1 Overview Emergency communication technology is a communication technology that comprehensively utilizes existing communication methods to achieve rapid deployment of communications and mobile responses. It was first used in the military, and played a major role in combat readiness operations, field construction, emergency rescue, and emergency response. With the development of embedded technology and multimedia communication technology, the application of handheld multimedia 1 communication terminal in emergency communication makes the system more flexible and flexible, and at the same time allows various information to be transmitted more effectively in the system. In emergency communication systems, there are usually multiple wired and wireless communication methods simultaneously. Various wired communication methods usually use the TCP / IP protocol, and Ethernet cards are typical of such devices. In terms of wireless communication, various shortwave radio stations are more suitable for harsh communication environments. At the same time, various new types of network radio stations using Ad hoc networking have also been widely used in recent years, which requires multimedia communication terminals to have the ability to communicate through a variety of lower-level communication devices. Therefore, designing a handheld multimedia communication terminal suitable for a variety of communication devices has broad application prospects. For the above three types of devices, this paper introduces a software implementation of a multimedia communication terminal suitable for a variety of communication devices. The terminal software is designed using an object-oriented method, and the interface of the communication module is reasonably abstracted, and different communication devices are encapsulated into a communication module with a unified interface. In this way, when different communication devices are used, the terminal only needs to select the corresponding communication module to perform normal communication. 2 Software design of multimedia communication terminal The hardware platform of the terminal uses the embedded processor PXA250 produced by Intel Corporation as the core processor, with a processing capacity of up to 400MHz, and also supports a very rich peripheral interface, including USB HOST interface, CF interface, serial port, Ethernet and so on. As shown in Figure 1, the terminal supports multiple communication services such as voice, video, still image, short message, electronic whiteboard, and file transmission. Each communication service is completed by an independent service module. The business module is controlled by the user interface and returns feedback information of various controls to the user interface. The business module hands over the generated various data packets and its target address to the communication module; the communication module hands over the received data packet and source address to the corresponding business module. Obviously, the channel characteristics of different communication devices are different, and corresponding communication modules need to be designed for different communication devices. If all communication modules have a unified interface and provide services of the same nature to the business module, then the business module will be completely reusable. Therefore, a reasonable abstraction of the interface of the communication module will make the structure of the software clearer, which is helpful to improve the reusability of the code, thereby reducing the complexity of software design. When abstracting the interface of the communication module, two principles need to be followed. First of all, for different communication devices, this unified interface can be used for encapsulation. This terminal supports three kinds of communication equipment: Ethernet card, Internet radio and short wave radio. When communicating via Ethernet, the terminal can use the TCP / IP [2] protocol stack provided by the operating system to communicate using TCP and UDP. Among them, the TCP protocol provides reliable byte stream transmission services, while the UDP protocol provides unreliable data packet transmission services. The multimedia communication terminal is connected to the Internet radio through the USB interface, and the TCP / IP protocol stack on the Internet radio is applied in a remote control mode, and TCP and UDP can also be used for communication. Generally, network stations are networked through Ad Hoc. Because the TCP connection has low transmission efficiency on the Ad hoc network, usually only UDP is used for data transmission. The shortwave radio station is connected to the multimedia communication terminal through a serial port to provide a bit link with random error codes and burst error codes. With reference to ITU-T H.223 [3] protocol, using various error correction codes and error detection codes, an unreliable data packet transmission service can also be realized on the bit link. Secondly, the functional scope of the communication module and the business module should be properly defined, and under the premise of ensuring the effective use of the channel, the code reuse should be improved as much as possible. For several problems that need to be solved in the communication process, it needs to be divided according to this principle. First, since multiple media services can communicate concurrently, data packets from different services need to be identified so that the receiving end can identify the service type of the data packets. When using UDP to transmit data packets, the identification of the service can be achieved by selecting different UDP ports for different services, or by adding identification bytes in the data packet. When communicating with short-wave stations, the service identification can be realized when data is multiplexed to obtain higher communication efficiency. Therefore, the method of service identification is related to the adopted communication device and needs to be implemented in the communication module. Second, data packets of different services have different quality requirements during transmission. Signaling of various services needs to achieve reliable transmission; data packets of static image, short message, electronic whiteboard and file transmission services also need to be reliably transmitted; voice and video communication services can withstand a certain amount of packet loss Some errors in the package can be tolerated. Therefore, the reliable transmission of data packets should be implemented in the business module through ARQ [4]; the communication module decides whether to submit the data packet with an error code to the business module according to the type of business. Finally, the optimal allocation of communication resources should be implemented in the communication module, giving priority to ensuring the bandwidth resources required for real-time communication services. Figure 1 Structure diagram of multimedia communication terminal According to the above analysis, the communication module provides unreliable data packet transmission services, identifies the business, optimizes the allocation of communication resources according to the type of business, and decides whether to submit the erroneous data packet to the business module; the business module uses ARQ Ensure the reliable transmission of data packets. When programming in C ++ language, the abstract communication module interface can be defined as a base class. Communication modules suitable for Ethernet, network radio, and shortwave radio are all derived from this base class. The following introduces the design of communication modules for three types of communication devices. 2.1 Communication module suitable for Ethernet When communicating via an Ethernet card, UDP data packet transmission can be achieved through socket programming [5]. As shown in Figure 2, two sockets are used in the communication module, which are responsible for sending and receiving data packets. The communication module actively reads the data packet from each business module, and adds a byte of business identification before the data packet. In order to prioritize the bandwidth resources required for voice and video communications, the communication module preferentially reads the data packets from the voice and video service modules and sends them. The data packet with the identification added is sent to the target terminal through the send socket. The flow control module controls the number of bytes sent to the Ethernet per second at a preset rate. When receiving a data packet, the UDP receiving socket judges the service type according to the first byte of the data packet, and writes the data packet into the receiving buffer of the corresponding service module. The UDP protocol guarantees that there will be no error codes in the data packet, so the function of error code judgment is omitted in this communication module. The communication module suitable for Ethernet is the most intuitive and simplest one. Figure 2 Communication module suitable for Ethernet cbg oil vapes Shenzhen Yingyuan Technology Co.,ltd , https://www.yingyuanvape.com
