Abstract:
The present invention relates to communication technologies, and discloses a method, an apparatus, and a system for transmitting a signal to ensure normal communication between the calling party and the called party. The method includes: A first node sends Global Text Telephone (GTT) signal encoded in a Cellular Text Telephone Modem (CTM) format to a second node after the first node receives the GTT signal and if transmission between the first node and the second node is based on an Internet Protocol (IP) network. The present invention is primarily applicable to the GTT service.

Description:
CROSS-REFERENCE TO RELATED APPLICATIONS 
     This application is a continuation of International Application No. PCT/CN2009/075602, filed on Dec. 15, 2009, which claims priority to Chinese Patent Application No. 200810189297.5, filed on Dec. 30, 2008, both of which are hereby incorporated by reference in their entireties. 
    
    
     FIELD OF THE INVENTION 
     The present invention relates to communications technologies, and in particular, to a method, an apparatus, and a system for transmitting signal. 
     BACKGROUND OF THE INVENTION 
     The Text Telephone Type (TTY)/Text telephone Devices for the Deaf (TDD) service provides mobile text voice communication for the deaf. Currently, TTY standards include V.21, EDT, and Baudot standards. 
     In the process of implementing a TTY service, text information (in the T.140 format) of a user is encoded by using the Baudot standard and sent to a network. The inventor finds that after the Nb/Nc interface between one Mobile Switching Center (MSC) and another MSC is IP-based, a Baudot signal needs to be transmitted in the IP network if a transmission method in the prior art is applied. However, because the Internet Protocol (IP) network generally employs a compressed voice codec format such as Adaptive Multi-Rate (AMR) which provides a very high compression ratio, the Baudot signal is impaired drastically, so that it is hard to recover the Baudot signal correctly on the called side. 
     SUMMARY OF THE INVENTION 
     The embodiments of the present invention provide a method, an apparatus, and a system for transmitting a signal to enable normal communication between the calling party and the called party. 
     The embodiments of the present invention are based on the following technical solution: 
     A method for transmitting a signal includes: 
     sending, by a first node, a Global Text Telephone (GTT) signal encoded in a Cellular Text Telephone Modem (CTM) format to a second node after the first node receives the GTT signal and if transmission between the first node and the second node is based on an IP network. 
     An apparatus for transmitting a signal includes: 
     a receiving unit, configured to receive a GTT signal; and 
     a sending unit, configured to send the GTT signal encoded in a CTM format to a next hop of the GTT signal if determining that transmission between the apparatus and the next hop is based on an IP network. 
     An apparatus for transmitting a signal includes: 
     a receiving unit, configured to receive a GTT signal encoded in a CTM format; and 
     a converting unit, configured to convert the GTT signal in the CTM format into the GTT signal in a format of an international text telephone modem standard, where the format of the international text telephone modem standard includes V.21, EDT, Bell 103, V.23, DTMF, V.18, and Baudot. 
     A system for transmitting a signal includes: 
     an end office of mobile services, configured to send a GTT signal encoded in a CTM format to a gateway office if determining that transmission between the end office of mobile services and the gateway office is based on an IP network; and 
     the gateway office, configured to receive the GTT signal in the CTM format from the end office of mobile services. 
     An apparatus for transmitting a signal includes: 
     a receiving unit, configured to receive a GTT signal; 
     a converting unit, configured to determine that the GTT signal received by the receiving unit are in a format of an international text telephone modem standard, and convert the GTT signal into a GTT signal in a CTM format; and 
     a sending unit, configured to send the converted GTT signal in the CTM format, and send a service message that carries an identifier of the GTT signal in the CTM format, where the identifier indicates that the GTT signal is in the CTM format. 
     By using the method, apparatus and system for transmitting a signal under the present invention, after the first node receives the GTT signal, the first node sends the GTT signal encoded in a CTM format to the second node if the transmission between the first node and the second node is based on an IP network. Therefore, after the Nb/Nc interface is IP-based, because the CTM signal has strong anti-interference abilities, the impact caused by the IP network on the GTT signal is mitigated, and normal communication is ensured between the calling party and the called party. 
