Design of Digital System without Center Call Control Protocol Based on ARM7 and MX618 Chips

With the rapid development of various digital technologies, the analog centerless system is already stretched under the current technical environment, and its comprehensive digital transformation will be an important work content in the field of private network wireless communication in the future. There is no official version of the digital standard of the call control protocol. The call control protocol introduced in this article has been applied to the product development of digital wireless walkie-talkies. It runs stably, and its functions need to be further expanded and improved according to actual needs.

introduction

The centerless mobile communication system is an important part of my country’s professional mobile communication system. It uses simplex intercom mode to work, and the working frequency is between 915.012 5~916.087 5 MHz. The system has many technical characteristics such as centerless networking, digital selective calling, automatic connection, multi-access user multi-channel sharing, and link decentralized control. It has broad practical application prospects and in-depth development potential. The network structure diagram of this system is shown as in Fig. 1. At present, my country’s 900 MHz centerless mobile communication system is still in the analog phase with analog voice plus digital signaling as the dominant technology. The digitization research ON the centerless call control protocol is still in a blank.

Design of Digital System without Center Call Control Protocol Based on ARM7 and MX618 Chips

The digitization of the centerless system is to digitize the voice and control signaling and transmit them in the form of binary code stream. This system platform uses CML’s MX618 chip for voice encoding, MX7041 chip for 4FSK baseband modulation of voice and signaling, and ARM7 processor as the system’s operational control core. In this development environment, this article takes the digital centerless call control layer protocol as the research object, and proposes and designs a set of call control protocol digital solutions based on the centerless system. This solution has been applied to the development of embedded system of digital non-centered walkie-talkies.

1. Overview of Digital Centerless Call Control Protocol

The ETSITS 102490-DPMR standard describes the protocol layered structure of the centerless system, as shown in Figure 2. The call control protocol is at the third layer of the protocol stack of the non-central system, above the data link layer and below the application layer, which is the control core of the non-central system.

Design of Digital System without Center Call Control Protocol Based on ARM7 and MX618 Chips

It provides basic call establishment, hold, and disconnection for the entire system; point-to-point calls and group calls; selection of call partners; support for functions such as late entry and call transfer. At the same time, it provides interfaces for the data link layer that carries data and voice services and the application layer that implements specific functions, and it is necessary to provide a complete upper and lower layer general interface to reserve space for future protocol upgrades.

The design of the call control protocol includes the design of the call control process, the call control signaling, and the later program realization coding, which then forms an independent call control protocol function module.

2. Design of call control process

According to the working principle of digital non-centered walkie-talkies, a set of call control procedures in line with practical applications are designed. The call control process describes a series of actions and events generated by the communication terminal from the establishment of the call to the end of the call.

The mobile station A and mobile station B in the standby state are waiting on the control channel. When mobile station A presses the call button to call mobile station B, A broadcasts the call establishment request signaling for B on the control channel and transfers to the selected communication channel to receive it. After receiving the call establishment request signaling, if B agrees to establish a connection, it will switch to the call channel identified in the signaling, and at the same time send confirmation signaling to A on this call channel. After A receives the confirmation signaling, both parties enter the state of being able to talk. If A presses PTT, it will send voice header frame + voice frame to B.

After the PTT is released, a tail frame is sent to indicate the end of party A’s conversation. At this time, B receives the voice frame until the tail frame is received, and both A and B enter the state of being able to talk. After that, the two parties repeat the above process to talk. Once A presses the disconnect button, the disconnect signal will be sent to B, and both parties return to the standby state. This process is shown in Figure 3.

Design of Digital System without Center Call Control Protocol Based on ARM7 and MX618 Chips

3. Design and implementation of call control protocol

Call control signaling and channel-associated signaling are respectively transmitted on the control channel and the call channel. According to the non-centralized multi-channel addressing mobile communication system, the control signaling transmission channel (control channel) is completely separated from the call channel. It integrates the control signaling of several call lines and occupies a common control channel to transmit control signaling.

3.1 Channel

Control Channel The control channel is mainly used for broadcast transmission of call setup signaling and as a transmission channel for signaling for other purposes. The service system of the control channel adopts the principle of the order waiting system. After the previous signal is transmitted in the control channel, the subsequent signal can be transmitted normally. Otherwise, wait until it is determined that the control channel is idle.

