Abstract:
In a system in which several data links are available for the transmission while one respective sending unit is allocated to the data links to temporarily store data that is to be transmitted via the respective data link, data packets containing non-real-time critical data are subdivided into fragments of variable sizes prior to forwarding to a sending unit. Data packets containing real time-critical data are preferably forwarded to a sending unit without being fragmented. Additionally, a minimum fragment size can be predefined for the fragmentation process.

Description:
CROSS REFERENCE TO RELATED APPLICATIONS 
     This application is the US National Stage of International Application No. PCT/EP2005/054249, filed Aug. 30, 2005 and claims the benefit thereof. The International Application claims the benefits of German application No. 102004052692.3 DE filed Oct. 29, 2004, both of the applications are incorporated by reference herein in their entirety. 
     FIELD OF INVENTION 
     The present invention relates to a method for transmitting data that is available in the form of data packets, wherein a plurality of transmission links are available for transmitting and the transmission links have each been assigned a sending unit for buffering data requiring to be transmitted over the respective transmission link. 
     BACKGROUND OF INVENTION 
     The data requiring to be transmitted is frequently assigned different service or priority classes so that correct data transmitting—for example within a pre-specified realtime constraint—can be insured. In such a scheme low priority data will then be transmitted over a transmission link only if there is no higher priority data to be transmitted. The consequence thereof in some cases is that low priority data will block a link for a certain period of time because the higher priority data was not yet available for transmission at the time the decision was made about sending the lower priority data. 
     Particularly in the case of links having a low data transmission rate, for example ISDN (Integrated Services Digital Network) payload data channels or slow uplinks of ADSL (Asymmetrical Digital Subscriber Line) connections, this situation results in unacceptable delays for the higher priority data. 
     SUMMARY OF INVENTION 
     A known approach to reducing the delay is to employ a data transmission protocol that makes fragmenting of the data requiring to be transmitted available on the sending side and defragmenting of the transmitted data available on the receiving side. A known protocol that allows fragmenting or, as the case may be, defragmenting of said type is the packet-oriented IP (Internet Protocol) protocol known for transmitting data in computer networks, or the ML-PPP (Multi-Link Point-to-Point Protocol) protocol. The data requiring to be transmitted is therein fragmented prior to sending and only then prioritized on the basis of the fragments. The consequence thereof is that the maximum blocking time will be limited to the duration of transmission of one fragment. 
     A data packet requiring to be transmitted is therein customarily divided up into as many fragments as there are links available. Although that will cause the delay for each individual data packet to be minimized, fragmenting is often unnecessarily high so that overhead data for fragment management will be generated unnecessarily and management resources consequently occupied unnecessarily. The problem can furthermore arise of not fragmenting highly enough, in particular when only a few links are available, so that the delay will continue exceeding a desired threshold. 
     U.S. Pat. No. 6,778,495 B1 discloses a method and a device for transmitting data packets wherein “best effort” or, as the case may be, non-realtime-critical data packets are fragmented and transmitted over two communication links. A round-robin technique is provided as the scheduling method for distributing or “balancing” the load in the links. 
     EP 1 303 083 A1 describes a packet scheduler wherein voice or video data packets are assigned to a premium traffic class and low priority data packets are assigned to a low priority traffic class. A queue is provided in a packet buffer in each case for low priority traffic and for premium traffic and the received data packets are sorted into the respective queue accordingly. The low priority data packets can further be divided in the queue for low priority traffic based on the high priority data packets to be transmitted. 
     EP 1 202 508 A1 discloses a method and an apparatus for fragmenting NDSI (non-delay sensitive information) wherein the fragmentation is performed based on parameters that are contained in the received DSI (delay sensitive information). The particular parameters are variables such as sample rate, information compression, amount of overhead information, number of channels, etc. NDSI and DSI are transmitted over a single communication link. 
     An object of the invention is hence to disclose a method and a sending unit by means of both of which it will be possible to achieve data transmitting matched to the prevailing transmission conditions. 
     Said object is achieved in terms of a method and by a sending unit as described by the independent claims 
     The inventive method is based on a system in which a plurality of transmission links are available for transmitting and the transmission links have each been assigned a sending unit—referred to frequently in the relevant literature as a queue—for buffering data requiring to be transmitted over the respective transmission link. 
     According to the invention, data packets containing non-realtime-critical data are therein divided up into fragments of variable size before being forwarded to a sending unit and the distribution of the fragments over the transmission links is selected based on an amount of data currently stored in the sending unit and on a data transmission rate that is available on the corresponding transmission link. The data packets containing realtime-critical data are advantageously forwarded to a sending unit with no fragmenting. 
     A major advantage of the invention is that the inventive method can be implemented in a simple manner in already existing systems and is compatible with existing transmission standards. 
     Advantageous developments of the invention are described in the dependent claims. 
     According to an embodiment of the present invention the fragment size and the distribution of the fragments over the transmission links are selected additionally taking into account a settable maximum waiting time between storing of the data into the sending unit and transmitting over the corresponding transmission link. In this case the fragment size is set in such a way that a configurable delay will not be exceeded for the data requiring to be transmitted. 
     A minimum fragment size can advantageously be specified. In this way unnecessary overhead data for fragment management is prevented in a simple manner in the absence of realtime-critical data requiring to be transmitted. 
     According to a further embodiment of the present invention realtime-critical data is transmitted only over certain transmission links. That will make it possible to increase the waiting time on the remaining transmission links transmitting the non-realtime-critical data. That will likewise prevent the generation of overhead data for fragment management and hence additionally save processing resources. That will in turn enable less expensive control units (processors) to be used having comparably less processing power. 
    