    
    
     
       BRIEF DESCRIPTION OF THE DRAWINGS 
       To illustrate the technical solution under the present invention more clearly, the following describes the accompanying drawings involved in description of the embodiments of the present invention. Apparently, the accompanying drawings described below are exemplary only and not exhaustive. Persons of ordinary skill in the art can derive other drawings from such accompanying drawings without any creative effort. 
         FIG. 1  is a flowchart of a method for transmitting a signal in a second embodiment of the present invention; 
         FIG. 2  is a flowchart of a method for transmitting a signal in a third embodiment of the present invention; 
         FIG. 3  shows a system for transmitting a signal in a fourth embodiment of the present invention; 
         FIG. 4  shows an apparatus for transmitting a signal in a fifth embodiment of the present invention; 
         FIG. 5  shows an apparatus for transmitting a signal in a sixth embodiment of the present invention; 
         FIG. 6  shows a structure of an apparatus for transmitting a signal in the sixth embodiment of the present invention; and 
         FIG. 7  shows an apparatus for transmitting a signal in a seventh embodiment of the present invention. 
     
    
    
     DETAILED DESCRIPTION OF THE EMBODIMENTS 
     To make the technical solution under the present invention clearer, the following describes the embodiments of the present invention with reference to the accompanying drawings. Apparently, the embodiments provided below are not exhaustive, and persons of ordinary skill in the art can derive other embodiments of the present invention from such embodiments without any creative effort. 
     In the first embodiment of the present invention, after the first node receives the GTT signal, the first node sends a MT signal encoded in a CTM format to the second node if the transmission between the first node and the second node is based on an IP network, so as to mitigate the impact caused by the IP network on the GTT signal and ensure normal communication between the calling party and the called party. 
     In order for the second node to correctly know that the received GTT signal is in the CTM format, the first node may send a service message that carries an identifier of the GTT signal in the CTM format to the second node. The identifier indicates that the GTT signal transmitted between the first node and the second node are in the CTM format. 
     In the embodiments of the present invention, the service message may be extended to carry the identifier of the GTT signal in the CTM format. The service message may be extended in at least the following modes: 
     Mode 1: An Initial Address Message (IAM) or a Session Initiation Protocol Invite (SIP-I INVITE) message carries the identifier of the GTT signal in the CTM format. The identifier indicates that the GTT signal transmitted between the first node and the second node are in the CTM format. 
     In practice, a Bits P field in a Forward Call Indicators field in the IAM message or SIP-I INVITE message may be extended to carry the identifier of the GTT signal in the CTM format. In this field, “1” indicates that the GTT signal is in the CTM format, and “0” is a reserved value; or, “0” indicates that the GTT signal is in the CTM format, and “1” is a reserved value. 
     Mode 2: An Address Complete Message (ACM) or a SIP-I 180/183 message carries the identifier of the GTT signal in the CTM format. The identifier indicates that the GTT signal transmitted between the first node and the second node are in the CTM format. 
     In practice, a Bits F field in an Option Backward Call Indicators field in the ACM or SIP-I 180/183 message may be extended to carry the identifier of the GTT signal in the CTM format. In this field, “1” indicates that the GTT signal is in the CTM format, and “0” is a reserved value; or, “0” indicates that the GTT signal is in the CTM format, and “1” is a reserved value. 
     In practice, other messages or other fields in the foregoing messages may be extended to carry the identifier of the GTT signal in the CTM format. 
     The first node is an end office of mobile services and the second node is a gateway office, or, the first node is a gateway office and the second node is an end office of mobile services. The end office of mobile services refers to an MSC with which a GTT user is registered, and the gateway office refers to the switching equipment for processing services between two networks and may be an MSC. Moreover, one end of the gateway office is connected to the IP network, and the other end is connected to a Time Division Multiplexing (TDM) network such as a Public Switched Telephone Network (PSTN). 
     If the first node is an end office of mobile services and the second node is a gateway office, when the first node sends the GTT signal in the CTM format to the second node, the first node determines that the received GTT signal is in the CTM format, and then sends the signal to the gateway office, and sends a service message that carries an identifier of the GTT signal in the CTM format to the gateway office. 
     Afterward, the gateway office determines an encoding format of the GTT signal according to the identifier, and converts the GTT signal in the CTM format into the GTT signal in a format of an international text telephone modem standard, where the format of the international text telephone modem standard includes V.21, EDT, Bell 103, V.23, DTMF, V.18, and Baudot. 
