Principles of Bit Errors – I

Errors occurring to data during the transmission process are called Bit Errors (BER). It is a key parameter that is used in assessing systems that transmit digital data from one location to another. Specifically, it is the error of a single number between receiving and sending data signals.

BER is critical for radio data links, Optical data systems (fiber), Ethernet. The basis of BER are same in all of these systems even though there are some differences in functioning of these systems, how they work and the way in which Bit Error rate is affected. . A good knowledge of the BER also enables other features of the link to be adjusted (such as the power and bandwidth, .. ) to enable the required performance to be obtained.

Below are relevant ITU-T Recommendations related to Bit Errors, control, Design & specifications:

  G.821   Specifies the Bit Error performance parameter connected with 64k bps, on the basis of bit.

  G.826   Specifies high bit rate channel Bit Error performance parameter, on the basis of block.

  G.828   Specifies Bit Error performance event, parameter and indices of international Constant Bit Rate (CBR) synchronous digital channel. It is applicable to lower order and higher order paths borne by SDH, that is, channels VC-n and VC-n-Xc. It is also applicable to the VC-n borne by PDH that meets the G.832 recommendations. The difference from G.826 is that the index defined by it is more strict than that of G.826.

  M.2100   It is a maintenance recommendation. It allows a short-term test. The test result can indicate whether the G.826 long-term requirement is met. In most cases, if the index recommended by M.2100 is satisfied, it can also meet the requirement of G.826.

In simple words, BER is important to consider in Design & Operation for any system that transmits data over a network of some form where noise, interference and phase jitter may cause degradation of the digital signal.

The working principle of BER assessment is a bit different from other forms of assessment. BER method assesses the full E2E performance of a system including the transmitter, receiver and the medium between the two. In this way, bit error rate, BER enables the actual performance of a system in operation to be tested, rather than testing the component parts and hoping that they will operate satisfactorily when all are connected together for form a system. Bit errors optimization is critical for stable network operations. To understand the Bit Errors in deep, we need a solid grip on Factors that control BER, BER Events, Reporting Points of BER, Testing & so on…

Bit Error Rate (BER) Calculation

Bit Error is simply the calculation of how many bits have been damaged during transmission out of the total. This definition can be translated into a simple formula.

The formula for calculation of Bit Error Rate is:

If the medium between the transmitter and receiver is good and the signal to noise ratio is high, then the bit error rate will be very small – possibly insignificant and having no noticeable effect on the overall system However if noise can be detected, then there is chance that the bit error rate will need to be considered.

The main reasons for the degradation of a data channel and the corresponding bit error rate, BER is noise and changes to the propagation path (where radio signal paths are used). Both effects have a random element to them, the noise following a Gaussian probability function while the propagation model follows a Rayleigh model. This means that analysis of the channel characteristics are normally undertaken using statistical analysis techniques.

For fibre optic systems, bit errors mainly result from imperfections in the components used to make the link. These include the optical driver, receiver, connectors and the fibre itself. Bit errors may also be introduced as a result of optical dispersion and attenuation that may be present. Also noise may be introduced in the optical receiver itself. Typically these may be photodiodes and amplifiers which need to respond to very small changes and as a result there may be high noise levels present.

Bit Error Control Factors

BER depends on a number of factors. Some of them are controllable & some are out of system design control. By manipulating the variables that can be controlled it is possible to optimize our system to provide the optimum performance levels. These measures are best to be undertaken in the network design phase of the project so that the performance parameters can be adjusted at the initial design concept stages. Important factors that control BER are in below:

Interference  The interference levels in a system are generally induced by external factors and can not be changed by the system design. However it is possible to set the bandwidth of the system. By reducing the bandwidth the level of interference can be reduced. However reducing the bandwidth reduces the maximum data throughput.

Transmitter Power  It is also possible to increase the power level of the system so that the power per bit is increased. This has to be balanced against factors including the interference levels to other users and the impact of increasing the power output on the size of the power amplifier and overall power consumption and battery life, etc.

Lower order modulation  Lower order modulation schemes can be used, but this is at the expense of data throughput.

Bandwidth  Another approach that can be adopted to reduce the Bit Error Rate is to reduce the bandwidth. Lower levels of noise will be received and therefore the signal to noise ratio will improve. Again this results in a reduction of the data throughput attainable.

It is critical to balance all the available factors to achieve a satisfactory bit error rate. But normally it is not possible to achieve all the requirements and some compromises & trade-offs are always required. The BER can be improved by choosing a strong signal strength (upto a certain level because after this level cross-talk occurs which results in even more bit errors), by choosing a slow and robust modulation scheme or line coding scheme and by applying channel coding schemes such as redundant forward error correction codes.

Relationship of BER & Measurement Points

It is important to understand the different points/parts of a Transmission system especially SDH. The optical synchronous transmission equipment makes an overall system check for bit errors by sections and layers, including B1 (regenerator section bit error), B2 (multiplex section bit error), B3 (higher-order path bit error) and V5 (lower-order path bit error). Their relation is illustrated below:

From the above figure, RST, MST, HPT and LPT indicate regenerator section terminal, multiplex section terminal, higher-order path terminal and lower-order path terminal respectively. B1, B2, B3 and V5 bit error are detected between these terminals.

If only the lower-order path has bit errors, no bit error can be detected at higher-order path, multiplex section and regenerator section. If the regenerator section has bit errors, bit errors will occur to multiplex section, higher-order path and lower-order path.

The relation between bit errors: Generally, if there are higher-order bit errors, there will be lower-order bit errors. For example, if there is a B1 bit error, there will be bit errors of B2, B3 and V5. Contrarily, there may not be higher-order bit errors when lower-order bit errors exist. For example, if there is a V5 bit error, there may not be bit errors of B3, B2 and B1.

Because higher-order bit error will lead to lower-order bit error, we should process the bit errors by the sequence of higher-order and lower-order.

Bit Error Performance Events

Important Bit Error Performance Events are listed in below table:

Error Block (EB) is the set of consecutive bits on channel, namely, a group of bits. Each bit belongs to and only belongs to a unique block. Treat a group of bits as an integrity. One or multiple bit errors are called block error, namely, the term Errored Block adopted in the G.826 recommendations. Background Block Error (BBE) refers to block error out of SES. ES is the Bit error second. In the G.821 it is defined as one or multiple errored bits in a second. Severed bit error seconds (SES) is bit error rate in a second if it is ≥10-3 as defined in G.821. Severed errored block second: In G.826 it is defined as errored blocks in a second, ≥30℅ or at least one error. Unavailable Second (UAS) is a period that starts from the first second of ten consecutive SESs.

Continued ….

Principles of Bit Errors – II