 digital signal as output. This process can be per- formed using different algorithms. The algorithms for converting voice into a digital stream are referred to as audio codecs (see Figure 1-10). The most widely used is the G.711 codec, also known as Pulse Code Modulation (PCM), which produces 64-kbps streams. Strengths of Digital Transmission Digital transmission presents many advantages over analog transmission. First, digital equipment is cheaper. Stream manipulations can be performed by relatively cheap com- puters, while analog switches need expensive electromechanical gear. Since the first digital exchange was installed in 1960 in the U.S., the price of com- puters has dropped drastically. Second, digital transmission quality is higher. Analog transmission suffers from signal attenuation, that is, the strength of the signal decreases with distance. Exchanges employ ampli- fiers to keep signal strength above an acceptable level, but to do so, they have to amplify the whole signal. (Remember that exchanges are unable to examine the contents of analog transmissions). Amplifying the signal as a whole has the unfortunate result of also amplifying noise, producing a dis- tortion in the signal received at the destination. By contrast, digital systems use repeaters rather than amplifiers. A repeater decodes the stream received and reproduces it on the outgoing side. Thus the outgoing signal is completely new, and its strength is inde- pendent of the strength of the signal in the previous link. No noise is added to the signal in a digital exchange by the transmission process. Weaknesses of Digital Transmission Regarding delay, digital systems perform worse than analog systems. The processing of digital signals is Figure 1-10 Digital transmission 01110010010011101 of voice. Signalling in the Circuit-Switched Network always slower than the transmission of an electromagnetic wave by an ana- logue exchange. We will see that the multiplexing mechanism used in dig- ital transmissions (TDM) also introduces some delays, sin