Abstract:
A method of transmitting data in a communication system. Data packets are transmitted from a first node to a second node on a first channel. An acknowledgement packet is transmitted from the second node to the first node on a second channel in response to receiving a number of packets on the first channel. The number of data packets that the acknowledgment packet is sent in response to is adjustable.

Description:
CROSS-REFERENCE TO RELATED PATENT APPLICATIONS 
     The present application claims priority to Great Britain Application No. 0611249.4, filed Jun. 7, 2006, the specification, drawings, claims and abstract of which is incorporated herein by reference in its entirety. 
     FIELD OF THE INVENTION 
     This invention relates to the communication of data, and particularly to a method for communicating data over asymmetric channels. 
     BACKGROUND OF THE INVENTION 
     This section is intended to provide a background or context to the invention that is recited in the claims. The description herein may include concepts that could be pursued, but are not necessarily ones that have been previously conceived or pursued. Therefore, unless otherwise indicated herein, what is described in this section is not prior art to the description and claims in this application and is not admitted to be prior art by inclusion in this section. 
     A variety of protocols are available for the communication of a data message over a communications link. In numerous protocols the message is divided into packets at the transmitter end; the packets are conveyed individually over the communications link; and at the receiver end the packets are combined to re-form the message. Each packet normally comprises a payload, which represents the portion of the message that the packet carries, and control data, which is carried in the packet 
     One example of a packet protocol is TCP (transmission control protocol). This protocol is widely used as the transport layer protocol in internet communications. The performance of TCP is however significantly reduced in asymmetric data communication systems, wherein a forward channel carries information at much higher speed than a reverse channel. 
     One such asymmetric system is the DVB-H (Digital Video Broadcasting via Handheld Terminals) system. DVB-H is used to provide high quality video broadcasting services to hand held terminals. It has also been proposed to use DVB-H for downloading game data to hand held devices. DVB-H uses a downlink channel which can transport data from a server to a device at speeds high as 5-30 Mbit/s. When using TCP it is necessary to send acknowledgement packets (ACK) from the device to the server for every data packet received on the downlink channel. The server will only continue to send data upon receipt of ACK packets. The ACK packets are transmitted in the uplink direction on a low bandwidth in a cellular network such as the GPRS network with an average data rate of 10 Kbit/s. 
     The DVB-H downlink bandwidth is approximately 500˜3000 times greater than the uplink channel which results in a large bandwidth asymmetry. Typically the maximum length of a general TCP data packet sent on the downlink channel is about 1500 bytes, and the length of an ACK packet sent on the uplink channel is 40 bytes. Since the ratio of the packet length between TCP data packet and ACK packet (1500/40=37.5) is much less than the ratio of throughput between downlink channel and uplink channel (500˜3000), this means that the high speed data downloading will cause a large number of ACK packets to be buffered in the hand held device because the data rate of the uplink channel can not satisfy the generating rate of the ACK packets. 
     As a result of the large bandwidth asymmetry, the blocked ACK packets increase the round trip time (RTT) of the TCP connection and degrade the transmission throughput. Furthermore, the buffered ACK packets will also occupy a lot of resources of the hand held device which will impact on the device performance and other communication processes. 
     The document “Shekhar et al., Performance Optimisation of TCP/IP over Asymmetric Wired and Wireless links” describes a buffer management solution called SAD (Smart Ack Dropper) for performance optimization of TCP in asymmetric networks. The basic idea of the SAD solution is to monitor the Ack queue status and maintain an Ack sequence number table at the communication node in order to suppress the number of Ack packets belonging to the same flow. 
     It is an aim of embodiments of the present invention to overcome at least the problems identified above. 
     SUMMARY OF THE INVENTION 
     According to a first aspect of the invention there is provided a method of transmitting data in a communication system comprising transmitting data packets from a first node to a second node on a first channel and transmitting an acknowledgement packet from the second node to the first node on a second channel in response to receiving a number of packets on the first channel, wherein the number of data packets that the acknowledgment packet is sent in response to is adjustable. 
     According to a second aspect of the present invention there is provided a communication system for transmitting data between a first node and a second node comprising means for transmitting data packets from a first node to a second node on a first channel and means for transmitting an acknowledgement packet from the second node to the first node on a second channel in response to receiving a number of packets on the first channel, wherein the number of data packets that the acknowledgment packet is sent in response to is adjustable. 
     According to a third aspect of the present invention there is provided a device for transmitting data in a communication system comprising means for receiving data packets from a network node on a first channel and means for transmitting an acknowledgement packet from to the first network node on a second channel in response to receiving a number of packets on the first channel, wherein the number of data packets that the acknowledgment packet is sent in response to is adjustable. 
     According to a fourth aspect of the present invention there is provided a device for transmitting data in a communication system comprising a receiver arranged to receive data packets from a network node on a first channel and a transmitter arranged to transmit an acknowledgement packet from to the first network node on a second channel in response to receiving a number of packets on the first channel, wherein the number of data packets that the acknowledgment packet is sent in response to is adjustable. 
     These and other advantages and features of the invention, together with the organization and manner of operation thereof, will become apparent from the following detailed description when taken in conjunction with the accompanying drawings, wherein like elements have like numerals throughout the several drawings described below. 
    
