WO1995006372A1

WO1995006372A1 - Method for controlling a modem connection in the event of disturbance

Info

Publication number: WO1995006372A1
Application number: PCT/FI1994/000375
Authority: WO
Inventors: Jari Torkkel
Original assignee: Nokia Telecommunications Oy
Priority date: 1993-08-27
Filing date: 1994-08-26
Publication date: 1995-03-02
Also published as: AU7501194A; FI933780A0; FI93914B; US5903602A; FI93914C; EP0721716A1

Abstract

The invention relates to a method for controlling a connection, especially a wireless connection, between two modems in the event of disturbance, said method comprising initiating a so-called retrain procedure in the event of disturbance when either one of the modems receives a signal of unsatisfactory quality, in which retrain procedure the modems transmit predetermined retrain signals to each other in order to achieve synchronism between them. For reliable and rapid disconnection of a connection, the progress of the retrain procedure is monitored, and every time the procedure is stuck in its internal state for a period of time longer than a predetermined limit, the modem is commanded to initiate a start-up procedure known per se, by means of which the connection between the modems was originally established.

Description

Method for controlling a modem connection in the event of disturbance

The invention relates to a method for con- trolling a connection, especially a wireless connection, between two modems in the event of disturbance, said method comprising initiating a so-called retrain procedure in the event of disturbance when either one of the modems receives a signal of unsatisfactory quality, in which retrain procedure the modems transmit predetermined retrain signals to each other in order to achieve synchronism between them. The term wireless connection is used herein to refer to a connection implemented in such a manner that it is at least partly wireless.

A wireless radio network is originally intended for transmission of speech but it is also suitable for transmission of data. Data transmission in a radio net¬ work is carried out by means of a modem in a terminal and a mobile phone, e.g. a cellular phone. It is possible to use, for example, a modem positioned in a portable computer or a modem designed for a conventional telephone network. The modem is connected, for example, to a cellular phone by a separate adapter, which, for instance, adjusts the transmission level.

Figure 1 is a block diagram of equipment known per se for transmitting data between a computer 11 (typically a portable computer) or a terminal with a similar function and another terminal (not shown) con- nected to it through a radio network RN. The equipment comprises a modem 12, which is connected to the computer 11 and, via the above-mentioned adaptor 19, to a mobile phone 13. The modem 12 comprises an interface unit 14 (e.g. conventional RS-232 serial interface), to which the terminal 11 is connected, an error correction unit 16 connected to the interface unit, a data pump 17 con¬ nected to the error correction unit, a line interface unit 18 connected to the data pump 17, and a control processor 15 for controlling the operation of the modem. The actual modem part consists of the data pump 17, which Modulates digital signals supplied from the terminal 11 to a form suitable for an analog telephone network and DEModulates signals supplied from the tele¬ phone network to a form suitable for the terminal equip- ment. The data pump 17 is connected to the telephone network (adapter) through the analog line interface unit 18. The control processor 15 of the modem controls the data pump and performs error correction and all other functions of the modem, such as communication with the computer through the interface unit 14. The error correction unit 16 receives data transmitted through the interface unit, packs the data in frames of a fixed size, and transmits the frame through the data pump. At the reception end, the data pump 17 converts the received analog signal into digital form, whereafter the receiving error correction unit examines the received frame and detects any possible errors, which are corrected by means of retransmission.

In view of data transmission in a mobile tele- phone network, the most significant component of the modem is the data pump 17. The CCITT (at present the ITU-T) has defined a number of modulation standards of different rates for use in a general switched telephone network. These standards are illustrated in the follow- ing table. Recom¬ Rate Modula¬ Carrier Symbol mendation (bit/s) tion fre¬ rate method quencies (1/s) (Hz)

V.21 0-300 FSK 1080/1750 300

V.23 1200/75 FSK 1700/420 1200/75

V.22 600 QAM 1200/2400 600 1200

V.22 Ms 1200 QAM 1200/2400 600 2400

V.32 4800 QAM and 1800 2400 9600 TCM

V.32 Ms 4800 QAM and 1800 2400

7200 TCM

9600

12000

14400

The abbreviations used in the third column of the table above have the following meanings: FSK = Frequency Shift Keying, QAM = Quadrature Amplitude Modu¬ lation, and TCM = Trellis Code Modulation.