    
     
       BRIEF DESCRIPTION OF THE DRAWINGS 
       An exemplary embodiment of the invention is explained in more detail below with reference to the drawing, in which: 
         FIG. 1 : is a structural schematic of the main functional units involved in the inventive method. 
     
    
    
     DETAILED DESCRIPTION OF INVENTION 
       FIG. 1  shows an inventive sending unit SE to which a transmission line ÜL is connected via an interface SS. Said transmission line ÜL comprises a plurality of transmission links, with four transmission links L 1 , . . . , L 4  being shown in the present exemplary embodiment. Said transmission links L 1 , . . . , L 4  are, for example, ISDN payload data channels—referred to frequently in the relevant literature also as B (bearer) channels—each having a transmission rate of 64 kbits per second. Each transmission link L 1 , . . . , L 4  has been assigned a sending unit Q 1 , . . . , Q 4  in which data requiring to be transmitted over the corresponding transmission link L 1 , . . . , L 4  is buffered. The data buffered in a sending unit Q 1 , . . . , Q 4  is scheduled to be transmitted as soon as possible over the corresponding transmission link L 1 , . . . , L 4 , with transmitting no longer able to be delayed. For better understanding the sending units Q 1 , . . . , Q 4  are referred to below—as is customary in the relevant literature—as queues Q 1 , . . . , Q 4 . 
     The data requiring to be transmitted over the transmission line ÜL is present in the sending unit SE in the form of data packets. A distinction is herein made between first data packets NDP containing non-realtime-critical data and second data packets EDP containing realtime-critical data. Shown in the present exemplary embodiment are a first data packet NDP and a plurality of second data packets EDP, with three second data packets EDP having been assigned to a first voice connection VS 1 —referred to frequently in the relevant literature as a voice stream—and three further second data packets EDP having been assigned to a second voice connection VS 2 . 
     For transmission over the transmission line ÜL, the first and second data packets NDP, EDP are distributed among the queues Q 1 , . . . , Q 4  in accordance with the method described below. 
     The second data packets EDP of the first voice connection VS 1  are buffered in the third queue Q 3  before being transmitted over the third transmission link L 3 . The second data packets EDP of the second voice connection VS 2  are buffered in the fourth queue Q 4  before being transmitted over the fourth transmission link L 4 . For transmission over the transmission line ÜL, the first data packet NDP can be buffered in both the first and the second queue Q 1 , Q 2 . The first data packet NDP is for this purpose subdivided into fragments F of variable size, with the specific size being dependent on the respective queue Q 1 , Q 2 . When the first data packet NDP is being divided up among the queues Q 1 , Q 2 , the queue Q 1 , Q 2  appearing most suitable for the next fragment F is chosen therefor. That is as a rule the queue Q 1 , Q 2  having the shortest what is termed “sending queue”. 
     In the present exemplary embodiment a first fragment F 1  has already been buffered in the first queue Q 1  and a second fragment F 2  has already been buffered in the second queue Q 2 , with the first fragment F 1  being larger than the second fragment F 2 , which is to say the amount of data currently buffered in the first queue Q 1  and released for transmission over the first transmission link L 1  is larger than the amount of data buffered in the second queue Q 2  and released for transmission over the second transmission link L 2 . As an example, the first fragment F 1  is 80 bytes in size and the second fragment F 2  is 40 bytes in size. 
     As already explained above, the transmission links L 1 , . . . , L 4  each have a transmission rate of 64 kbits per second. If it is assumed that the maximum delay for a second data packet containing realtime-critical data is 20 ms, then the maximum amount of data requiring to be buffered in a queue Q 1 , . . . , Q 4  will be 160 bytes. Said maximum amount of data requiring to be buffered in a queue Q 1 , . . . , Q 4  is referred to frequently as the “queue limit”. The queue limit is therein dependent on the codecs used for transmission and the necessary transmission quality, etc. 
     Thus in the present exemplary embodiment a fragment F that is 80 bytes in size could be buffered in the first queue Q 1  and a fragment F that is 120 bytes in size could be buffered in the second queue Q 2 . The second queue Q 2  is consequently the more suitable queue for the next fragment F so that a third fragment F 3  that is 120 bytes in size will be formed from the first data packet NDP and buffered in the second queue Q 2 . 
     Known protocols that allow fragmenting of data packets are the packet-oriented IP (Internet Protocol) protocol for transmitting data in computer networks and the ML-PPP (Multi-Link Point-to-Point Protocol) protocol. If the IP protocol is used, what are termed RTP (Realtime Transport Protocol) data packets are employed as the second data packets EDP. Fragmenting based on the IP protocol will in contrast to fragmenting based on the ML-PPP protocol result in a larger packet header so that processing will be more complex. 
     Through a fixed assignment to a queue Q 3 , Q 4  of second data packets EDP assigned to a voice connection VS 1 , VS 2 , due to the sequence maintained within a voice connection it will on the one hand be possible when the ML-PPP protocol is used to transmit the realtime-critical data without headers so that the fragmenting overhead will be reduced and processing capacity saved and, on the other hand, it will be possible when the IP protocol is used to employ compression techniques that use redundancy in successive headers, in particular in RTP headers. 
     Queues can furthermore be reserved only for transmitting first data packets NDP. As the queue limit provided for transmitting realtime-critical data does not have to be maintained for said queues, the queue limit can be increased. That will result in a further reduction in the fragmenting overhead and a saving in processing capacity.