s 640 Kbps Residential 2/1.5 Mbps 2/1.5 Mbps SDSL, G.shdsl 2.3 Mbps 2.3 Mbps G.dmt ADSL 8 Mbps 1.5 Mbps Residential 0.1-2 52 Mbps 6 Mbps Residential (34 Mbps) (34 Mbps) *SME = small and medium size enterprises. Introduction to Telecommunications Network Engineering switch. The two B-channels of ISDN BRI are multiplexed to offer a dedi- cated 128 Kbps of bandwidth for data only. This technology does not pro- vide a simultaneous voice channel as do other DSL technologies, but it operates over longer distances than higher speed technologies. 6.4.3.4 High-Bit-Rate DSL A conventional primary rate transmission PCM system operating at a 1.544- or 2.048-Mbps data rate over twisted-pair copper cable uses two cable pairs, one for each transmission direction. In a typical cable, signal attenuation together with crosstalk (interference from other systems in the cable) restricts the transmission distance and a regenerator is required after about each 1.5-km cable section. These conventional 1.544- and 2048-Mbps systems use AMI and HDB-3 encoding. The high-bit-rate DSL (HDSL) increases the section length and thus reduces the need for intermediate repeaters. This technology uses 2B1Q (two bits are transmitted in each four-level symbol) encoding that has superior spectral and distance characteristics. HDSL uses two (or some- times three cable pairs) and thus it is not a consumer access technology. It provides the same data rate for E1 or T1 in both directions and is suitable for small and medium size enterprises (SMEs) where upstream traffic has equal volume. HDSL systems use two cable pairs for full-duplex transmission. The data rate is divided between pairs. In one pair, to one direction, it is only half of the data rate of conventional systems that use different cable pairs for each transmission direction. Further improvement is achieved with the help of an efficient line code. The line code in use is 2B1Q, which means that each pair of bits is coded into one quaternary symbol with four values to the line. This is the same li