As appears from the table, the modems according to recommendation V.32 Ms offer a high rate and are thus a reasonable option even in radio networks. A V.32 Ms connection is established between modems by a start-up procedure described on page 10 of recommenda¬ tion V.32 Ms. There are different procedures for the calling and the answering channel (modem). The estab¬ lishment of the connection is described more closely in the above-mentioned recommendation, pages 14 to 16, which are referred to for a more detailed description. In practice, the start-up and the retrain procedure are almost identical, as the retrain procedure forms the synchronization part of the start-up procedure. If the modems lose synchronism during the con¬ nection, the so-called retrain procedure, i.e. resyn- chronization of the modems, is carried out. The operating principle of the retrain procedure is illustrated in the figure on page 11 of the above- mentioned recommendation. A more detailed description of the retrain procedure is given on page 16 of the above-mentioned recommendation.

In a radio network there are often situations where modems lose synchronism, and the above-mentioned retrain procedure must be carried out. Such situations may occur in a wireless network, such as a cellular network, for the following reasons:

- Synchronism is lost on account of change of base stations. - Noise is suddenly present on the connection, which prevents data from being transmitted at the current rate.

- A cellular connection from a moving car brings about changes in the field, and radio waves reflected from buildings cause interference.

- The quality of the line is low. (Operations in cellular networks are always performed within the area of a certain base station. The quality of the line is high near the base station but gets lower as the distance from the base station grows. In the area where base stations are changed the quality is usually low. )

- The internal signalling (change of base stations, transmission level, field strength) carried out on a speech channel of a cellular network causes breaks in data transmission. Depending on the duration and frequency of the signalling, unnecessary resynchron- izations, which weaken the performance, may take place on the data connection.

- With a moving car, there are always shadow regions, in which the connection does not work in practice. The connection is, however, restored when the shadow region is left behind. A normal data connection is disconnected at such locations unless special measures are taken to prevent it. A shadow region may also occur, for instance, at traffic lights, in which case the car may remain in the shadow region for rather a long time, even dozens of seconds.

- The (wireless) network disconnects the con¬ nection. In a normal telephone network the retrain procedure usually succeeds, and the modems return to the data transmission state. In a radio network, however, the quality of the connection varies, which causes problems. Modems designed for use in a normal telephone network usually disconnect the connection after two or three retrain attempts. In a mobile telephone network it is essential that the connection is not disconnected on account of disturbances in the network. In a normal case, the connection is disconnected by means of LINK DISCONNECT messages from a V.42 or an MNP error corrector or a GSTN CLEARDOWN sequence defined in recom¬ mendations V.32 and V.32 Ms. If the (wireless) network disconnects the connection, the modems proceed to the retrain state and after a given time disconnect the connection as the retrain procedure does not succeed (the far end is no longer on the line). The given time must be extended even to several minutes so as to prevent unnecessary disconnection of the connection.

In a mobile telephone network, the change of base stations or the shadow regions due to the environ- ment may thus lead to loss of synchronism between data pumps, wherefore the data pumps start the retrain procedure in order to be able to return to the data transmission state. Disturbances in the network may, however, lead to failure of the retrain procedure, in which case the procedure is started over again. When the quality of the connection is improved, the retrain procedure usually succeeds, and the modems return to the data transmission state. Very often, however, the retrain procedure does not succeed for some unaccount¬ able reason no matter how high the quality of the mobile telephone connection is. In that event, there is no other alternative than to disconnect the connection and to establish it all over again. As appears from the above, the problems caused in data transmission by wireless connections are such that the current methods do not provide a reliable and sufficiently rapid way to maintain or disconnect a con¬ nection. The object of the present invention is to obviate this drawback. This is achieved with a method according to the invention, which is characterized in that the progress of the retrain procedure is monitored, and every time the procedure is stuck in its internal state for a period of time longer than a predetermined limit, the modem is commanded to initiate a start-up procedure known per se, by means of which the connection between the modems was originally established.

