Abstract:
The invention relates to a method whereby the control unit produces a sequence of blocks, each comprising a header and data to be transmitted. Each block is transmitted to the destination terminal with a level of error protection which is selected from among several pre-defined levels, the selected level being indicated in a piece of signalling information that accompanies the block transmitted. The header of each block comprises an acknowledgement control field which is activated intermittently by the control unit in order to request an acknowledgement of blocks from the terminal. A higher level of protection is selected for transmitting blocks having a header with an activated acknowledgement control field than for transmitting blocks having a header with a non-activated acknowledgement control field.

Description:
BACKGROUND OF THE INVENTION 
   The present invention relates to techniques for transmitting packets in acknowledged mode from a master control unit to a slave terminal. 
   The invention relates more particularly, among these techniques, to those in which the control unit, which produces a sequence of blocks each comprising a header in addition to the user data, uses a “polling” mechanism to request acknowledgement of packets from the remote terminal. The header of each block then comprises an acknowledgement control field activated intermittently by the control unit so as to request acknowledgement of blocks from the terminal. In response to such a request, the terminal returns a message in which a certain number of previous blocks are acknowledged, positively and/or negatively. 
   A technique of this kind is used, in particular, for the downlinks (from the network to the mobile terminals) in GPRS (“General Packet Radio Service”) networks that have been developed to allow the transmission of data in packet mode in GSM (“Global System for Mobile communications”) type cellular networks. 
   The mechanism for acknowledging the packets may be disturbed when a block whose header has an activated acknowledgement control field is poorly received by the slave terminal. In this case, the terminal does not execute the acknowledgement request, so that the control unit remains uncertain as to the blocks that have been correctly received. 
   Often, the acknowledgement mechanism is used within the framework of an automatic repeat protocol (ARQ, “Automatic Repeat reQuest”) in which the unit sending the packets uses a send window of specified length, positioned onward of the first block which has not yet been positively acknowledged. If an acknowledgement request message has not been correctly received by the remote terminal, the send window may remain blocked at an old position, thereby leading to unnecessary repetitions of already received packets and to significant delays in the transmission of the new packets. 
   An object of the present invention is to propose an efficient method for transmitting data in packet mode. 
   Another object is to reduce the risks of blocking of the send windows utilized in certain ARQ mechanisms. 
   SUMMARY OF THE INVENTION 
   The invention thus proposes a method for transmitting data in acknowledged mode between a control unit and a terminal, wherein the control unit produces a sequence of blocks each comprising a header and data to be transmitted, and the blocks are transmitted to the terminal, each block being transmitted with a level of protection against errors selected from several predefined levels, the selected level being indicated in signaling information accompanying the transmitted block. The header of each block comprises an acknowledgement control field activated intermittently by the control unit so as to request an acknowledgement of blocks from the terminal. According to the invention, a higher level of protection is selected for the transmission of at least one block whose header has an activated acknowledgement control field than for the transmission of the blocks the acknowledgement control field of whose header is not activated. 
   The process uses differentiated protection of the transmitted blocks, depending on whether or not they contain an acknowledgement request. The better protection of the acknowledgement requests, which are transmitted in the band with the user data, makes it possible to avoid a large proportion of the blockages that acknowledgement mechanisms might give rise to. 
   Most often, protection against transmission errors is ensured by a channel coder using a convolutional code or a block code. Different levels of protection are then effected by adjusting the coding rate: introduction of additional redundancy symbols, modification of the structure of the code used, alteration of the degree of puncturing of the code, etc. 
   Other methods may be used to adapt the level of protection against errors, for example methods of adaptive control of the signals transmission power. 
   Another aspect of the present invention concerns a packet control unit, comprising means of producing at least one sequence of blocks each comprising data to be transmitted and a header including an acknowledgement control field, means for transmitting the blocks of the sequence to a terminal, means of selecting a level of protection against errors for the transmission of each block of the sequence, from several predefined levels, and means of intermittent activation of the acknowledgement control field in the header of the blocks of the sequence so as to request an acknowledgement of blocks from the terminal to which the block is transmitted. The means of selection are devised to select a higher level of protection for the transmission of at least one block whose header has an activated acknowledgement control field than for the transmission of the blocks the acknowledgement control field of whose header is not activated. 

   
     BRIEF DESCRIPTION OF THE DRAWINGS 
       FIG. 1  is a diagram of a GPRS-type network to which the invention may be applied; 
       FIG. 2  is a schematic diagram of a packet control unit of such a network, adapted to the implementation of the invention. 
   

