Abstract:
During the transmission of data packets received on a wired connection via a first interface via a second interface designed for the wireless transmission of data, the problem exists that between receiving the data and transmitting the data via the second interface a relatively large delay time can arise which must be bridged by means of a suitably dimensioned buffer memory. To enable the maximum delay time to be further reduced, the necessary processing of the received IEEE 1394 bus packets is already performed section by section immediately after a received data packet has arrived. After the complete number of bus packets falling within a transmission frame has been received, there is therefore no longer any lengthy processing time required and the probability of missing the time slots reserved in the next transmission frame is significantly reduced. The buffer memory for bridging the delay time period can therefore be correspondingly smaller.

Description:
This application claims the benefit, under 35 U.S.C. 119 of German Patent Application 10343458.5, filed Sep. 19, 2003. 
   The invention relates to the technical field of digital data communications, to be precise in such a way that data is received in packets via a first interface, is converted into the format according to a second interface, and is output via the second interface. 
   BACKGROUND OF THE INVENTION 
   Devices in the home, such as typical home entertainment equipment for example, particular mention also being made of a personal computer which nowadays can also be considered home entertainment equipment, or also other household devices, for example “white goods” appliances, are increasingly being networked. This networking encompasses not only systems in which the devices are networked by means of wires, that is to say with appropriate cable connections between the devices, such as with the aid of the IEEE 1394 bus system for example, but also systems in which the devices are wirelessly networked. Particular mention is made at this point of the so-called HIPERLAN/2 system which has become established alongside the IEEE 802.11x system. The HIPERLAN/2 system enables the networking of devices in the home. It provides a number of approximately 20 MHz wide channels in the 5 GHz range, which in turn are subdivided according to a TDMA method (Time Division Multiplex Access). 
   The modulation method corresponds to an OFDM method (Orthogonal Frequency Division Multiplex), so that the least possible interference occurs during multipath receiving. The maximum data rate is in the range of 54 Mbit/s. Video data streams and audio data streams can consequently be transmitted in the HIPERLAN channels. 
   The HIPERLAN system has already been specified in an ETSI/BRAN standard. An overview of the system may be found in the document ETSI TR 101 683 V1.1.1 (2000/02) Broadband Radio Access Network (BRAN); HIPERLAN Type 2; System Overview. The key component for the invention is described in the document ETSI TS 101 493-1 V1.1.1 (2000/04); Broadband Radio Access Networks (BRAN); HIPERLAN Type 2; Packet Based Convergence Layer; Part 1 and Part 3. Further components of the system, such as the physical layer and data link control layer (DLC) in particular, are published in other documents. These may all be obtained from the European Telecommunications Standards Institute in F-06921 Sophia-Antipolis Cedex, France. 
   For networking devices in the home, one scenario which uses a combination of both networking systems, namely firstly wired and secondly wireless, has proved to be expedient. If data between the two different systems is also to be exchanged among the systems, a bridge circuit is consequently required for this purpose; on the one hand said circuit is therefore connected to the wired system and on the other hand it also has an interface to the wireless transmission system. Such a bridge circuit is therefore equipped with two interfaces, firstly for the wired transmission system and secondly for the wireless transmission system. 
   The invention relates to such a bridge unit. In particular, the invention solves a problem that occurs when the IEEE 1394 bus system is used as the wired system and the aforementioned HIPERLAN/2 system is used as the wireless transmission system. A separate specification was created for this application. This is called ETSI TS 101 493-3 V1.1.1 (2000/09); Broadband Radio Access Networks (BRAN); HIPERLAN Type 2; Packet Based Convergence Layer; Part 3; IEEE 1394 Specific Convergence Sublayer (SSCS). It is described in Annex C.3 of the aforesaid document that the maximum transmission delay with isochronous data transmission from the IEEE 1394 bus system into the HIPERLAN/2 system is 6.1 ms. Accordingly, 2.1 ms of this arises from the fact that in the worst case it is necessary to wait first for 17 IEEE 1394 bus packets to arrive before the data for a MAC transmission frame (Medium Access Control) has been collected. A further 2 ms time delay can arise from the fact that the time slots reserved for the station are available just when the received data needs to be made available by the DLC layer for transmission. In the extreme case, it is then necessary to wait for the next MAC transmission frame. 2 ms of delay time can thus also arise if the transmission frame was newly divided just at the moment when the data is available, and afterwards the time slots allotted to the station are positioned at another point of the MAC transmission frame. 
