Abstract:
In a NAK-based window transmission protocol where the transmitter retains transmitted data frames in a retransmission queue, one or more transmitted data frames are discarded as the number of data frames approaches the available queue capacity. A message, such as a control frame, is transmitted to the receiver indicating which of the retained data frame copies were discarded. The queue stall avoidance mechanism operates at the transmitter, which has direct knowledge of its queue utilization. By avoiding a queue stall, the transmitter may continue to receive incoming data packets for transmission to a receiver. That is, the transmitter can be configured to prioritize sending new data over retaining previously transmitted data for support of data retransmissions.

Description:
RELATED APPLICATIONS  
       [0001]     This application claims priority to Provisional U.S. Patent Application 60/542,731 filed Feb. 6, 2004, which is incorporated herein by reference. 
     
    
     BACKGROUND  
       [0002]     The present invention relates generally to the field of wireless communication systems and in particular to a system and method for transmitting and receiving data frames in a NAK-based window protocol.  
         [0003]     Modern communications systems, including wireless systems, use a layered architecture, with separate protocols for performing defined functions at each layer. The layered protocol approach allows upper level logical operations to be implemented without regard to the lower level physical operations of transmission and reception, error correction and the like.  
         [0004]     The IS-2000 CDMA wireless communications standard defines a protocol known as the Radio Link Protocol (RLP) for implementing physical layer communications between a Radio Access Network and a Mobile Station. RLP is unaware of higher layer framing; it operates on a featureless octet stream, delivering the octets in the order received. RLP has procedures to substantially reduce the error rate exhibited by CDMA traffic channels. There is no direct relationship between higher layer packets and the RLP data frames; a large packet may span multiple data frames, or a single RLP data frame may contain all or part of several small higher layer packets.  
         [0005]     RLP is a NAK-based window protocol. That is, the receiver does not acknowledge each received data frame. Rather, the receiver receives and processes data frame in order (according to sequence numbers associated with each data frame), sending a Negative AcKnowledgement (NAK) when a data frame is “missed”—either not received or received with unrecoverable errors. The receiver may send a NAK with the sequence number of the missed data frame when one or more data frames are received that are not in contiguous sequential order to previously received data frames. The NAK is a request to re-transmit the missing data frame. However, the NAK need not be sent immediately; the receiver may continue to receive data frames ahead of the missed frame, hoping to receive the missed frame in an un-requested retransmission. The receiver may also optionally occasionally transmit a “fill” frame, indicating all data frames up to a particular sequence number have been correctly received.  
         [0006]     To comply with the NAK protocol, the transmitter must retain a copy of each transmitted data frame in a retransmission queue, against the possibility of receiving a NAK request. Once a NAK is received, the transmitter knows that the receiver has correctly received all data frames previous to the data frame identified in the NAK request. Thus, only upon receiving a NAK or fill frame can the transmitter discard copies of previously transmitted data frames, freeing space in its retransmission queue. For any fixed size retransmission queue—and in particular for the fixed queue size imposed by the RLP protocol, as discussed herein—if the receiver properly receives all transmitted data frames, or does not promptly NAK a missing frame, and does not transmit a fill frame, the transmitter may completely fill the retransmission queue with copies of transmitted data frames. Once this occurs the transmitter must stall, and cannot accept new data frames for transmission to the receiver.  
