ed pair copper wire was generally not used. The bandwidth of the copper wire has capacity for data communications at much faster rates than 56 kbps. As noted above the medium was already being used for inter-exchange transmis- sion rates of 1.5 Mbps, as well as ISDN voice communications which operated in the 80-120 kHz range of the spectrum. New systems needed to be developed to take advantage of this latent capacity. Twisted Pair Copper Wire Characteristics Twisted pair copper wire has a specific frequency response bandwidth. The path loss of 26 AWG (0.6 mm) twisted pair wire as a function of frequency is drawn in Figure 9.2. As shown, the path loss for signals at low frequencies, such as that of voice, is very small, and is Fixed Broadband Communications Systems 100 Hz 1 kHz 10 kHz 100 kHz 1 mHz 10 MHz Frequency Figure 9.2 Twisted pair wire path loss per kilometre for 26 AWG medium as a function of frequency. Source: Ref. [5] in the order of a few decibels per kilometre. However as carrier frequency increases, path loss increases such that a signal at 10 MHz loses power by a factor of approximately 70 dB every kilometre. Clearly twisted pair copper wire is more suitable to low frequency, low speed data communication applications. Nevertheless, the path loss is small enough in the range of up to several megahertz to support high data rate communications over short distances. The maximum length for such communication is calculated from path loss, line distortions, interference from other lines and frequency of operation. Digital Subscriber Line A number of technologies were developed to utilise the twisted pair copper wire spectrum at frequencies higher than that used for voice. The most popular were T-Carrier and E-Carrier technologies developed in the US and in Europe for multiplexing several voice channels over a high capacity inter-exchange link. Also known as T-1 and E-1, these technologies used twisted pair copper wires to transmit 24 (32 for E-1) digital voice channels and later on data at a rate of 1.544 