ARRANGEMENT FOR ENHANCING DATA TRANSMISSION IN A DIGITAL CELLULAR RADIO NETWORK

Abstract: The invention relates to an arrangement for enhancing data transmission in a digital cellular radio network. The arrangement comprises a radio data terminal (A), a network terminating unit (IWF) connected to a mobile exchange (MSC) and comprising a data modem (MOD1), and a remote data modem (MOD2). The radio data terminal (A) and the remote data modem (MOD2) comprise data compression and decompression. The digitatl transmission connection between the radio data terminal (A) and the data modem (MOD1) is a non-transparent asynchronous data connection of a cellular radio system, over which compressed data is transmitted using the error correction of the cellular system. The error correction protocol of the remote modem is used for the transmission of compressed data over the modem connection between the data modem (MOD1) and the remote data modem (MOD2). At the beginning of a data session the data modem transmits compression parameters to the radio data terminal; these parameters are determined on the basis of a handshaking procedure between the data modem and the remote data modem.

Description: Arrangement for enhancing data transmission in a digital cellular radio network Field of the Invention The invention relates to an arrangement for enhancing data transmission in a digital cellular radio networ . Background of the Invention In the European mobile telephone system GSM it is possible to establish a circuit switched data trans¬ mission connection or traffic channels so as to enable a data terminal connected to the data interface of a mobile radio unit to transfer data to and from a data modem in a mobile exchange and further by means of a modem link to and from another data modem and a data terminal connected to it. The maximum transmission rate of such data transmission is determined by the maximum capacity of the data channel established over the radio path, e.g. 9600 bit/s in the GSM system. In many appli¬ cations there exists, however, a need for considerably higher transmission rates of user data. Disclosure of the Invention An object of the present invention is to enhance data transmission in a digital cellular radio network. According to the invention, an arrangement for enhancing data transmission in a digital cellular radio network, said arrangement comprising a radio data terminal; a network terminating unit connected to a mobile exchange and comprising a data modem; a remote data modem; and call control means for establishing a digital transmission connection between the radio data terminal and the data modem and a modem connection between the data modem and the remote data modem, is characterized in that the radio data terminal and the remote data modem comprise means for transmitting data in a compressed form, the digital transmission connec- tion between the mobile radio data terminal and the data modem is a non-transparent asynchronous data connection of a cellular radio system, over which connection the compressed data is transmitted using the error correc¬ tion of the cellular system, and the data modem of the network terminating unit comprises means for trans¬ mitting compressed data over said modem connection by using the error correction protocol of the remote modem and for transmitting compression parameters to the radio data terminal at the beginning of a data session, said parameters being determined on the basis of a hand¬ shaking procedure between the data modem and the remote data modem. According to the invention, user data is trans¬ mitted between a radio data terminal and a remote data modem in a compressed form, whereby any possible redundancy has been removed from the data stream, and thus the amount of data to be transmitted over the radio link is reduced. The typical maximum compression ratios obtained by compression algorithms are 2:1 or 4:1, which allows a user data stream of up to 19200/38400 bit/s to be adapted in a data channel of 9600 bit/s in a cellular radio system. The actual compression ratios achieved by the compression algorithms are, however, highly depend¬ ent on the type of user data. The transmission of compressed data requires a completely error-free transmission channel, as even a minor transmission error corrupts the decompression algorithm at the reception end. It is therefore necessary to use as efficient error correction as possible over the entire transmission connection in order to prevent transmission errors. An obvious solution is to use, in addition to compression, the same error correction algorithm in the radio data terminal as in the remote data modem of the other end, whereby the intermediate transmission connection would only be a transparent "tube" through the radio path. However, the error correction uses the transmission capacity to some extent, wherefore the maximum capacity of the data channel over the radio path cannot be utilized solely for the transmission of user data. According to the invention, this is solved so that the digital transmission connection over the radio path between a mobile radio data terminal and a data modem in a network terminating unit is a non-transparent asynchronous transmission connection, where the error correction protocol of the radio system, optimized for correcting errors in the radio link, can thus be employed automatically. As the error correction of the radio system does not use the capacity of the data channel, the invention allows the entire transmission capacity to be used for the transmission of compressed