sible speed is proportional to the distance from the exchange or from a remote access unit. (Low speed) ADSL can be used over distances up to 5.5 km. In Europe ~90 % of all subscriber lines are shorter than 3 km. 13.12.3 SHDSL Single pair high bit rate digital subscriber line (SHDSL) delivers high-speed digital infor- mation for data and voice services over copper lines. The transmission is symmetric (downstream speed and upstream speed are equal), providing a rate of up to 2.3 Mbit/s. Understanding Changing Telecommunications - Building a Successful Telecom Business Local exchange Filter Filter (PSTN/ ISDN) Max length 0.3-1.5 km 13-52 Mbit/s Internet 1.6-19 Mbit/s (data) Figure 13.33 Very high bit rate digital subscriber line (VDSL). Example SHDSL is also known under its standard name G.SHDSL or ITU-T standard G.991.2. SHDSL delivers greater bandwidth over longer distances than current 2B1Q-based SDSL and traditional HDSL systems and enables operators to provide symmetric, high- speed data transmission over existing copper pairs. The standard is a multi-rate DSL technology that allows data rates from 192 kbit/s to 2312 kbit/s and can transport T1 (1544 kbit/s, E1 (2048 kbit/s), ISDN, ATM and IP signals. Symmetric solutions fit small and medium enterprises with roughly the same traffic volume in and out. 13.12.4 VDSL An even faster technique is the very high bit rate digital subscriber line (VDSL). See Figure 13.33. Here the standard defines the full bit rate to 26 Mbit/s in each direction for symmetric traffic, or different speeds downstream and upstream for asymmetric traffic. 13.13 FIBRE-BASED SYSTEMS There was a lot of talk of fibre to the home (FTTH) during the 1980s. However, fibre was fairly expensive and the pressure from the subscribers was modest. Figure 13.34 shows the dramatic cost reduction during the last few years. Since then many competing solutions have been developed, as seen from Figure 13.34. An increasingly popular fibre-based solution uses Ethernet in layer 2. It can also offer home access. 