. 8 , at step  808 , the sub-frame payload data blocks of the sub-frames in the re-ordering buffer are arranged in order based on sub-frame sequence number. At step  809 , if needed, sub-frames are concatenated and/or divided back to generate reassembled data of the original packet data size and then the reassembled data is provided. 
     The described exemplary embodiments may be preferred when implementing the present invention in existing networks, since few changes are employed to the MAC layer mechanisms, with most changes between the MAC data service interface and higher layers (e.g., application layer). Other embodiments might be implemented with sub-framing and selective acknowledgment operations within the MAC layer entirely. 
     For the described embodiments herein, the checksum calculations employed to verify integrity might use a cyclic redundancy check (CRC) for the MAC header and sub-frames. Instead of CRC verification, Forward Error Correction (FEC) might also be employed for the RIC and the SFCSs. In addition, the number of erroneous sub-frames in a frame provides a measure of channel quality and might be employed for other applications, such as automatic rate selection and/or antenna diversity mechanisms. 
     While the exemplary embodiment of the present invention is described for WLANs operating in accordance with an edition of the IEEE 802.11 standard, the present invention is not so limited. One skilled in the art may extent the teachings herein to other types of local area networks having a unit within a network of fixed communication points. 
     The present invention can be embodied in the form of methods and apparatuses for practicing those methods. The present invention can also be embodied in the form of program code embodied in tangible media, such as floppy diskettes, CD-ROMs, hard drives, or any other machine-readable storage medium, wherein, when the program code is loaded into and executed by a machine, such as a computer, the machine becomes an apparatus for practicing the invention. The present invention can also be embodied in the form of program code, for example, whether stored in a storage medium, loaded into and/or executed by a machine, or transmitted over some transmission medium, such as over electrical wiring or cabling, through fiber optics, or via electromagnetic radiation, wherein, when the program code is loaded into and executed by a machine, such as a computer, the machine becomes an apparatus for practicing the invention. When implemented on a general-purpose processor, the program code segments combine with the processor to provide a unique device that operates analogously to specific logic circuits. 
     It will be further understood that various changes in the details, materials, and arrangements of the parts which have been described and illustrated in order to explain the nature of this invention may be made by those skilled in the art without departing from the principle and scope of the invention as expressed in the following claims.