     When the gateway office converts the GTT signal in the CTM format into the GTT signal in the format of the international text telephone modem standard, the gateway office makes a judgment first, namely, the gateway office judges whether the transmission between the gateway office and a third node is based on a TDM network by judging whether the third node belongs to a PSTN. If the third node belongs to a PSTN, it is determined that the transmission between the gateway office and the third node is based on a TDM network. Therefore, the gateway office converts the GTT signal in the CTM format into the GTT signal in the format of the international text telephone modem standard. The third node is the next hop of the GTT signal (namely, a message receiver). 
     If the first node is a gateway office and the second node is an end office of mobile services, the first node sends the GTT signal in the CTM format to the second node in this way: The gateway office determines that the received GTT signal is in the format of the international text telephone modem standard, and converts the GTT signal into a GTT signal in the CTM format, and then sends the GTT signal in the CTM format to the end office of mobile services. 
     By using the method for transmitting a signal in the first embodiment of the present invention, after the first node receives the GTT signal, the first node sends the GTT signal encoded in the CTM format to the second node if the transmission between the first node and the second node is based on an IP network. Therefore, after the Nb/Nc interface is IP-based, because the CTM signal has strong anti-interference abilities, the impact caused by the IP network on the GTT signal is mitigated, and normal communication is ensured between the calling party and the called party. 
     The following describes the signal transmission process in the case that a GTT terminal serves as a calling party and a called party. 
     The second embodiment deals with the process of implementing the GTT service, assuming that the GTT terminal serves as a calling party. 
     As shown in  FIG. 1 , the method for implementing the GTT service in the second embodiment includes the following steps: 
     Step  11 : The GTT terminal initiates a call, and sends a Setup message to a Mobile Switching Center Server (MSS) 1 through a Radio Access Network (RAN). The Setup message indicates that the transmitted GTT signal is in the CTM format. 
     Step  12 : After receiving the Setup message, the MSS1 adds a calling endpoint T 1  through an ADD (T 1 )[3gctm] message, and requests the MGW1 to allocate resources. The message carries a threegctm packet. In the threegctm packet, a Text Transport parameter indicates that the transmitted GTT signal is in the CTM format. 
     Steps  13 - 14 : The MSS1 judges whether the transmission between the MSS1 and the next-hop node is IP-based transmission. If so, the MSS1 adds the threegctm packet into the ADD (T 2 )[3gctm] message, through which an outgoing endpoint T 2  is added. In the threegctm packet, the Text Transport parameter indicates that the transmitted GTT signal is in the CTM format. Meanwhile, the MSS1 sends an IAM message to the Gateway office Mobile Switching Center Server (GMSS) connected to the PSTN. The IAM message carries an identifier of the GTT signal in the CTM format. 
     Step  15 : After receiving the IAM message, the GMSS determines that the transmitted GTT signal is in the CTM format, and adds an incoming endpoint T 3  through an ADD (T 3 )[3gctm] message. The message carries a threegctm packet. In the threegctm packet, the Text Transport parameter indicates that the transmitted GTT signal is in the CTM format. 
     Steps  16 - 17 : After determining that the called party is outside the local office, the GMSS initiates an outgoing trunk procedure. If the office direction is a PSTN office direction, the GMSS adds an incoming endpoint T 4  through an ADD (T 4 )[txp] message which carries a txp packet. The txp packet indicates that the transmitted GTT signal is in the Baudot format. The GMSS sends the IAM message to the PSTN. 
     The third embodiment deals with the process of implementing the GTT service, assuming that the GTT terminal serves as a called party. 
     As shown in  FIG. 2 , the method for implementing the GTT service in the third embodiment includes the following steps: 
     Steps  21 - 22 : After receiving the IAM message from the PSTN, the GMSS connected to the PSTN does not know whether the transmitted GTT signal is in the CTM format, and hence adds an endpoint T 1  of an ordinary attribute through an ADD(T 1 ) message. 
     Step  23 : The GMSS knows that the called party is outside the local office, but does not know whether the transmitted GTT signal is in the CTM format, and hence adds an endpoint T 2  of an ordinary attribute through an ADD(T 2 ) message. 
     Step  24 : The GMSS sends the IAM message to the MSS1. 
