ITTO970311A1 - DEVICE AND PROCEDURE FOR THE TRANSMISSION OF DIGITAL SIGNALS, ESPECIALLY ON DECT SYSTEMS. - Google Patents

Description

DESCRIPTION of the industrial invention entitled .- "Device and method for the transmission of digital signals, particularly on DECT type systems"

TEXT OF THE DESCRIPTION

The present invention relates in general to the transmission of digital signals and has been developed with particular attention to the possible application to the transmission of data in the context of communication systems preferably operating according to the DECT standard. This statement is not to be understood in a limiting sense since the invention is advantageously applicable to any system of the mobile radio type. The same invention also applies to both the private application environment (PBX) and the public application environment.

The DECT standard (acronym for Digital Enhanced Cordless Telecommunications, i.e. improved cordless digital telecommunications) was developed and finalized at European level in order to facilitate the use in telecommunications networks of cordless terminals capable of communicating in the context of a picocell system with respective base stations.

For a general overview of the subject, one can usefully refer to the work "The New DECT Standard for Cordless Communications" by Hans van der Hoek published on pp. 77 to 80 of the February 1993 issue of Telecommunications magazine, or again in the article "DECT-Cordless Functionality in New Generation Alcatei PABXs" by v. Werbus, A. Veloso and A. Villanueva, published on pp. 172 to 180 of the Electrical Communication magazine - second quarter 1993.

The present invention faces the problem of allowing the transmission of digital signals (and in particular of data signals) on a system operating according to a standard such as DECT or similar. In this perspective, the term "digital signals", as used in the present description and in the claims annexed thereto, generally indicates digital signals not directly associated with the voice signal.

On a superficial analysis, the aforesaid problem does not emerge in its relevance, since the DECT standard is based precisely on the transmission of digital signals. This also in the case of voice signals, after coding them according to an ADPCM scheme with a transmission speed (bit rate) of 32 Kbit / sec. Such a system would therefore appear to be easily adaptable to the transmission of digital signals such as data signals through the insertion and, respectively, the extraction of the data flows downstream and upstream of the coding / decoding sections of the voice signal.

The existing DECT infrastructures, however, are almost exclusively prepared for the transmission of signals in voice, and the voice transmission channel of the DECT proves to be poorly protected against perturbative phenomena such as sudden attenuation and / or propagation on multiple paths of the signal. In the transmission of the voice signal, the aforesaid perturbation phenomena, typical of the context of application of a mobile radio system such as DECT, are in fact tolerable. Although restricted in band, the voice signal has in fact intrinsic redundancy qualities such as to make it "robust" against the aforementioned perturbative phenomena, in the sense that these, as they occur in the current use of the system, are not usually such to be engraved on the voice signal until it is unintelligible.

On the other hand, the situation is quite different in the case of digital signals such as data signals (including signals relating to images, etc.): if placed directly on the digital channel of the DECT system, the latter would be strongly altered by the aforementioned perturbative phenomena to the point of practically rendering communication is impossible, at least in the vast majority of situations of current employment.

Basically, the invention aims to overcome the aforementioned drawback, allowing, for example, the reliable transmission of digital signals such as data signals within a "DECT type" system.

The term "of the DECT type", as used in the present application, must be understood in the sense that the invention applies not only to systems corresponding exactly to the DECT standard as it is precisely defined by current regulations, but also to communication systems operating in substantially similar ways. Therefore, these systems are exposed, when used for the transmission of digital signals, to the occurrence of the same drawbacks described above. Reference made in the context of the present description and, if applicable, the appended claims, to the DECT standard; it must therefore be understood as such as to include within the scope of the invention also such substantially similar systems.

What described below applies both to the case in which a single connection or call is used, and in the case in which several connections are used in parallel, between which it is possible to divide the data flow relating to a user. The protocol can essentially operate on multiple channels by splitting and recombining the data at the two ends. For simplicity, the case of data transport on a single connection will be described below.

The invention will now be described, purely by way of non-limiting example, with reference to the attached drawings, in which:

- Figure 1 illustrates, in the form of a block diagram, the general structure of a terminal inserted in a DECT system and capable of operating according to the solution according to the invention,

Figure 2 illustrates, in greater detail, again in the form of a block diagram, the implementation of the invention in the context of a. terminal such as that shown in Figure 1, and

Figure 3 illustrates further details of implementation of the invention.

