Mbps (2.048 Mbps for E-1) between operator exchanges and later on to mostly business subscribers' premises [6]. These technologies utilised a much wider frequency spec- trum of twisted pair copper wires and beyond the 4 kHz used for voice. T-1 and E-1 were marketed to business subscribers on a lease basis as data backhaul lines for inter-office and office-computer centre connectivity. The cost of leasing a T-1 was quite high and in the order of $2000 per month because repeaters and other electrical equipment were required every 2 km. This was because symmetrical data transmission rates in downstream and upstream for voice communications had to be maintained. DSL technologies grew from T-1/E-1. The principle was the fact that the twisted pair copper wire spectrum has several useful megahertz of bandwidth over short distances and therefore higher data rates may be supported on telephone subscriber lines using this bandwidth. Broadband Telecommunications Technologies and Management Voice Used for DSL upstream and downstream Guard 4 kHz Channel bandwidth Frequency 25 kHz 1.1 MHz Figure 9.3 ADSL frequency band allocation If symmetrical communication is not required, as is the case for data applications, then repeaters are not needed every 2 km and therefore many subscribers served by a telephone exchange may be able to connect to the internet at high transmission rates without a need for new infrastruc- ture roll-out. Given the path loss characteristics shown in Figure 9.2, possible transmission rates over different distances for a system bandwidth can be calculated using Shannon's theorem. Several DSL standards have been developed and standardised under the ITU G.99x series of standards since the early 1990s. The most common standards have been asymmetric DSL (ADSL) and its evolutions. Asymmetric refers to the fact that upstream and downstream trans- mission rates are unequal. The difference between these standards is mainly the overall range of frequencies used and the spectrum allocated to upstream and downstream t