DEVICE FOR AND METHOD OF TRANSMISSION OF DIGITAL SIGNALS, IN PARTICULAR IN DECT-TYPE SYSTEMS

Abstract: Digital signals such as data signals (IP) are transmitted over a digital channel (for instance the one used for voice service according to the DECT standard or the like) where a point-to-point communication protocol is active, by organising such data signals into HDLC type frames which preferably have a constant length, the value of which can be configured upon setting up the connection. In the presence of receptor errors and in particular of errors considered as non correctable, a mechanism of re-transmission with selective repetition of said frames is actuated. Said mechanism is based on windows whose sizes can be configured upon setting up the communication in view of system performance optimisation. The device autonomously manages a mechanism for the segmentation and re-composition of the data transmitted by the upper layer (IP) in a manner that is totally transparent to that layer.

Description: DEVICE FOR AND METHOD OF TRANSMISSION OF DIGITAL SIGNALS, IN PARTICULAR IN DECT-TYPE SYSTEMS The present invention relates in general to the transmission of digital signals and it has been developed with particular attention to its possible application to data transmission within communication systems preferably operating according to DECT (Digital Enhanced Cordless Telecommunications) standard. This statement is not to be taken in a limiting sense since the invention is advantageously applicable to other mobile communication systems. The same invention is also applicable to both the private environment (PABXs) and the public environment. DECT standard was developed at the European level in order to facilitate the use, within telecommunication networks, of cordless terminals capable of communicating with respective base stations within a pico-cell system. For a general overview of the subject, one can usefully refer to the papers "The New DECT Standard for Cordless Communications" by Hans van der Hoek, Telecommunications, February 1993, pages 77 - 80, or "DECT-Cordless Functionality in New Generation Alcatel PABXs" by V. Werbus, A. Veloso and A. Villanueva, Electrical Communication, 2nd Quarter 1993, pages 172 through 180. The present invention tackles the problem of allowing the transmission of digital signals (and more specifically of data signals) over a system operating according to a standard such as DECT standard or the like. In such a perspective the term "digital signals", as used in the present description and in the accompanying claims, indicates in general the digital signals not directly associated to the voice signal and the terms "digital signals" and "data signals" will be used with the said meaning throughout the specification. At a first glance, the aforesaid problem does not emerge in all its importance, since DECT standard is indeed based on the transmission of digital signals. This also holds true in the case of voice signals, which are transmitted after coding according to an ADPCM technique with a bit rate of 32 Kbit/sec. Transmission of digital signals such as data signals by inserting and, respectively, extracting the data flows downstream and upstream of the voice signal coding/decoding sections would therefore appear an obvious extension allowing use, for data signal transmission, also of existing DECT terminals nearly exclusively conceived for the transmission of voice signals,. Yet the voice transmission channel of DECT systems proves poorly protected against perturbing phenomena such as the sudden signal attenuation and/or multiple path propagation. The aforesaid perturbing phenomena, typical of the application context of a mobile communication system such as DECT, are in fact tolerable in voice signal transmission. Though its bandwidth is narrow, the voice signal presents such intrinsic redundancy qualities as to become "robust" against the aforesaid perturbing phenomena, in the sense that such phenomena, as they occur in the current use of the system, usually do not affect the voice signal to such an extent as to make it unintelligible. The situation is however wholly different in the case of data signals (including image signals, etc.): if such signals are inserted directly onto the voice channel of a DECT system, they would be affected by the aforesaid perturbing phenomena to such an extent that the communication is made practically impossible, at least in the majority of the current use situations. Thus, DECT system provides for a specific and more protected channel for data signal transmission, but access to said channel is not provided for in said terminals mainly intended for voice transmission. Essentially, the object of the invention is to overcome the aforesaid drawbacks, by allowing, for instance, the reliable transmission of digital signals such as data signals on the voice channel of a "DECT-type" system. The expression "DECT-type", as it is used in the present application, is to be meant in the sense that the invention applies not only within systems exactly conforming to DECT standard, but also to communication systems operating in essentially similar ways. Such systems suffer, when used for the transmission of digital signals, of the same drawbacks described above. The reference to DECT standard made in the present description