In Figure 1 (taken from the article by Werbus et al. Cited in the introductory part of the present description) the numerical reference 1 generally indicates a mobile terminal inserted in a telecommunications system operating according to the DECT standard (or similar). Terminal 1 is in fact comparable to a normal cordless telephone capable of communicating with a base station arranged for connection to one or more mobile terminals.

The basic components of terminal 1 are:

- voice signal encoding / decoding group 2 (ADPCM encoding / decoding) connected respectively to a microphone 3 and a loudspeaker 4,

- control unit 5 in charge of the selection and signaling functions when the called / calling number is displayed, usually associated with a keypad and a display unit (not shown),

- group 6 acting as the MAC level management logic (Medium Access Control Layer, i.e. level or layer for controlling access to the transmission medium) and as a decoder for the generation of DECT frames,

- modulator 7 (usually operating according to a GMSK or π / 4-DQPSK modulation, with different choices, for example, in the European and US standard),

- radio frequency synthesizer 8,

- transmit radio frequency stage 9a, - receive radio frequency stage 9b, and - antenna assembly 10.

The interconnection criteria of the various modules referred to above and the relative operating modes are to be considered widely known in the art and such as not to be illustrated here, also because they are not relevant in themselves for the purposes of understanding the invention.

For the purposes of further illustrating the invention, the set of modules 5 to 10 can in fact be seen as a sort of "black box" 11 (figure 2) which in fact forms the respective portion of the transmission channel in the device and in addition to the voice unit (2 to 4) it is connected to two further units 12, 13 which are characteristic of the configuration according to the invention. The latter communicate with the black box 11 through two bidirectional communication lines 120, 130, for example configured as RS232 / V.24 serial interface lines. It is however evident that the aforesaid lines could be of a completely different type, providing, for example, the transmission of the bytes on eight wires.

The units 12, 13, the characteristics of which will be better illustrated in the following, are intended to exchange with the black box 11 the data relating to the signaling information and to the user information, named according to current terminology, respectively plan C and plan U .

The communication between the units 12 and 13 - in fact capable of being integrated, at least in part, in the processing means (typically a personal computer or personal computer or PC) used by the user for the transmission of data on the system - and unit 11 could also be made using a single interface, for example with a single line, such as a line of the RS232 / V.24 type, capable of conveying both signaling information and user information.

However, the use of two interfaces proves to be preferential, since the black box 11 is able to implement the entire DECT stack on the terminal up to the MAC layer. In this way it is therefore possible to keep separate the functions relating to plane C, which can be implemented as a section of the portable part (or PP) of the DECT terminal (this is precisely the meaning of the dotted line that surrounds, in figure 2, the black box 11 and the unit 12), and the functions of the plane U, preferably allocated on the user's laptop and kept as much as possible under the control of the user's computer applications.

In particular, the functions performed by the unit 13 of figure 2 can be implemented entirely on the user's computer or divided between this and a dedicated intermediate apparatus (adapter) or finally be placed on the terminal (blocks 11 and 13 of figure 2 partially integrated ). The methods for dividing the functions described below are therefore not to be understood in a limiting sense.

The implementation choice described here can be justified considering the fact that the black box 11 and the control processing structure in which the unit 12 is implemented are actually vital for the operation of the portable part of the DECT terminal even as a simple telephone terminal . Once the computer related to the user's floor (floor u) is disconnected, the control of the remaining functional group (black box 11 and processing structure in which the unit 12 is implemented) can function as a portable part of the DECT and a user can access it to make simple voice calls, for example using a generic access profile (GAP = Generic Access Profile). As is known, this denomination indicates a subset of the DECT protocol which allows the interworking of the apparatuses for a basic telephone service. In this regard, reference can be made to the ETS 300444 standard.

For the communication through the communication line 120, the unit -12 makes use of a management module 122 to which, according to a typical stack configuration, a three-layer structure, indicated respectively as DLC, NWK and IWU, is connected. These layers and the relative interface at the MAC layer (included in the black box 11 of Figure 2) are used to make voice calls and / or data calls.

The DLC layer is essentially the one that oversees the Data Link Control. The upper NWK layer essentially oversees the network management (NetWork layer). The IWU level, even higher, manages the interface functions with the user: it is essentially the level of interworking at the application level, used to access functions not described in the DECT standard and made available by the central body.

The unit 13, on the other hand, is connected to the line 130 through a driving group 132 which interfaces a corresponding stack structure comprising three layers or levels: PPP +, IP and TCP.