user data between a radio terminal equipment and a data modem in a network terminating unit. According to the invention, the error correction required over the modem connection is achieved by utilizing identical error correction protocols in the data modem of the network terminating unit and in the remote data modem at the other end of the modem connection. In the invention the compression functions are thus distributed to the radio data terminal and the error correction of the modem connection to the data modem, which in no way particip¬ ates in the compression. At the beginning of the modem connection, however, the data modem negotiates the com¬ pression parameters to be used in the data transmission by means of handshaking with the remote data modem, and transmits them to the radio data terminal. Brief Description of the Drawings The invention will be described in more detail by means of illustrative embodiments with reference to the accompanying drawings, in which Figure 1 is a block diagram which illustrates the structure of a cellular radio system, Figure 2 is a block diagram which illustrates a system configuration for data transmission according to the invention, Figures 3A and 3B illustrate the disposition of the compression function in a mobile station and correspondingly in terminal equipment, Figure 4 is a block diagram which illustrates a mobile station and a connection of a data terminal thereto, and Figure 5 is a block diagram which illustrates a mobile exchange implemented according to the inven¬ tion. Detailed Description of the Invention The arrangement according to the present inven- tion for implementing a facsimile service is applicable to any digital cellular radio system. It is particularly suitable for use in the European digital mobile tele¬ phone system GSM and corresponding digital systems, such as DCS1800 and PCN (Personal Communication Network). In the following, the preferred embodiment of the invention will be described in connection with the GSM system; it is, however, not restricted to this system. Figure 1 is a schematic view of part of the GSM cellular radio system. In a cellular radio system the area covered by the system is divided into radio cells. Two base station controllers BSC1 and BSC2 are connected to a mobile exchange MSC by digital transmission connec¬ tions 12. Base stations BTS1, BTS2 and correspondingly BTS3 and BTS4 are further connected to the base station controllers BSC1 and BSC2 by digital transmission con¬ nections 11. Each BSC and the base stations associated with it form a base station system BSS. Typically the radio coverage area of a base station BTS forms a radio cell. Each base station BTS1-BTS4 has a predetermined number of physical radio channels. A signal (physical channel) of the GSM system consists of TDMA frames, each of which contains preferably eight time slots by means of which logical channels are transmitted. The logical channels contain traffic channels for calls (voice and data) to be set up with mobile stations MS located within the cell, and control channels for signalling performed with the mobile stations located within the cell. Over the traffic channels it is possible to establish a speech connection, a V.110 rate-adjusted 64 kbit/s full-duplex data connection or a 9.6/4.8/2.4 kbit/s data connection. Special purpose adaptors are typically required at both ends of the data connection for adapting the data connection to terminal equipment and/or other transmission connections/systems. The adaptor connected to terminal equipment is usually called a terminal adaptor, and the adaptor at the net¬ work end a network terminating unit. In the GSM mobile phone system such a network terminating unit is located in a mobile exchange MSC. Figure 2 shows a data transmission arrangement in which radio terminal equipment A consists of a mobile station 3 (MS) and data terminal equipment DTE1 connect¬ ed thereto. A data connection as described above is established on a logical traffic channel from the radio terminal equipment A to a mobile exchange 8 (MSC) by means of the radio link between the mobile station MS and the base station system 6 (BSS) . The network adaptor IWF in the mobile exchange MSC comprises a data modem MODI. The line interface (modem interface) of the modem is connected to a conventional 2-wire line (2w), which in the example is connected through a public switched telephone network 9 (PSTN) to a remote modem 10, which in turn is connected to another remote terminal equip¬ ment B comprising a remote modem 10 (M0D2) and a data terminal DTE2 connected thereto. The data modem MOD of the network terminating unit and the remote data modem 10 signal in the normal manner over the modem connection established between them. The data terminal equipment DTE1 controls the operation of the data modem MODI over the GSM data connection, transmits data to the data modem MODI, and correspondingly receives it. Normally the bottle-neck of the end-to-end data connection between radio terminal equipment A and remote terminal equipment B is the GSM data connection, the maximum transmission rate of which is typically 9600 bit/s. In the present invention the data transmission is enhanced by providing the terminal equipment A and B with data compression and decompression, and by trans¬ mitting data over an end-to-end link A-B in a compressed form. The transmission of compressed data is very sensitive to transmission errors, and therefore it is absolutely necessary to have error correction. Conven¬ tional compressing modems connected to a public switched