 they can tolerate higher interference than analog systems. If the network is analog, a digital message has to be modulated into the frequency band of the analog telecommunications channel. This reduces the capacity available for the user. For example, a voice channel in the digital telephone network has a data capacity of 64 Kbps. If we use it via an analog Introduction to Telecommunications Network Engineering subscriber loop with a voice-band modem, the data rate is restricted in prac- tice to approximately 30 Kbps. With a digital subscriber line (DSL) (e.g., ISDN), the user data are exactly the same 64 Kbps used inside the network. Digital systems are ideal for control via software because digital circuits operate in a numerical way. Integrated software makes systems flexible and new functions needed for new services are easier to implement. Intelligent network services, reviewed in Section 2.10, are good examples of these new services. As another example, we would not have cellular telephone service if we did not have digital software-controlled systems in the network. The digital processing of information makes better utilization of chan- nels possible; for example, several digital broadcast television channels fit into the band of one analog broadcast channel. In Chapter 4 we will see that digi- tal signals tolerate higher disturbances than analog signals and this is one rea- son behind the better frequency efficiency. Low-cost multiplexing (no analog filtering and modulation circuitry required) and efficient use of optical trans- mission media make high-capacity digital systems feasible. Optical systems transmit digital signals as a series of short light pulses. The distortion of these digital pulses does not influence the quality of the message because distorted pulses are regenerated, which eliminates distortion. All types of analog signals can be converted into digital signals. When this is done, the digital network is able to carry any information. Bits are han- dled in the same way whether they repre