, and absolute power at (a) the input of the amplifier and (b) the output of the system. The attenuation of the cable is 2.5 dB/km. Problem 3.20 Explain the meaning and purpose of the decibel units dB, dBm, and dBm0. Problem 3.21 Sound pressure level is defined as decibels, L, = 2010g P/ dB, where P = sound pressure (in Pascals) and Po = 20 uPa = (20 micropascals, reference level). The threshold for human hearing is about 0 dB, and threshold for pain about 140 dB. How many times stronger is the sound pressure of the strongest sound that we can hear without pain compared to the weakest one? Problem 3.22 Explain the term digital milliwatt. Problem 3.23 Draw the analog waveform generated by a PCM decoder with a digital milli- watt as the input signal of the decoder. Use Figure 3.14 to determine the approximate analog signal value. What is the periodic time and what is the frequency of the analog signal produced? Introduction to Telecommunications Network Engineering References [1] Ericsson Telecom, Understanding Telecommunications, Vol. 1, Lund, Sweden: Ericsson Telecom, Telia, and Studentlitteratur, 1997. Sklar, B., Digital Communications: Fundamentals and Applications, Upper Saddle River, NJ: Prentice Hall, 1988. Bray, J., and C. F. Sturman, Bluetooth Connect Without Cables, Upper Saddle River, NJ: Prentice Hall, 2001. Transmission Transmission is the process of transporting information between end points of a system or network. As we have seen in previous chapters, the end-to-end communication distance is often very long and there are many electrical sys- items on the line. These systems, network elements such as exchanges, are connected to the other elements with connections provided by the transmis- sion systems. In this chapter we discuss the basic restrictions and require- ments for transmission and the characteristics of various transmission media and equipment used in the telecommunications core network. The transmis- sion systems for access networks for high-data-rate customer access to the Internet are di