The data communication network includes many systems and related equipment, and may have digital transmission systems, packet switching, DDN, frame relay, ATM, Internet systems, public switched telephone networks, local telephone cables, and routers, gateways, bridges, multiplexers , Frequency band or baseband modem, user terminal equipment, etc. During system connection, installation and commissioning, and network acceptance in network construction, test instrumentation is indispensable for maintenance, inspection, judgment, and troubleshooting after the network is built. Data communication test instruments mainly include communication test, transmission error test, transmission channel test and interface test.
1. Procedure analysis (test) instrument
Data communication is the communication between people and computers or between computers and computers. To achieve effective, fast and reliable communication, a set of communication protocols is required, which is called a communication protocol or communication protocol. To this end, a seven-layer model of open system interconnection has been proposed internationally. Each layer has its specific function, and the upper layer uses the services provided by the functions of the lower layer. In order to enable devices in the network to work together and achieve correct and effective communication, they are required to follow a common protocol throughout the communication process, including any details that may occur. Therefore, the procedure test is very important in data communication.
The functions that the procedure test analyzer may provide are summarized as follows:
â‘´. Online monitoring function
Connect the procedure test analyzer to the interface to monitor the data transmitted in both directions, used to monitor the working status, track the procedure and find errors.
-Interface status monitoring.
-Capture data in real time to disk or buffer for further recall and analysis.
-Event trigger function, that is, when the defined event occurs, it will cause the corresponding action of the instrument.
-Filter function to monitor data that meets user-defined conditions.
-Real-time decoding, which decodes the data stream frame by frame.
-Statistics, including statistics of traffic and events.
⑵. Simulation function
To be able to simulate "users" to test the relevant performance of the network; simulate "networks" to test the corresponding functions of user equipment. Built-in simulation application program, automatically generate simulation of corresponding regulations. It can automatically answer inquiries, establish and maintain link management, perform flow control and network link building and disconnection. The user can define the parameters of the procedure.
When simulating, you can use the real-time function, display the simulation and real-time status at the same time, perform decoding and statistics of each layer, and also capture data.
⑶. Analysis function
It can analyze statistics in real time or send back analysis afterwards, and can edit, list and print the analysis results.
-Analyze network transmission errors by different elements, such as source address, destination address, address pair, protocol, etc.
-Real-time "focusing and zooming" from the decoded data state is to select the data stream related to an element for detailed analysis.
-Display of various statuses confirmed by the user.
-State diagram with time interval scale.
-Schedule statistics. Including frame distribution statistics, utilization rate, business flow, errors, etc.
â‘·. expert system
Capture the events confirmed by the user, analyze the network according to the rules established by the user, identify the events, analyze the problems, point out the reasons, and propose solutions. Generally, an expert system is only available in a high-end procedure analyzer.
⑸. Multi-port real-time monitoring
Used to test the parameters of the Internet connection, such as the transmission delay between the Internet, the delay of the router, and the display of the operating status.
⑹. Filter function
Cooperate with monitoring, analysis and expert system to screen out the data of interest.
⑺. Report generation and printing functions
When equipped with a procedure tester, the following questions are available for reference:
A type of procedure tester can test a variety of procedures, which can be applied to wide area networks such as packet switching, frame relay, ATM, and LANs such as Ethernet, token network, FDDI, ASYNC, SYNC (BSC), HDLC, SDLC, X.25, FR, ATM, TCP / IP, PPP, SLIP, etc., to choose the instrument that can support the various regulations involved in your system, you can not buy the optional options of the regulations. Many models of instruments have extended functions, and corresponding modules and software packages can be purchased when needed in the future.
The physical interface and its speed must meet your needs.
The high-end procedure analyzer is fully functional and the price is relatively high (hundreds of thousands of yuan), which is suitable for being configured in the provincial or regional maintenance center. The basic maintenance unit can be equipped with general-grade instruments, which also have monitoring, simulation, capture and analysis functions, which can meet the needs of general maintenance and opening users; can be programmed by users according to needs; can be matched with the high-end procedure analyzer of the maintenance center If you need further analysis, you can save the captured data for later recall, and then do more perfect work. Many general grade test instruments are portable instruments, the price is not high (generally the price is tens of thousands of yuan), easy to use, very suitable for users to open and use in daily maintenance.
