In this paper, we optimize the use of existing DSL low-power sleeping modes (L2 and L3) in order to improve the energy efficiency of DSL access networks. Given that switching on a DSL line can cause instability by increasing the amount of time-varying crosstalk in the cable bundle, and that it takes energy and time to switch a DSL line on and off, we develop a method to optimally choose the appropriate sleeping state based on the line and traffic characteristics. We further develop and prove the structural properties of the optimal policy for switching to the appropriate sleeping state, allowing transitions between submodes with different power and transmit rate characteristics. We also present techniques that guarantee stable sleep mode operation. Using a realistic DSL simulator, we demonstrate the three-way tradeoff among energy consumption, delay performance, and stability. The increased flexibility of control introduced by our approach improves the energy-delay Pareto optimal tradeoff, and results in a more energy efficient and stable DSL operation compared to existing power saving policies.

Switching off a DSL line to a low-power sleeping state is becoming an important method to enhance energy efficiency of DSL broadband access networks. Although a low-power (L2) state and an off (L3) state are already defined in DSL standards, they have not been fully exploited due to concern about the resulting time-varying crosstalk. In this paper, we develop a method to optimally choose the appropriate sleeping state based on the modem's switching cost characteristics and the power consumption incurred during each sleeping state. We further develop the optimal policy for switching to the appropriate sleeping state, allowing transitions between operating modes with different power and transmit rate. Numerical results on our realistic DSL simulator show that more flexibility in control, introduced by our approach, improves the energy-delay Pareto optimal tradeoff, and results in a more energy efficient and stable DSL system compared to the existing power saving policies.

Energy efficient and stable operation of the DSL broadband access infrastructure has become an essential part of the emerging trend towards green communications. One promising means to obtain energy savings is the use of low power "sleep modes", putting DSL modems to sleep when they are not used. Executing the optimal sleeping policies is, however, not straightforward since turning the modem ON and OFF consumes both energy and time, and it also impacts the stability of the DSL network. We present an analytic framework providing optimal sleeping policies that achieve a Pareto-optimal tradeoff between energy consumption and delay performance. Furthermore, we present mechanisms achieving stable sleep mode operation that improve overall stability of DSL systems. Using a realistic DSL simulator, we demonstrate the three-way tradeoff between energy consumption, delay performance and stability.