     Step  25 : After receiving the IAM message, the MSS1 does not know whether the transmitted GTT signal is in the CTM format, and hence adds an endpoint T 3  of an ordinary attribute through an ADD(T 3 ) message. 
     Step  26 : The RAN sends a Call Confirm message to the MSS1, indicating whether the transmitted GTT signal is in the CTM format. 
     Steps  27 - 28 : The MSS1 adds another called endpoint T 4  through an ADD(T 4 )[3gctm] message which carries a threegctm packet. In the threegctm packet, the Text Transport parameter indicates that the transmitted GTT signal is in the CTM format. Afterward, the MSS1 modifies the attribute of the endpoint T 3  through a MOD(T 3 )[3gctm] message which carries a threegctm packet. In the threegctm packet, the Text Transport parameter indicates that the transmitted GTT signal is in the CTM format. 
     Step  29 : After the called party rings, the MSS1 notifies the GMSS through an ACM message (ACM[CTM]). The ACM message carries an identifier of the GTT signal in the CTM format. 
     Steps  210 - 211 : After receiving the ACM message, the GMSS modifies the attribute of the endpoint T 2  through a MOD(T 2 )[3gctm] message which carries a threegctm packet. The threegctm packet indicates that the transmitted GTT signal is in the CTM format. Afterward, the GMSS modifies the attribute of the endpoint T 1  through a MOD(T 1 )[txp] message which carries a txp packet, indicating that the transmitted GTT signal is in the Baudot format. 
     In the case that the MSCs are interconnected through a SIP-I protocol, the procedure that occurs when the GTT terminal serves as a calling party or a called party is the same as the procedure described in the second embodiment and the third embodiment each except for the following difference: When the GTT terminal serves as a calling party, the IAM message in the second embodiment is replaced with a SIP-I INVITE message; when the GTT terminal serves as a called party, the ACM message in the third embodiment is replaced with a SIP-I 180/183 message. 
     Therefore, through the method for transmitting a signal in the second embodiment and the third embodiment above, after the Nb/Nc interface is IP-based, because the CTM signal has strong anti-interference abilities, the impact caused by the IP network on the GTT signal is mitigated, and normal communication is ensured between the calling party and the called party. 
     Persons of ordinary skill in the art should understand that all or part of the steps of the method under the present invention may be implemented by a program instructing relevant hardware. The program may be stored in a computer readable storage medium. When the program runs, the steps of the method specified in any of the embodiments above can be performed. The storage medium may be a magnetic disk, a Compact Disk-Read Only Memory (CD-ROM), a Read Only Memory (ROM), or a Random Access Memory (RAM). 
     As shown in  FIG. 3 , a system for transmitting a signal is provided in the fourth embodiment of the present invention. The system includes: an end office of mobile services  31 , configured to send a GTT signal encoded in a CTM format to a gateway office  32  if determining that transmission between the end office of mobile services and the gateway office is based on an IP network; and the gateway office  32 , configured to receive the GTT signal in the CTM format from the end office of mobile services. 
     The end office of mobile services  31  is further configured to send a service message that carries an identifier of the GTT signal in the CTM format to the gateway office, where the identifier indicates that the GTT signal transmitted between the end office of mobile services and the gateway office are in the CTM format. The gateway office  32  is further configured to receive the service message that carries the identifier of the GTT signal in the CTM format. 
     The gateway office  32  is further configured to determine the encoding format of the GTT signal according to the identifier, and convert the GTT signal in the CTM format into a format of an international text telephone modem standard, where the format of the international text telephone modem standard includes V.21, EDT, Bell 103, V.23, DTMF, V.18, and Baudot. 
     Through the system for transmitting a signal in the fourth embodiment, after the Nb/Nc interface is IP-based, because the CTM signal has strong anti-interference abilities, the impact caused by the IP network on the GTT signal is mitigated, and normal communication is ensured between the calling party and the called party. 
     As shown in  FIG. 4 , an apparatus for transmitting a signal is provided in the fifth embodiment of the present invention. The apparatus includes: a receiving unit  41 , configured to receive a GTT signal; and a sending unit  42 , configured to send the GTT signal encoded in a CTM format to a next hop of the GTT signal if determining that transmission between the apparatus and the next hop is based on an IP network. In practice, the apparatus shown in  FIG. 4  may be an MSC or another device of similar functions. 
