y. At a few decibels better value for the S/N, the transmission is almost error free. The S/N values in the Table 4.2 Examples of Error Rates and Mean Times Between Errors for a 64-Kbps Channel Mean Time S/N (dB) Error Rate Between Errors 1.5 ms 150 ms 15 seconds 26 minutes 10-10 2 days 10-12 6 months 10-14 50 years Introduction to Telecommunications Network Engineering Figure 4.19 curve and Table 4.2 are examples and are based on certain assumptions. The actual S/N value in decibels at a certain error rate of a specific system depends on the system characteristics and how the S/N is defined and measured. However, the shape of the error curve is the same as in Figure 4.19 and the threshold value is usually between 8 and 20 dB. When the S/N of a digital system decreases, errors occur more and more frequently and when the error rate becomes too high, information is lost. An error rate of 1 X 10-3 is standardized to be the worst allowed com- munication quality for PCM speech in the telecommunications network. If the error rate becomes worse, ongoing calls are cut off. Data are transmitted in large packets and if a packet contains one or more errors it needs to be retransmitted. As a rule of thumb, we can say that data transmission requires an error rate of 1 X 10-5 or better, otherwise retransmissions slow down the end-to-end transmission data rate. 4.6 Multiplexing Multiplexing is a process that combines several signals for simultaneous transmission on one transmission channel. Most of the transmission systems in the telecommunications network contain more capacity than is required by a single user. It is economically feasible to utilize the available bandwidth of optical fiber or coaxial cable or a radio system in a single high-capacity sys- tem shared by multiple users. The main principles of multiplexing are described in the following sections. 4.6.1 Frequency-Division Multiplexing (FDM) and TDM FDM modulates each message to a different carrier frequency. The modu- lated messages are transmitted through the same