and, where necessary, in the accompanying claims, is thus to be meant as including within the scope of the invention also such essentially similar systems (e. g., the so called PHS system). Thus, the invention provides a device for the transmission of digital signals on at least one transmission channel of a mobile communication system where a point-to-point communication protocol is active, comprising conversion means capable of operating in at least one direction of conversion between said digital signals and respective signals suitable for being transmitted on said transmission channel for carrying out the conversion of said digital signals into respective signals suitable for being transmitted on said transmission channel or the opposite conversion, said signals suitable for being transmitted on said transmission channel corresponding to said digital signals organised into HDLC- type frames adapted for error correction on the radio interface. The invention also provides a method of transmission of digital signals on at least one transmission channel of a mobile communication system where a point-to-point communication protocol is active, comprising the operation of effecting a conversion, in at least one direction, between said digital signals and respective signals suitable for being transmitted on said transmission channel, said signals suitable for being transmitted on said transmission channel corresponding to said digital signals organised in HDLC-type frames adapted for error correction at the radio interface. The following description applies to the case both of a single connection and of multiple parallel connections among which the data stream relating to a user can be subdivided. The protocol can essentially operate over multiple channels by subdividing and recombining the data at the two ends. For the sake of simplicity, the case of data transport over a single connection shall be described hereafter. The invention shall now be described, purely by way of non limiting example, with reference to the accompanying drawings, where: Figure 1 shows, in the form of a block diagram, the general structure of a terminal of a DECT system, capable of operating in accordance with the solution according to the invention, - Figure 2 shows, in greater detail and also in the form of a block diagram, the embodiment of the invention within a terminal such as the one shown in Figure 1 , and Figure 3 shows additional embodiment details of the invention. In Figure 1 (which is derived from the paper by Werbus et al. mentioned in the introductory part of the present description) numerical reference 1 indicates in its entirety a mobile terminal of a telecommunications system operating according to DECT standard or the like. Terminal 1 is essentially a conventional cordless telephone capable of communicating with a base station to which one or more mobile terminals can be connected. The basic components of terminal 1 are: unit 2 for coding/decoding the voice signal (ADPCM coding/decoding), which unit is connected respectively to a microphone 3 and to a loudspeaker 4, controller 5, tasked with dialling and signalling and usually associated with a keyboard and a display unit (not shown), - unit 6 acting as management logic of the MAC (Medium Access Control) layer and as DECT frame generator and decoder, modulator 7 (usually operating according to a GMSK or π/4-DQPSK modulation technique, with different choices, for instance, in the European and in the US standard), radio frequency synthesizer 8, radio frequency transmission stage 9a, radio frequency reception stage 9b, and - antenna unit 10. The criteria for the interconnection of the various components mentioned above and the related operating modes are to be considered widely known in the art and they do not require illustration herein, also because they are not relevant, in themselves, for the comprehension of the invention. To the aims of the invention, the whole of units 5 to 10 can actually be seen as a sort of "black box" 11 (Figure 2) which implements in the device the respective portion of transmission channel and is connected, in addition to the voice part (units 2 to 4), to two additional units 12, 13, typical of the invention. The characteristics of such units shall be better illustrated hereinafter. Units 12, 13 are destined to exchange with block 11 the data pertaining to signalling information and to user information, denominated according to the current terminology C-plane and U-plane, respectively. Such units can be integrated, at least in part, into the processing means (typically a personal computer) employed by the user to transmit data over the system. Units 12, 13 communicate with block 11 through two bi-directional communication lines 120, 130, for instance two RS232 V.24 serial interface lines. It is in any case evident that the aforesaid lines may be of a wholly different kind, providing, for instance, for the parallel byte transmission over eight wires. Communication between units 12 and 13 and block 11 could also take place by using a single interface, for instance with a single RS232/V.24 line, able to convey both signalling information and user information. The use of two interfaces has however proved preferable, in that block 11 is able to implement the entire DECT protocol up to the MAC layer on the terminal. In