The PPP + layer is essentially responsible for managing the point-to-point protocol and is based on the Point to Point Protocol (PPP) standard. The Internet Protocol (IP) layer is typical of an Internet Protocol. Finally, the Transmission Control Protocol (TCP) layer is responsible for managing the transmission protocol.

When a user wishes to use the terminal for the transmission of digital signals, ie data, it is sufficient for him to connect his personal computer to the line 130 thus preparing the portable part PP of the terminal to receive data.

The architecture described above has two further advantages.

In the first place, it allows the distribution of the operating loads on different processors, since the relevant load linked to the elaboration of plan U has no influence on the processor dedicated to the functions of plan C. This allows to realize the solution by carrying out an update of a DECT portable part originally designed for voice service only. Furthermore, the solution is compatible with a series of different allocations of functions between the DECT portable part, the user's computer and intermediate devices (see in this regard the considerations made previously regarding the implementation of the units 12 and 13).

A further advantage is achieved in terms of flexibility, since the architecture considered does not prevent and indeed lends itself well to the merger of the two units 12, 13 into a single machine, so as to obtain a fully integrated voice and data terminal.

Figure 3 illustrates in the form of a functional diagram the criteria implemented at the PPP + level within the unit 13. In practice, the PPP + level realizes the communication between the IP level and the lines 130 through the driving group 132.

The digital signals coming out of the terminal, therefore coming from the IP level, pass into a module 201, essentially acting as a multiplexer, which orders the aforementioned digital signals originally organized in frames at IP level in lower level frames substantially similar to HDLC frames (High Level Data Link Control = control of a high-level data link - see the ISO / IEC 3309 standard) whose length is preferably chosen as a fixed value and such as to be adapted to the PPP + protocol described here. The experiments carried out up to now by the Applicants have indicated the choice of a length of less than 240 bytes as particularly advantageous. The size of the frames can in any case be varied when the data connection is established.

The HDLC frames thus obtained are then subjected, in module 202 to an operation of insertion of a cyclic redundancy control code (CRC) and then pass through a transmitter module 203, the function of which will be better illustrated below, towards a location 204 whose function is essentially that of acting as a buffer memory in view of the communication with the line 130 through the group 132. It will be appreciated that, in principle, at the output of the transmitter module 203 the HDLC frames are configured as a whole as unformatted textures.

In a dual way, the digital signals coming from the line 130 arrive in the buffer memory 204 to then be extracted, also here in the form of unformatted frames, in view of the passage to a receiver module 206 which reconfigures them as HDLC frames. All this in view of the passage through an error control module 207. Hence the digital signals subjected to error control, with positive results (therefore validated or, possibly, subjected to an error correction operation - type known), always configured in the form of HDLC frames, pass to a demultiplexer module 208 which reconstitutes the frames at the IP level destined to propagate towards the IP level itself.

The error control module 207 has a feedback on the transmitter module 204 whose function is to send messages to the source of the received data which respectively reflect the correctness or the incorrectness of the data received. Upon recognition of an erroneous (possibly uncorrectable) transmission, the data source is activated (in a manner which is widely known per se) so as to transmit again the signals disturbed during transmission. A retransmission mechanism based on the principle of selective repetition is thus implemented which provides that only those HDLC frames are retransmitted in which the presence of errors has been detected in the context of windows comprising a number of configurable HDLC frames in view of performance optimization. system. The experiments carried out so far by the Applicants have shown that the choice, for this parameter, of a value between 16 and 32 is particularly advantageous.

In this regard, it is important to draw attention to the fact that at the PPP + level, unlike the conventional PPP protocol, a segmentation and reassembling function (Segmentation And Reassembling or SAR) is implemented - which allows the use of packet lengths at the IP level greater than the HDLC frame length (for example HDLC frames can be of the order of 4 or 8 times shorter than IP frames) increasing the exploitation efficiency of the DECT channel. Through this function, the independence of the frame length on which PPP + operates from the frame length of the next higher level protocol (IP or similar) is obtained.

In particular, the aforementioned SAR function is managed in transmission by module 203, which acts on the basis of the indications contained in the HDLC frames received by module 207.

The same frames received also give information on the status of the frames received from the other side of the communication system, through a mechanism of the type currently called "piggy backing". This information, together with the above, must be transmitted from module 207 to module 203.