2. Error code tester
The quality of the data circuit is measured by the error rate. Transmission errors can be divided into two types: discrete errors and burst errors, which have different effects on different systems. Nowadays feedback feedback retransmission error control methods are used more frequently. The quality of the system is measured by the transmission efficiency. The more retransmissions occur when an error occurs, the lower the efficiency. Discrete errors have a greater impact on the transmission efficiency than burst errors. And for those real-time transmission systems, the impact of burst errors is sometimes greater. For this reason, the error rate, word error rate, group error rate, and error-free seconds are commonly used to count errors from different angles.
The bit error meter is a frequently used meter in data communication maintenance. Choose the applicable instrument according to the following indicators (mainly the first three items): data rate, interface type (V.24, V.35, X.21, RS-449, RS-530, G.703, etc.), data format (Synchronous, asynchronous, etc.), working mode (DCE and DTE), test pattern (511, 2047, FOX, etc.), clock selection, flow control selection, test time control, error record statistical analysis ability, error insertion ability, etc.
It should be pointed out that the procedure analyzer generally has an error test function, so that there is no need to repeatedly equip the error detector.
3. Data transmission channel test instrument
Connections between nodes such as X.25 packet switching, DDN, frame relay, ATM, Internet, etc., and connection of routers, gateways, bridges, multiplexers, frequency band or baseband modems, user terminals and other user equipment to data communication nodes, And the connection between the user equipment via the dedicated line, etc., need to pass various types of data transmission channels. The quality of the data transmission channel directly affects the communication quality of the data network, and its testing is also very necessary.
Data transmission channels can be divided into digital channels and analog channels.
â‘´. Digital channel test instrument
With the application of SDH, 2.048Mb / s E1 and 155.52Mb / s STM-1 (OC-3) digital channels are now mainly used. With the development of broadband networks, the digital channels used for data transmission have to go higher. Speed ​​development. With the rapid increase in the number of 2.048Mb / s E1 users and relays, it is necessary to equip the E1 channel tester in the data communication operation and maintenance department. For higher-speed digital channel test instruments, except for large data communication centers or separate locations with digital transmission centers, I do n’t think it is necessary to equip with digital transmission centers repeatedly.
For data transmission, the E1 channel test instrument should have the following main functions:
G.703 E1 error test, test and analyze the data transmission quality of the circuit. Use 29-1, 211-1, 215-1 and other random codes in accordance with ITU-T O.151, O.152, O.153 and all "0", all "1", "1" "0" alternate Fixed code patterns, test bit errors, coding errors, framing errors, CRC errors, E-bit errors and bipolar errors, with ITU-T G.821 analysis capabilities (or add G.826, M2100 / 550 analysis ability).
The time slot can be dropped and inserted. The data transmission quality of a certain time slot or FE1 can be tested without interfering with other time slot services. Test and analyze N & TImes; 64kb / s data errors.
Online monitoring of E1 line signal quality. Including signal level and frequency measurement, pulse mode analysis and error monitoring.
Frame and timing sliding test, used to analyze the synchronization problem of E1 line.
Real-time status and alarm monitoring, alarm function test.
Unframed and framed (including PCM-30, PCM-30 CRC, PCM31, PCM31 CRC) structures can be tested to suit the application of different frame structures.
It can be terminated test, bridge test and connected to the detection terminal with protection for testing. Convenient for testing at different access points.
Some portable or handheld E1 channel testers that meet the above requirements are very practical and easy to use, and are very popular with O & M personnel.
⑵. Analog channel test instrument
Analog channels include telephone exchange networks, local telephone cables, and long-distance analog leased lines. Some characteristics of analog channels have an impact on data transmission. These characteristics are called "damage factors" and mainly include total attenuation, attenuation frequency distortion, and group delay distortion , Nonlinear distortion, noise, quantization noise, phase jitter, frequency deviation, echo, impulse noise, instantaneous interruption, phase mutation and amplitude mutation, etc.
The impairment parameters required for different types of analog channels are different. For the local telephone cable line, its group delay distortion is very small and can be ignored; because no active transmission equipment is involved, non-linear distortion, quantization noise, phase jitter, frequency deviation, phase mutation and amplitude mutation will not occur. The carrier channel does not need to test quantization noise. There is also no need to consider phase jitter, frequency deviation, phase mutation and amplitude mutation for PCM voice circuits and program-controlled telephone switching circuits. The test echo is only considered for two-wire circuits. Among the damage parameters to be measured, some are the main parameters that must be measured, and some are the parameters to be tested according to specific conditions.