this way it is therefore possible to maintain the functions pertaining to the C-plane - which can be implemented as a section of the portable part (terminal) of the DECT, as shown by the dash and dot line which surrounds, in Figure 2, block 11 and unit 12 - separated from the functions of the U-plane, preferably located on the user's portable computer and maintained insofar as possible under the control of the applications of the user's computer. More particularly, the functions carried out by unit 13 in Figure 2 can be implemented entirely on the user's computer or split between the latter and a dedicated intermediate apparatus (adapter) or also located on the terminal (blocks 11 and 13 in Figure 2 partially integrated). The procedures described hereafter for dividing the functions should thus not be taken in a limiting sense. The implementation choice described herein can be justified considering that block 11 and the control processing structure where unit 12 is implemented are actually essential for the operation of the portable part of the DECT even as a simple voice terminal. Once the computer related to the U-plane is disconnected, the control of the remaining functional units (block 11 and the processing structure where unit 12 is implemented) can operate as portable part of the DECT and a user can access it in order to make simple voice calls, for instance by using a Generic Access Profile (GAP). As is well known, this denomination indicates a sub- set of the DECT protocol which allows apparatus interworking for a basic telephone service. In that regard, reference can be made to standard ETS 300 444. For communication through communication line 120, unit 12 uses a management module 122 (implementing the functions of the so-called Lower Layer Management Entity of the protocol) which cooperates, according to a typical stack configuration, with a structure with three layers, indicated as DLC, NTW and IWU respectively. These layers and the related interface at the MAC layer (included in block 11 in Figure 2) are used to make voice and/or data calls. Layer DLC (Data Link Control) is essentially to provide and supervise the data link. Upper layer NTW (NetWork Layer) is essentially entrusted with network management. Uppermost layer IWU manages user interface functions: essentially this layer is the interworking layer at the application level and is employed for accessing functionalities not described in the DECT standard and made available by the exchange devices. Unit 13 instead is connected to line 130 through a driving unit 132 which interfaces a corresponding stack structure comprising three layers: PPP+, IP and TCP. Layer PPP+ is essentially tasked with managing the point-to-point protocol and it is based on PPP (Point to Point Protocol) standard. Layer IP (Internet Protocol) is the typical one of an Internet protocol. Lastly layer TCP (Transmission Control Protocol) is tasked with managing the transmission protocol. Reference numerals 121, 131 denote co-ordination entities of the C- plane and U-plane, respectively; said entities are known in se and have no interest for the invention. When a user desires to use the terminal to transmit digital signals , i. e. data signals, it is sufficient for him/her to connect his/her personal computer to line 130 thus setting the terminal for data handling. The architecture described above presents two additional advantages. First, it allows distributing operation loads on different processors, since the heavy load connected with U-plane processing has no influence on the processor dedicated to C-plane functions. This allows implementing the solution by updating a DECT portable part originally designed only for the voice service. Moreover the solution is compatible with a series of different function allocation to the DECT portable part, the user's computer and intermediate apparatuses (see in this respect the considerations made above with regard to the implementation of units 12 and 13). An additional advantage is obtained in terms of flexibility, since the architecture considered herein does not prevent, and in fact is well suited to, combining the two units 12, 13 into a single machine, thus obtaining a fully integrated voice and data terminal. Figure 3 shows in the form of a functional diagram the criteria implemented at layer PPP+ within unit 13. In practice, layer PPP+ accomplishes communication between the IP layer and lines 130 through driving unit 132. The digital signals exiting the terminal, thus coming from layer IP, pass into a module 201, acting essentially as a multiplexer, which arranges the data signals originally organised into IP frames into lower level frames which can essentially be assimilated to HDLC frames (High Level Data Link Control - see ISO/IEC 3309 standard). The HDLC-type frame length (and in particular, the information field length) preferably is fixed and is suited to protocol PPP+ described herein. The experiments conducted so far by the Applicants have indicated as particularly advantageous the choice of a length of less than 240 bytes. The sizes of the frames can in any case be varied upon setting up the data connection. The HDLC-type frames thus obtained are then associated in module 202 with a cyclic redundancy control (CRC) code and then are made to pass, through a transmitter module 203 whose