Capacity analysis of RT-based VDSL2 copper access networks

Attenuation and crosstalk are the dominant problems in high-speed communications over copper-based digital subscriber lines (DSL). In order to minimize attenuation and maximize the data rate (or capacity) of twisted pair lines, xDSL operators try to keep twisted pair lines as short as possible. This is often achieved by deploying xDSL technologies (i.e., VDSL2) close to the end users using remote terminals (RT). However, since xDSL lines deployed from an RT can share a cable binder with lines deployed from the central office (CO), the resulting crosstalk may cause severe performance degradation for lines deployed from the CO. This is often termed as the near-far problem. In order to reduce crosstalk resulting from the near-far configuration, the transmit power on RT lines may be allocated to optimize for minimal impact on the CO lines. Power Backoff (PBO) spectrum management techniques at the RT may be used in these situations Although this may alleviate the near-far problem, reducing transmit power of RT based xDSL systems will not remove crosstalk created within the binder group. Therefore, implementing vectorization techniques to cancel crosstalk between lines is highly recommended, especially for short lines In this paper we calculate potentially achievable data rates in the downstream direction for very short (RT deployed) very-high data-rate digital subscriber lines. Our findings are supported by laboratory measurements of attenuation and far-end crosstalk (FEXT), which take into account downstream PBO (DPBO) and vectoring techniques. The presented results provide useful bounds for developers, xDSL providers, and relevant regulatory agencies.