telephone network PSTN automatically comprise error correction. The remote terminal equipment B can thus be implemented with normal commercial equipment comprising data compression and error correction, e.g. in accord¬ ance with CCITT Recommendation V.42bis. When the GSM data connection is established as a so-called trans- parent data connection and the radio terminal equipment A is provided with a similar data compression and error correction as the terminal equipment B, the entire data connection is provided with data compression and error correction. In this case, however, the error correction uses part of the transmission capacity of the GSM data connection and reduces thus the advantage gained by com¬ pression. Moreover, the error correction employed does not necessarily suit in the best possible manner for the correction of errors occurring over the radio path. Such configuration further requires an end-to-end synchronous connection, wherefore the GSM connection must also be synchronous, and the radio terminal equipment A should be designed for such synchronous transmission. However, both the present subscriber terminals and those under development for cellular radio networks support prim¬ arily asynchronous data transmission solutions. In the present invention this is solved by distributing the functions in such a manner that data compression and decompression are performed in the radio terminal equipment A, and the error correction required for the modem connection is performed in the data modem MODI of the network terminating unit. The transmission connection between the radio terminal equipment A and the data modem MODI of the network terminating unit is thus a so-called non-transparent asynchronous GSM data connection over which compressed data is transmitted using the error correction of the GSM system, said error correction being optimized for the radio path. The error encoding of the GSM system does not use the capacity of the GSM data connection, and therefore the entire capacity of the channel can be used for the transmission of compressed user data. The error correction in the data modem MODI of the network terminating unit corresponds to the error correction in the remote data modem M0D2 and performs error correction for the modem connection. The modem MODI does not participate in the actual data compression in any way. By means of handshaking performed at the beginning of the modem session, the modems MODI and M0D2 agree with each other on the compression parameters to be used during the data session. The modem MODI trans¬ mits these compression parameters over the GSM data connection to the compression algorithm of the radio terminal equipment A. The compression/decompression function can be disposed in several different ways in the radio terminal equipment A. In Figure 3A the compression/decompression is performed in the terminal adaption function TAF of a mobile station MS, between the DTE interface INT1 and RLP (radio link protocol) and the rate adaption functions L2RC0P. This entails modifications in a con¬ ventional MS. Figure 4 shows a mobile station MS and data terminal equipment DTE connected thereto. The MS 33 comprises a radio unit 36, call control 39, a user interface 40 (e.g. keyboard and display), mobility management 41, radio path management (signalling functions) 42, and a terminal adaptor 37 providing a V.24 interface. The radio unit comprises e.g. a radio transceiver 44 connected to an antenna 55, and a channel coding and decoding unit 43. By means of a two-way switch unit 38, the unit 43 can be connected either to a telephone unit, comprising e.g. a speech codec 56 and a telephone receiver 34, or to a terminal adaptor 37. The terminal adaptor 37 comprises a unit 46 which performs data compression/decompression and adaption for the GSM data connection. The unit 46 is connected to the two-way switch 38 via an I/O circuit 45 and to a data terminal 31 via an I/O circuit 47, which forms the interface 53 (e.g. V.24). Another alternative is to dispose the com¬ pression/decompression function between the application programme and the serial interface as shown in Figure 3B. In this case the MS does not have to be modified. Figure 5 illustrates one way of applying the invention to a mobile exchange MSC. For the sake of clarity, Figure 5 shows only the switching field 83 and call control 82 included in the exchange, but in practice an MSC contains a large number of various devices. The switching field may be an analog or digital switching device which selectively connects speech and data connections between transmission lines 12 from the base station system BSS and transmission lines 13 from a mobile exchange. An example of such digital telephone exchanges is Telenokia DX 220 MSC. A network adaptor 80A (IWF) is connected in parallel with the switching field 83 by connecting a signal port PI to the port 0 n .