Call channel The call channel is responsible for transmitting the voice and data of the two parties after the call is established. At the same time, the call channel is also used to transmit channel-associated signaling. The channel associated signaling includes a series of necessary control signaling during the call. The service system of the call channel adopts the principle of call loss control. After two terminal devices establish a connection and occupy a call channel, this channel can no longer be used by other terminals until the terminal disconnects and releases the second call channel.

As shown in Figure 4, the control channel with a frequency of 915.02.5 MHz is only used for the communication parties to transmit call establishment signaling, while the call channel can transmit disconnection signaling, response signaling, and other channel-associated signaling. The mobile station switches between the two channels, stays on the call channel during a call, and returns to the control channel when the call ends or on standby.

Design of Digital System without Center Call Control Protocol Based on ARM7 and MX618 Chips

When the channel spacing of the mobile communication system without center multi-channel access is 12. 5 kHz, the system has a total of 158 channels. Among them, the first channel 915.012 5 MHz is the control channel, all call signaling is sent out on this channel, and the remaining 157 are call channels. The flow of call control is carried through by the transmission of control signaling in the control channel and the call channel. Therefore, how to formulate control signaling and how to handle all state transition events in the call signaling process has become the main content of the call control protocol design.

3.2 Design of call control signaling without center

Call control signaling is a control signal for various state transitions in a centerless system, and transmits system messages and commands. Call control signaling is divided into call setup request signaling, call response signaling, disconnect signaling, voice origination signaling, and voice termination signaling according to functions.

3.2.1 Frame structure of control signaling

The basic data transmission unit of a centerless system is a frame. Call control signaling is composed of two frame structures, the first frame and the last frame.

The header frame contains most of the control information in the signaling, and its frame structure is as follows:

Design of Digital System without Center Call Control Protocol Based on ARM7 and MX618 Chips

Preamble: used for receiver synchronization.

Frame synchronization: identification and synchronization of the first frame.

Header frame type: 4 b, identifies the header frame type. It is the main identification of the header frame and the main identification of signaling. Its value is: 0000 call initiation signaling; 0001 call establishment request signaling; 0010 disconnect request signal Order; 0011ACK signaling; 0100 prohibits sending and restoring signaling; 0101 prohibits sending and receiving signaling.

Receiver ID: The ID number of the called party’s mobile station, which is obtained by encoding a 7-digit call sign through the air interface.

Sender ID: The ID number of the calling party’s mobile station, which is obtained by encoding a 7-digit call sign through the air interface.

Communication mode: used to distinguish voice communication or data transmission.

Communication format: used to distinguish between full call communication and point-to-point communication.

Call information: 11 b, used to distinguish the definition of single call, group call and ACK signaling.

The tail frame supplements the head frame to form a complete signaling or a separate signaling for the end of communication. The frame structure is as follows:

Design of Digital System without Center Call Control Protocol Based on ARM7 and MX618 Chips

Tail frame synchronization code: 4 b, used for tail frame discovery and synchronization, fixed as 7D DF F5.
End frame type: 2 b, the main identification of the end frame. 00 represents a normal tail frame; 01 represents a tail frame with status information.
ACK request: 2 b, identifies whether the called party needs to send an ACK signal.
Tx wait: 4 b. Identify whether it takes a period of time to force the PTT of the user who received the tail frame to be invalid, so as to allow the user to send an insertion request.
Status information: 5 b, user-defined according to needs, a total of 32 types of status information.

3.2.2 Contents of call control signaling

Call establishment request signaling is control information that is broadcast to the called party when the user initiates a call. The composition of the signaling is: header frame (setup request) + tail frame.

Call response signaling is feedback control information sent to the calling party after the called party receives the call setup request signaling. The signaling structure is: header frame (ACK).

Voice initiation signaling precedes voice transmission and is used to identify that the actual voice is coming. Its structure is: header frame (communication start).

Voice termination signaling indicates that voice transmission has come to an end, and its position is after the voice data block. The structure of voice termination signaling is: tail frame (end of communication).

