VoIP telephone message transmission method for high-speed magnetic levitation vehicle-ground communication system

Abstract

The invention discloses a VoIP telephone message transmission method of a high-speed maglev train-to-ground communication system, with a high-speed maglev train or a ground control center as a signal sender and the other party as a signal receiver; the method comprises the following steps: S1, transmitting the VoIP telephone message to the communication device of the signal sender through the TCP / IP protocol; S2, parsing the VoIP telephone message and allocating an IP address; S3, processing the application layer content of the VoIP telephone message; S4, transmitting the framed signal to the base station of the signal receiver through a non-TCP / IP protocol; S5, parsing the received framed signal to determine whether it is RTP / RTCP or SIP: if it is the RTP / RTCP protocol, directly perform VoIP packaging; if it is the SIP protocol, decapsulate and package it, and then perform VoIP packaging; S6, transmit the message to the corresponding device according to the destination port or signaling requirement. The invention realizes the transmission of VoIP dispatch telephone in a high-speed maglev train-to-ground communication system of a non-TCP / IP protocol based on data structure unpacking and packaging of different protocols.

Description

Technical Field

[0001] The invention belongs to the technical field of communication, and in particular relates to a VoIP telephone message transmission method of a high-speed magnetic levitation vehicle communication system. Background Art

[0002] The high-speed maglev train is equipped with an IP dispatch telephone, which is mainly used as a special telephone for train dispatchers to command train operations. The main ways of train dispatch calls are: calls between different positions on the same train, calls between the train and the ground control center, and calls between different trains. Among them, calls between different positions on the same train, because they are in the same network segment, are directly transmitted using TCP / IP messages based on VoIP (Voice of IP), just like the commonly used message transmission based on VoIP. In the other two cases, in addition to the use of the VoIP protocol, they also need to be realized through the high-speed maglev train's train-to-ground communication system.

[0003] Currently, the commonly used VoIP telephone message transmission is completed in the network based on the TCP / IP protocol. At the application layer, it is mainly based on SIP / RTP (RTCP) to complete signaling control and voice media stream transmission, at the transport layer it is based on the UDP protocol, the network layer is the IP protocol, and finally to the data link layer it is completed by the HDLC protocol.

[0004] The communication process of VoIP dispatch phone is as follows Figure 1 As shown. The specific transmission process is as follows: VoIP protocol must first use SIP to establish a connection. SIP is an application layer signaling protocol, usually composed of a start line, a message header, and an optional message body. To establish a call between two IP phones, the phone initiator, the server, and the phone receiver need to complete a series of operations such as request, response, and ringing. At the same time, based on SIP message transmission, the number of bytes sent each time is not a fixed frame length. During the call, the language signal that has been compressed through analog-to-digital conversion and language encoding is packaged at the application layer based on RTP and RTCP protocols. Among them, RTP is mainly responsible for the data transmission of the media stream, and RTCP completes the transmission control of the media stream. The data format and size based on RTP and RTCP are relatively fixed. After completing the encapsulation of the application layer, it is sent to the transport layer and the network transport layer in turn for UDP packetization and IP packetization. After a simple HDLC protocol framing, it can be transmitted in the network. On the contrary, the phone receiver or server will complete the data processing of the received signal corresponding to each layer, until the SIP or RTP protocol format of the application layer, complete the decoding and restore the signaling or voice signal.

[0005] The high-speed maglev train-to-ground communication system has a dedicated service interface for IP dispatching calls. However, due to the limited capacity of the service interface and the message transmission of non-traditional TCP / IP protocols, it cannot be directly connected to VoIP messages. Therefore, it is necessary to repackage the VoIP messages and perform address management to complete the message transmission in the train-to-ground communication system. Moreover, since RTP / RTCP and SIP are application layer protocols, VoIP voice calls based on NAT technology also need to complete the transition from RTP and SIP to NAT, that is, first transmit from the intranet to the public network, then transmit from the public network to the target public network, and then the target public network transmits to its intranet. Similar IP address management and allocation is required on the existing high-speed maglev train-to-ground communication system to realize the message transmission of IP telephone services in the non-TCP / IP high-speed maglev train-to-ground communication system.