       SUMMARY  
       [0007]     In one aspect, the present invention relates to a method of preventing transmitter queue stalls in a transmitter. Data frames are successively transmitted to a receiver, each data frame identified by a sequence number. A copy of each transmitted data frame is retained in a queue for possible retransmission, until receiving a message, such as a control frame, from the receiver indicating the data frame was correctly received, or until a maximum time for retransmission is exceeded. One or more the retained data frame copies are discarded responsive to the number of retained data frame copies approaching the available queue capacity, and a message, such as a control frame, is transmitted to the receiver indicating which of the retained data frame copies were discarded.  
         [0008]     In another aspect, the present invention relates to a transmitter. The transmitter includes an input queue for receiving new data to be transmitted to a receiver and circuitry for transmitting data frames to the receiver. The transmitter also includes memory, such as a re-transmit queue, for storing copies of transmitted data frames not acknowledged as received by the receiver, and a controller operative to discard one or more the copies of transmitted data frames when the copies approach an available capacity of the memory. This method of operation permits the transmitter to send new data instead of waiting for re-transmit queue availability.  
         [0009]     In yet another aspect, the present invention relates to a method of receiving frames transmitted by a transmitter and delivering the frames to a higher protocol layer. Data frames are received from a transmitter, each frame identified by a sequence number. Received, sequential data frames are delivered to the higher protocol layer. Received, non-sequential data frames are stored in an out-of-order queue. Upon receiving a message from the transmitter, such as a control frame, including a flush sequence number and indicating that one or more transmitted frame copies retained by the transmitter for possible retransmission were discarded, all received data frames up to the flush sequence number are delivered to the higher protocol layer and the beginning of the out-of-order queue is reset to the flush sequence number.  
         [0010]     In still another aspect, the present invention relates to a receiver. The receiver includes a receiver for receiving data frames from a transmitter, each the data frame having a sequence number. The receiver also includes an output for delivering received data frames to a higher protocol layer, and memory for storing received, non-sequential data frames prior to delivery to the higher protocol layer, pending the receipt of intervening sequential data frames. In addition, the receiver includes a controller operative to deliver the non-sequential data frames to the higher protocol layer responsive to detecting that memory used for storing non-sequential data frames is at or near a capacity limit. The controller is further operative to deliver non-sequential data frames to higher protocol layers upon receipt of a message from the transmitter that copies of the intervening sequential data frames retained by the transmitter for possible retransmission have been discarded. 
     