function shall be described better farther on, to a location in a memory 204 which is essentially to serve as buffer memory for communication with line 130 through unit 132. It will be appreciated that, as a general rule, at the output of transmitter module 203 the HDLC-type frames will have to be made compatible with the DECT frame structure and thus they are generally configured as non-formatted frames. In a dual manner, the digital signals coming from line 130 arrive at buffer memory 204 to be extracted, also in the form of non-formatted frames, in view of their passage to a receiver module 206 which reconfigures them as HDLC-type frames and passes such frames to an error control module 207. Hence the digital signals subjected to error control with positive outcome (thus validated or, if need be, subjected to an error correction operation of a type known in the art), still in the form of HDLC-type frames, pass to a demultiplexer module 208 which reconstitutes IP frames destined to propagate towards the IP layer. Error control module 207 presents a feedback on transmitter module 203. The feedback function is to allow transmission, towards the source of the received data, of messages indicating the correctness or incorrectness, respectively, of the data received. Upon recognition of an erroneous transmission (possibly non correctable), the data source is activated (in a way widely known in itself) to re-transmit the signals perturbed during transmission. Thus, a re- transmission mechanism is performed based on a selective repetition principle, which provides for the re-transmission solely of those HDLC-type frames where the presence of errors has been detected within windows comprising a number of HDLC-type frames that can be configured so as to optimise the system performance. The experiments conducted so far by the Applicants have shown that the choice of a value ranging from 16 to 32 for this parameter is particularly advantageous. In this regard it is important to point out that, at the PPP+ level, unlike the conventional PPP protocol, a segmentation and re-assembly (SAR) function is performed which allows using IP frame lengths exceeding the HDLC-type frame length (for instance, HDLC-type frames can be 4 to 8 times shorter than IP frames) thereby increasing the exploitation efficiency of the DECT channel. Through this function, the frame length on which PPP+ operates is made independent from the frame length required of the protocol at the layer immediately above (IP orthe like). In particular, the aforesaid SAR function is managed at the transmission side by module 203, which acts according to indications contained in the HDLC- type frames received by module 207. The frames received also provide information about the status of the frames received at the other end of the communication system, through a mechanism commonly denominated "piggy backing". Such information, together with the aforesaid indications, is transmitted from module 207 to module 203. The aforesaid segmentation and re-assembly mechanism is also effected, in an obviously complementary manner, at the receiving side, in module 206. Once the presence of a non recoverable error is detected, the aforesaid re-transmission mechanism is activated. Specifically, module 207 notifies module 203 that a PPP+ frame considered erroneous must be re-transmitted and - to that end - module 203 temporarily retains within it all PPP+ frames into which one IP frame has been segmented. Conversely, in case of a PPP+ frame not affected by errors or affected by a recoverable error, the HDLC-type frames resulting from the correction are sent towards demultiplexer module 208 in the terms described above. The latter entity accomplishes IP frame extraction from the HDLC-type frames by eliminating the HDLC flags, the FCSs (frame control sequences), the address and control fields, and then passes the information field, in the IP frame format, to layer IP. Upon reception of a data packet (DECT frame) from line 130, driving unit 132 copies that packet into a location of upper level memory 204 and notifies also the reception of a packet. Protocol PPP+ accepts the data (at the receiver module level) and identifies the start of the HDLC-type frame. This is done since the channel used procedures do not guarantee constant start and stop positions for all HDLC-type frames received. As a consequence it is necessary to identify a start-of-frame flag, as well as a corresponding stop-of-frame flag, and to copy them into another memory location in order to inform the error control module 207 which, in turn, accepts these data (in HDLC format) and currently performs the error detection/correction procedure, (with selective re-transmission, if necessary, in the terms specified above) including the various operations related with checking the flags and the payload data as well as with computing the CRC. At the transmission side, multiplexer module 201 receives IP frames from upper layer IP and multiplexes them, according to HDLC rules, thereby forming HDCL frames. It then informs transmitter module 203 of the availability of the data to be sent. Transmitter module 203 at the same time detects whether a message has been received from error control module 207. In the negative, it sends the data to driving unit 132 (through memory 204) thereby informing