[0006] The present invention is aimed at the transmission of VoIP dispatch telephone messages in a high-speed maglev train-to-ground communication system, realizes the compatibility of IP telephone messages with the train-to-ground communication system, solves the problems of inconsistent protocols, uncertain frame lengths, and address management when VoIP telephone services are transmitted in the maglev train-to-ground communication system, and has important application value and engineering significance in the field of high-speed maglev train-to-ground communication. Summary of the invention

[0007] The purpose of the present invention is to overcome the shortcomings of the prior art and provide a method for transmitting VoIP telephone messages in a high-speed maglev vehicle-to-ground communication system by designing the frame structure of the vehicle-to-ground communication system service interface according to the signaling data and call data of the VoIP telephone, unpacking and assembling the data structures based on different protocols, thereby realizing the transmission of the VoIP dispatch telephone in the high-speed maglev vehicle-to-ground communication system of the non-TCP / IP protocol.

[0008] The object of the present invention is achieved through the following technical scheme: a VoIP telephone message transmission method of a high-speed maglev train-to-ground communication system, wherein communication is performed between a high-speed maglev train and a ground control center, with the high-speed maglev train or the ground control center as the signal sender and the other party as the signal receiver; the method comprises the following steps:

[0009] S1, transmitting the VoIP telephone message of the signal sender to the communication device of the signal sender through the TCP / IP protocol;

[0010] S2. The communication device of the signal sender parses the VoIP telephone message and allocates an IP address;

[0011] S3, processing the application layer content of the parsed VoIP telephone message, including the following sub-steps:

[0012] S31, determining the space size of the VoIP service interface;

[0013] S32, determine the protocol type, and process according to the different protocol types:

[0014] If the application layer of the VoIP telephone message is an RTP / RTCP media stream data packet, there is no need to unpack the data packet, and the data packet is directly sent to the VoIP service interface framing part of the vehicle-to-ground communication system for framing;

[0015] If the application layer of the VoIP telephone message is a SIP signaling data packet, first determine whether the length of the data packet exceeds the size of the VoIP service interface in the vehicle-to-ground communication system. If it exceeds, unpack the data packet according to the service interface size; then send it to the VoIP service interface framing part of the vehicle-to-ground communication system for framing;

[0016] S4, the signal sending base station transmits the framed signal to the signal receiving base station through a non-TCP / IP protocol;

[0017] S5. The deframing part of the VoIP service interface of the signal receiving base station parses the received framed signal to determine whether it is RTP / RTCP or SIP: if it is RTP / RTCP protocol, it directly performs VoIP packaging and reconstructs the VoIP telephone message; if it is SIP protocol, it decapsulates the received message and packages the decapsulated message, then performs VoIP packaging and reconstructs the VoIP telephone message;

[0018] S6. Transmit the message to the corresponding device according to the destination port or signaling requirement.

[0019] The VoIP telephone message is composed of voice media stream or signaling, which is encapsulated by RTP / RTCP or SIP at the application layer, then encapsulated and framed by UDP at the transport layer, encapsulated by IP at the network layer, and finally encapsulated at the data link layer, and then encapsulated by OSI four layers to form a VoIP data packet.

[0020] In step S2, the method for allocating IP addresses is: transmitting from the IP phone of the signal sender to the communication device of the signal sender, the target IP address is the IP address of the communication device of the signal sender, and the source IP address is the IP phone of the signal sender; the source IP address of the SIP or RTP / RTCP-based message in the application layer is set to the intranet address of the IP phone of the information sender, and the target IP address is set to the intranet address of the scheduling server of the information receiver or the IP phone of the information receiver; during transmission, the IP address of the IP phone or server needs to be converted into the address of the vehicle-mounted communication device or the ground communication device to complete the conversion of the TCP / IP network transmission address and the non-TCP / IP network transmission address.

[0021] The specific implementation method of step S31 is: determine the length of the VoIP service interface in the vehicle-to-ground communication system based on the size after the RTP protocol completes the application layer encapsulation; then determine the length of its identifier based on the design of the frame structure; then the size of the entire service interface is the sum of the length of the VoIP service interface and the length of the identifier.