    
     
       BRIEF DESCRIPTION OF THE DRAWINGS 
       In the following the present invention will be described with reference to the accompanying drawings in which: 
         FIG. 1  is a schematic representation of a DVB-H system; 
         FIG. 2  is a flowchart showing the steps of an algorithm used to adjust the ACK packet generation rate in accordance with an embodiment of the present invention; and 
         FIG. 3  is a flowchart showing the adaptive ACK generation process according to an embodiment of the invention. 
     
    
    
     DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENTS 
     Embodiments of the invention are described in relation to a broadcast network and a cellular network. However, the present invention is not restricted thereto, but any other bandwidth asynchronous systems such as some extremely asynchronous ad-hoc networks that can also be enhanced by applying the present invention. 
     Reference is first made to  FIG. 1  which shows a schematic representation of a DVB-H system. Data such as digital video broadcasting data is downloaded from a data server  10  to a DVB-H supported hand held device  16  via a multiplexer  12  and DVB-H transmitter  14  belonging to a broadcast network  22 . The data is sent using TCP as data packets on a downlink wireless channel to hand held device  16 . 
     The hand held device is arranged to send ACK packets acknowledging the received TCP packets to the server  10 . ACK packets are sent on an uplink channel of the cellular network  24  via a base station  18  and the UMTS network  20 . The hand held device  16  includes a TCP sink in a TCP module  26 . The function of the TCP sink will be described hereinafter. 
     The device  16  is arranged to generate one ACK for receiving every K data packets. K is defined as the ACK generation rate. The ACK is sent as a cumulative acknowledgement relating to the receipt of the K data packets. In accordance with an embodiment of the invention the TCP sink is arranged to dynamically adjust the ACK packet generation rate K in accordance with the measured throughput ratio of the downlink DVB-H and the uplink cellular channel. The TCP sink is therefore arranged to monitor the TCP data packets arriving on the downlink DVB-H channel and the ACK packets sent on the uplink cellular channel. 
       FIG. 2  shows the steps of an algorithm used to adjust the ACK packet generation rate in accordance with an embodiment of the present invention. 
     At S 1  of the algorithm the TCP sink monitors an uplink buffer in the device  16  at the wireless interface and determines the number of ACK packets stored in the buffer. When the number of ACK packets exceeds a predefined threshold the algorithm continues to S 2 . 
     At S 2  the TCP sink enables the ACK generation algorithm. 
     At S 3  the TCP sink measures the number of ACK packets N sent on the uplink channel and the number of TCP data packets M arriving on the DVB-H downlink channel in a predetermined time period T. 
     Based on the measurement M and N, the optimal value of the ACK generation rate K is then obtained by:
 
 K=M/N   (1)
 
