Application of Microcomputer in Bit Error Rate Test of Communication Lines

    Abstract: On the basis of elaborating the principle of bit error rate testing of communication lines, a new method of bit error rate testing using a microcomputer system is introduced, as well as the design and implementation of the system's software and hardware.

    Keywords: communication bit error rate 511 code sequence test

1 Communication line bit error rate test

1.1 Testing principles and methods

In digital communications, the bit error rate is a major criterion for testing the working quality of data transmission equipment and its channels. In practical applications, the m sequence whose primitive polynomial is 1+X5+X9 is generally used, and its period is 2 9 -1=511, which is commonly known as the 511 code sequence. The 511 code sequence is actually a pseudo-random code sequence. The principle of using it to test the bit error rate of communication lines is shown in Figure 1.

In actual work, a bit error rate tester (hereinafter referred to as a bit error meter) is generally used to test the bit error rate of communication lines. In order to solve the signal attenuation caused by long-distance transmission, two modems (MODEM) are commonly used in actual communications to connect the receiving and sending data terminals. Therefore, there are three types of bit error rate tests in practical applications: self-test, loop test and pair test. way, as shown in Figure 2.

1.2 Defects in using a bit error meter to test the bit error rate

When it is necessary to conduct a bit error rate test on a certain channel, the transmitting and receiving cables of the data terminal must be unplugged at both ends of the communication and then connected to the bit error meter. The disadvantages are obvious: First, the operation is inconvenient. Testing to restore the line status requires plugging and unplugging the cable twice, which is time-consuming and troublesome; secondly, when testing the bit error rate of multiple channels at the same time, multiple error detectors are necessary; thirdly, plugging and unplugging the cable multiple times will cause Severe cable wear or poor contact affects the reliability of the equipment.

2 Use microcomputer system to test bit error rate

2.1 Implementation principle

In view of the above disadvantages, we designed a new bit error rate test method and applied it to the data transmission center of our unit (30 communication channels). The principle of the new method is to use a microcomputer, supplemented by self-developed software and hardware. The system replaces the bit error meter and conducts centralized control and testing of all channels. When the test system is not working, it is only connected to the communication channel and does not have any impact on the data transmission in the channel; when one or several When the channel is tested for bit error rate, the connection between the channel and the data end is cut off under the control of the software, and it is connected to the test system. The microcomputer is used to test the bit error rate of the opposite end. After the test is completed, the software restores the line to its original state, such as As shown in Figure 3.

2.2 Hardware design

Since the data transmission center has as many as 30 communication channels, in order to control flexibly and conveniently, we designed a peripheral line switching board and a control board based on the ISA bus to realize group selection from 32 full-duplex communication channels. 8 channels for bit error rate testing. The method is to use 4 channels as a group, and only one channel in each group can be selected for testing. At most, 8 channels (one in a group) can be selected to participate in the test. In order to realize the reception and transmission of 511 code sequences, we purchased a bit error test board based on the ISA bus. This board is designed to have 8 full-duplex channels and supports a variety of communication protocols including dual synchronous transmission methods; And there is a clock control circuit on the board, which can easily select the test code rate. However, in practical applications, we design the system to use an external clock decoding method, that is, the rate at which the system sends and receives 511 code sequences is determined by the transmission rate of the MODEM, which can support a maximum rate of 14.4Kbits/s. As shown in Figure 4.