[0022] In the step S32, the signal after the VoIP service interface framing part is framed includes a header, a data content part and a tail;

[0023] The first part includes the frame header, function type, destination address, source address and data content size, with special characters as the frame header; the second part is the function type, indicating whether the data is based on RTP, RTCP or SIP protocol; at the same time, the first part also needs to determine the device destination address and source address, and its source address and destination address are determined according to the source IP address and destination IP address of its application layer and through table lookup;

[0024] The data content includes the frame header, total number of slices, slice sequence number, valid data length and header CRC check;

[0025] The trailer includes a frame check and a frame trailer consisting of special characters.

[0026] The beneficial effects of the present invention are as follows: the present invention designs the frame structure of the service interface of the train-to-ground communication system according to the signaling data and call data of the VoIP phone, so that the variable frame length VoIP message is converted into a fixed frame length message for transmission. Based on the data structure unpacking and assembly of different protocols, the transmission of the VoIP dispatch phone in the high-speed maglev train-to-ground communication system of the non-TCP / IP protocol is realized, and the problem that the VoIP dispatch phone is incompatible with the service interface of the current high-speed maglev train-to-ground communication system due to too long bytes, uncertain purpose, and IP address management is solved. By using the method of the present invention, the VoIP dispatch phone can be used normally and stably on the train. BRIEF DESCRIPTION OF THE DRAWINGS

[0027] Figure 1 For the VoIP transmission process;

[0028] Figure 2 It is the interface for high-speed maglev services;

[0029] Figure 3 It is a flow chart of VoIP message transmission;

[0030] Figure 4 For VoIP message processing, (1) for the frame structure of VoIP message, (2) for the parsing of VoIP message, (3) for the splitting and reassembly of SIP protocol data packets, (4) for the dispatching telephone service interface frame structure;

[0031] Figure 5 It is used for VoIP message address management. DETAILED DESCRIPTION

[0032] In high-speed maglev trains, the dedicated telephones used by train dispatchers to direct train operations are IP phones based on the VoIP (Voice of IP) network model. Figure 1 The figure shows the VoIP telephone transmission process, which consists of users and proxy servers. To talk to each other, users need to use the SIP signaling protocol: first, a connection is established through the proxy server, that is, user 1 initiates a request to the proxy server, and the server then turns to user 2. After completing a series of ringing responses and other processes, user 1 and user 2 start to establish a call. The call transmission process is to sample and compress the voice information through AD, and then send it after RTP encapsulation and IP protocol encapsulation. The receiving part is to process IP encapsulation and RTP data packets. Finally, it becomes voice information after decoding, decompression and DA conversion. The above data transmission is all based on the TCP / IP protocol network.

[0033] The high-speed maglev train-to-ground communication system has different service interfaces for different services. Figure 2 As shown, service interface 1 is for train diagnosis, service interface 2 is for dispatching telephone, and so on to service interface n. Each service interface specifies a different frame structure and byte size according to different needs.

[0034] The present invention mainly completes the processes of unpacking and packaging of related protocols, designing service interface frame structure, and managing and allocating IP addresses based on the structural features of RTP (RTCP) and SIP protocols in VoIP, VoIP transmission architecture, and the characteristics of the communication service interface of the maglev train. In the communication system service of the high-speed maglev train, the present invention is based on the RTP (RTCP) / SIP protocol message frame structure such as Figure 4 As shown in (1), its VoIP transmission uses TCP / IP network transmission in the car or on the ground, while the vehicle-to-ground or vehicle-to-vehicle communication is based on the service interface under the wireless communication system. As for the space allocated to the VoIP service interface, since the message size composed of the SIP protocol is uncertain and is generally much larger than the space of the system's service interface, it is necessary to unpack and send the messages based on the SIP protocol and receive the packets. The message size of the RTP and RTCP protocols is relatively fixed, but since there are multiple protocol transmissions in the transmission process, it is necessary to design a special frame structure for the service interface so that the receiving end can identify the function of the service interface and perform subsequent processing. And since the VoIP telephone transmission process also needs to complete IP address crossing in vehicle-to-ground communication, a special design is required. The technical solution of the present invention is further explained below in conjunction with the accompanying drawings.

