METHOD FOR HANDLING DATA TRANSMISSION FRAMES OF VARIABLE LENGTH WITH A CHANNEL CONTROLLER AND FOR WRITING THEM TO A CYCLIC BUFFER MEMORY

Abstract: The invention pertains to a method for handling data transmission frames of variable length in the interface between two networks, and for writing them into a cyclic buffer memory (3) wherein the data transmission frames consist of a header field (H) containing at least control data, a data field (D) and an end field of variable length. To obtain flexible buffering, the buffer memory (3) is divided into more than one logical sections, each having a header field and a data field. The indicated real length of an incoming frame is taken into account in the buffer memory operations in such a manner that the channel encoder (4) is interrupted for each frame section which have come from the channel to the area of the buffer, whereby the channel encoder reads said frame sections to its own memory, whereby the last interrupt related to said frame is made on the basis of the indicated real length of the frame when the last section of said frame is in the buffer.

Description: Method for handling data transmission frames of variable length with a channel controller and for writing them to a cyclic buffer memory The present invention pertains to a method for handling data transmission frames of variable length with a channel controller and for writing them to a cyclic buffer memory wherein the data transmission frames consist of a header field containing at least control data, a data field and an end field of variable length. In the interface between the PCM and GSM networks communication takes place by using TRAU frames which are defined in the GSM specifications and which possess the following properties: - recurrence interval 20 ms - contains data relating to one traffic communication ( oice/data channel) - reserves one PCM time slot - transparent to the PCM channel - contain 320 bits of control information and data. In order for the channel encoder receiving the TRAU frames to be capable of operating in compliance with the GSM specifications, the frames must arrive at the channel encoder correctly phased. Owing to time slips of clocks, maintenance of the correct phasing requires that the TRAU frame be shortenable or extendable by four bits, i.e. the length of the TRAU frames be in fact 320 ± 4 bits. The channel encoder is informed of whether the TRAU frame concerned has been extended or shortened by control bits located at the very beginning of the frame. The situation described above is problematic, because TRAU frames may have a length varying four bits in both directions while the length of the buffer section is invariable. Several extended or shortened TRAU frames then cause the frame to slip within the buffer section, whereby the controller interrupts the channel encoder at the wrong time, either too late (TRAU frame shortene ) or too early (TRAU frame extende ), i.e. when the buffer section is full. The only right moment for interrupt would be when the buffer has a complete TRAU frame. For instance in a GSM base station slip causes unnecessary delay in the speech signal. The present invention provides a solution to problems in the interface between the the PCM and GSM networks caused by non-ideal behaviour of the clocks controlling said networks. To achieve this, the method of the invention is characterised in that the buffer memory is divided into more than one logical sections, each section comprising a header field and a data field, and that at least the following steps are performed by means of the channel controller and a channel encoder in respect of an incoming frame: - reading an amount of information corresponding to the length of one buffer section by means of the channel controller from the beginning of the frame, - writing the frame section which has been read to one section of the buffer memory, and interrupting the channel encoder, whereby the channel encoder reads the frame section from the buffer memory to its own storage space, - recognising the real length of the frame indicated in the header field of the frame by means of the channel encoder, - writing the required length, calculated using the indication of the length of the frame, by the channel encoder to the header field of a second buffer section, - writing the frame section coming from the channel in a corresponding manner by the channel controller to an area of the buffer, and interrupting the channel encoder correspondingly, whereby the channel encoder reads the frame sections to its own memory, - making the last interrupt related to said frame on the basis of the indicated real length of the frame when the last section of said frame is in the buffer. The last interrupt related to said frame occurs, thanks to the extension/shortening of one buffer section, exactly when a complete frame is in the buffer. The buffer memory is preferably divided into two sections, each having a header and a data field. In this way a buffer of 'flexible' length is realised, whereby slip of the TRAU frame within the buffer is avoided, and the channel encoder is interrupted at the right moment. In the services provided by the GSM network this can be noticed as elimination of speech delays caused by slip of the frame. In the following the invention will be described in more detail by means of an example with reference to the attached drawings, wherein Figure 1 schematically shows a TRAU frame, Figure 2 illustrates handling of TRAU frames in the interface between the networks, Figures 3a and 3b show buffer memory areas arranged according to the prior art and according to the invention. Figure 1 schematically shows a TRAU frame. It comprises initial and end control areas Cl and C2, a data area D, and the total length thereof may vary ± 4 bits, which is indicated by two blocks provided with the text '4 bit' . The actual communication takes place as illustrated by Figure 2. A controller 1 takes care of picking up TRAU frames belonging to the traffic communication from a PCM channel 2. The controller 1 reads first (arrow A) its own control data from the header field of a buffer 3, writes (arrows Bl...Bn) a TRAU frame to the buffer i.e. DP-RAM. There may be 16 buffer memories 3 connected to the same controller, as indicated by arrows Bl-Bn. After this the controller 1 interrupts (arrow C) a DSP processor i.e. a channel decoder 4, which reads (arrows E) the TRAU frame to its own memory. The memory area of the buffer 3 is divided into two sections: a header field H, which assists the controller in writing to memory, and an actual data area D, to which the TRAU frame is written. The header field H, formatted by the channel encoder 4, comprises e.g. an indication of the length of the data area, which the controller always reads before starting to write the TRAU frame to the data area, as indicated above. Having filled the data area D, the controller 1 interrupts the channel encoder 4 according to the prior art, which, owing to a TRAU frame of variable length, may arrive at the wrong time, as described above. The controller used, such as a Rockwell 8071 controller, is characterised in that the buffer section used has a length which can be dynamically regulated. Figure 3a illustrates the memory area of the buffer 3 arranged according to the prior art and comprising a header field H and a data area D of fixed length. Figure 3b illustrates the memory area of the buffer 3 arranged in accordance with the invention and wherein the header field H and data area D are divided into two sections HI and H2, and Dl and D2, respectively. The overall length of the two data areas is half of the normal length of a TRAU frame. In this case data contained in one TRAU frame is thus written to the buffer sections e.g. in four different stages I- IV for instance such that in stages I and III data is written to the first buffer section and in stages II and IV to the second buffer section. The beginning of a TRAU frame may be phased to the beginning of either one of the buffer sections. In the example of a buffer divided into two sections, the following steps are carried out according ' to the invention in respect of an incoming frame by means of the channel controller 1 and channel encoder 4: - an amount of data corresponding to the length of one frame is read by the channel controller from the first section of the frame, - the section of the frame which has been read is written to a section Dl of the buffer memory, and the channel encoder is interrupted, whereby the channel encoder reads the frame section from the buffer memory to its own storage space, - the real length of the frame indicated in the header field of the frame is recognised by the channel encoder; - the required length calculated using the indication of the length of the frame is written by the channel encoder to the header field H2 of the second buffer section, - the section of the frame which has come from the channel to the area D2 of the buffer is written by the channel controller in a corresponding manner, and the channel controller is correspondingly interrupted, whereby the channel encoder reads the rame sections to its own memory, - whereby the last interrupt relating to said frame is made on the basis of the indicated real length of the frame when the last quarter of said frame is in the buffer. In such a case the last interrupt relating to said frame occurs, thanks to the extension/shortening of one buffer section, exactly when a complete frame is in the buffer. In other words, as the channel encoder makes an interrupt after each filled buffer section, one of these interrupts occurs when the entire frame is ready in the buffer sections. After receiving the first interrupt, the channel encoder 4 has 10 ms time to read the first quarter of the TRAU frame to its own memory, interpret e.g. the indication of the length of the TRAU frame from the control bits contained therein, and modify the length indication of the buf er in the header information of the second buffer section accordingly before the controller 1 writes the second part of the TRAU frame. When the data areas (Dl-D2)*2 correspond to the real length of the entire TRAU frame, the last interrupt relating to said TRAU indicates that even the last data contained in said frame is ready in the buffer 3. It is obvious for a person skilled in the art that the different embodiments of the invention are not limited by the example described above but may vary within the scope of the attached claims.

