ficantly to overall transmission rates. However at distances of more than 3 km, these subcarriers have very low SINRs and therefore do not contribute much extra transmission rate capability. ADSL2++ A further evolution of ADSL standard is ADSL2++ (also known as ADSL4) which further extends transmission bandwidth from 2.2 to 3.75 MHz. VDSL further extends transmission bandwidth to 12 MHz and incorporates a new frequency plan with a mixture of downstream and upstream frequency allocations. The transmission rates can be as high as 55 Mbps in the downstream and 3 Mbps in the upstream. Fixed Broadband Communications Systems Frequency 25 138 kHz kHz VDSL2 Frequency Figure 9.16 Frequency plan and possible transmission rates for downstream (DS) and upstream (US) for VDSL and VDSL2 VDSL2 VDSL2 extends system bandwidth to 30 MHz with a mixture of downstream and upstream frequency allocations. The transmission rates can be as high as 100 Mbps in the downstream and 100 Mbps in the upstream. VDSL and VDSL2 frequency plans and transmission rates are shown in Figure 9.16. The SINR for the downstream of a VDSL system with a DS1, DS2, and US1 and US2 configuration can reach as much as 60 dB. This supports a maximum of 15 bits per symbol/ carrier, which combined with a 4 kilo symbol/sc/carrier, results in a 60 kbps per carrier trans- mission rate. Latest Transmission Rates ADSL and VDSL maximum transmission rates are a function of available bandwidth and SINR. Higher bandwidths of up to 300 MHz have been considered for VDSL systems for short range communications, and theoretical calculations of capacity have been reported by Ericsson with possible transmission rates of as high 3500 Mbps [7]. Using a 30 MHz bandwidth VDSL2, Ikanos Communications demonstrated a 300 Mbps VDSL system using a single twisted pair copper wire in July 2013 [8]. These rates are limited by the length of copper medium and FEXT level between users within the same cable bundle. However, given these limitations it is possible to provide very high transmission r