   In order to bridge this delay time, it is therefore necessary to implement a correspondingly large buffer memory in the bridge circuit. It is even recommended to dimension the buffer memory to be so large that even the delay time of 8 ms can still be buffered. 
   SUMMARY OF THE INVENTION 
   The object of the invention is to reduce the maximum delay time between data arrival and data transmission. 
   The problem is solved in that the necessary reformatting for the transmission of the data via the second interface is performed section-by-section and not, as proposed in the standard, after the last data arriving in the time-slot pattern of a HIPERLAN transmission frame has been received. When the last data to be transmitted has arrived, most of the processing of this data has already been completed, so that it is then only necessary to process a smaller section of data. This then no longer requires as much processing time and the maximum possible delay time is reduced to 4.1 ms. The measure thus has the advantage that a much smaller buffer memory must be implemented, since the maximum delay time to be buffered is reduced. 
   Other advantageous measures and further developments of the method according to the invention are set out in the dependent claims. One particularly advantageous measure is, if a reserved time slot in a transmission frame has been missed, to fill up the still available remaining reserved time slots of said transmission frame with the reformatted data that is already available. The time slots of the transmission frame that have not yet been missed are therefore not simply allowed to pass by uselessly, in that so-called “dummy data” is inserted into them, but rather they are filled with already processed payload data which then does not need to be transmitted again in the next transmission frame. 
   Another advantageous measure consists of using a time-slot pattern that has been advanced by a given amount instead of the time-slot pattern of the transmission frame for internal processing of the received data packets. By virtue of this measure it is possible to avoid such a large additional time delay, arising because of slight deviations in the clock signals for the two different interfaces as a result of receiving a further data packet via the first interface, that the first reserved time slot in the next transmission frame is missed for the transmission of the data. If the reserved time slots are situated in the latter part of the transmission frame, there is no risk of said time slots being missed. The problem exists primarily when the reserved time slots are positioned in the front part of the frame, that is to say in particular whenever they are the first permitted reservable time slots. By advancing the internal processing time, it is however necessary to increase the size of the buffer memory again slightly so that the advance time can also be buffered. 
   Advantageous measures for a device for carrying out the method are set out in Claims  7  to  12 . 

   
     BRIEF DESCRIPTION OF THE DRAWINGS 
     Exemplary embodiments of the invention are described in greater detail in the description below and are illustrated in the drawings, in which: 
       FIG. 1  shows an application of the invention with a set-top box and a television set; 
       FIG. 2  shows an outline block circuit diagram of a bridge circuit for connecting a wired and a wireless transmission system; 
       FIG. 3  shows a block circuit diagram of the digital IC in the bridge circuit; 
       FIG. 4  shows a layer model for the bridge circuit; 
       FIG. 5  shows a representation of the individual processing steps for the reformatting of the data from the IEEE 1394 data packet format into the HIPERLAN/2 data packet format; 
       FIG. 6  shows a more detailed representation of the process steps for resegmenting the received data; 
       FIG. 7  shows a representation of the normal case of data processing, as proposed in the standard; 
       FIG. 8  shows a representation of data processing in the special case where receiving a seventeenth data packet begins already during the transmission frame; 
       FIG. 9  shows a representation of the section-by-section processing of the received data packets according to the invention; and 
       FIG. 10  shows the format of a HIPERLAN/2 MAC transmission frame. 
   

   DETAILED DESCRIPTION OF PREFERRED EMBODIMENTS 
     FIG. 1  shows the application of data transmission from a set-top box for digital television which is located in one room to a television in another room. The digital television signals are fed to the set-top box  21  via a satellite receiving antenna  20 . Said set-top box forwards the received data stream to a transmitting unit  22  located in the room, which is often also referred to as a wireless box (WB) and represents a bridge to the wireless transmission system. The transmitting unit  22  is connected to a transmitting antenna, via which it emits the data stream (it can include video and audio data) wirelessly by means of electromagnetic waves. The radio signals are received by a second wireless box, for example one located in another room, namely by the receiving station  23 , which is equipped with a receiving antenna for this purpose. The receiving unit  23  has a wired connection to a television set  24 , which is likewise internally equipped with a decoding unit for digital television signals that decodes the received video and/or audio signals and forwards them for displaying or for output over the loudspeakers respectively. 