The aforementioned segmentation and reassembly mechanism is also carried out, in an evidently complementary way, upon receipt, in module 206.

Once the presence of a non-recoverable error has been detected, the aforementioned retransmission mechanism is provided. In particular, module 207 signals to module 203 that a frame (packet) at the PPP + level considered to be erroneous must be returned and - for this purpose - module 203 itself (temporarily) keeps inside it all the PPP + packages in which a packet to IP layer has been segmented. On the contrary, in case of PPP + frame not affected by error or affected by a recoverable error, the HDLC frames deriving from the correction function are sent towards the IP layer in the terms described above. The latter entity carries out the extraction of the IP frames starting from the HDLC frames by eliminating the HDLC identifiers, the FCS addresses, the control fields and then passing the information field, in the IP frame format, to the IP level.

Upon receipt of a data packet starting from the line 130, the management of the group 132 copies the same into the upper level memory position 204, also notifying that a packet has been received.

The PPP + protocol accepts the data (at the receiver module level) and identifies the start of the HDLC frame. This is done because the channel usage methods do not guarantee constant start and end positions (start and stop) for all HDLC frames received.

Consequently, it is necessary to identify a frame start flag, as well as a corresponding frame end indicator by copying them to another memory location so as to inform the control module of the error 207 which, in turn, accepts these data (in HDLC format) to currently implement the detection procedure - error correction (with selective retransmission, if necessary, within the terms specified above) including the various control operations of the indicators and useful data as well as the calculation of the CRC.

During transmission, the multiplexer module 201 receives the IP frames from the upper IP layer and the multiplexes, according to the HDLC rules, forming HDLC frames. It then informs the transmitter module 203 of the availability of data to be sent. The transmitter module 203 detects at the same time whether a message has been received from the control module of the error 207. If not, it sends the data (received by the multiplexer module 201) to the interface management, thus informing the level physical existence of data to be transmitted.

In summary, therefore, the set of PPP + modules actually integrates a protocol that performs the segmentation of higher-level packets in transmission (for example at the IP level) in the lower-level frames of the HDLC type and autonomously manages any retransmission of said frames in accordance with a correction mechanism. In reception, the set of PPP + modules carries out, starting from the received HDLC frames, the reconstruction of the higher level packet (for example IP), in a totally transparent way with respect to the latter. This functionality has been identified with the term segmentation and recomposition (SAR).

The modes of operation described above benefit from having sufficiently large window sizes when using the selective repeat mode. The window dimensions indicated above, ranging from 16 to 32, have proved to be particularly advantageous. Moreover, the reference to them must not be interpreted as limiting the scope of the invention. However, it is preferable to have configurable window sizes so as to be able to better adapt to the specific service requirements and to the environmental conditions in which the underlying access system and the mechanism described herein operate. This can be done when establishing the connection for data transfer.

Naturally, the principle of the invention remaining the same, the details of construction and the embodiments may be widely varied with respect to what has been described and illustrated, without thereby departing from the scope of the present invention. In particular, even if the present description has been made with reference to the bidirectional transmission (with consequent conversion, within the set of modules indicated with PPP +, of the digital signals at the IP level into digital signals capable of being transmitted on the system channel DECT and vice versa), it is quite clear that the same solution can also be used only for one-way transmission, that is, both for the transmission (input) of IP data in the DECT system, and for the reception (extraction) of data IP from the DECT system. In this regard, it should also be noted that the segmentation and reassembly (SAR) function described above can be activated or not depending on the size of the PPP and IP packets, the same function being an important aspect for obtaining the best performance on DECT.

Claims (28)

