There's life in those old copper wires yet: Dynamic Spectrum Management (DSM) aims to reduce the crosstalk among DSL in bundles of copper wire for vastly improved benefits through coordination. Ars Technica writes about how a consortium of hardware vendors and phone companies are pushing for a rethink of DSL. In this vision, DSL could carry more bandwidth than a fiber strand, with the extra advantage that the copper wire would already be in place. DSM could allow FTTN--fiber to the node--where the long haul is fiber to a point near the homes, and then the very short distance are carried over DSL with DSM. We'll see.

I recall that when I first heard about DSL back in 1996, and started talking to vendors about how it would be implemented, that crosstalk was the big issue. It was assumed that crosstalk might make it impossible to have more than a handful of DSL-enabled copper pairs in any binder, the sheath of bundled pairs that traverse distances from central office to cross-connect points.

The solution to this was, in part, a switch from the early, cheap CAP (carrierless amplitude/phase) modulation, which treated the entire swath of spectrum allotted to DSL as one big modem signal, more or less, to DMT (discrete multi-tone) modulation, which is quite close to OFDM (orthogonal frequency division multiplexing) in some of its particulars. Notably, DMT and OFDM both channelize a swath of frequency into smaller subchannels, each of which acts effectively like a separate modem connection, which can increase or decrease the symbol rate as the provisioning or quality of the line allow. (OFDM as implemented in 802.11 also allows the number of subchannels to change based on signal quality.)

With DSL, DMT allows interference to be avoided by reducing symbol rates or turning off particular subchannels that won't produce good results. In a binder, you can have DMT-based signals coping with each other and making sure that the worst interference only degrades overall performance rather than blocking all transmission. It's funny how close this wired technology is to our fancy new Wi-Fi (OFDM starting in 802.11a in 5 GHz and 802.11g in 2.4 GHz) and WiMax (OFDMA in 802.16-2005).