Disconnect signaling is a call end signal sent when a communication party wishes to end a call, and is used to inform the other party that the call is about to end. The signaling structure of disconnection is: first frame (disconnect) + last frame + first frame (disconnect) + last frame.

3.3 Application of state machine to realize call control protocol

The digital centerless call control protocol uses the design scheme of the finite state machine to describe and implement. The finite state machine overcomes the shortcomings of inflexible sequential control methods of pure hardware digital systems. It can construct sequential logic modules with good performance and solve complex program flow problems in a simple, event-driven manner. Its reliability advantage is obvious, and it is a general solution for control programs in the embedded field.

The design of the state in the communication process is the key to the realization of the call control layer protocol. A complete and reasonable state transition process is also an important prerequisite for program realization. The state of the call control layer should include each typical situation in the call control flow cycle. And it should be possible to switch between states, there should be no unreachable states or dead states that cannot enter the process again, and redundant states that are meaningless for the characteristics to be described should also be avoided.

3.3.1 User A and B communication process state event machine

According to the characteristics of call control process and call control signaling and the technical specifications of the centerless system, the call control state machine has designed a total of 5 states from S0 to S4. S0 (standby state) indicates the default state when the mobile terminal is idle; S1 (initiating call state) is the call waiting state after the call originating party dials, and S1 is the in-progress state of the called party after the called party receives the call establishment request. Judge the waiting state; S2 (call established state) indicates that the call has been successfully established and is waiting for the next call; S3 (speaking state) is the state in which the user presses the PTT button of the walkie-talkie to transmit the voice of the call; S4 (receiving state) ) Is the state that the user is listening to the voice of the other party’s call. After the call, both parties return to S0 (standby state). The state machine considers the technical details in the actual application of call control, and realizes the predictable transfer of each state under various actions and event excitations. The specific content of the state machine is shown in Figure 5 and Figure 6 (Note: After resetting and removing the wire, each state is converted to the standby state).

Design of Digital System without Center Call Control Protocol Based on ARM7 and MX618 Chips

3.3.2 Implementation of the call control program

The call control program is developed in ARM7 processor using embedded C language. Set the variable ccl_state to store the code of the current state, and next_state to store the code of the next state. Use the Switch_Case statement and the if condition statement for event judgment to realize a complex multi-condition and multi-branch call control state machine.

Design of Digital System without Center Call Control Protocol Based on ARM7 and MX618 Chips

The ARM timer was added in the design process of the program. Timers play a very critical role in embedded systems. In order to save frequency resources, the non-central system has added the call time limit function. After the call time limit starts, a timer needs to be started. After the timer expires, an IRQ interrupt is generated and the call is terminated. In order to increase the reliability of the program and prevent the occurrence of a dead state, a judgment action on the timer overtime event should also be added in the program. Once each state (except the standby state) is in the state of waiting for the next action, it is necessary to immediately start the timer to judge the timeout. For example, when a call is established, the answering time of the other party is detected by a timer, and when it times out, it returns to the S0 standby state.

4. Feasibility and reliability analysis of call control protocol

The three parts of call control process, call control signaling structure, and call control state machine are inseparable, and constitute the basic elements of the call control protocol. The call flow design takes into account the non-central system and the traditional walkie-talkie workflow.

The call control signaling draws on and improves the signaling frame structure of the data link layer in the ETSI TS 102 490-DPMR standard. The call control state machine uses UML model to design five transition states triggered by communication events and actions. Logically, this call control protocol fully simulates various working scenarios of call control in practical applications, and can theoretically complete the functions of the call control system correctly. In the actual embedded development process, taking advantage of the high-speed operation advantages of the ARM7 processor, the control signaling that requires strict bit error rate in data transmission adds error control coding at the data link layer, CRC cyclic redundancy check, and Chinese Clear error correction coding, interleaving suppression of continuous burst interference error correction coding, to ensure the accurate transmission of control signals.

In addition, an anti-dead state timer interrupt is added to the call control program, which improves the reliability of the program’s operation. This call control protocol has been practically applied to the prototype development of laboratory digital non-centered walkie-talkies, and the operation is stable.