[0035] like Figure 3 As shown, the VoIP telephone message transmission method of the high-speed maglev train-to-ground communication system of the present invention communicates between the high-speed maglev train and the ground control center, with the high-speed maglev train or the ground control center as the signal sender and the other party as the signal receiver. In this embodiment, the train is used as the signal sender; the method comprises the following steps:

[0036] S1. Transmit the VoIP phone message of the IP phone in the car to the vehicle communication device through the TCP / IP protocol; the dispatch phone also needs to design the frame structure and space size according to its needs. First, determine the space size of the dispatch phone: the VoIP-based phone message transmission method is based on the RTP (RTCP) protocol in the call media stream. The RTCP protocol controls the media stream information, and the signaling part is based on the SIP protocol. Figure 4 As shown in (1), the VoIP telephone message consists of a voice media stream or signaling, which is encapsulated using RTP / RTCP or SIP at the application layer, then encapsulated and framed using UDP at the transport layer, encapsulated using IP at the network layer, and finally encapsulated at the data link layer, and then encapsulated through the four layers of OSI to form a VoIP data packet.

[0037] S2. The vehicle-mounted communication equipment parses the VoIP phone message and allocates the IP address; because the IP phone is based on the RTP or SIP protocol at the application layer, which contains its target IP address and source address. However, the air interface signals of the vehicle-mounted base station and the ground base station of the high-speed maglev vehicle-ground communication system are non-TCP / IP transmission communication signals, and cannot directly transmit the network message after IP encapsulation based on VoIP. Therefore, it is impossible to directly set the source IP address and target IP address of each layer to the source IP phone and the target IP phone, so it is necessary to allocate IP addresses; in the network transmission process, the addressing method is mainly through the IP addresses of the transport layer and the network layer, so the target IP address and source IP address of the transport layer and the network layer depend on the transmission path, and the method of allocating IP addresses is as follows: since it is a two-way communication, the IP address allocation method at both ends of the sender and the receiver is the same, with the train as the signal sender, transmitted from the on-board IP phone to the on-board communication equipment, the target IP address is the IP address of the on-board communication equipment, and the source IP address is the on-board IP phone; the source IP address of the SIP or RTP / RTCP-based message in the application layer is set to the intranet address of the on-board IP phone, and the target IP address is set to the intranet address of the ground dispatch server or the ground dispatch IP phone; during transmission, the IP address of the phone or server needs to be converted into the address of the on-board communication equipment or the ground communication equipment to complete the conversion of the TCP / IP network transmission address and the non-TCP / IP network transmission address. The receiving end is the ground communication equipment, which completes the message analysis and repackaging, and then completes the TCP / IP network protocol message transmission according to its content. Figure 5As shown, the vehicle-mounted IP phone B in car A calls the ground IP phone E in D. It is necessary to use the signaling based on the SIP protocol between the vehicle-mounted phone B in car A, the server F in D and the IP phone G in D, complete the connection between the two IP phones through a series of handshakes, and then complete the call through the media stream of the RTP protocol. First, the vehicle-mounted IP phone needs to use the network TCP / IP protocol to transmit messages. The source IP address of its network layer and transport layer is 192.168.23.4 (1004), which is the actual IP address of the IP phone in network transmission. The target IP address is 192.168.23.5 (1005), which is the IP address used for actual transmission of the vehicle-mounted communication device. On the vehicle communication equipment, there are two ways to transmit VOIP messages. One is based on TCP / IP message transmission, and its transmission direction is from the vehicle communication equipment to the IP phone. Therefore, the source address is the actual IP address of the vehicle communication equipment, and the target IP is the IP address of the vehicle phone. The other is based on the non-TPC / IP protocol message transmission of the vehicle-to-ground communication system. Its transmission content is based on the application layer of VOIP, and its address is the pre-defined physical device address. For example, the source address on the vehicle communication equipment is 0x010001, and the target device address is 0x0f000f, which is the physical address of the ground communication equipment. The ground communication equipment is similar to the vehicle communication equipment, and there are two different message transmission methods. Subsequently, the TCP / IP message transmission of the server and the IP phone is completed according to the function and type of VOIP. After completing the message processing, the vehicle communication equipment and the ground communication equipment continue to build and parse the service interface. In the actual transmission process, the IP address of the application layer does not need to be considered, but the service interface function is determined as the dispatch phone and the source address according to the source IP address of its application layer. Therefore, VOIP message transmission can be completed without considering the target IP address and source address in the application layer.

