Introduction Traditional mobile communication systems are centrally controlled, relying on pre-deployed network equipment for operation. However, this is unsuitable when a centralized control approach is not feasible. In such cases, a network capable of rapid, automatic, temporary network deployment and mobile nodes is needed. Mobile Ad hoc networks are special-purpose peer-to-peer networks that utilize wireless communication technology. Nodes act as routers to each other, communicating through forwarding. Nodes can move. Compared to traditional mobile communication systems, they do not require a fixed network and offer advantages such as rapid and flexible deployment, high mobility, robustness, and low cost, making them particularly suitable for military, disaster relief, and electronic classroom applications. This type of network has become an effective communication network in the field of wireless communication and has achieved widespread application, such as the 802.11 system and the US integrated land, sea, and air digital communication system. In mobile Ad hoc networks, the network topology changes dynamically due to the mobility of communication nodes. During the development phase of Ad hoc networks, verifying the correctness of network communication protocols requires extensive field testing on actual physical channels, posing significant challenges to network communication experiments. Network environment simulators were developed to address this issue. They allow for development without relying on actual physical channels and without considering physical layer transmission protocols, focusing instead on MAC and network layer protocols. This accelerates system development, facilitates debugging and maintenance, and provides a simple and effective device for communication network simulation experiments. Functional Requirements of the Network Environment Simulator The Ad Hoc network experimental system based on the network environment simulator consists of a network environment simulator and a 16-node network controller. The network environment simulator simulates a real Ad Hoc network environment, while the network controller simulates the nodes in the Ad Hoc network. The network environment simulator is specifically designed for mobile Ad hoc networks, which possess self-organizing and adaptive characteristics, including adaptive topology detection, adaptive topology update, adaptive routing, and automatic network management functions. During the development phase of Ad hoc networks, MAC layer multiple access protocols are also required, with CSMA and TDMA being commonly used. The network environment simulator must also provide corresponding support for MAC layer access protocols. To adapt to these characteristics, the network environment simulator should have the following functions: It should be able to arbitrarily change the network topology, and the movement speed of network nodes should be simulated according to real-world conditions; it should simulate a wireless transmission environment, realize data transmission between nodes, and implement the characteristics of wireless channels, adding different error rates to the transmission paths of each communication node; it should be able to provide time references and time synchronization information for nodes in the network to support TDMA or frequency hopping access protocols; it should support multi-frequency hierarchical distributed network topologies, simulating multiple channels between nodes in the network; it should be able to track the data packets received and sent by each node, providing support for debugging the network layer and MAC layer; it should provide an interface displaying relevant information, showing the network topology, network node transmission and reception status, transmission and reception history, and channel information; System Hardware Design The network environment simulator mainly consists of a main control MCU, a bus driver module, a multi-serial port expansion module, an LCD display module, and a keyboard module. Its system block diagram is shown in Figure 1. The main control MCU communicates with the multi-serial port expansion module and LCD display module via the EBI bus. To enhance the MCU's bus driving capability, a bus driver module is placed between the main control MCU and the peripherals. The keyboard module is connected to the MCU's I/O port. The main control MCU controls the peripheral circuits and handles the entire simulator's workflow. The multi-serial port expansion module expands the network environment simulator's ports. The LCD display module displays the network topology, node transmission and reception status, node collision status, the length of data transmitted and received by nodes, and the node's transmission and reception history. The keyboard module enables node movement, changing the network topology. The main control MCU module is the core processing unit of the network environment simulator, controlling the peripheral display module, serial port expansion module, and keyboard control module, and handling the entire simulator's workflow. The MCU uses Atmel's 32-bit ARM microprocessor AT91RM9200. The AT91RM9200 chip is based on the ARM920T core, a 32-bit RISC processor that operates at 180MHz and has a processing speed of up to 200MIPS. Since the AT91RM9200 only has 16KB of internal SRAM, which is far from sufficient for the application's requirements, external SDRAM and FLASH memory need to be added. The external storage chips used are ICSI's IC42S32200L SDRAM chip and MICRON's MT28F640J3 FLASH chip. The IC42S32200L has a 32-bit data bus and 64Mbit of storage, while the MT28F640J3 has a 16-bit data bus and 64Mbit of storage. Serial Port Expansion Module The network environment simulator needs to provide multiple interfaces to network nodes. Here, we use the RS-232 standard serial port as the communication port. The AT91RM9200 only has four serial ports (one of which is a DEBUG serial port), so we need to add external serial ports. In the serial port expansion module, we use TI's TL16C554A as the serial port expansion chip. The multi-serial port expansion of the embedded system based on AT91RM9200 + TL16C554A is the main part of the network environment simulator hardware. The interface connection between TL16C554A and the main control MCU is shown in Figure 2. TL16C554A is a 4-channel asynchronous transceiver integrated chip manufactured by TI. The main features of TL16C554A are as follows: it consists of four TL16C550A asynchronous communication units with logic control, each channel is relatively independent; it can reach a baud rate of up to 1M, with a programmable baud rate generator for flexible selection of transmission and reception frequencies; each channel independently controls the transmission and reception of data, with independent modem control signals, and both data and control buses are driven by tri-state TTL; it has a fully programmable serial data format, with data bit length set to 5, 6, 7 or 8, stop bit length set to 1 or 2, and parity mode set to even parity, odd parity, or no parity. The LCD display module provides a human-machine interface, displaying network topology, node movement, node transmit/receive status, node transmit/receive history, and the channels used for data transmission and reception. On the LCD, the left area displays the node transmit/receive history and channels. Each node has a corresponding elongated window, with the history and channels displayed in a flowing manner within the window. The upper half of the window shows the node's transmit/receive history, and the lower half shows the channel