    
     BRIEF DESCRIPTION OF DRAWINGS  
       [0011]      FIG. 1  is a functional block diagram of a wireless communication system.  
         [0012]      FIG. 2  is protocol layer diagram for the U m  reference point.  
         [0013]      FIG. 3  is a functional diagram of the RLP transmitter and receiver buffers. 
     
    
     DETAILED DESCRIPTION  
       [0014]      FIG. 1  illustrates an exemplary wireless communication network generally referred to by the numeral  10 . In an exemplary embodiment, network  10  is based on cdma2000, 1xEV-DO/DV standards as promulgated by the Telecommunications Industry Association (TIA), although the present invention is not limited to such implementations. Here, network  10  communicatively couples one or more mobile stations (MSs)  12  to the Public Switched Telephone Network (PSTN)  14 , the Integrated Data Services Network (ISDN)  16 , and/or a Public Data Network (PDN)  18 , such as the Internet. In support of this functionality, the network  10  comprises a Radio Access Network (RAN)  20  connected to a Packet Core Network (PCN)  22  and an IS-41 network  24  across an A interface.  
         [0015]     The RAN  20  typically comprises one or more Base Station Controllers (BSCs)  26 , each including one or more controllers  28  or other processing systems, with associated memory  30  for storing necessary data and parameters relating to ongoing communications activity. Generally, each BSC  26  is associated with one or more Base Stations (BSs)  32 . Each BS  32  comprises one or more controllers  34 , or other processing systems, and assorted transceiver resources  36  supporting radio communication with MSs  12 , such as modulators/demodulators, baseband processors, radio frequency (RF) power amplifiers, antennas, etc.  
         [0016]     BSs  32  may be referred to as Base Transceiver Systems (BTSs) or Radio Base Stations (RBSs). In operation, BSs  32  transmit control and traffic data to MSs  12 , and receive control and traffic data from them over the U m  interface. BSC  26  provides coordinated control of the various BSs  32 . The BSC  26  also communicatively couples the RAN  20  to the PCN  22 .  
         [0017]     The PCN  22  comprises a Packet Data Serving Node (PDSN)  38  that includes one or more controllers  40 , or other processing systems, a Home Agent (HA)  42 , and an Authentication, Authorization, and Accounting (AAA) server  44 . Typically, the PCN  22  couples to the PDN  18  through a managed IP network  46 , which operates under the control of the network  10 . The PDSN  38  operates as a connection point between the RAN  16  and the PDN  18  by establishing, maintaining and terminating Point-to-Point Protocol (PPP) links, and further provides Foreign Agent (FA) functionality for registration and service of network visitors. HA  42  operates in conjunction with PDSN  38  to authenticate Mobile IP registrations and to maintain current location information in support of packet tunneling and other traffic redirection activities. Finally, AAA server  44  provides support for user authentication and authorization, as well as accounting services.  
         [0018]     The BSC  26  also communicatively couples the RAN  20  to the IS-41 network  24 . The IS-41 network  24  includes a Mobile Switching Center (MSC)  48  accessing a Home Location Register (HLR)  50  and Visitor Location Register (VLR)  52  for subscriber location and profile information. The MSC  48  establishes circuit-switched and packet-switched communications between the RAN  20  and the PSTN  16  and ISDN  16 .  
         [0019]     The protocol layer structure of the U m  interface—defining communications between the RAN  20  and the MSs  12 —is depicted in  FIG. 2 . At the upper layers, the PCN  22  provides Packet Data Service, and the IS-41 network  24  provides Voice Service and Circuit Switched Data, as described above. The IS-2000 Layer 3, also called the Signaling Layer, comprises base station and mobile station interoperability procedures and associated signaling that allow the end user to receive service.  
         [0020]     The Link Access Control (LAC) layer provides transport services over logical channels for Layer 3 signaling. The logical channels may be dedicated or common. Dedicated channels are mapped to physical channels assigned to a single user, such as for transmitting voice communications. Common channels are mapped to physical channels shared by multiple users, and may carry system overhead traffic, paging messages, and the like. The LAC encapsulates Layer 3 messages into LAC Protocol Data Units (PDU) that are subject to fragmentation and reassembly to make them suitable for transport by the lower layers.  
         [0021]     The Media Access Control (MAC) layer maps the logical channels to physical channels and coordinates the use of physical resources. The MAC also enforces the negotiated Quality of Service (QoS) level. The MAC implements a multiplexing and demultiplexing function to allow access to the medium to both PDUs received from the LAC and data units received from user applications. The MAC ensures reliable transmission of data—nearly error-free and in order—through the Radio Link Protocol (RLP) and other protocols. The RLP is a specialized form of selective-repeat Automatic Repeat Request (ARQ) protocol, defined in the IS-707 standard, which is incorporated by reference herein in its entirety. While not explicitly handled by the MAC layer, the RLP is functionally and conceptually part of the MAC layer. As depicted in  FIG. 2 , the RLP  60  receives data from the PDSN  38 , storing the data in a buffer or queue  66 . The RLP communicates by data and control frames to a peer RLP  70  in a mobile station  12 .  