Claims: Claims : 1. A method for handling data transmission frames of variable length in the interface between two networks with a channel controller and for writing them to a cyclic buffer memory (3) wherein the data transmission frames consist of a header field (H) containing at least control data, a data field (D) and an end field of variable length, c h a r a c t e r i s e d in that the buffer memory (3) is divided into more than one logical sections, each section comprising a header field (HI, H2) and a data fieid (Dl, D2), and that at least the following steps are performed by means of the channel controller (1) and a channel encoder (4) in respect of an incoming frame: - reading an amount of information corresponding to the length of one buffer section by the channel controller (1) from the beginning of the frame, - writing the frame section which has been read to one section (Dl) of the buffer memory, and interrupting the channel encoder (4), whereby the channel encoder reads the frame section (Dl) from the buffer memory (3) to its own storage space, - recognising the real length of the frame indicated in the header field (Cl) of the frame by means of the channel encoder, - writing the required length, calculated using the indication of the length of the frame, by the channel en¬ coder (4) to the header field (H2) of a second buffer section, - writing the frame section coming from the channel in a corresponding manner by the channel controller (1) to an area (D2) of the buffer, and interrupting the channel encoder (4) is correspondingly, whereby the channel encoder reads the frame sections to its own memory, - making the last interrupt related to said frame on the basis of the indicated real length of the frame when the last section of said frame is in the buffer. 2. A method according to claim 1, c h a r a c ¬ t e r i s e d in that said buffer memory (3) is divided into two sections and that each section has its own header field and data field (HI, H2; Dl, D2).

Inventors: KOIVU VESA, KOIVU, VESA
Applicants: NOKIA TELECOMMUNICATIONS OY
Application year: 1992
Main classification: H04L12
IPC codes: H04J3, H04L12, H04L29, H04Q11