     In order for the next hop to correctly know that the received GTT signal is in the CTM format, the sending unit  42  is further configured to send a service message that carries an identifier of the GTT signal in the CTM format to the next hop. The identifier indicates that the GTT signal transmitted between the apparatus and the next hop are in the CTM format. 
     As in the first embodiment, the service message may be an IAM, a SIP-I INVITE message, an ACM, or a SIP-I 180/183 message, and the message is extended in the same way as in the first embodiment. 
     Through the apparatus for transmitting a signal in the fifth embodiment, after the Nb/Nc interface is IP-based, because the CTM signal has strong anti-interference abilities, the impact caused by the IP network on the GTT signal is mitigated, and normal communication is ensured between the calling party and the called party. 
     As shown in  FIG. 5 , an apparatus for transmitting a signal is provided in the sixth embodiment of the present invention. The apparatus includes: a receiving unit  51 , configured to receive a GTT signal in the CTM format; and a converting unit  52 , configured to convert the GTT signal in the CTM format into a GTT signal in a format of an international text telephone modem standard, where the format of the international text telephone modem standard includes V.21, EDT, Bell 103, V.23, DTMF, V.18, and Baudot. 
     In addition, when the sender of the GTT signal in the CTM format also sends a service message that carries an identifier of the GTT signal in the CTM format, the receiving unit  51  is further configured to receive the service message that carries the identifier of the GTT signal in the CTM format, where the identifier indicates that the GTT signal transmitted between the apparatus and the next hop are in the CTM format. In this case, the converting unit  52  is specifically configured to determine an encoding format of the GTT signal according to the identifier of the GTT signal in the CTM format, and convert the GTT signal in the CTM format into a GTT signal in a format of an international text telephone modem standard. 
     As shown in  FIG. 6 , the apparatus in fifth embodiment may further include: a judging unit  53 , configured to judge whether the transmission between the apparatus and a third node is based on a TDM network, where the third node is the next hop of the GTT signal. Accordingly, the converting unit  42  converts the GTT signal in the CTM format into the GTT signal in the format of the international text telephone modem standard if the judgment result of the judging unit is positive. 
     In practice, the apparatus shown in  FIG. 5  or  FIG. 6  may be a gateway office or another device of similar functions. 
     Through the apparatus for transmitting a signal in the sixth embodiment, after the Nb/Nc interface is IP-based, because the CTM signal has strong anti-interference abilities, the impact caused by the IP network on the GTT signal is mitigated, and normal communication is ensured between the calling party and the called party. 
     As shown in  FIG. 7 , an apparatus for transmitting a signal is provided in the seventh embodiment of the present invention. The apparatus includes: a receiving unit  71 , configured to receive a GTT signal; a converting unit  72 , configured to convert the MT signal into a GTT signal in a CTM format if determining that the GTT signal received by the receiving unit are in a format of an international text telephone modem standard; and a sending unit  73 , configured to send the GTT signal in the CTM format, and send a service message that carries an identifier of the GTT signal in the CTM format, where the identifier indicates that the GTT signal is in the CTM format. 
     In practice, the apparatus shown in  FIG. 7  may be a gateway office or another device of similar functions. 
     Through the apparatus for transmitting a signal in the seventh embodiment, after the Nb/Nc interface is IP-based, because the CTM signal has strong anti-interference abilities, the impact caused by the IP network on the GTT signal is mitigated, and normal communication is ensured between the calling party and the called party. 
     In conclusion, by using the method, apparatus, and system for transmitting a signal in the embodiments of the present invention, after the first node receives the GTT signal, the first node sends the GTT signal encoded in the CTM format to the second node if the transmission between the first node and the second node is based on an IP network. Therefore, after the Nb/Nc interface is IP-based, because the CTM signal has strong anti-interference abilities, the impact caused by the IP network on the GTT signal is mitigated, and normal communication is ensured between the calling party and the called party. 
     The previous descriptions are merely exemplary embodiments of the present invention, but not intended to limit the protection scope of the present invention. Any modifications, variations or replacements that can be easily derived by those skilled in the art shall fall within the protection scope of the present invention. Therefore, the protection scope of the present invention is subject to the appended claims.