VDSL duplexing scheme

Written on 3:10 PM by ooe

The duplexing scheme is the method employed to achieve two-way transmission on a single pair of wires. Like ADSL, VDSL uses frequency division duplexing (FDD) to achieve two-way transmission on a single cable. FDD was chosen over time division duplexing (TDD or ‘ping-pong’). TDD was seriously considered for VDSL because the proportion of capacity allocated to upstream and downstream can be easily varied in software. But to operate correctly, TDD modems sharing the same cable must all be synchronised to avoid severe crosstalk. It transpired that this is a serious disadvantage in the unbundled environment because adopting TDD would have required the industry and regulators to agree a synchronisation plan in each country as well as a frequency plan (which would still be required) – not an attractive prospect.

This was the first-ever DSL standards decision to be influenced by regulatory factors. Now, all key DSL standards decisions must take into account regulatory implications – a new dynamic which came into play again in the choice of VDSL frequency plan.