   In the application illustrated, it is assumed that the wireless transmission of the data takes place in compliance with the so-called HIPERLAN/2 transmission system. The transmitting and receiving units are accordingly designed in accordance with said transmission system. The HIPERLAN/2 transmission system is designed to transmit data at up to 54 Mbit/s in the 5 GHz range. As described in the introduction, the HIPERLAN/2 transmission system is standardized and accordingly is described in detail in the associated description sections of the standard. For the disclosure of the invention, therefore, reference is expressly made to the standard. Reading of the following sections is particularly recommended:
         ETSI TR 101 683 V1.1.1 (2000/02) Broadband Radio Access Network (BRAN); HIPERLAN/2 System Overview   ETSI TS 101 493-1 V1.1.1 (2000/04) Broadband Radio Access Networks (BRAN); HIPERLAN/2; Packet Based Convergence Layer; Part 1: Common Part, and   ETSI TS 101 493-3 V1.1.1 (2000/09) Broadband Radio Access Networks (BRAN); HIPERLAN/2; Packet Based Convergence Layer; Part 3: IEEE 1394 Service Specific Convergence Sublayer (SSCS).       

     FIG. 2  shows an outline block circuit diagram of the transmitting unit  22 . This block circuit diagram shows on the one hand a digital IC  25  and on the other hand an analog IC  26 . The digital IC  25  contains the component for preprocessing the data for the wireless transmission. Said IC will be discussed in greater detail below. The analog IC  26  represents a front-end IC in relation to the air interface. Located in this part are the transmission amplifier and the means for generating the HF signals that are then forwarded to the transmitting antenna. 
     FIG. 3  shows an outline block circuit diagram of the digital IC  25 . Reference numeral  30  designates a part of an IEEE 1394 interface. This part corresponds to the data link control layer of the IEEE 1394 bus system. Several different ICs are commercially available to implement the IEEE 1394 data link control layer, which are also sold as IC cores and thus can also be integrated in a larger digital circuit. The digital IC  25  is additionally equipped with three components which together form the so-called packet-based convergence layer according to the HIPERLAN/2 standard. These components are SSCS  31  (service-specific convergence sublayer), CPCS  32  (common part convergence sublayer) and SAR  33  (segmentation and reassembly). All three components will be described in greater detail below. Said three components are all connected via an internal data bus  35  to further units. These include on the one hand a memory unit  34  (RAM), a microprocessor unit  38  and a block DLC  36  which is provided for implementing the data link control layer provided in the standard for the HIPERLAN/2 interface. Finally, connected downstream of the DLC block  36  is a block  37  for implementing the bit transmission layer—that is to say the physical layer of the HIPERLAN/2 transmission system. Further units, such as a unit for calculating an error correction code for example, may also be integrated in the digital IC  25 , but are not explained in greater detail below. It is possible to use an AMBA bus, for example, as an internal data bus  35 . 
   The block division for the digital IC  25  is an expression of the layer model with a so-called convergence layer which is provided in the HIPERLAN/2 standard and shown in  FIG. 4 . The reference numeral  37  designates the physical layer of the HIPERLAN/2 system. Accordingly, the reference numeral  36  designates the data link control layer (DLC). The aforementioned layer PBCL (packet-based convergence layer) is denoted by reference numeral  39 . The HIPERLAN/2 system is also open for the transmission of data in so-called smaller units that are referred to as cells, for example transmission in accordance with the ATM standard. In this case, a so-called cell-based convergence layer  40  would be implemented. Reference numeral  41  designates the higher layers. 
     FIG. 5  shows part of said layer model again, but the division of the packet-based convergence layer into three different sublayers is additionally represented and the processing steps that take place in the respective layers are illustrated. The data received via the 1394 interface  30  in accordance with  FIG. 3  is collected in the RAM  34 . After a data packet has arrived in the RAM  34 , said data is first processed in the SSCS block  31 . In this block, the received IEEE 1394 data packets assembled as a service data unit (SDU) are converted into a so-called protocol data unit (PDU). This conversion does not change the size of the assembled data block (see  FIG. 5 ). However, an updating or recalculation of certain parameters is performed. Such parameters are, for example, time information for synchronization between the IEEE 1394 clock signal and the relevant clock signal for the HIPERLAN/2 system. In this respect, reference is made to the description in the standard for details, see the cited document ETSI TS 101 493-3 V1.1.1 (200/09). 
