A Brief Discussion on E1 Line Maintenance Technology and Applications

I. Overview<?xml:namespace prefix = o ns = "urn:schemas-microsoft-com:office:office" />

With the acceleration of informatization, digital communication is comprehensively transforming our traditional communication methods and communication quality. Regardless of the launch and operation of any new service, it cannot be separated from the guarantee of strong, effective, and highly reliable transmission. Consequently, the issue arises of how to measure and ensure the transmission quality of the system. The Bit Error Rate Tester is an effective tool that can achieve this function, and by detecting the bit error damage performance indicators that reflect the working quality of data transmission equipment and their channels, it performs transmission quality analysis. In telecommunications operations, project acceptance, scientific research, equipment production, teaching experiments, and other aspects, the bit error rate tester is an indispensable auxiliary tool for communication measurement and line maintenance.

II. Basic Concepts of Bit Error Rate Testing

1. Bit Error

Bit error refers to the situation where code elements are corrupted during signal transmission due to the influence of line transmission and equipment transmission.

2. Bit Error Performance Indicators

(1) Recommendation G.821 specifies the bit error performance events, parameters, and indicators for N×64kbit/s circuit-switched digital connections used for voice services or "bearer channels" for data-type services.

Data and indicators analyzed by G.821: Errored Seconds (ES), Error-Free Seconds (EFS), Severely Errored Seconds (SES), Unavailable Seconds (UAS)

(2) Recommendation G.826 specifies the bit error performance events, parameters, and indicators for international constant bit rate digital paths operating at the primary rate and above.

Data and indicators analyzed by G.826: Errored Blocks (EB), Errored Seconds (ES), Severely Errored Seconds (SES), Background Block Errors (BBE), Unavailable Seconds (UAS)

(3) Recommendation M.2100 is specifically used to clearly express connection quality, enabling service providers to conduct long-term test analysis.

Data and indicators analyzed by M.2100: Errored Seconds (ES), Severely Errored Seconds (SES), Unavailable Seconds (UAS)

III. Common Methods of Bit Error Rate Testing

The Bit Error Rate Tester is primarily designed for the measurement of E1 channels, but most instruments also have the functions of terminal equipment performance testing and operation monitoring. Since error performance testing is the main content of E1 channel measurement, some instruments also add data communication bit error testing functions by utilizing the error testing resources that must be built into the instrument.

1. E1 Transmission Channel

The digital network can be a PDH network or an SDH network; the multiplexing equipment may be PDH digital multiplexing equipment (MUX/DEMUX) or SDH terminal multiplexing equipment; there are many types of terminal equipment, such as data multiplexers, digital cross-connect (DXC) equipment, and various interface converters. The 2048kbit/s E1 interface is used between the terminal equipment and the multiplexing equipment.

2. Several Common E1 Channel Test Modes, Test Methods, and Main Test Parameters

(1) Out-of-service test mode (or offline test mode):

Test prerequisites: No actual data signals are transmitted in the line, but the E1 line and related multiplexing equipment and terminal equipment are all in a normal and available state.

Bit error rate tester connection method: Connect the transmitter Tx and receiver Rx to the receiver Rx and transmitter Tx of the equipment simultaneously.

Use one bit error rate tester to connect directly to the multiplexing equipment through the G.703 interface, simulating the terminal equipment to send and receive data. This terminal equipment simulation test is mostly used in situations where the terminal equipment is a data multiplexer.

In the out-of-service test mode, the following tests can be performed:

● Line frequency, frequency deviation.
● Measurement of line frequency deviation tolerance (Offset Tolerance).
● Editing, monitoring, and analysis of frame overhead bits.
● Payload setting and testing. Unframed, framed with Channel Associated Signaling (CAS), and framed with Common Channel Signaling (CCS) modes can be selected.
In the framed mode, it is also possible to specify which timeslots form the N×64kbit/s data channel connected to the internal pattern generator and error detector of the instrument for bit error testing.
● Round Trip Delay (RTD) testing. RTD can be performed in unframed or N×64kbit/s modes.
● Error testing, with G.821, G.826, and M.2100 statistics and analysis.
● Processing of test results. Test results can be stored, or output to a printer or computer.