         [0022]     The RLP is a negative acknowledgement (NAK)-based ARQ protocol, described above. RLP operation—data transmission and retransmission—proceeds according to parameters that are negotiated when the service is connected (e.g., at call establishment). RLP parameters are contained in a data structure known as the RLP Block of Bits (RLP-BLOB). These parameters include the number of NAKs per round, the number of rounds, and optionally an estimated Round Trip Time (RTT), or the round-trip delay between the Base Station  32  and the Mobile Station  12 . For Forward Packet Data Channel (F-PDCH) operation, the RLP-BLOB may additionally include a DELAY_DETECTION_WINDOW (DDW) parameter. The RTT is determined by a SYNC procedure between the BS  32  and the MS  12  at the outset of the service connection.  
         [0023]     Once the RLP parameters are initialized, data transfer may proceed. The RLP data frame sequence space comprises a 12-bit value, for 4096 sequence numbers. Each data frame is identified by a sequence number, and transmission and reception of data frames is tracked by pointers to queues maintained in the RLP transmitter and receiver. The queues are conceptually configured as circular buffers, as depicted in  FIG. 3 . The four pointers have the following meanings:  
         [0024]     L_V(S): 12-bit sequence number of the next data frame to be transmitted.  
         [0025]     L_V(R): 12-bit sequence number of the next expected new data frame.  
         [0026]     L_V(N): 12-bit sequence number of the next data frame needed for sequential delivery.  
         [0027]     L_V(N) PEER : An estimate of the peer RLP&#39;s L_V(N).  
         [0028]     The transmitter advances L_V(S) as it transmits data frames to the receiver. Upon receipt of a new data frame, if the received data frame is the next sequential data frame (by sequence number) to those previously received in sequential order, the receiver advances L_V(N). If the new data frame falls between L_V(N) and L_V(R), the data frame is stored in the receiver queue and no pointer is updated. If the new data frame is beyond the most advanced data frame received (by sequence number), the receiver advances L_V(R) to the sequence number of the received data frame. If a received data frame has a sequence number less than L_V(N) (as defined below), the receiver interprets it as a retransmission of an already properly received, sequential data frame, and discards it.  
         [0029]     The receiver maintains a pointer L_V(N) PEER , which is the transmitter&#39;s best estimate of the value of the pointer L_V(N) at the receiver. The data frames between L_V(N) PEER  and L_V(S) comprise the retransmission queue. This queue stores copies of previously transmitted data frames, against the possibility of receiving a NAK request to retransmit one or more of the data frames.  
         [0030]     Mathematical and logical operations on the sequence numbers are conducted modulo-4096 (i.e., 2 12 ). In particular, greater-than and less-than operators are defined. The following ranges of sequence numbers are considered to be greater than or less than a data frame sequence number N:  
         [0031]     N is greater than the sequence numbers (N-2048)%4096 to (N-1)%4096  
         [0032]     N is less than the sequence numbers (N+1)%4096 to (N+2047)%4096  
         [0033]     That is, the preceding half of the circular buffer is defined as less than N; the following half of the circular buffer is defined as greater than N. Because the receiver will discard any received data frame with a sequence number less than L_V(N), and because the definition of “less than” comprises the preceding half of the buffer, the transmitter can never advance L_V(S) more than 2048 sequence numbers ahead of L_V(N) PEER . Assuming L_V(N) PEER  accurately mirrors the receiver&#39;s L_V(N), to do so would mean the transmitter would be transmitting data frames that the receiver would consider less than L_V(N) and discard. As the data frames stored by the transmitter between L_V(N) PEER  and L_V(S) comprise the retransmission queue, the retransmission queue is effectively limited to half of the potential buffer size, or 2048 data frames. In practice, actual memory limitations at the transmitter may restrict the retransmit queue size to some value less than 2048 data frames.  
         [0034]     Under poor channel conditions that generate a significant number of NAKs, or under fairly low data rates that allow for the relatively frequent sending of fill frames, a 2048-element retransmission queue may be sufficient. However, under the high data rates possible on the Packet Data Channels (PDCH), particularly under good channel conditions, the transmitter may experience a queue stall condition, wherein 2048 data frames are transmitted without receiving a NAK or fill frame. For example, assuming an average data throughput of 1 Mbps, the 2048-element retransmission queue will fill in: 
 