[0038] S3, processing the application layer content of the parsed VoIP telephone message, including the following sub-steps:

[0039] S31, determine the space size of the VoIP service interface; VoIP telephone message transmission is transmitted under different protocols. Since the number of bytes of the subsequent transport layer and network layer of the RTP (RTCP) protocol is fixed in size and will not modify the content of the application layer, the length of the VoIP service interface in the vehicle-to-ground communication system is determined by the size after the application layer is encapsulated based on the RTP protocol; then the length of its identifier is determined according to the design of the frame structure; the size of the entire service interface is the sum of the length of the VoIP service interface and the length of the identifier. Since the number of bytes added due to encapsulation in the transport layer, network layer and data link layer is fixed, generally 50 bytes. In the application layer, if it is a media stream data packet composed of the RTP protocol, its number of bytes is also a fixed number of bytes, generally 214 bytes. The bytes of the RTCP data packet are smaller, generally only half of the RTP size. The number of bytes of the signaling data packet composed of the SIP protocol is longer and not fixed according to its content, generally between 400 and 1200 bytes. Since the data transmitted by the service interface is mainly the application layer of VoIP, the data space size of the VoIP service interface in the high-speed maglev train-to-ground communication system is determined by the size after the application layer is encapsulated based on the RTP protocol, that is, the size of the service interface is set to: 214 bytes, and the identifier is designed to have a total of 11 bytes, so the frame size of the corresponding VoIP service in the high-speed maglev train-to-ground communication system is: 225 bytes.

[0040] S32, determine the protocol type, and process according to the different protocol types:

[0041] After setting the service interface size, the media stream data based on the RTP and RTCP protocols can be transmitted all at once. Therefore, if the application layer of the VoIP telephone message contains RTP / RTCP media stream data packets, there is no need to unpack them. Instead, the data packets can be directly sent to the VoIP service interface framing part of the vehicle-to-ground communication system for framing.

[0042] However, the signaling data based on the SIP protocol may not be transmitted at one time. Therefore, if it exceeds the space size of the service interface, it is necessary to fragment the signaling data based on the SIP protocol. Therefore, if the application layer of the VoIP telephone message contains a SIP signaling data packet, since it involves splitting the data, the VoIP service interface is redesigned according to the 4-byte identifier and 210-byte data content. First, determine whether the length of the data packet exceeds the size (210 bytes) allocated to the SIP signaling data packet by the VoIP service interface in the vehicle-to-ground communication system. If it exceeds, the data packet will be unpacked; then it will be sent to the framing part of the VoIP service interface in the vehicle-to-ground communication system for framing. After unpacking, each piece of data is split into a fixed and appropriate size. If the number of the last piece does not meet the size requirement, it is also sealed as one piece for processing. After unpacking the signaling data packet based on the SIP protocol, it is necessary to add an identifier so that it can be restored to the original message when the subsequent packet is received. The specific unpacking method is as follows: Figure 3 and Figure 4 (2) and (3) are shown. After completing the SIP protocol and packaging it to the network layer, the signaling data packet is divided into pieces with a byte size of 210 bytes per piece. If the size of the last piece is less than 210 bytes, the bytes are filled with 0. The 4-byte identifier is designed as follows: the frame header occupies 8 bits, which is designed as: 0x80 (1000_0000), the total number of packets is 4 bits wide, that is, there may be 0 to 15 packets in total, and the packet sequence number also occupies 4 bits. Each sequence number represents the number of packets; for example, 0101_0100, that is, a total of 5 packets are transmitted at one time, and this packet is the 4th packet. The effective signaling length is 8 bits wide, representing the length of the effective signaling data in the signaling. Except for the last packet, the value is 1101_0010. The last packet is assigned a value according to its actual effective signaling data length, for example, 0_1011_1110, which means that the effective data of the last packet is 190 bytes. The last bit is the 7-bit header CRC check code, and 1 bit of 0 represents the last bit of the identifier.