5 Conclusion

With the rapid development of various digital technologies, the analog centerless system is already stretched under the current technical environment, and its comprehensive digital transformation will be an important work content in the field of private network wireless communication in the future. There is no official version of the digital standard of the call control protocol. The call control protocol introduced in this article has been applied to the product development of digital wireless walkie-talkies. It runs stably, and its functions need to be further expanded and improved according to actual needs.

introduction

The centerless mobile communication system is an important part of my country’s professional mobile communication system. It uses simplex intercom mode to work, and the working frequency is between 915.012 5~916.087 5 MHz. The system has many technical characteristics such as centerless networking, digital selective calling, automatic connection, multi-access user multi-channel sharing, and link decentralized control. It has broad practical application prospects and in-depth development potential. The network structure diagram of this system is shown as in Fig. 1. At present, my country’s 900 MHz centerless mobile communication system is still in the analog phase with analog voice plus digital signaling as the dominant technology. The digitization research on the centerless call control protocol is still in a blank.

Design of Digital System without Center Call Control Protocol Based on ARM7 and MX618 Chips

The digitization of the centerless system is to digitize the voice and control signaling and transmit them in the form of binary code stream. This system platform uses CML’s MX618 chip for voice encoding, MX7041 chip for 4FSK baseband modulation of voice and signaling, and ARM7 processor as the system’s operational control core. In this development environment, this article takes the digital centerless call control layer protocol as the research object, and proposes and designs a set of call control protocol digital solutions based on the centerless system. This solution has been applied to the development of embedded system of digital non-centered walkie-talkies.

1. Overview of Digital Centerless Call Control Protocol

The ETSITS 102490-DPMR standard describes the protocol layered structure of the centerless system, as shown in Figure 2. The call control protocol is at the third layer of the protocol stack of the non-central system, above the data link layer and below the application layer, which is the control core of the non-central system.

Design of Digital System without Center Call Control Protocol Based on ARM7 and MX618 Chips

It provides basic call establishment, hold, and disconnection for the entire system; point-to-point calls and group calls; selection of call partners; support for functions such as late entry and call transfer. At the same time, it provides interfaces for the data link layer that carries data and voice services and the application layer that implements specific functions, and it is necessary to provide a complete upper and lower layer general interface to reserve space for future protocol upgrades.

The design of the call control protocol includes the design of the call control process, the call control signaling, and the later program realization coding, which then forms an independent call control protocol function module.

2. Design of call control process

According to the working principle of digital non-centered walkie-talkies, a set of call control procedures in line with practical applications are designed. The call control process describes a series of actions and events generated by the communication terminal from the establishment of the call to the end of the call.

The mobile station A and mobile station B in the standby state are waiting on the control channel. When mobile station A presses the call button to call mobile station B, A broadcasts the call establishment request signaling for B on the control channel and transfers to the selected communication channel to receive it. After receiving the call establishment request signaling, if B agrees to establish a connection, it will switch to the call channel identified in the signaling, and at the same time send confirmation signaling to A on this call channel. After A receives the confirmation signaling, both parties enter the state of being able to talk. If A presses PTT, it will send voice header frame + voice frame to B.

After the PTT is released, a tail frame is sent to indicate the end of party A’s conversation. At this time, B receives the voice frame until the tail frame is received, and both A and B enter the state of being able to talk. After that, the two parties repeat the above process to talk. Once A presses the disconnect button, the disconnect signal will be sent to B, and both parties return to the standby state. This process is shown in Figure 3.

Design of Digital System without Center Call Control Protocol Based on ARM7 and MX618 Chips

3. Design and implementation of call control protocol

Call control signaling and channel-associated signaling are respectively transmitted on the control channel and the call channel. According to the non-centralized multi-channel addressing mobile communication system, the control signaling transmission channel (control channel) is completely separated from the call channel. It integrates the control signaling of several call lines and occupies a common control channel to transmit control signaling.

3.1 Channel

Control Channel The control channel is mainly used for broadcast transmission of call setup signaling and as a transmission channel for signaling for other purposes. The service system of the control channel adopts the principle of the order waiting system. After the previous signal is transmitted in the control channel, the subsequent signal can be transmitted normally. Otherwise, wait until it is determined that the control channel is idle.