   DESCRIPTION OF PREFERRED EMBODIMENTS 
   The GPRS network illustrated in  FIG. 1  is built on a GSM infrastructure, and conventionally divided into a network core, also called Network and Switching Subsystem or NSS, and a radio-access network also called Base Station Subsystem or BSS. 
   For the packet service, the switches of the NSS are called GPRS support nodes or GSNs. A distinction is made between the SGSNs (Serving GSNs)  5 , which are linked to the BSS by way of an interface called Gb, and the GGSNs (Gateway GSNs, not represented) which serve as a gateway with external packet transmission networks, such as the Internet, for example. 
   A general description of the radio interface, called Um, between the mobile stations (MS)  10  and the base stations (BTS)  20  of the BSS is provided in the technical specification ETSI TS 101 350, “Digital cellular telecommunications system (Phase 2+); General Packet Radio Service (GPRS); Overall description of the GPRS radio interface; Stage 2 (GSM 03.64, version 6.3.0, Release 1997)”, published by ETSI (European Telecommunications Standards Institute) in July 1999. 
   Each base station  20  is supervised by a base station controller or BSC  21  by way of an interface called Abis. In order to manage the transmission of GPRS packets, the BSS further comprises an entity  22  called packet control unit or PCU. The locating of the PCU within the BSS is not standardized. In the example represented in  FIG. 1 , the PCU  22  is situated between the BSC  21 , with which it communicates via an interface called Agprs, and the NSS, with which it communicates via the interface Gb. 
     FIG. 2  illustrates a possible structure of a PCU  22  situated between an SGSN  5  and a BSC  21 , as in the example of  FIG. 1 . The reference  40  designates the Gb interface controller for the link with the SGSN  5 . 
   The Gb interface is of asynchronous type. It is based on the frame relay (FR) protocol, as well as on a protocol called BSSGP (BSS GPRS Protocol.) which transports routing and quality-of-service information between the BSS and the SGSN. The Gb interface controller  40  provides the physical link with the SGSN  5 , as well as carrying out the procedures specific to the FR and BSSGP protocols. 
   The links between the PCU  22  and the BTSs  20  via the Agprs interface are of synchronous type. Consequently, the data manipulated by the PCU  22  between the Gb interface controller  40  and the Agprs interface controller  42  transit via a buffer memory  41  where packet queues are recorded. 
   Between the PCU  22  and the BTS  20 , the information is carried by 320-bit frames of TRAU (Transcoder/Rate Adapter Unit) type, at the rate of one frame every 20 ms. These TRAU frames are formatted and processed by a module  44  and transmitted by way of synchronous interface circuits  45  which form MIC subchannels at 16 kbit/s with the BTSs  20 . Several 16-kbit/s subchannels can be multiplexed on the Agprs interface and separated by the BSC  21  for routing to the BTSs. A module  46  of the Agprs interface controller  42  implements the radio protocols of layer 2 of the OSI model, that is to say the RLC/MAC (Radio Link Control/Medium Access Control) protocols described in the European Standard ETSI EN 301 349, “Digital cellular telecommunications system (Phase 2+); General Packet Radio Service (GPRS); Mobile Station (MS)—Base Station System (BSS) interface; Radio Link Control/Medium Access Control (RLC/MAC) protocol (GSM 04.60, version 6.8.1, Release 1997)”, published by ETSI in October 2000. 
   The RLC sublayer forms the interface with the upper-layer protocol, called LLC (Logical Link Control). It carries out the segmentation and the reassembling of LLC protocol data units (LLC-PDUs), which are exchanged asynchronously on the Gb interface. It produces RLC data blocks to which the MAC sublayer adds a one-byte MAC header. 
   In the downlink direction, from the PCU to the MSs, the MAC header of each RLC/MAC block includes:
         a three-bit USF (Uplink State Flag) field, serving to indicate which mobile station is authorized to use an uplink resource corresponding to the downlink resource on which the RLC/MAC block is transmitted;   a three-bit acknowledgement control field, including a one-bit S/P (Supplementary/Polling) subfield indicating whether the acknowledgement control field is active (S/P=1) or inactive (S/P=0) and a two-bit RRBP (Relative Reserved Block Period) subfield uniquely specifying an uplink block in which the mobile station addressed should transmit an acknowledgement message;   a two-bit Payload Type field, specifying the type of RLC block following (data, control, etc).       

   It is the transmission of RLC data blocks which is of interest here. Each of these blocks includes an RLC header following the MAC header byte. This RLC header especially includes the following information:
         temporary flow identity (TFI), consisting of five bits identifying the temporary block flow (TBF), from which the RLC data of the block originate. A TBF is a connection supporting the unidirectional LLC-PDU transfer on physical data channels. A TBF is temporary, that is to say that it is maintained only during the data transfer;   a block sequence number BSN of seven bits, which contains the sequence number of the RLC/MAC block, modulo 128.       