[0043] The identifier is designed as follows: 1. The highest bit is the frame header, which usually uses a special character as its frame header; 2. The total number of sub-packets and the sub-packet sequence number of each packet; 3. The effective data length of each sub-packet is given. Generally, except for the effective length of the last sub-packet data, which is not necessarily the maximum value, the other sub-packet data will be filled with a fixed maximum value, and the number of the last sub-packet may not be enough. The effective data of the last sub-packet is determined by the length of the effective data. In addition, the header checksum is added to determine whether there is an error in the data transmission process of the frame. If there is an error, it will be retransmitted.

[0044] The service interface needs to re-encapsulate the data into frames to adapt to the high-speed maglev communication system. The signal after the VoIP service interface framing part is framed includes the header, data content part and tail, such as Figure 4 (4) As shown. The header includes the frame header, function type, destination address, source address and data content size, with special characters as the frame header; the second part is the function type, indicating whether the data is based on RTP, RTCP or SIP protocol; at the same time, the header also needs to determine the device destination address and source address, and its source address and destination address are determined according to the source IP address and destination IP address of its application layer and through table lookup, so that the subsequent transmission processing can know the source and destination of the data;

[0045] The data content includes the frame header, total number of slices, slice sequence number, valid data length and header CRC check;

[0046] The trailer includes a frame check and a frame trailer consisting of special characters.

[0047] In this embodiment, the data content is a packet of data after RTP (RTCP) or SIP packetization, and then the header and tail form the whole service interface, and the header and tail occupy 11 bytes in total. The header is designed to use a 16-bit special character as the frame header, for example: 0x5a5a. The function type is then determined by a 2-bit length, that is, 00 (default), 01 represents the adopted RTP protocol, 10 represents the sampled RTCP protocol, and 11 represents the adopted SIP protocol. The destination address and source address each occupy 24 bits. The number of each dispatch phone or server on each train or each base station is made into a table in advance, and the source and destination of the call are determined according to the address of the application layer, and filled by looking up the table. The tail is mainly determined by the 7-bit CRC check and the 6-bit frame tail. The frame tail also uses special characters: 0b00_0000, representing the end of the entire frame.

[0048] S4. The vehicle-mounted base station transmits the framed signal to the ground base station through a non-TCP / IP protocol; the vehicle-mounted communication device determines whether the service interface is a dispatch phone based on the source IP, and determines that the target of the service interface is the ground communication device based on the target IP of the original VoIP application layer. The protocol type of the service interface is RTP or SIP based on the content of the application layer, and is indicated in the service interface through an identifier. The data part of the service interface is based on the application layer of the RTP / SIP protocol, and is the data after the corresponding unpacking and redesign.

[0049] S5. The ground processing process is opposite to the process of sending messages in the car. The deframing part of the VoIP service interface of the ground base station parses the received framing signal and the received frame structure. It first determines the subsequent network layer and transport layer target IP according to the function of its service interface, determines the source IP of the transport layer / network layer according to its transmission path, and determines its type based on the identifier, whether it is RTP or SIP; if it is the RTP protocol, it directly performs VoIP packaging and reassembles the VoIP phone message; if it is the SIP protocol, it decapsulates the received message and assembles the decapsulated message, and then performs VoIP packaging and reassembles the VoIP phone message; the process of transmitting the message from the ground equipment to the IP phone in the car is the same. Finally, through the overall process, the connection is established according to the signaling, and the media stream completes the voice data interaction, so that the normal call of the VoIP dispatch phone can be completed.

[0050] S6. Transmit the message to the ground IP phone or ground server according to the destination port or signaling requirements.

[0051] Those skilled in the art will appreciate that the embodiments described herein are intended to help readers understand the principles of the present invention, and should be understood that the protection scope of the present invention is not limited to such specific statements and embodiments. Those skilled in the art can make various other specific variations and combinations that do not deviate from the essence of the present invention based on the technical revelations disclosed by the present invention, and these variations and combinations are still within the protection scope of the present invention.