Call channel The call channel is responsible for transmitting the voice and data of the two parties after the call is established. At the same time, the call channel is also used to transmit channel-associated signaling. The channel associated signaling includes a series of necessary control signaling during the call. The service system of the call channel adopts the principle of call loss control. After two terminal devices establish a connection and occupy a call channel, this channel can no longer be used by other terminals until the terminal disconnects and releases the second call channel.

As shown in Figure 4, the control channel with a frequency of 915.02.5 MHz is only used for the communication parties to transmit call establishment signaling, while the call channel can transmit disconnection signaling, response signaling, and other channel-associated signaling. The mobile station switches between the two channels, stays on the call channel during a call, and returns to the control channel when the call ends or on standby.

Design of Digital System without Center Call Control Protocol Based on ARM7 and MX618 Chips

When the channel spacing of the mobile communication system without center multi-channel access is 12. 5 kHz, the system has a total of 158 channels. Among them, the first channel 915.012 5 MHz is the control channel, all call signaling is sent out on this channel, and the remaining 157 are call channels. The flow of call control is carried through by the transmission of control signaling in the control channel and the call channel. Therefore, how to formulate control signaling and how to handle all state transition events in the call signaling process has become the main content of the call control protocol design.

3.2 Design of call control signaling without center

Call control signaling is a control signal for various state transitions in a centerless system, and transmits system messages and commands. Call control signaling is divided into call setup request signaling, call response signaling, disconnect signaling, voice origination signaling, and voice termination signaling according to functions.

3.2.1 Frame structure of control signaling

The basic data transmission unit of a centerless system is a frame. Call control signaling is composed of two frame structures, the first frame and the last frame.

The header frame contains most of the control information in the signaling, and its frame structure is as follows:

Design of Digital System without Center Call Control Protocol Based on ARM7 and MX618 Chips

Preamble: used for receiver synchronization.

Frame synchronization: identification and synchronization of the first frame.

Header frame type: 4 b, identifies the header frame type. It is the main identification of the header frame and the main identification of signaling. Its value is: 0000 call initiation signaling; 0001 call establishment request signaling; 0010 disconnect request signal Order; 0011ACK signaling; 0100 prohibits sending and restoring signaling; 0101 prohibits sending and receiving signaling.

Receiver ID: The ID number of the called party’s mobile station, which is obtained by encoding a 7-digit call sign through the air interface.

Sender ID: The ID number of the calling party’s mobile station, which is obtained by encoding a 7-digit call sign through the air interface.

Communication mode: used to distinguish voice communication or data transmission.

Communication format: used to distinguish between full call communication and point-to-point communication.

Call information: 11 b, used to distinguish the definition of single call, group call and ACK signaling.

The tail frame supplements the head frame to form a complete signaling or a separate signaling for the end of communication. The frame structure is as follows:

Design of Digital System without Center Call Control Protocol Based on ARM7 and MX618 Chips

Tail frame synchronization code: 4 b, used for tail frame discovery and synchronization, fixed as 7D DF F5.
End frame type: 2 b, the main identifier of the end frame. 00 represents a normal tail frame; 01 represents a tail frame with status information.
ACK request: 2 b, identifies whether the called party needs to send an ACK signal.
Tx wait: 4 b. Identify whether it takes a period of time to force the PTT of the user who received the tail frame to be invalid, so as to allow the user to send an insertion request.
Status information: 5 b, user-defined according to needs, a total of 32 types of status information.

3.2.2 Contents of call control signaling

Call establishment request signaling is control information that is broadcast to the called party when the user initiates a call. The composition of the signaling is: header frame (setup request) + tail frame.

Call response signaling is feedback control information sent to the calling party after the called party receives the call setup request signaling. The signaling structure is: header frame (ACK).

Voice initiation signaling precedes voice transmission and is used to identify that the actual voice is coming. Its structure is: header frame (communication start).

Voice termination signaling indicates that voice transmission has come to an end, and its position is after the voice data block. The structure of voice termination signaling is: tail frame (end of communication).

Disconnect signaling is a call end signal sent when a communication party wishes to end a call, and is used to inform the other party that the call is about to end. The signaling structure of disconnection is: first frame (disconnect) + last frame + first frame (disconnect) + last frame.