≈2048*46*8/1 Mbps=754 msecs. 
 
         [0035]     This number will be even smaller under peak operating conditions of 3 Mbps over F-PDCH. In a queue stall, the transmitter is unable to transmit new data frames to the receiver until the receiver NAKs, allowing the transmitter to advance L_V(N) PEER  and discard some of the copies of previously transmitted data frames. The queue stall may force the transmitter to discard incoming data frames and allow higher layer protocols to deal with the error. This queue stalling and discarding of new data may not be the best option for applications desiring certain QoS behaviors.  
         [0036]     There are several ways to address the queue stall problem. For example, a queue stall will not occur when the number of NAKs per round parameter is set to 1 and if the Round Trip Time (RTT)+Delay Detection Window (DDW)+REXMIT_TIMER is less then 754 ms for the 1 Mbps data rate. Even lower DDW &amp; REXMIT_TIMER values will be required to support higher data rates. However, lower values of DDW have the negative effect of inducing unnecessary NAKs under regular operating scenarios, and hence this is not a desirable option. As another alternative, the receiver could use implementation based schemes to prevent queue stalls by not waiting for the full NAK rounds, based on implementation-specific criteria. This solution places the decision at the receiver, which does not have any knowledge of the buffering at the transmitter, and hence may lead to incorrect decisions in many scenarios.  
         [0037]     According to the present invention, the transmitter may prevent queue stalls by advancing the L_V(N) PEER  pointer when the retransmission queue approaches the available capacity. This effectively discards the earliest (by sequence number) retransmission data frame copies stored in the retransmission queue. If the protocol allows, the transmitter then supplies the new value of the L_V(N) PEER  pointer to the receiver, so the receiver may update the value of its corresponding L_V(N) pointer.  
         [0038]     In one embodiment, the transmitter supplies the updated L_V(N) PEER  value (referred to herein as L_V(N) NEW ) to the receiver in a Skip Frame. As defined herein, a Skip Frame has the format of a fill frame, with the seq-number field containing the sequence number for the new proposed L_V(N) pointer. The fields of a Skip Frame are defined as follows:  
                                                   Field   Length (bits)                           SEQ   8           CTL   4           SEQ_HI   4           Padding   Variable                      
 
         [0039]     SEQ—This field contains the least signification 8 bits of the new proposed L_V(N).  
         [0040]     CTL—This field shall be set to ‘1111’. This identifies that the frame as a fill frame type.  
         [0041]     SEQ_HI—This field shall contain the most significant 4 bits of the new proposed L_V(N).  
         [0042]     Padding—Padding bits. As required to fill the remainder of the frame. These bits shall be set to ‘0’.  
         [0043]     The transmitter-controlled queue stall avoidance solution of the present invention requires cooperative processing by the receiver upon receipt of a Skip Frame. In particular, the receiver should take the following actions, depending on the value of the new proposed L_V(N) in the Skip Frame. First, the receiver extracts the value L_V(N) NEW  from the Skip Frame according to the following equation: 
 
 L   —   V ( N ) NEW ={SEQ+[SEQ_HI*256]}
 
         [0044]     The receiver then processes data frames as follows (wherein the operators &lt;=, &gt; and &gt;= are modulo-4096 as described above):  
         [0045]     If L_V(N) NEW &lt;=L_V(N), the Skip Frame is discarded. In this case, the transmitter has discarded only retransmission data frames that were already received in sequential order by the receiver, and delivered to a higher protocol layer.  
         [0046]     If L_V(N) NEW &gt;L_V(N), the receiver delivers all received data frames whose sequence number is less than L_V(N) NEW  to the higher protocol layer. The receiver also removes all sequence numbers less than L_V(N) NEW  from the NAK-list. Any missing data frames will be addressed by error handling routines in the higher protocol layer(s) (e.g., TCP). Finally, the receiver sets L_V(N) to the value of L_V(N) NEW .  
         [0047]     If L_V(N) NEW &gt;=L_V(R), the receiver sets L_V(R) to the value of L_V(N) NEW .  
         [0048]     In this manner, the transmitter may place a greater priority on the delivery of new data frames than on the retention of copies of previously transmitted data frames retained against the possibility of a required retransmission. This capability may be desired or required in many high-data rate applications such as audio or video streaming that can tolerate some data loss but have low latency requirements for the delivery of new data frames. The present invention places control of the queue stall avoidance at the transmitter, where the queue resource allocation is known, and can be best optimized.  
         [0049]     Although the present invention is explicated herein with reference to an RLP transmitter and RLP receiver, the present invention is not so limited, and may be advantageously applied in any NAK-based communication system with fixed size buffers. More generally, although the present invention has been described herein with respect to particular features, aspects and embodiments thereof, it will be apparent that numerous variations, modifications, and other embodiments are possible within the broad scope of the present invention. Accordingly, all variations, modifications and embodiments are to be regarded as being within the scope of the invention. The present embodiments are therefore to be construed in all aspects as illustrative and not restrictive and all changes coming within the meaning and equivalency range of the appended claims are intended to be embraced therein.