   The data converted in this way is subsequently processed in the CPCS block  32 . As shown in  FIG. 5 , the data that has arrived, which is now designated CPCS-SDU, is organized in this sublayer. This is done by appending so-called “padding data” at the end of the data unit and appending finally a concluding information item, which is referred to in the standard as a trailer. This can also be seen in  FIG. 5 . As many padding bits are appended until the resulting data unit CPCS-PDU has a length of an integral multiple of 48 bytes. In the concluding information (trailer), it is then entered how much payload data is included in the protocol data unit. The resulting data unit is subsequently processed in the SAR block  33 . In accordance with the HIPERLAN/2 standard, when it enters said sublayer the data unit is termed the SAR-SDU data unit ( FIG. 5 ). The data unit is segmented in this layer. The SAR layer is designed to process SAR-SDUs that have a variable length, however with the restriction that it must be an integral multiple of 48-byte data blocks. As a result of the segmenting, the data unit SAR-SDU is divided into 48-byte segments. A control information item of 12 bits is set at the beginning of a data segment. Apart from the SAR stop bit, said added control information is however reserved for future applications and otherwise has no control function. 
     FIG. 6  shows the segmenting process in detail. It can be seen there that the SAR stop bit is set to 0 everywhere except for the segment containing the last payload data. It thus indicates which is the last data segment in the processed data unit. 
   The lower part of  FIG. 5  further illustrates the processing step in the DLC layer  36 . The data segments are supplied to the DLC layer individually. Processing of the data segment, which is designated the DLC-SDU from the start in the DLC layer, is complex and cannot be explained here in detail. As shown in  FIG. 5 , header and trailer information is added at the beginning and at the end respectively of each data segment. The trailer information may consist, for example, of an error correction code (FEC) which is recalculated in this layer. Alternatively, an error detection code (CRC) can be calculated. The header information contains for example information designating the type of the data (dummy/payload data). For further information about the processing of the data packets in the DLC layer, refer to the document ETSI TS 101 761-1 V1.1.2.1 (200/11) Broadband Radio Access Networks (BRAN); HIPERLAN Type 2; Data Link Control (DLC) Layer; Part 1: Basic Data Transport Functions. 
     FIG. 7  now shows the normal case of processing data packets received via an IEEE 1394 interface for forwarding via the HIPERLAN/2 interface. A transmission frame according to the HIPERLAN/2 transmission system has a length of 2 ms. This time period is indicated by vertical bars. Ideally exactly 16 IEEE 1394 bus packets are received in this period, since the transmission of the data is here segmented into 125 μs long bus cycles as standard. This is shown in the top part of  FIG. 7 . It is shown there that exactly 16 bus packets go into the transmission frame N. This applies to the isochronous data transport provided for in the IEEE 1394 standard. Accordingly, an IEEE 1394 bus packet is transmitted at time intervals of 125 μs. The start of a MAC transmission frame according to the HIPERLAN/2 standard is marked in  FIG. 7  as BCH (broadcast channel) in each case. 
     FIG. 10  also shows the format of a HIPERLAN/2 MAC transmission frame. According to this figure, a transmission frame can have a number of broadcast channels (BCH  1  to BCH N), frame control channels (FCH  1  to FCH N), downlink channels (DL  1  to DL N), uplink channels (UL  1  to UL N) as well as random access channels (RCH  1  to RCH N). 
   The associated time slots of the transmission frame that are reserved for the transmitting station are indicated in the fourth part of  FIG. 7  as AS (allocated slots). 
   In the second part of  FIG. 7  it is shown in the transmission frame N+1 that the previously collected 1394 bus packets are processed en bloc, that is to say the processing takes place there as described above in connection with  FIG. 5 . If the reserved time slots AS have a sufficient distance from the start of the transmission frame N+1, the collected and processed data can already be transmitted via the air interface in the next transmission frame N+1. The third part of  FIG. 7  further indicates that a certain amount of time can elapse between the completion of the processing according to  FIG. 5  and transmission of the data via the air interface in order to prepare for the data transmission. It is shown in the transmission frame N+2 that the reserved time slots for the transmitting station have been shifted to a different position by the so-called “scheduler”. According to the HIPERLAN/2 standard, this is possible during operation and would be notified in the frame control channel (FCH). 
     FIG. 8  shows the special case where the start of a seventeenth IEEE 1394 bus packet has also been received in the transmission frame N. This can occur due to slight deviations between the two clock systems for the IEEE 1394 bus system on the one hand and for the HIPERLAN/2 transmission system on the other hand. It would then not be possible to perform the SSCS-SDU data processing already after the sixteenth data packet has arrived. It is necessary to wait first for the seventeenth bus packet to arrive. It is therefore shown in the second part of  FIG. 8  that the data processing is delayed in comparison with the previous case. As a result, the data can no longer be fully processed when the time slots AS reserved for the transmitting station are ready for transmission. In this case, a delay in data transmission would occur if the data processing were performed in compliance with the standard. The received data could then not be-transmitted until the next but one data transmission frame N+2. Since, moreover, a seventeenth bus packet has also been received, the space in the assigned time slots in one transmission frame would not be sufficient, and it would even be necessary to transmit the rest of the received data in transmission frame N+3, which is likewise illustrated in  FIG. 8  by means of grey hatching. 