3.3 Application of state machine to realize call control protocol

The digital centerless call control protocol uses the design scheme of the finite state machine to describe and implement. The finite state machine overcomes the shortcomings of inflexible sequential control methods of pure hardware digital systems. It can construct sequential logic modules with good performance and solve complex program flow problems in a simple, event-driven manner. Its reliability advantage is obvious, and it is a general solution for control programs in the embedded field.

The design of the state in the communication process is the key to the realization of the call control layer protocol. A complete and reasonable state transition process is also an important prerequisite for program realization. The state of the call control layer should include each typical situation in the call control flow cycle. And it should be possible to switch between states, there should be no unreachable states or dead states that cannot enter the process again, and redundant states that are meaningless for the characteristics to be described should also be avoided.

3.3.1 User A and B communication process state event machine

According to the characteristics of call control process and call control signaling and the technical specifications of the centerless system, the call control state machine has designed a total of 5 states from S0 to S4. S0 (standby state) indicates the default state when the mobile terminal is idle; S1 (initiating call state) is the call waiting state after the call originating party dials, and S1 is the in-progress state of the called party after the called party receives the call establishment request. Judge the waiting state; S2 (call established state) indicates that the call has been successfully established and is waiting for the next call; S3 (speaking state) is the state in which the user presses the PTT button of the walkie-talkie to transmit the voice of the call; S4 (receiving state) ) Is the state that the user is listening to the voice of the other party’s call. After the call, both parties return to S0 (standby state). The state machine considers the technical details in the actual application of call control, and realizes the predictable transfer of each state under various actions and event excitations. The specific content of the state machine is shown in Figure 5 and Figure 6 (Note: After resetting and removing the wire, each state is converted to the standby state).

Design of Digital System without Center Call Control Protocol Based on ARM7 and MX618 Chips

3.3.2 Implementation of the call control program

The call control program is developed in ARM7 processor using embedded C language. Set the variable ccl_state to store the code of the current state, and next_state to store the code of the next state. Use the Switch_Case statement and the if condition statement for event judgment to realize a complex multi-condition and multi-branch call control state machine.

Design of Digital System without Center Call Control Protocol Based on ARM7 and MX618 Chips

The ARM timer was added in the design process of the program. Timers play a very critical role in embedded systems. In order to save frequency resources, the non-central system has added the call time limit function. After the call time limit starts, a timer needs to be started. After the timer expires, an IRQ interrupt is generated and the call is terminated. In order to increase the reliability of the program and prevent the occurrence of a dead state, a judgment action on the timer overtime event should also be added in the program. Once each state (except the standby state) is in the state of waiting for the next action, it is necessary to immediately start the timer to judge the timeout. For example, when a call is established, the answering time of the other party is detected by a timer, and when it times out, it returns to the S0 standby state.

4. Feasibility and reliability analysis of call control protocol

The three parts of call control process, call control signaling structure, and call control state machine are inseparable, and constitute the basic elements of the call control protocol. The call flow design takes into account the non-central system and the traditional walkie-talkie workflow.

The call control signaling draws on and improves the signaling frame structure of the data link layer in the ETSI TS 102 490-DPMR standard. The call control state machine uses UML model to design five transition states triggered by communication events and actions. Logically, this call control protocol fully simulates various working scenarios of call control in practical applications, and can theoretically complete the functions of the call control system correctly. In the actual embedded development process, taking advantage of the high-speed operation advantages of the ARM7 processor, the control signaling that requires strict bit error rate in data transmission adds error control coding at the data link layer, CRC cyclic redundancy check, and Chinese Clear error correction coding, interleaving suppression of continuous burst interference error correction coding, to ensure the accurate transmission of control signals.

In addition, an anti-dead state timer interrupt is added to the call control program, which improves the reliability of the program’s operation. This call control protocol has been practically applied to the prototype development of laboratory digital non-centered walkie-talkies, and the operation is stable.

5 Conclusion

With the rapid development of various digital technologies, the analog centerless system is already stretched under the current technical environment, and its comprehensive digital transformation will be an important work content in the field of private network wireless communication in the future. There is no official version of the digital standard of the call control protocol. The call control protocol introduced in this article has been applied to the product development of digital wireless walkie-talkies. It runs stably, and its functions need to be further expanded and improved according to actual needs.

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