d to ensure minimal voice/DSL mutual interference. Next is the upstream spectrum of 25 to 138-250 kHz. The remainder of the spectrum is used for down- stream transmissions. The total bandwidth has evolved through different versions of the DSL standard and its annexes. ADSL2 ADSL2 incorporates better channel coding, reduced frame overhead, and supports multiple wire pairs, thereby multiplying the capacity. ADSL2+ ADSL2+ extends overall bandwidth from 1.1 to 2.2 MHz, thereby doubling the maximum pos- sible transmission rate in the downstream. The upstream spectrum remains the same in a number of variants of the standards, or doubled in some others. Figure 9.14 shows the signal to noise ratio (SNR) for the downstream of an ADSL system and possible bits that each carrier supports at that possible SNR level. ADSL transmission rates are a function of received SINR. Received signal power dimin- ishes as the distance between CO and CPE increases. As discussed above copper wire path loss is a function of carrier frequency, and higher frequencies experience higher path loss. This means that at short distances, all subcarriers experience low path loss, and therefore SINR is high across all subcarriers. At long distances, higher subcarriers experience large path loss, and therefore can contribute little to overall ADSL capacity. Broadband Telecommunications Technologies and Management Upstream Downstream Frequency 25 kHz 138 kHz 1104 kHz Upstream Downstream ADSL2+ Frequency 25 kHz 278 kHz 2208 kHz Figure 9.14 Variant of frequency allocation for ADSL and ADSL2+ standards ADSL2+ ADSL2 Distance from central office (km) Figure 9.15 ADSL, ADSL2 and ADSL2+ downstream transmission rates as a function of distance Downstream transmission rates are shown for ADSL, ADSL2 and ADSL2+ in Figure 9.15. At distances less than 2 km from a CO, downstream transmission rates of ADSL2+ are signif- icantly higher compared with ADSL2 and ADSL. This is because extra subcarriers associated with increased bandwidth from 1.1 to 2.2 MHz contribute signi