Instead of establishing a completely new con- nection to the other end every time the retrain procedure fails, it is the idea of the invention to attempt to establish only a new data connection without disconnecting the actual telephone connection if the retrain procedure indicates that the connection as such is in order. The solution according to the invention allows the disconnection time to be considerably shortened. If the entire telephone connection is disconnected, the retrain procedure cannot proceed, wherefore the con- nection can be disconnected in a clearly shorter time than before, e.g. after 60 s. If the retrain procedure proceeds, it is obvious that the connection as such is in order but its quality is temporarily unsatisfactory, wherefore the connection is not disconnected but re- establishment of only the data transmission connection is attempted.

In the following, the invention will be described in greater detail by way of example and with reference to Figure 2 in the accompanying drawings, in which

Figure 1 is a block diagram of equipment for implementing a wireless modem connection, and

Figure 2 is a state transition diagram illus¬ trating the method according to the invention. When the modems proceed to the retrain state after either of them has detected that the quality of the received signal is not satisfactory, the data con¬ nection is disconnected and the data received from the error corrector has no significance. In this case, the decision concerning disconnection of the connection must be made on the basis of data obtained from the data pump. The most reliable data on the state of the con¬ nection is obtained by monitoring the progress of the retrain procedure in detail. Figure 2 illustrates more closely the principle of the monitoring process accord¬ ing to the invention.

If the retrain procedure does not proceed in the normal manner (and the modems are thus not able to return to the data transmission state), there are two alternative ways of proceeding: 1. The retrain procedure does not proceed at all.

2. The retrain procedure, proceeds but not to the end; instead, it always returns to the initial state.

In the case of alternative 1, the modem does not hear the modem of the other end, wherefore no internal transitions in the retrain procedure take place (cf. CCITT (at present the ITU-T) recommendation V.32 Ms). A reason for this may be that (a) the physical line (telephone connection) is disconnected, or (b) the data pump of the modem is somehow "stuck" in one state. The situation can be checked by commanding the modem to start the above-mentioned start-up procedure, by which the connection between the modems was originally established.

The following timers and counters are used for monitoring the retrain procedure:

RETRAIN_TIMER = a timer which measures the total time the modem has remained in the retrain state,

RETRAIN_IDLE_TIMER = a timer which measures the time the modem has remained in the initial state of the retrain procedure at a time (between the start-up procedures), RETRAIN_TOTAL_IDLE_TIMER = a timer which measures the total time the modem has remained in the initial state of the retrain procedure,

RETRAIN_COUNT = a counter which counts the number of retrain attempts (an attempt when the retrain procedure proceeds from its initial state to another one of its internal states),

START-UP_C0UNT = a counter which counts the number of start-up procedures, i.e. the number of the attempts to re-establish the data connection of the entire physical connection. When a transition to the retrain procedure is detected, all the above-mentioned timers and counters are reset (step 21, Figure 2). In the initial state 22 of the retrain procedure, the calling modem (ORG) transmits the AA segment according to recommendation V.32 Ms and the answering modem (ANS) transmits the AC segment. From the initial state it is possible to proceed only to two different states: either the retrain procedure starts to proceed, in which case it proceeds to state 23 (another internal state of the retrain procedure), or a process for disconnecting the con¬ nection is started and the procedure proceeds to the disconnection state 24.

If it is detected that the retrain procedure starts to proceed, the counter RETRAIN_COUNT is incremented by one. The detection may be based, for example, on the fact that the calling modem ORG stops transmitting the AA segment (ORG AA) or the answering modem ANS stops transmitting the AC segment (without that the state 24 for disconnecting the connection is proceeded to). If the retrain procedure proceeds in the normal manner, it proceeds thereafter to the data trans¬ mission state 25, whereby data transmission between the modems can be continued again. (In Figure 2, the events that bring about each transition are indicated above the horizontal line beside each transition, whereas the events following the transition are indicated below the line. )

If the retrain procedure does not proceed to the end, the modem returns to the initial state 22 at some step of the retrain procedure. This can be detected, for example, when the calling modem (ORG) starts again to transmit the AA segment and the answer¬ ing modem (ANS) the AC segment (0RG=AA or ANS=AC). In this case, the timers RETRAIN_IDLE_TIMER and RETRAIN_TOTAL_IDLE_TIMER are reset (step 23a).

