ated by a laser or light emitting diode. One common telecommunications technique used by optical fibre systems is on-off keying where reception of a pulse of light - 'on' - over a symbol period indicates a and no-light - 'off' - indicates a 0. Some optical fibre systems use a technique where 1 is represented by a strong power pulse of light, and a 0 by a weak power pulse of light. This is a classic ASK system as shown in Figure 7.2 where two different amplitudes are used to communicate a bit of data. +1.0 V +0.5 V -0.5 V -1.0 V Figure 7.10 PAM-5 signal levels as used in 1000 BASE-T Ethernet systems Digital Modulation 1.0E+00 1.0E-01 4-PAM 8-PAM 2-PAM 1.0E-02 1.0E-03 1.0E-04 1.0E-05 2345678910 11 12 13 Eb/No (dB) Figure 7.11 BER performance of 2-, 4- and 8-PAM modulation techniques. Source: Ref. [10] Case Study 7.1: Ethernet Ethernet was developed by Robert Metcalfe and his colleagues at the Xerox Palo Alto research laboratories in 1973 as a communications protocol to enable computers to connect with each other. The initial network used coaxial cables and modems for transmission rates of 100 kbps to 10 Mbps over distances of 100 m to more than 10 km [11]. With the growth of the internet, faster transmission rates became necessary and grad- ually faster Ethernet protocols were developed. These systems have been developed through the IEEE 802.3 working group and standards and specify a data transmission on a number of different media, including copper wires, optical fibres, and twisted pair copper wires categories 5, 6 and 7. A list of the standards, date, transmission rates and medium are shown in Table 7.1. Table 7.1 Ethernet standards Standard Tx Rate Medium Experimental Ethernet 0.1-10 Mbps Co-axial cable 802.3 10Base5 10 Mbps Co-axial cable 802.3u 100Base-Tx 100 Mbps Co-axial cable 802.3y 100Base-T2 100 Mbps Twisted pair 802.3ab 1000Base-T 1 Gbps Twisted pair 802.3ae 10GBase-SR 10 Gbps Fiber 802.3an 10GBase-T 10 Gbps Twisted pair 802.3ba 100GBase 40-100 Gbps Fiber 802.3bq 40Gbase-T 40 Gbps Twisted pair Source: 