2.3 Software design

2.3.1 Application introduction

The software of the microcomputer test system currently used is developed under the DOS6 platform using DC++3.1. Since the bit error rate test of multi-channel channels is random, in order to adapt to this multi-task operation, the multi-path technology of the Win32 system was simulated during the design process, and the software was designed to include a main thread and multiple sub-threads. The main thread is responsible for system initialization, human-machine interface, record display and sub-thread scheduling. Each sub-thread is independently responsible for performing a bit error rate test on a certain channel, and its number is related to the number of channels selected to be tested. When it is necessary to test the bit error rate of a channel, the main thread performs the following operations:

·Send control information to the line switch through the control board, connect the corresponding channel to the error test board, and cut off the connection between the channel and the data terminal;

·Initialize the status of the corresponding channel on the bit error test board;

·Create a sub-thread to start the sending, receiving and comparison of 511 codes, and start timing display;

·The sub-thread communicates with the main thread and sends the test results to the main thread in real time for display and recording. The displayed content includes the channel name, test time, synchronization status, error count and final result error rate;

·When the test is completed, the main thread will reset the corresponding channel and restore the line to its original state.

2.3.2 Generation of 511 code sequence

The primitive polynomial that generates the 511 code sequence is 1+X5+X9, and its initial value is any 9-bit binary number that is not all "0". The generation method is shown in Figure 5.

That is, bit a0 and bit a5 are XORed and used as the next bit input channel. This cycle is repeated 511 times to obtain a set of 511 code sequences (one cycle).

2.3.3 Transmission of 511 code sequence

Since during the actual bit error rate test, what is transmitted on the line is a continuous equally spaced 511 code bit stream sent at a given code rate, that is, no control characters or bit ratios are allowed to be inserted into the bit stream. The synchronization of each bit is accomplished by clock signals (TxC, RxC) transmitted together with the data (TxD, RxD).

Considering that the 511 code sequence of one cycle is not an integer byte, in order to facilitate the calculation of the microcomputer, a code sequence of 8 cycles, that is, 511 bytes, is generated in advance and placed in a special sending buffer. According to the 511 code generation principle, in an 8-cycle code sequence, any adjacent 16-bit code group must be unique. Therefore, a 16-bit code group can be selected as a synchronization character from the 511-byte code sequence, and the communication protocol of the error test board can be set to the supported dual synchronization (16-bit synchronization character) transmission method. The sending of 511 codes mainly relies on hardware. The software only sets the address of the sending buffer and the number of bytes to be sent during initialization, and then starts loading and sending; after the first 511-byte bit stream is sent, the The hardware automatically loads and continues to send until the test is stopped, thereby achieving continuous and uninterrupted sending of the 511 code sequence.

2.3.4 Reception and comparison of 511 codes

After setting the dual synchronization working mode, use the 16-bit synchronization character in Section 2.3.3 to synchronize the received bit stream. Once synchronized, a 511-byte bit stream is received, and then compared with the pre-generated 511-byte code sequence, and the number of bit errors is calculated. For the convenience of processing, two receive buffers are set for each tested receive channel, each with a length of 511 bytes, and are processed alternately. Since the 511-byte bit stream received after synchronization is directly driven into Xiangqing's receive buffer (DMA reception) by hardware, it is very efficient. Therefore, the system unlocking increased by setting two receive buffers is relatively small. is trivial.

2.4 Utilization effects

Using the new microcomputer test system, we conducted self-tests, loop tests and comparison tests with a bit error meter on one channel and multiple channels, and artificially added interference during the test process to determine the system's adaptability. sex. The test results show that the new bit error rate test method is completely reliable and feasible.

Compared with the method of using a bit error meter to test the bit error rate of communication lines, the method described in this article has the following advantages:

·The control is flexible and convenient, no need to plug and unplug cables, simplifying the operation;

·The function of grouping and selecting 1 to 8 channels for testing from 32 channels not only reduces the complexity of hardware design, but also can adapt to the needs of different problems. By selecting a bit error test board with enough channels, efficient bit error rate testing can be achieved at both ends of the communication.

·When testing the bit error rate, the channel under test will not affect the normal operation of other channels. When the system is not working, it will not have any impact on the data transmission of all channels, and the reliability is high.

In order to better adapt to the needs of multi-channel bit error rate testing, the multi-path technology of Windows NT and VC++ can be fully utilized to design the microcomputer test system to run under Windows NT 4 (workstation version), which can improve the performance of the test system. , and facilitates software maintenance.