   The MAC sublayer furthermore manages the multiplexing of the blocks arising from the various TBFs which are active on the available physical channels, arbitrating among the various mobile users via a planning mechanism (“scheduling”). 
   The RLC/MAC entity of the destination mobile station receiving the downlink data blocks from a TBF updates for this flow a reception state variable V(R) which indicates the BSN following the highest BSN received on this TBF. The number V(R)-1 (modulo 128) thus points to the end of a reception window whose length is k=64 RLC/MAC blocks. On receipt of a “polling” command, that is to say of a block whose MAC header has the bit S/P=1, the MS returns in the uplink block specified by the RRBP subfield, an acknowledgement message PDAN (“Packet Downlink Ack/Nack”) which comprises in particular:
         a field SSN (“Starting Sequence Number”) of seven bits containing the current variable V(R) for the TBF; and   a field RBB (“Receive Block Bitmap”) of k=64 bits indicating those of the blocks of the reception window that have been correctly received. A positive acknowledgement of the block BSN=(SSN-i) mod 128 is indicated by the value 1 of the bit of rank i (1≦i≦k) of the RRB bitmap, and a negative acknowledgement by the value 0.       

   On receipt of the PDAN message, the PCU updates for the TBF an acknowledgement state variable V(A) which contains the BSN of the oldest block that has not been positively acknowledged, as well as a table V(B) with k inputs indicating the respective acknowledgement states (positive acknowledgement/negative acknowledgement/acknowledgement not received) of k consecutive blocks following the one designated by V(A), these k consecutive blocks forming a send window. The state variables V(A) and V(B) are deduced directly from the SSN and RBB fields received in the last PDAN message. The RLC/MAC protocol does not authorize the transmission of blocks other than inside the send window thus managed by the PCU. Outside of this window, the transmission of the blocks is inhibited. 
   So as not to delay the transmission of the new blocks, it is advisable to avoid the phenomena of blocking of the send window on an obsolete position. Such blocking may in particular occur when errors affect the transmission of the “polling” commands sent by the PCU. To avoid this, it is proposed that a protection specific to the downlink data blocks whose MAC header has the bit S/P=1 be applied. 
   In the case of the GPRS, a variable level of protection can be selected block by block within a TBF, by the choice of a coding scheme (CS) from among four schemes CS-1 to CS-4 specified in the European Standard ETSI EN 300 909, Digital cellular telecommunications system (Phase 2+); Channel coding (GSM 05.03, version 6.2.1, Release 1997), published by ETSI in August 1999. 
   The scheme CS-4 does not use any error-correction coding, that is to say that the coding rate is equal to 1: only a block check sequence BCS is adjoined to the data blocks. The schemes CS-1 to CS-3 use a convolutional code with rate ½ after the addition of the BCS sequence. No puncturing is carried out in the 
   CS-1 scheme (which offers the highest level of protection), while puncturing is applied in the CS-2 and CS-3 schemes so that they give rise to overall coding rates of about ⅔ and of about ¾, respectively. 
   The CS-1 (1≦i≦4) channel coding is applied at the level of the physical-layer protocol, that is to say in the BTSs in the case of the downlinks. Each coded RLC/MAC block is composed of 456 bits and is transmitted in corresponding time intervals of four TDMA frames on a carrier frequency, the successive TDMA (“Time-Division Multiple Access”) frames each being split into eight time intervals to ensure time-division channel multiplexing. 
   A pattern of eight signaling bits is inserted into each coded frame (two bits per time interval) so as especially to indicate which coding scheme has been applied by the transmitter. 
   These signaling bits are extracted from the coded block received by the addressee, in order to allow it to identify the coding scheme. The receiver then carries out the appropriate decoding of the block which will give rise to a positive acknowledgement if it is successful and if the decoded BCS is consistent with the content of the block. 
   The coding scheme applied to the downlink is determined in a way which is known in itself by the PCU on the basis of measurements of reception quality on the radio link, according to link-adaptation mechanisms which seek to achieve an objective in terms of rate of error-affected blocks so as to optimize the raw throughput. The scheme selected is inserted into the TRAU frame carrying the block so as to be applied by the BTS. 
   Each time a coding scheme other than CS-1 is determined by the link adaptation mechanisms, the RLC/MAC layer selects a scheme which is more robust to errors for each block whose MAC header has the bit S/P=1. In particular the CS-1 scheme (maximum level of protection) may be adopted systematically for these blocks which contain acknowledgement requests. 
   It should be noted that this systematic selection of the CS-1 scheme might not be applied when the TBF is in the termination phase and when all the RLC data of this TBF have been sent at least once. Specifically, the 1997 release of the RLC/MAC protocol of GPRS prescribes the resending of the data in the same code as the first send. For example, if only blocks already sent in CS-4 are to be resent in the termination phase of the TBF, the repetitions will also be coded in CS-4 even if their MAC header contains S/P=1. However, certain link adaptation mechanisms tend to favor a more robust coding scheme at the end of a TBF, so that the risks of window blocking remain small.