   The processing method according to the invention deviates from the process envisaged in the standard. It is illustrated schematically in  FIG. 9 . According to the IEEE 1394 standard, a 1394 bus packet can contain a complete MPEG transport packet with 188 bytes. With all additional information of the bus packet, the latter then has a length of 224 bytes. According to the invention, immediately after it has been received, said data packet is passed to the processing in the packet-based convergence layer  39 . The processing steps in the sublayers SSCS, CPCS and SAR are thus performed directly after the respective bus packet has been received. Processing of the received bus packet takes place concurrently with the receiving of further bus packets. For this purpose, the individual processing steps are supplied with their own clock signals and are controlled by the central microprocessor unit  38 . For the sake of simplicity, in the second part of  FIG. 9 , the section-by-section processing of each received bus packet is represented in such a way that it always takes place between the reception of two successive bus packets. This is not however intended to give the false impression that, if necessary, this processing cannot be completed also during reception of the next bus packet. 
   The completed segmented data sections can each be forwarded immediately to the data link control layer (DLC)  36  since they are processed separately in the data link control layer in each case. Once the sixteenth data packet has arrived in the transmission frame N, therefore, no lengthy processing time arises for the reformatting of the data. It is then ensured that the reserved time slots for the transmitting unit can already be filled in the following transmission frame N+1 with the received data from the previous transmission frame. This reduces the maximum delay time envisaged by the HIPERLAN/2 standard by 2 ms to 4.1 ms. 
   If necessary, the time slots in the next transmission frame can also be missed if a seventeenth bus packet already falls within the first transmission frame. In order to ensure immediate transmission of the data in the next transmission frame here as well, as a further measure, it is possible to advance the time-slot pattern for the processing of the received bus packets slightly. Said time-slot pattern no longer matches the 2 ms time-slot pattern of the transmission frame. Although the 2 ms time interval is observed, the two time-slot patterns are slightly offset from one another. This measure is shown in  FIG. 8 . The dashed line there indicates the advance time of the processing time-slot pattern in each case. If a seventeenth bus packet was then also started in relation to the advanced processing time-slot pattern, this means that said packet has certainly arrived but before the end of the actual transmission frame, and consequently the processing can still be completed with certainty before the first reserved time slots become valid. The processing time-slot pattern must be advanced so far that a complete IEEE 1394 bus packet can still be received and additionally there is still enough time remaining to process this bus packet. The time slots for BCH, FCH, ACH and Preamble provided at the beginning of a transmission frame make a minimum time of 20+36+12+8=76 μs available. In order to enable any begun data packet still to be received and processed in this time, the advance time should therefore be about 49 μs. 
   There is still a problem that if the first reserved time slot has been missed, according to the HIPERLAN/2 standard, so-called dummy data must be transmitted in it. In the HIPERLAN/2 standard said data is termed dummy LCHs. The solution according to the invention now permits the free remaining reserved time slots still available to be utilized for transmitting payload data instead of the dummy data. Since the data of the received bus packets has already been immediately processed and prepared for transmission via the air interface, said data can then also be inserted into the available remaining reserved time slots. This is shown in the fourth part of  FIG. 9 . In said figure, there is a grey-hatched area in the area of the reserved time slots AS into which already processed data of the current transmission frame can be inserted. An implemented solution can be such that it is checked every time after four LCH channels whether data from the current transmission frame has been fully processed and it is then inserted into the remaining LCH time slots. This solution enables the available bandwidth to be better utilized so that further resources can be used for control information, e.g. for compensating synchronization differences. 
   If only the first described method of the section-by-section processing of the bus packets is implemented, in the case of an MPEG2 transport stream, the buffer memory can be dimensioned for approximately 70 fewer LCH time slots, which is equivalent to a size of 4480 bytes. In the case of a DVC transport stream, this advantage can even be 163 LCH packets, equivalent to 10,432 bytes. 
   The invention was explained with reference to the example of the transmission of data received via the IEEE 1394 bus and forwarded via a HIPERLAN/2 interface. It is however not solely restricted to this application. The invention can be deployed anywhere where data packets are received via a first interface and are transmitted again via a second interface in a different time-slot pattern.