Now, the transmission of data through the carrier circuit is very rare. With the application of X.25, DDN, FR, and ATM networks, there are not many data users who use long-distance PCM channels. With the widespread use of the Internet, people are no longer willing to carry out point-to-point data transmission through the telephone exchange network. Therefore, the focus of current analog channel maintenance and testing has shifted from users to X.25, DDN, FR, ATM nodes or Internet ISP user lines.
On the other hand, the data transmission rate on the subscriber line has evolved from the low speed of the audio modem to 64 / 128kb / s baseband or ISDN, N & TImes; 64kb / s baseband, up to 2.048Mb / s HDSL and 8Mb / s ADSL. The increase of transmission rate and the widening of transmission frequency band put forward higher requirements on the quality of local telephone cable subscriber line.
Therefore, we must pay attention to the maintenance and measurement of local telephone cable subscriber lines, and equip them with corresponding instruments. With the development of high-speed broadband data services, this will become increasingly urgent and important.
The local telephone cable subscriber line test instrument shall have the following functions:
Testing of DC loop resistance, insulation resistance (between wires and conductors to ground) and working capacitance of subscriber lines.
These are the most basic tests used to check the quality of cable wires over the years. Use the DC loop resistance to calculate the line length and determine the location of the mixed line; use the working capacitor to determine the location of the open circuit. For data lines, it is not enough to use these tests, but the following tests are also required.
Testing of line transmission attenuation and attenuation frequency characteristics
Because the signals transmitted by ISDN, baseband MODEM, HDSL and ADSL on the local telephone cable line have corresponding bandwidths, the bandwidth of the test instrument should also have certain requirements. ADSL requires more than 1100kHz.
Test for idle noise.
There are two noise test methods, sending a test tone (820 Hz or 1004 Hz or 1020 Hz) method and not sending a test tone method. Since the audio frequency band MODEM, ISDN, baseband MODEM, HDSL and ADSL signals have their own corresponding frequency bandwidth, only the noise falling in this frequency band will affect the data transmission, so the instrument should have a corresponding frequency band noise filter, they are D -Filter (audio), E-filter (ISDN and baseband modem with 64 / 128kb / s 2B1Q coding method), F-filter (HDSL), G-filter (ADSL). The signal-to-noise ratio can be calculated or read from the meter.
Line impedance / return loss test.
Test the line impedance / return loss within the corresponding test bandwidth.
Impulse noise test.
According to the required bandwidth, use D (or O.71) or E or F or G filters to test the impulse noise count at different threshold levels.
During the completion test of the line project and the selection of data line pairs, the line crosstalk (mainly near-end crosstalk) must also be tested. This requires a wideband frequency selection function, and generally requires additional corresponding instruments.
The subscriber line test instruments on the market now tend to be portable or palm-type, the price is not expensive, and it is very convenient to use.
This kind of instrument can also be used for PCM speech channel test, testing its attenuation, attenuation frequency distortion, idle circuit noise, quantization noise (single frequency method), impulse noise and echo and other main parameters. Some also have dial-up and DC hold functions, and can also be used to test the telephone switching network, making it more widely used, called "analog line tester" or "transmission line tester". Equipped with such instruments can be used for various analog channels for data transmission, especially the measurement of subscriber lines.
Four, interface test
Commonly used V.24 (RS-232), V.35, V.36, X.21, RS449, RS530 and other interface standards. The electrical characteristics standards are V.28, V.10, V.11, etc., which are divided into two types: double-flow balance and double-flow unbalance. Commonly used interface tester, used to indicate the status of each interface line, provide positive or negative voltage for the interface line for status test, connection test of the interface line, connect the oscilloscope or logic analyzer to observe the interface signal waveform and time relationship, and connect the procedure analyzer Wait.
The interface test box is a commonly used small test equipment, but it is still very useful in solving specific interface problems.
With the development of CPU technology and the application of large-scale integrated circuits and digital signal processing technology, data communication test instruments have now developed in the direction of new technology, miniaturization, multi-function, low price, and ease of use. The provision of these instruments will significantly improve the maintenance level and maintenance quality.
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