If it is detected that the retrain procedure does not proceed from the initial state, the modem is commanded to start the start-up procedure again. This is effected by loading those sequences (commands) which denote a transition to the start-up procedure into the data pump of the modem. Such a "kick" is performed every time the RETRAIN_IDLE_TIMER has exceeded its limit, which may be e.g. 15 seconds as shown in Figure 2. At the same time the counter START-UP_C0UNT is incremented by one, and the RETRAIN_IDLE_TIMER is reset for new measurement. As the start-up and the retrain procedure are in practice identical for the most part (they contain identical synchronization parts, but the start¬ up procedure further includes listening to the answer tone, which starts the procedure, cf. the above- mentioned recommendation), the starting up of the start¬ up procedure is indicated in Figure 2 with an arrow 22a returning to the retrain initial state 22.

If, however, the total time the modem has remained in the retrain state or in the retrain initial state 22 becomes too long or the number of above- mentioned "kicks" or retrain attempts becomes too high, the disconnection state 24 is proceeded to, i.e. the process for disconnecting the connection is started in a manner known per se (defined in the recommendations). The limits which start the transition to the disconnec¬ tion state may be, for example, as follows: RETRAIN TIMER = 300 seconds,

RETRAIN_TOTAL_IDLE_TIMER = 60 seconds,

RETRAIN_COUNT = 20,

START-UP_C0UNT = 10.

In principle, it should be possible to carry out the above-mentioned process in both the calling and the answering modem, as either one of them may be "stuck" in one state. However, depending on the practical realization, it may also be sufficient if the method according to the invention is implemented only in the answering modem.

The method according to the invention can be implemented quite easily in the equipment according to Figure 1 by providing the equipment with the counters and timers described above. The other necessary modi- fications concern only the software of the modem.

Although the invention has been described above with reference to the examples illustrated in the accom¬ panying drawings, it is obvious that the invention is not limited to the examples but can be modified within the scope of the inventive concept disclosed above and in the appended claims. For instance, although the invention is primarily intended for wireless trans¬ mission connections, the same principle can also be used for connections of a fixed telephone network. In addition, the modems to which the present method is applied do not necessarily have to be modems according to recommendation V.32 Ms, but it is also possible to use modems according to other compatible, previous and future recommendations. In addition to the initial state of the retrain procedure, it is also possible to monitor other internal states of the retrain procedure, and perform a "kick" according to the invention if a modem is "stuck" in these states.

Claims

1. A method for controlling a connection, especially a wireless connection, between two modems (12) in the event of disturbance, said method comprising initiating a so-called retrain procedure in the event of disturbance when either one of the modems receives a signal of unsatisfactory quality, in which retrain procedure the modems transmit predetermined retrain signals to each other in order to achieve synchronism between them, c h a r a c t e r i z e d in that the progress of the retrain procedure is monitored, and every time the procedure is stuck in its internal state (22) for a period of time longer than a predetermined limit, the modem is commanded to initiate a start-up procedure known per se, by means of which the connection between the modems was originally established.

2. A method according to claim 1, c h a r a c ¬ t e r i z e d in that if the number of commanded start- up procedures exceeds a predetermined limit, the entire connection between the modems is disconnected.

3. A method according to claim 1, c h a r a c ¬ t e r i z e d in that if the total time the procedure remains in the internal state (22) exceeds a predeter- mined limit, the entire connection between the modems is disconnected.

4. A method according to claim 1, c h a r a c ¬ t e r i z e d in that for monitoring the progress of the retrain procedure, at least the initial state (22) of the retrain procedure is controlled.