the physical layer of the existence of data to be transmitted. In summary, then, the set of the PPP+ modules integrate a protocol which in transmission performs the segmentation of the upper level frames (for instance IP frames) into lower level HDLC-type frames and autonomously manages the possible re-transmission of said frames in accordance with a correction mechanism. In reception the set of the PPP+ modules effects, starting from the HDLC-type frames received, the reconstruction of the upper level frame (for instance an IP frame), in a manner that is totally transparent to layer IP. This function has been denoted segmentation and re-assembly (SAR). It is advantageous for the operating modalities described above to have sufficiently large window sizes when the selective repetition mode is used. The window sizes indicated above, ranging from 16 to 32 frames, have proven particularly advantageous. The reference to such lengths must not however be interpreted as limiting the scope of the invention. It is in any case preferable that the window sizes are configurable, so that it is possible to suit to specific service requirements and to the environmental conditions in which the underlying access system and the mechanism described herein are to operate. This can be effected upon setting up the data transfer connection. Of course, without altering the principle of the invention, the embodiment details may vary widely with respect to what has been described and illustrated, without thereby departing from the scope of the present invention. In particular, although the present description has been made with reference to bi-directional transmission (with subsequent conversion, within the set of modules indicated as PPP+, of the digital signals from the IP layer into digital signals suitable of being transmitted on the voice channel of the DECT system and vice versa), it is wholly evident that the same solution is suitable for use even for unidirectional transmission only, i. e. both for the transmission (injection) of IP data into the DECT system and for the reception (extraction) of IP data from the DECT system. In this regard it should be noted that the segmentation and re-assembly function described above may be activated or not depending on the size of the PPP+ and IP frames, the same function constituting an important aspect to obtain the best performance on the DECT system.

Claims: CLAIMS 1. Device for the transmission of digital signals (IP) on at least one transmission channel (11) of a mobile communication system where a point-to-point communication protocol is active, characterised in that it comprises conversion means (PPP+) capable of operating in at least one direction of conversion between said digital signals (IP) and respective signals suitable for being transmitted on said transmission channel (11) for carrying out the conversion of said digital signals into respective signals suitable for being transmitted on said transmission channel (11) or the opposite conversion, said signals suitable for being transmitted on said transmission channel corresponding to said digital signals (IP) organised into HDLC-type frames adapted for error correction on the radio interface. 2. Device as claimed in claim 1 , characterised in that said conversion means (PPP+) are arranged to operate on HDLC-type frames presenting a length having at least one of the following characteristics: said length is fixed; said length is less than about 240 bytes; said length can be selectively configured upon setting up a connection for the transmission of said digital signals. 3. Device as claimed in claim 1 or 2, characterised in that said conversion means (PPP+) comprise a module (207) for the correction of errors of said HDLC-type frames through a selective re-transmission mechanism. 4. Device as claimed in any one of the previous claims, characterised in that said conversion means (PPP+) are arranged to perform transmission of said digital signals (IP) as said HDLC-type frames through a segmentation and reassembly of related signal packets. 5. Device as claimed in claims 3 and 4, characterised in that said conversion means (PPP+) are arranged to perform the segmentation and re-assembly according to indications provided by said error correction module (207). 6. Device as claimed to any one of claims 1 through 5, characterised in that said conversion means (PPP+) comprise, in the direction of conversion of said digital signals (IP) into said signals suitable for being transmitted on said transmission channel (11), a module (202) for the introduction of a code for transmission error detection. 7. Device as claimed in claim 6, characterised in that said conversion means (PPP+) comprise, in the direction of conversion of said signals suitable for being transmitted on said transmission channel (11) into said digital signals (IP), a module (207) for the transmission error detection by exploiting said code. 8. Device as claimed in claims 3 and 7, characterised in that said error detection module (207) is arranged to perform said error correction. 9. Device as claimed in claims 7 and 8, characterised in that said error detection module (207) is arranged to determine, upon detection of a transmission error, the transmittal towards said transmission channel (11) of a message commanding the re-transmission of the frame where the presence of an error has been detected. 