Tackling the cupboard monsters

Your always-on broadband gateway is munching through nearly 90kWh of energy a year, but is there anything you can do about it? E&T investigates.

It sits under the stairs, blinking away to itself day and night. But that's not all it is doing. Unless you diligently switch the thing off - and who's going to do that when broadband is so widely used for overnight downloads and as a substitute for a regular phone line - your Internet gateway is drawing a steady 10W or more, day and night. For much of that time, it's just bouncing keep-alive packets back and forth, and fending off the odd port probe from a hacker's PC. You, and the planet, are paying the price of this high-energy idleness.

The European Union (EU) and manufacturers are now working to find ways to cut that constant electrical drain and give consumers the option to feel better about having an always-on Internet connection.

"Energy efficiency has become a big issue for service providers, particularly in the EU," says Duncan Bees, chief technology officer of the Home Gateway Initiative, which puts together specifications for the equipment. The specifications cover the network interfaces that home gateways should support and the services they are expected to handle, such as audio content and IPTV.

The EU's reaction to the increasing uptake of broadband equipment has been to publish a series of codes of conduct on power consumption. The third version emerged at the end of last year, in response to complaints from industry that the second release did not take into account the reality of today's hardware designs.

"Both the Broadband Forum and the Home Gateway Initiative came together to provide comments to the EU," says Kevin Foster, member of the board of the Broadband Forum. "What resulted was a much richer code of conduct, more grounded in reality in terms of what stretching targets could be set and what may be achievable."

The code provides near-term targets for both active and idle power. One striking aspect is that, for many of the communications technologies used in home gateways, the idle and active power targets are not that different. For Wi-Fi, even the near-term target only sees a drop of 50 per cent from active to idle. Ethernet fares better: manufacturers reckon they can get down to 30 per cent of active power when idling. But ADSL power consumption barely changes.

Low-power mode

Low-power modes do exist in ADSL, but they have turned out to be harder to use than the people who drafted the original standards expected. The equipment can move from its regular L0 mode to one called L2, which reduces the transmission power and can also simplify how the signals are encoded. Both come at the cost of bit-rate and have not, so far, proved popular.

Network operators are not keen on using the low-power modes in their head-end equipment. "L2 needs some improvements, particularly when equipment comes out of L2 mode. It can go in [to the mode] quite gracefully but it comes out quite quickly. That can result in sudden changes in crosstalk," says Foster.

The change in crosstalk upsets other lines and can cause ADSL modems to have to spend several seconds retraining, which disrupts users' Internet connections. Service providers are not keen on something that might lead to helpline calls and complaints about poor connections in online forums.

L2 was designed on the assumption that the link would have to come out of its drowsy state quickly to deal with traffic such as voice-over-IP packets. But, by bringing the link back to full active status by increasing power usage in small steps, it's possible to limit the impact on crosstalk. Operators can program in different L2 power levels to minimise the changes in signal strength, so groups such as the Broadband Forum are working on recommendations on how to use them.

"Rather than coming back instantly, if the system came back in two, three, four milliseconds, most users would not see that as a problem," Foster explains. He adds that it's not necessary to drop power right down. "You can drop the power by 10dB, which provides a significant gain - it reduces the power needed by about ten times. When it comes back from -10dB to 0dB, you don't have the massive transitions you would get if it dropped to -30dB and then back."

The Home Gateway Initiative is putting more focus on home connections, as L2 provides limited power reductions for the modem under the stairs. "These modes are designed predominantly at the head-end. There is not much in the standard to help on the customer-premises equipment side," says Bees.

At the moment, the Home Gateway Initiative is trying to work out what changes it could recommend to manufacturers.

"We are in the thick of the analysis," Bees says. "If you look at the power budget of the gateway, there are several major points of consumption. The areas that we are focusing on are the Ethernet interfaces, the CPU, and other internal processing functions. We are talking with some of the Wi-Fi group about the power savings that might be available, as the home gateway is basically a Wi-Fi access point.

"You really need to understand what services are running on the gateway: what is needed in terms of wake-up times and the background traffic requirements."

Idling CPUs

It's not easy to find ways to save power with the wireless interfaces on home gateways. At short ranges, the power consumption of the receiver circuit dominates. They have to be continually attentive and often consume more power than the transmitters - the reverse of the situation with the head-end equipment on ADSL, where signals are powerful enough to traverse long distances.

Another source of waste energy is due to CPUs idling away when nothing is really happening. The gateway should be able to put its CPUs to sleep whenever nothing is happening on the network. But it's rare for a home network to go to sleep.

"You need a kind of quiescent mode with a minimum bandwidth capability, intelligently sensing with one eye open," says Bees. But the picture is complicated by the amount of background traffic on the network. It does not have to be passed at maximum speed, but it takes time and power to work out whether a packet is just a keep-alive notification or the first chunk of a real-time video feed.

"The aim is to get to the best practice. We will keep reviewing the numbers and find the right balancing point," Bees explains.

In the short term, overall gateway power could go up rather than down. Mobile operators want to put femtocells in the home, but this adds yet another network interface on top of Wi-Fi and Ethernet.

Bees says: "When we add femto, it will definitely change the power equation. We haven't tackled it in detail but we are aware it has higher power consumption than others."

As with the traditional home gateway, much of the power consumption is due to the silicon rather than the protocol, according to Will Franks, chief technology officer of femtocell specialist Ubiquisys.

"The basestation is the one that is always on. There are some optimisations you can do but not as many as you can do in the handset," says Franks.

Femtocells work over much shorter ranges than macrocells. The regular signals that pass between the basestation and the handset when calls are not being made need just milliwatts of transmit power. The energy - some 6W to 8W - goes mainly into the signal processors that decode the 3G signals. Franks says Moore's Law will help cut this consumption. Some functions will also migrate into hardware that can be run at a low clock speed, which tends to result in less consumption.

Ultimately, manufacturers will move to software defined radio systems that will enable Wi-Fi, 3G and other network processing to be integrated into a single piece of silicon. Such combo processors should consume less power than the array of silicon needed in the designs that are being prepared now. As vendors work out when it is safe to power the processors down, or run them more slowly, their overall power consumption should gradually shrink.

The key is to get to the point that Foster terms 'always available'. Instead of equipment being fully-on just in case something happens, the software needs to be smarter about working out when full power is needed, when it is possible to snooze between packets, and when it is safe to run the processors more slowly.

"We live in an always-on world. We need an always-available world," he says.

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