Design of digital system for call control protocol without center based on ARM7 and MX618 chip

introduction

The centerless mobile communication system is an important part of my country's professional mobile communication system. It works in a simplex intercom mode with an operating frequency between 915.0125 and 916.0875 MHz. The system has many technical features such as centerless networking, digital selective calling, automatic connection, multi-user multi-channel sharing, and link decentralized control. It has broad practical application prospects and in-depth development potential. The network structure diagram of the system is shown in Figure 1. At present, my country's 900 MHz centerless mobile communication system is still in the analog stage with analog voice plus digital signaling as the dominant technology. The digital research on centerless call control protocol is still blank.

The digitization of the centerless system is to digitally encode 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 computing control core. In this development environment, this paper takes the digital centerless call control layer protocol as the research object, and proposes and designs a call control protocol digitalization solution based on the centerless system. This solution has been applied to the embedded system development of digital centerless intercom.

1. Overview of Digital Centerless Call Control Protocol

ETSI TS 102490-DPMR standard describes the protocol layered structure of the decentralized system, as shown in Figure 2. The call control protocol is located at the third layer of the decentralized system protocol stack, above the data link layer and below the application layer, and is the control core of the decentralized system.

It provides the entire system with support for basic call establishment, maintenance, and disconnection; point-to-point calls and group calls; call partner selection; late entry, call transfer, etc. It also provides interfaces for the data link layer that carries data and voice services and the application layer that implements specific functions, and needs to provide complete upper and lower layer general interfaces to reserve space for future protocol upgrades.

The design of the call control protocol includes the design of the call control process, call control signaling and the subsequent program implementation coding, which then forms an independent call control protocol functional module.

2. Design of call control process

According to the working principle of digital centerless intercom, a call control process that meets practical application is designed. The call control process describes a series of actions and events generated by the communication terminal from call establishment to call termination.

Mobile station A and mobile station B in standby state are waiting on the control channel. When mobile station A presses the call button to call mobile station B, A broadcasts a call establishment request signaling for B on the control channel and switches to the selected call channel to receive it. After receiving the call establishment request signaling, if B agrees to establish a connection, it switches to the call channel identified in the signaling and sends a confirmation signaling to A on this call channel. After A receives the confirmation signaling, both parties enter the talkable state. If A presses PTT, it will send a voice header frame + voice frame to B.

After releasing PTT, the last frame is sent to indicate that the call of party A ends. At this time, B receives the voice frame until the last frame is received, and both A and B enter the talkable state. After that, the two parties repeat the above process to talk. Once A presses the disconnect key, a disconnect signal is sent to B, and both parties return to the standby state. This process is shown in Figure 3.

3. Design and implementation of call control protocol

Call control signaling and channel-associated signaling are transmitted on the control channel and call channel respectively. According to the decentralized multi-channel selection 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 public control channel to transmit the control signaling.

3.1 Channel

Control channel is mainly used for broadcast transmission of call establishment signaling and as a channel for sending signaling for other purposes. The service system of the control channel adopts the principle of sequential waiting. The subsequent signaling can be transmitted normally only after the previous signaling is transmitted in the control channel. Otherwise, it waits until it is determined that the control channel is idle.

The call channel is responsible for transmitting the voice and data of both parties after the call is established. At the same time, the call channel is also used to transmit channel-associated signaling. Channel-associated signaling includes a series of necessary control signaling during the call process. The service system of the call channel adopts the call loss system principle. 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 secondary call channel.

As shown in Figure 4, the control channel with a frequency of 915.0125 MHz is only used for the communication parties to transmit call establishment signaling, while the call channel can transmit disconnection signaling, answer signaling and other channel-related signaling. The mobile station switches between the two channels, staying on the call channel during a call, and returning to the control channel when the call ends or when it is on standby.

When the channel spacing of the centerless multi-channel selection mobile communication system is 12.5 kHz, the system has a total of 158 channels. The first channel 915.012 5 MHz is the control channel, and all call signaling is sent on this channel. The remaining 157 are call channels. The call control process is connected 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 become the main content of call control protocol design.

3.2 Design of Centerless Call Control Signaling

Call control signaling is the control signal for various state transitions in a decentralized system, transmitting system messages and commands. Call control signaling is divided into call establishment request signaling, call answer signaling, disconnect signaling, voice origination signaling, and voice termination signaling according to its function.

3.2.1 Control signaling frame structure

The basic data transmission unit of the decentralized system is the frame. Call control signaling consists of two frame structures: the header frame and the tail frame.

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

Preamble: used for receiver synchronization.

Frame synchronization: identification and synchronization of header frames.

Header frame type: 4 b, identifies the header frame type, is the main identifier of the header frame, and is also the main identifier of the signaling. Its value is: 0000 call start signaling; 0001 call establishment request signaling; 0010 disconnection request signaling; 0011 ACK signaling; 0100 prohibition of sending and restoring signaling; 0101 prohibition of sending and receiving signaling.

Receiver ID: The called party's mobile station ID, obtained by encoding the 7-digit call sign over the air interface.

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

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

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

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

The tail frame is added after the head frame to form a complete signal or constitute the signal of the end of communication. Its frame structure is as follows:

Tail frame synchronization code: 4 bits, used for tail frame discovery and synchronization, fixed at 7D DF F5.

Tail frame type: 2 b, the main identifier of the tail frame. 00 indicates a normal tail frame; 01 indicates a tail frame with status information.

ACK request: 2 b, indicating whether the called party needs to send an ACK signal.

Tx wait: 4 b, identifies whether a period of time is required to force the PTT of the user who receives the tail frame to become 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

The call establishment request signaling is the control information sent to the called party in a broadcast mode when the user initiates a call. The signaling consists of: header frame (establishment request) + tail frame.

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

Voice origination signaling precedes voice transmission and is used to indicate that actual voice is about to arrive. Its structure is: header frame (communication start).

The voice termination signaling indicates that the voice transmission has come to an end. It is located after the voice data block. The structure of the voice termination signaling is: tail frame (communication end).

The disconnect signal is a call ending signal sent when one party wants to end the call, and is used to inform the other party that the call is about to end. The disconnect signal structure is: header frame (disconnect) + tail frame + header frame (disconnect) + tail frame.