10. Device as claimed in claims 8 and 9, characterised in that said error detection module (207) is arranged to determine the transmittal of the aforesaid message commanding re-transmission only in the presence of an error considered as not correctable. 11. Device as claimed in claim 9 or 10, characterised in that said re-transmission concerns an HDLC-type frame window whose length is selectively configurable at the set up of the connection for the transmission of said digital signals. 12. Device as claimed in claim 11 , characterised in that said window has a length ranging from 16 to 32 frames. 13. Device as claimed in any one of the previous claims, characterised in that the device uses a plurality of said transmission channels (11) associated with means to subdivide said digital signals into a corresponding plurality of flows each transmitted on a respective transmission channel (11), as well as with means to re-assemble said flows of digital signals after transmission. 14. Device as claimed in any one of the previous claims, characterised in that said transmission channel (11) is a voice channel of a DECT system and said digital signals are data signals. 15. Method of transmission of digital signals (IP) on at least one transmission channel of a mobile communication system (11) where a point-to-point communication protocol is active, characterised in that it comprises the operation of effecting a conversion, in at least one direction, between said digital signals (IP) and respective signals suitable for being transmitted on said transmission channel (11), said signals suitable for being transmitted on said transmission channel (11) corresponding to said digital signals (IP) organised in HDLC-type frames adapted for error correction at the radio interface. 16. Method as claimed in claim 15, characterised in that said HDLC-type frames are chosen with a length having at least one of the following characteristics: - said length is fixed; said length is less than about 240 bytes; said length can be selectively configured upon setting up the connection for the transmission of said digital signals. 17. Method as claimed in claim 15 or 16, characterised in that said conversion operation entails a function (207) of error correction of said HDLC-type frames through a selective re-transmission mechanism. 18. Method as claimed in any one of claims 15 through 17, characterised in that said conversion entails, in the passage from said digital signals (IP) to said HDLC-type frames, a function of segmentation and re-assembly of related signal packets. 19. Method as claimed in claim 17 and claim 18, characterised in that said segmentation and re-assembly function is managed according to said error correction function. 20. Method as claimed in any one of claims 15 through 18, characterised in that the operation of converting said digital signals (IP) into said signals suitable for being transmitted on said transmission channel (11) comprises the introduction (202) of a code for transmission error detection. 21. Method as claimed in claim 20, characterised in that the operation of converting said signals suitable for being transmitted on said transmission channel (11) into said digital signals (IP) comprises a transmission error detection phase exploiting said code. 22. Method as claimed in claim 21, characterised in that said error detection phase is associated with a transmission error correction function. 23. Method as claimed in claim 21, characterised in that, upon detection of a transmission error, a message is sent towards said transmission channel (11) for commanding the re-transmission of the frame where the presence of an error has been detected. 24. Method as claimed in claims 22 and 23, characterised in that said message commanding re-transmission is only emitted in the presence of an error considered as not correctable. 25. Method as claimed in claim 23 or 24, characterised in that said retransmission is based on a window of HDLC-type frames whose length is selectively configurable upon setting up the connection for the transmission of said digital signals. 26. Method as claimed in claim 25, characterised in that said window has a length ranging between 16 and 32 frames. 27. Method as claimed in any one of the claims from 15 through 26, characterised in that transmission of said digital signals takes place by using a plurality of said transmission channels (11) and in that said digital signals are subdivided in a corresponding plurality of streams, each transmitted on a respective transmission channel (11), said streams being re-assembled after transmission. 28. Method as claimed in any one of the claims from 15 through 27, characterised in that the transmission channel (11) is a voice channel for a DECT system and said digital signals are data signals.

Inventors: BETTI ALESSANDRO, FERRARI GIANFRANCO, GOBBI ROBERTA, MORENA GAETANO, BETTI, ALESSANDRO, FERRARI, GIANFRANCO, GOBBI, ROBERTA, MORENA, GAETANO
Applicants: BETTI ALESSANDRO, FERRARI GIANFRANCO, GOBBI ROBERTA, ITALTEL SPA, MORENA GAETANO, TELECOM ITALIA SPA, BETTI, ALESSANDRO, FERRARI, GIANFRANCO, GOBBI, ROBERTA, ITALTEL S.P.A., MORENA, GAETANO, TELECOM ITALIA S.P.A.
Application year: 1998
Main classification: H04J7
IPC codes: H04L1, H04L12, H04L29, H04W88