Claims

1. A VoIP telephone message transmission method for a high-speed maglev train-to-ground communication system, wherein communication is performed between a high-speed maglev train and a ground control center, with the high-speed maglev train or the ground control center as the signal sender and the other party as the signal receiver; characterized in that: The following steps are involved: S1, transmitting the VoIP telephone message of the signal sender to the communication device of the signal sender through the TCP / IP protocol; S2. The communication device of the signal sender parses the VoIP telephone message and allocates an IP address; S3, processing the application layer content of the parsed VoIP telephone message, including the following sub-steps: S31, determining the space size of the VoIP service interface; S32, determine the protocol type, and process according to the different protocol types: If the application layer of the VoIP telephone message is an RTP / RTCP media stream data packet, there is no need to unpack the data packet, and the data packet is directly sent to the VoIP service interface framing part of the vehicle-to-ground communication system for framing; If the application layer of the VoIP telephone message is a SIP signaling data packet, first determine whether the length of the data packet exceeds the size of the VoIP service interface in the vehicle-to-ground communication system. If it exceeds, unpack the data packet according to the service interface size; then send it to the VoIP service interface framing part of the vehicle-to-ground communication system for framing; S4, the signal sending base station transmits the framed signal to the signal receiving base station through a non-TCP / IP protocol; S5. The deframing part of the VoIP service interface of the signal receiving base station parses the received framed signal to determine whether it is RTP / RTCP or SIP: if it is RTP / RTCP protocol, it directly performs VoIP packaging and reconstructs the VoIP telephone message; if it is SIP protocol, it decapsulates the received message and packages the decapsulated message, then performs VoIP packaging and reconstructs the VoIP telephone message; S6. Transmit the message to the corresponding device according to the destination port or signaling requirement.

2. The VoIP telephone message transmission method of the high-speed magnetic levitation vehicle-to-ground communication system according to claim 1 is characterized in that: The VoIP telephone message is composed of voice media stream or signaling, which is encapsulated by RTP / RTCP or SIP at the application layer, then encapsulated and framed by UDP at the transport layer, encapsulated by IP at the network layer, and finally encapsulated at the data link layer, and then encapsulated by OSI four layers to form a VoIP data packet.

3. The VoIP telephone message transmission method of the high-speed magnetic levitation vehicle-to-ground communication system according to claim 1 is characterized in that: In step S2, the method for allocating IP addresses is: transmitting from the IP phone of the signal sender to the communication device of the signal sender, the target IP address is the IP address of the communication device of the signal sender, and the source IP address is the IP phone of the signal sender; the source IP address of the SIP or RTP / RTCP-based message in the application layer is set to the intranet address of the IP phone of the information sender, and the target IP address is set to the intranet address of the scheduling server of the information receiver or the IP phone of the information receiver; during transmission, the IP address of the IP phone or server needs to be converted into the address of the vehicle-mounted communication device or the ground communication device to complete the conversion of the TCP / IP network transmission address and the non-TCP / IP network transmission address.

4. The VoIP telephone message transmission method of the high-speed magnetic levitation vehicle-to-ground communication system according to claim 1 is characterized in that: The specific implementation method of step S31 is: determine the length of the VoIP service interface in the vehicle-to-ground communication system based on the size after the RTP protocol completes the application layer encapsulation; then determine the length of its identifier based on the design of the frame structure; then the size of the entire service interface is the sum of the length of the VoIP service interface and the length of the identifier.

5. The VoIP telephone message transmission method of the high-speed magnetic levitation vehicle-to-ground communication system according to claim 1 is characterized in that: In the step S32, the signal after the VoIP service interface framing part is framed includes a header, a data content part and a tail; The first part includes the frame header, function type, destination address, source address and data content size, with special characters as the frame header; the second part is the function type, indicating whether the data is based on RTP, RTCP or SIP protocol; at the same time, the first part also needs to determine the device destination address and source address, and its source address and destination address are determined according to the source IP address and destination IP address of its application layer and through table lookup; The data content includes the frame header, total number of slices, slice sequence number, valid data length and header CRC check; The trailer includes a frame check and a frame trailer consisting of special characters.

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