currently used for transmission and reception. The right area displays the network topology; nodes within the communication range are connected by solid lines. A schematic diagram of the LCD display is shown in Figure 3. The LCD display uses the EPSON S1D13806 graphics control chip, designed specifically for embedded systems, with built-in SDRAM and a maximum resolution of 800 x 601. The S1D13806 communicates with the AR91RM9200 via the EBI bus (20 address lines, 16 data lines). It has a built-in 1.28MB SDRAM, uniformly addressed by the system, serving as a display buffer. The control unit sends image data to the host interface unit via the system bus. According to the register settings, the data is sent to the built-in SDRAM, and the remaining work is handled by the S13806. This eliminates the need for MCU intervention and overcomes the bandwidth limitations of other access methods, effectively preventing image jitter and screen distortion. To control the S1D13806, the AT91RM9200's EBI bus registers should first be configured, defining the read/write signal pulse length, wait state, data flow time, byte access type, and data bus width. Then, the S1D13806's operating mode should be configured via the EBI bus. The 50MHz BUCLK clock required by the S1D13506 to drive the LCD display is provided by an external crystal oscillator. The 25MHz CLK1 and 12.5MHz CLK2 are provided by the programmable clock generator chip ICS1523. The AT91RM9200 controls the ICS1523 via the I2C bus, causing it to output the required CLK1 and CLK2 frequencies to adapt to different LCD or VGA specifications, improving the versatility of the display solution. Because the S1D13506 needs to wait 70ns to access RAM, the LCD display speed is relatively slow. This system does not require frequent switching of the graphical interface, mainly because the AT91RM9200 is a high-speed ARM processor well-suited for industrial control. Keyboard Control Module The keyboard control module is connected to the AT91RM9200's I/O ports. The AT91RM9200 uses a polling method to monitor for level changes on the corresponding I/O pins and then performs corresponding processing. The keyboard control module can control node movement and select the node's movement speed. Several network topologies can be preset, and a topology can be directly selected using the keyboard. Nodes can then move to the corresponding position at the selected speed. System Software Implementation The network environment simulator software consists of two modules: one module simulates the network environment, and the other module displays relevant information on the LCD. For each node in the network, the network environment simulator has a corresponding port, and each port in the program has a corresponding structure. Network Environment Simulation The network environment simulator mainly simulates physical layer wireless transmission, network topology changes, and the provision of synchronization signals. Simulated Wireless Channel Transmission Simulating physical layer transmission mainly involves three aspects: 1. Network nodes sending data, and other nodes should be able to receive the data when the receiving conditions are met; 2. Simulating the node data transmission rate; 3. Simulating bit errors on the transmission path between nodes. In a real network environment, for a node in a network to receive data from another node, the following conditions must be met: one node is in a transmitting state, the other is in a receiving state, both nodes are on the same channel, and the receiver should be within the power coverage range of the transmitter. The simulation in the network environment simulator works as follows: there are two types of data interaction between the network environment simulator and the nodes in the network: control information and data information. Network nodes use control information to set the corresponding status for the ports of the network environment simulator, while data information refers to the data sent and received between the network nodes and other nodes. The network environment simulator relies on the pin levels of the RTS and CTS pins of the serial port to determine the type of information, and uses the TX and RX pins of the serial port to send and receive data entities. During each processing step, the network environment simulator can send the data received from each network node to other network nodes that meet the receiving conditions, based on the relevant information previously sent by each network node. In the network environment simulator, we use the timer interrupt of the AT91RM9200 to simulate the data transmission rate between nodes. After each interrupt processing, one byte of data is sent to each port. Suppose we want to simulate a shortwave channel rate of 2.4Kbps–4.8Kbps. Using the timer interrupt of the AT91RM9200, an interrupt is generated every 1/300 or 1/150 second. Within the interrupt, we read the serial port data from each network node using a TL16C554A. This process is equivalent to the network node sending data, performing corresponding processing to determine which nodes in the network can receive it, and then sending data back to the corresponding network node's serial port using the TL16C554A. This process is equivalent to the network node sending and receiving one byte of data every 1/300 or 1/150 second, resulting in a transmission rate of 2.4Kbps–4.8Kbps. Errors can be added to any transmission path, and the error rate can be controlled. By inverting the data read from each port, the error rate requirement is achieved. By periodically controlling the error rate switch, different error rates can be obtained for the data along the transmission path. Synchronization Signal Provision The network simulator can send synchronization signals to each network node, providing support for debugging MAC layer protocols such as TDMA and frequency hopping. Nodes can use the data sent by the network environment simulator each time as a time base, ensuring that the time base of all nodes in the network is the same. Simultaneously, the network environment simulator can send a time synchronization signal to each node at a specific time to achieve synchronization, greatly facilitating the debugging of the TDMA protocol. LCD Information Display: The LCD can display relevant information. Basic drawing functions such as drawing points, lines, and rectangles are implemented first. These basic drawing units can be combined to display corresponding graphics. During the interrupt handling process of the network environment simulator, corresponding states are set, such as transmit/receive status, collision status, and node position coordinates. Then, the display status queue is updated. The display program is executed in the main program, and the graphics are drawn according to the information in the display status queue. Conclusion The developed network environment simulator with 16 ports can realize arbitrary connectivity between data terminals, supporting variable channel data transmission rates and different error rates along the transmission path. By simulating dynamic changes in the network topology through the network environment simulator, the data transmission and reception process between nodes and information on whether nodes collide can be clearly observed on the LCD. The network environment simulator provides a good platform for studying the self-organizing and adaptive functions of packet wireless networks and verifying network protocols. Based on this network environment simulator, the TDMA protocol and AODV routing protocol have been developed, and the correctness of the scheme has been verified.