CLAIMS 1. Device for the transmission of digital signals (IP) on at least one transmission channel (11) of a mobile radio system on which a point-to-point communication protocol is active, characterized in that it includes: - conversion means (PPP +) operating in at least one conversion direction between said digital signals (IP) and respective signals capable of being transmitted on said transmission channel (11), with said signals capable of being transmitted on said transmission channel ( 11) which correspond to said digital signals (IP) organized in HDLC-type frames, said HDLC-type frames being adapted for error correction on the radio interface. 2. Device according to claim 1, characterized in that said HDLC-type frames have 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 configurable upon establishing the connection for the transmission of said digital signals. 3. Device according to claim 1 or claim 2, characterized in that said conversion means (PPP +) comprise a module (207) for error correction of said HDLC-type frames by means of a selective retransmission mechanism. 4. Device according to any one of the preceding claims, characterized in that said conversion means (PPP +) perform the transmission on said HDLC-type frames of said digital signals (IP) by means of a segmentation and recomposition function of the relative signal packets. 5. Device according to claim 3 and claim 4, characterized in that said segmentation and recomposition function is managed according to indications derived from said module (207) for error correction. 6. Device according to any one of claims 1 to 5, characterized in that said conversion means (PPP +) comprise, in the direction of conversion between said digital signals (IP) and said signals capable of being transmitted on said transmission channel (11 ), a module (202) for introducing a transmission error detection code. 7. Device according to claim 5, characterized in that said conversion means (PPP +) comprise, in the direction of conversion between said signals capable of being transmitted on said transmission channel (11) and said digital signals (IP), a module (207) for detecting the transmission error on the basis of said code. Device according to claim 7, characterized in that said error detection module (207) is configured with an error correction function. 9. Device according to claim 7, characterized in that said error detection module (207) is configured, to determine, upon detection of a transmission error, the sending to said transmission channel (11) of a message command for the retransmission of the frame in which the presence of an error has been detected. Device according to claim 8 and claim 9, characterized in that said error detection module (207) is configured to cause the aforementioned retransmission command message to be sent only in the presence of an error considered non-correctable. 11. Device according to any one of claims 9 or 10, characterized in that said retransmission involves a window of frames of the HDLC type of selectively configurable length upon establishing the connection, for the transmission of said digital signals. 12. Device according to claim 11, characterized in that said window has a length between 16 and 32. 13. Device according to any one of the preceding claims, characterized in that the device itself uses a plurality of said transmission channels (11) with associated means for dividing said digital data into a corresponding plurality of streams each transmitted on a respective transmission channel (11) as well as means for recombining said digital data streams after transmission. Device according to any one of the preceding claims, characterized in that the transmission channel (11) is a speech channel of a DECT system. 15. Process for the transmission of digital signals (IP) on at least one transmission channel of a mobile radio system (11), characterized in that it comprises the operation of carrying out, in at least one direction, a conversion between said digital signals (IP ) and respective signals capable of being transmitted on said transmission channel (11), with said signals capable of being transmitted on said transmission channel (11) corresponding to said digital signals (IP) organized in frames of the HDLC type, said frames being adapted for error correction on the radio interface. 16. Process according to claim 15, characterized in that said HDLC-type frames are selected 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 is selectively configurable upon establishing the connection for the transmission of said digital signals. 17. Process according to claim 15 or claim 16, characterized in that said conversion operation involves an error correction function (207) of said HDLC-type frames by means of a selective retransmission mechanism. 18. Process according to any one of claims 15 to 17, characterized in that said conversion involves, in the passage to said HDLC-type frames starting from said digital signals (IP), a segmentation and recomposition function of relative signal packets. 19. Process according to claim 17 and claim 18, characterized in that said segmentation and recomposition function is managed in dependence on said error correction function. Method according to any one of claims 15 to 18, characterized in that the operation of converting said digital signals (IP) into said signals capable of being transmitted on said transmission channel (11) comprises introducing (202) a transmission error detection code. 21. Process according to claim 20, characterized in that the conversion operation of said signals capable of being transmitted on said transmission channel (11) into said digital signals (IP) comprises a step for detecting the transmission error based on said code. Method according to claim 21, characterized in that a transmission error correction function is associated with said error detection step. 23. Method according to claim 21, characterized in that when a transmission error is detected, a message for the retransmission of the frame in which the presence of an error has been detected is sent to said transmission channel (11). 24. Method according to claim 22 and claim 23, characterized in that said retransmission message is issued only in the presence of an error considered to be uncorrectable. 25. Process according to claim 23 or claim 24, characterized in that said retransmission is based on an HDLC-type frame window of selectively configurable length upon establishing the connection for the transmission of said digital signals. 26. Process according to claim 25, characterized in that said window has a length comprised between 16 and 32. 27. Process according to any one of claims 15 to 26, characterized in that it comprises the operation of using a plurality of said transmission channels (11) and in that, for the purpose of transmission on said plurality of channels (11), said digital signals are divided into a corresponding plurality of streams, each transmitted on a respective transmission channel (11), said streams of said plurality being recombined with each other after transmission. Method according to any one of claims 15 to 27 characterized in that the transmission channel (11) is a speech channel of a DECT system.