(2) In-service test mode (or online test mode, online monitoring mode):

Test prerequisites: Normal signal streams are transmitted in the line, and all equipment and lines are in a normal transmission and communication state.

Bit error rate tester connection method: The receiver Rx is bridged (tapped) onto the E1 interface, and the Rx is connected to the monitoring port through a T-connector (commonly known as a tee). It should be noted that the Rx end of the instrument should be set to high impedance before bridging, to avoid unnecessary impact on the service being carried.

In the online test mode, the following tests can be performed:
● Line frequency, frequency deviation
● Frame overhead bit monitoring and analysis.
● Payload monitoring.
● Real-time monitoring of CRC check bits.
● G.826 (only for frame structures with CRC-4) and M.2100 statistics and analysis of the E1 channel.
● Processing of test results.

IV. Common Faults and Possible Causes

During the initial stage of line commissioning and the later maintenance process of actual lines, faults of varying degrees will be encountered, resulting in the dedicated line provided to users being unable to connect normally or data transmission/reception being abnormal. Some of these faults are caused only by incorrect clock settings of a certain transmission device such as multiplexing equipment, while others may be caused by connection faults at a certain interface of the line...

At this time, when using the bit error rate tester for line commissioning testing or maintenance troubleshooting, the following points need to be noted:

First: Select the correct test connection method.

Second: Select the correct test parameters.

Third: Determine the correct test procedure to judge and locate faults in the shortest time with the fewest operations and connections.

The following introduces some common fault phenomena encountered during bit error rate testing, analyzes the causes, and provides resolution steps, hoping to inspire E1 channel maintenance personnel with more innovative methods in line testing and maintenance, as well as more flexible use of the bit error rate tester.

Fault 1: After all equipment is set up and connected to the actual transmission line, and the equipment power is turned on, it is found that services cannot be transmitted or the bit error performance indicators are poor during transmission.

Solution: In this situation, generally the transmission channel fault should be ruled out first, and then the user service terminal equipment issues should be checked.

Check that the line connections between all equipment are correct.

1. Check the parameter configurations of all multiplexing equipment and terminal equipment.

2. Use the bit error rate tester to select the out-of-service test mode, and select a duration sufficient to preliminarily judge the quality of the line.

3. In a certain direction, continuously change the loopback position and use the bit error rate tester for equipment and line troubleshooting testing.

Fault 2: When testing with the bit error rate tester, an E1 remote alarm is detected. What causes this?

Solution: In fact, the E1 remote alarm is sent by the remote end and then detected by the local end, so the remote alarm can be seen at the local end. It is an alarm signal sent back to the local end equipment because the opposite end equipment cannot receive the E1 signal, or the bit error rate is too high.

Main causes:

1. An open circuit occurs in the line;

2. Problems occur in the frame structure;

3. The line bit error performance indicators are too poor, and when the bit error rate exceeds the minimum bit error rate allowed by the line, such as 10-6, an alarm will also be caused.

Fault 3: LOF alarm occurs during transmission

Solution: LOF, i.e., Loss of Frame alarm, corresponds to the loss of frame synchronization of the E1 input signal. At this time, the clock settings of the entire network and the local end equipment should be checked to avoid clock setting conflicts or errors:

1. Check the clocks of the two systems:

2. Check the input jitter tolerance and output jitter of each;

3. Check the grounding issues of the two sets of equipment;

Introduction to the HCT-BERT/C E1 Data Bit Error Rate Tester

The HCT-BERT/C is a full-featured bit error rate tester that covers E1 channel bit error testing as well as data channel bit error testing. Its comprehensive E1 testing functions and test parameters, along with complete bit error statistical analysis, can maximally meet the needs of E1 line measurement and maintenance.

The product features an extra-large true color screen and provides a full Chinese graphical operation interface, lowering the threshold for instrument use and bringing the user closer to the instrument.

The intuitive and comprehensive LED display, simple and practical one-key operation, and large-capacity lithium battery design features all provide strong support for daily maintenance and emergency fault repair.

Application environments:

● E1 line bit error testing
● Framed and unframed end-to-end testing
● Framed monitoring
● Timeslot extraction and insertion
● Data line bit error testing
● DTE/DCE n×64k synchronous testing
● DTE/DCE low-speed (asynchronous 50-115.2K bps; synchronous: 150-128K bps)