-_ of the switching field 83, and a signal port P2 to the port 1^ on the opposite side of the switching field 83. Another network adaptor 80B (or more) can be connected correspondingly in parallel with the switching field 83. The network adaptor 80 comprises a data transmission service adaptor DASA close to the port PI, said adaptor adapting the GSM data connection to the V.24 interface 81. The control interface of a data modem MODI is connected to the V.24 interface 81, and the line or modem interface is con¬ nected to the port P2. In the following, the operation of the data transmission arrangement according to the invention will be described so that mobile originated and mobile terminated calls will be dealt with separately. As a special modem is required in the network terminating unit IWF when the compression arrangement of the invention is used, the MS must, in the case of a mobile originated call, in some way uniquely address this modem. One solution is to reserve a specific telephone number for the compressed connection. In this case the MSC routes the calls made to this number automatically to the special modem MODI of the network terminating unit 80A. When the MS is connected to the modem MODI via the switching field 83, the MS dials the number of the remote terminal equipment B with an AT command, as a result of which the MODI performs corresponding dialling towards its modem interface. Another possibility of implementing the addressing of the modem MODI is to use the Bearer Capability element of a call set-up message in a non- standard way. A simple way, and also the most natural one, is to set the modem type field of this message element to a value which is normally not in use while the other fields of the Bearer Capability element are set as for a normal 9600 bit/s asynchronous call. Five bits are reserved for this field, but only the first nine values are currently in use. The remaining 23 values are defined as "reserved". If it is assumed that these "reserved" values are intended for new modem standards and that they are used in numerical order (as they have been used so far), the last value (31) can be used safely for addressing the modem according to the invention. Thus, when the modem field is set to 31 in a call set-up message, the MSC knows that a data call according to the invention is requested and connects the call to the special modem according to the invention. In the case of mobile terminated calls, it is also possible to use a specific service number; a data connection can be established through the special modem of the invention by dialling this number. Alternatively, in the case of an incoming call to the GSM system, the modem type field of the Bearer Capability element can be used for addressing the modem, whereby the situation is the same as in a normal data call: the MSC does not know how to set the Bearer Capability element in the call set-up message unless the subscriber has a separate telephone number for data calls. The MS must therefore send the Bearer Capability element to the MSC in a call confirm message. The element indicates that the MS is set to the data mode with modem type value 31. At the beginning of a data call the modem MODI of the network terminating unit 80A performs handshaking and the V.42 detection and negotiation steps over the modem connection in compliance with the recommendations. Compression parameters according to Table 1 are stored in the modem MODI. The modem MODI negotiates about these parameters with the remote modem M0D2 by means of handshaking. The resulting compression parameters are transmitted to the radio terminal equipment A. Table 1 The attached figures and the associated description are intended merely to illustrate the present invention. The details of the invention may vary within the scope and spirit of the attached claims.

Claims: Claims : 1. An arrangement for enhancing data trans¬ mission in a digital cellular radio network, said arrangement comprising a radio data terminal (A); a network terminating unit ( IWF) connected to a mobile exchange (MSC) and comprising a data modem (MODI); a remote data modem (M0D2); and call control means for establishing a digital transmission connection between the radio data terminal (A) and the data modem (MODI) and a modem connection between the data modem and the remote data modem (M0D2) , c h a r a c t e r i z e d in that the radio data terminal (A) and the remote data modem (MOD2) comprise means for transmitting data in a compressed form, the digital transmission connection between the mobile radio data terminal (A) and the data modem (MODI) is a non-transparent asynchronous data connection of a cellular radio system, over which connection the com¬ pressed data is transmitted using the error correction of the cellular system, and the data modem (MODI) of the network terminat¬ ing unit ( IWF) comprises means for transmitting com- pressed data over said modem connection by using the error correction protocol of the remote modem and for transmitting compression parameters to the radio data terminal at the beginning of a data session, said para¬ meters being determined on the basis of a handshaking procedure between the data modem and the remote data modem. 2. An arrangement according to claim 1, c h a r a c t e r i z e d in that the radio data terminal (A) comprises radio terminal equipment (MS), such as a mobile phone, to the asynchronous data inter- face (INT1 ) of which data terminal equipment (DTE), such as a computer, is connected. 3. An arrangement according to claim 2, c h a r a c t e r i z e d in that compression and de- compression means (COMP) are disposed in the data terminal equipment (DTE) . 4. An arrangement according to any one of the preceding claims, c h a r a c t e r i z e d in that the data compression is in compliance with CCITT Recommenda- tion V.42bis. 5. An arrangement according to any one of the preceding claims, c h a r a c t e r i z e d in that the error correction employed by the data modem (MODI) is in compliance with CCITT Recommendation V.42.

Inventors: SUOMI ARTO, TERHO MIKKO, SUOMI, ARTO, TERHO, MIKKO
Applicants: NOKIA TELECOMMUNICATIONS OY, SUOMI ARTO, TERHO MIKKO
Application year: 1993
Main classification: H04L25
IPC codes: H04B7, H04Q7