Ber

Bit error rate In telecommunication transmission, the bit error rate (BER) is the percentage of bits that have errors relative to the total number of bits received in a transmission, usually expressed as ten to a negative power. For example, a transmission might have a BER of 10 to the minus 6, meaning that, out of 1,000,000 bits transmitted, one bit was in error. The BER is an indication of how often a packet or other data unit has to be retransmitted because of an error. Too high a BER may indicate that a slower data rate would actually improve overall transmission time for a given amount of transmitted data since the BER might be reduced, lowering the number of packets that had to be resent. A BERT (bit error rate test or tester) is a procedure or device that measures the BER for a given transmission.

Bit error ratio In telecommunication, an error ratio is the ratio of the number of bits, elements, characters, or blocks incorrectly received to the total number of bits, elements, characters, or blocks sent during a specified time interval. The error ratio is usually expressed in scientific notation; for example, 2.5 erroneous bits out of 100,000 bits transmitted would be 2.5 out of 105 or 2.5 × 10-5. The most commonly encountered ratio is the bit error ratio (BER) - also sometimes referred to as bit error rate. For a given communication system, the bit error ratio will be affected by both the data transmission rate and the signal power margin. Examples of bit error ratio are (a) transmission BER, i.e., the number of erroneous bits received divided by the total number of bits transmitted; and (b) information BER, i.e., the number of erroneous decoded (corrected) bits divided by the total number of decoded (corrected) bits. On good connections the BER should be below 10-9. The test time for a 95% confidence level at several speed links is shown here: • • • • • •

40 Gbit/s (STM-256 or OC-768): 1 s 10 Gbit/s (STM-64 or OC-192): 3 s 2.5 Gbit/s (STM-16 or OC-48): 12 s 622 Mbit/s (STM-4c or OC-12: 48 s 155 Mbit/s (STM-1 or OC-3): 3.2 min 64 Mbit/s (STM-1 or stnd) : 6.4 min

Equivalent isotropically radiated power In radio communication systems, Equivalent isotropically radiated power (EIRP) or, alternatively, Effective isotropic radiated power is the amount of power that would have to be emitted by an isotropic antenna (that evenly distributes power in all directions and is a theoretical construct) to produce the peak power density observed in the direction of maximum antenna gain. EIRP can take into account the losses in transmission line and connectors and includes the gain of the antenna. The EIRP is often stated in terms of decibels over a reference power level, that would be the power emitted by an isotropic radiator with an equivalent signal strength. The EIRP allows making comparisons between different emitters regardless of type, size or form. From the EIRP, and with knowledge of a real antenna's gain, it is possible to calculate real power and field strength values. EIRP(dBm) = (Power of Transmitter (dBm)) – (Losses in transmission line (dB)) + (Antenna Gain(dBi)) where antenna gain is expressed relative to a (theoretical) isotropic reference antenna. This example uses dBm, although it is also common to see dBW. Decibels are a convenient way to express the ratio between two quantities. dBm uses a reference of 1mW and dBw uses a reference of 1W. dBm = 10 log(power out / 1mW) and dBW = 10 log(power out / 1W) A transmitter with a 50W output can be expressed as a 17dBW output 16.9897 = 10 * log(50/1) The EIRP is used to estimate the service area of the transmitter, and to co-ordinate transmitters on the same frequency so that their coverage areas do not overlap. In built-up areas, regulations may restrict the EIRP of a transmitter to prevent exposure of personnel to high power electromagnetic fields however EIRP is normally restricted to minimise interference to services on similar frequencies

Atmospheric Absorption Electromagnetic waves are absorbed in the atmosphere according to wavelength. Two compounds are responsible for the majority of signal absorption: oxygen (O 2) and water vapor (H2O). The first peak occurs at 22 GHz due to water, and the second at 63 GHz due to oxygen. The actual amount of water vapor and oxygen in the atmosphere normally declines with an increase in altitude because of the decrease in pressure, so these graphs apply from sea level to around 1 km altitude. Total attenuation through the atmosphere at any frequency through unobstructed atmosphere is the sum of free space path loss, attenuation caused my oxygen absorption and attenuation caused by water vapor absorption. Rain attenuation, when present adds an additional element. For a chart of sunlight power density in the visible light and infrared / ultraviolet region of the electromagnetic spectrum, click here. So, AttenTotal = AttenFreeSpacePathLoss + AttenOxygen + AttenWaterVapor + AttenRain