eceiver to generate an appropriate decision). The BER is usually measured over long periods of time. The measurement can vary per unit time and, therefore, the mean BER is the quantity required for some types of system. However, for other types the burstiness of the errors is more important-that is, the distribu- tion of errors in time. This term is used because of the possibility of many errors at a specific interval of time and the chance of no or small amounts of errors at any other interval. Different distributions can be used to estimate the amount of errors in a system. Fur- thermore, if a system is simulated, specific distributions can be used to calculate the number of errors in a transmission configuration to minimize the amount of computation required. The most common distribution for digital systems is the Gaussian distribution. In some specific optical systems where spontaneous noise is considered, Chi square distributions produce results closer to the error estimation. Some typical objectives for BER are presented in Table 2.1. Databit error rates of the order of 10-9 mean one error in 109 bits sent. This measurement will depend on the require- ment of the link of error-free transmission. In some cases, such as voice and ADPCM, the restriction is less due to the methods used for error prediction. Latency Latency is a measurement of the time it takes a signal to completely arrive at the destination from the moment the first symbol (bit) was delivered by the source. This parameter is very important if a transmission network is considered for the propagation of a signal. In a Chapter 2 Data Transmission Table 2.1 Typical BER Objectives Digital Transmission Service Transmission Rate BER Objective (approx.) 16 kbit/s-2500 Mbit/s Voice: ADPCM 32 kbit/s Voice: Log-law PCM 64 kbit/s X 10-5 Video: Linear PCM 60 Mbit/s X 10-7 network the transmission is usually executed by sending noncontinuous messages, such as packets of information, which can be a set of bits. In this way, the transmission is more bandwidth