     At S 4  the ACK generation rate K is set according to equation 1. 
     At S 5  the TCP sink then employs a K-delayed ACK process to generate one ACK packet for every K continuously received data packets. The K-delayed ACK generation process of S 5  is shown in  FIG. 3 . 
     Referring to  FIG. 3 , at  100  the TCP sink sets the ACK packet counter to C=0. 
     At  200  the TCP sink receives a new data packet and determines the sequence number of the data packet, herein referred to as l_seq. The sequence number is included in the header of the data packet. 
     As discussed previously, the data server  10  will only continue to send data packets if it receives ACK packets from the device  16 . It has been found that the maximum delay between the transmissions of subsequent ACK packets should not exceed 500 ms in order to achieve good TCP granularity. In accordance with an embodiment of the invention the maximum delay guarantee is set at 200 ms. Accordingly, at  300  the TCP sink determines if the interval after the most recently generated ACK packet exceeds 200 ms. 
     If the interval after the most recently generated ACK packet does exceed 200 ms, the process continues to S 400 . 
     At S 400  the TCP sink immediately generates an ACK packet for the arriving data packet with the sequence number l_seq. The process then returns to  200 . 
     If the interval after the most recently generated ACK packet does not exceed 200 ms, the process continues to S 500 . 
     At S 500  the TCP sink determines if the received data packet with the sequence number l_seq is the next packet in the sequence. 
     If it is detected that the received packet was not the next expected packet in the sequence after a previously received packet, this indicates that there is a gap in the current data sequence and the process continues to S 400  where the TCP sink immediately generates an ACK packet for the arriving data packet with the sequence number l_seq. The process then returns to S 200 . 
     If it is detected that the received packet was the next expected packet in the sequence, the process continues to S 600 . 
     At S 600  the TCP sink detects if the received packet has the largest packet sequence number received thus far. If so this indicates that the packet is the latest in the sequence and the process continues to S 700 . If not this indicates that an earlier packet has arrived late and that there is a gap in the sequence and the process continues to S 400  where a new ACK is generated immediately. 
     At  700  the TCP sink checks the counter and determines if the counter is at a value K−1. If the counter is at a value K−1, this indicates that the data packet received is the Kth data packet received since the last ACK packet was sent and the process continues to S 800 . 
     At S 800  the TCP sink immediately generates an ACK packet for the arriving data packet with the sequence number l_seq. The process then returns to S 100  where the counter is reset to C=0. 
     If the counter is not yet at a value K−1, this indicates that the data packet received is not the Kth data packet received since the last ACK packet was sent and the process continues to S 900 . 
     At  900  the TCP sink is arranged to increase the counter by one. The process then continues to  200 . 
     Returning to  FIG. 2 , at S 6  the TCP sink checks the receiving status to predict if a timeout may occur. This may be achieved by timing the gaps between the receipt of data packets. According to an embodiment of the invention the TCP sink estimates the RTT (Round Trip Time) of the TCP connection in order to determine a RTO (Retransmission Time Out) value prediction. For example, the RTT estimation can be done just by simply letting the TCP sink send an echo ICMP (Internet Control Message Protocol) packet to the sender and measure the time interval before receiving the echo response from the TCP server. Based on measured RTT, the RTO value can be calculated according to a TCP standard specification. Various ways of predicting the RTO value are known in the art and will therefore not be described herein. The inventors of the present invention have found that the precision of RTO prediction does not have any key influence on the algorithm performance. 
     If at S 6  it is detected that new data packets have been received during the RTO period the algorithm continues to S 7 . 
     At S 7  the TCP sink checks if there are any blocked ACK packets in the uplink buffer. If there are no ACK packets blocked in the uplink buffer for a predefined period, this indicates that the ACK packet generation frequency is too low. The algorithm then continues to S 8 . 
     At S 8  the ACK generation rate K is halved. The process then continues to S 5 . 
     If there are some ACK packets in the uplink buffer the value of K does not need to be updated until a predetermined time when the value of K is reset by re-evaluating the uplink and downlink data rates. Thus if it is determined that there are ACK packets in the buffer, the algorithm continues to S 9  where the TCP sink maintains a K-updating timer. When the predetermined time for updating the value of K expires the algorithm continues to S 3  to update the value of downlink and uplink throughput ratio dynamically for optimal performance. 
     If at S 6  it is determined that no data packets have been received during the RTO period the algorithm continues to S 10 . 
     At S 10  the TCP sink sets K to a value of one thus generating an ACK packet to acknowledge the last data packet received since the last ACK packet was sent, and thereby cumulatively acknowledging all of the remaining previously unacknowledged data packets. 
     The adaptive ACK generation process will be disabled until the TCP congestion window is recovered. Accordingly, after S 10  the algorithm continues to S 1  whereby the adaptive ACK generation process is only restarted if the number of ACK packets in the uplink buffer reaches the threshold described previously. 
     According to an alternative embodiment of the present invention the value of K may be set using equation (1) as described in S 3  and then updated regularly by repeating the measurements of M and N at predetermined time intervals. 
     The required data processing functions may be provided via one or more data processor entities. All required processing may be provided in the TCP module  26  of  FIG. 1 . An appropriately adapted computer program code product, embodied in a computer-readable medium, may be used for implementing the embodiments, when loaded to a processor, for example for computations required when determining the value of K. The program code product for providing the operation may be stored on and provided via a carrier medium such as a carrier disc, card or tape. A possibility is to download the program code product via a data network. Implementation may be provided with appropriate software in a server. 
     The present invention is described in the general context of method steps, which may be implemented in one embodiment by a program product including computer-executable instructions, such as program code, executed by computers in networked environments. Generally, program modules include routines, programs, objects, components, data structures, etc. that perform particular tasks or implement particular abstract data types. Computer-executable instructions, associated data structures, and program modules represent examples of program code for executing steps of the methods disclosed herein. The particular sequence of such executable instructions or associated data structures represents examples of corresponding acts for implementing the functions described in such steps. 
     Software and web implementations of the present invention could be accomplished with standard programming techniques with rule based logic and other logic to accomplish the various database searching steps, correlation steps, comparison steps and decision steps. It should also be noted that the words “component” and “module,” as used herein and in the claims, is intended to encompass implementations using one or more lines of software code, and/or hardware implementations, and/or equipment for receiving manual inputs. 
     The Applicant draws attention to the fact that the present invention may include any feature or combination of features disclosed herein either implicitly or explicitly or any generalisation thereof, without limitation to the scope of any of the present claims. The foregoing description of embodiments of the present invention have been presented for purposes of illustration and description. It is not intended to be exhaustive or to limit the present invention to the precise form disclosed, and modifications and variations are possible in light of the above teachings or may be acquired from practice of the present invention. The embodiments were chosen and described in order to explain the principles of the present invention and its practical application to enable one skilled in the art to utilize the present invention in various embodiments and with various modifications as are suited to the particular use contemplated.