18 April 2009

Saving Energy and Money By Extending the Battery Life of Mobile Devices

The battery life of Wi-Fi capable mobile devices can be extended by enabling the Wi-Fi radio to enter a low-power “sleep” mode during period when the device neither needs to transmit or receive data. The longer the sleep time, the lower the battery drain. The difficulty is ensuring that sleep mode does not interfere with network performance, i.e., the device can wake-up in a timely manner.

Mobile device drivers and radio firmware employ a variety of pre-set times and trigger events to optimize entry into, and termination of, sleep time. The techniques employed typically vary by device and applications. For example, scanners typically have longer pre-set sleep times than laptops because the latter is assumed to have greater access to a recharger. The IEEE 802.11 standard includes a mandatory power save polling (PSP) feature whereby the Wi-Fi access point with which the device is associated must buffer data for that device while it is sleeping. Once the device awakens, the buffered data are delivered.

Following the transaction the device can return to the sleep mode if no additional data are to be sent or received. The PSP mechanism includes additional provisions that enable the access point to override sleep times and force the device to wake up at shorter intervals (called DTIM interval) even if there is no traffic to send or receive.

Battery life can be compromised as a result of two primary issues. Network performance problems, such as the failure to respond to ARP requests within the allocated time, or insufficient buffer storage within an access point, can reduce the DTIM interval and cause a mobile device to wake-up more often than necessary.

Additionally, broadcast and multicast Wi-Fi traffic chatter can prevent a mobile device from entering sleep mode, keeping it awake to check lest any of the chatter include packets intended for the device. In both scenarios battery life is compromised because the sleep mode cannot be utilized as intended.

To address these issues some vendors have implemented proprietary power-saving solutions that require software clients (Cisco CCX) or firmware hooks (Symbol). There are two fundamental issues with these approaches: they limit the range of available devices by locking customers into using only devices embedded with the proprietary technology; they require that the customer implement strict revision control over the client software and firmware to avoid incompatibilities or performance differences that exist between revisions.

Aruba has taken a standards-based approach to extending battery life by using infrastructure controls to manage off-the-shelf mobile devices without recourse to proprietary software or firmware. Three standards-based infrastructure controls are leveraged to equal or exceed the battery life achievable with proprietary solutions:

• Proxy-ARP: Mobility Controllers answer all ARP requests for devices with their radios in sleep mode, permitting longer DTIM intervals than could be supported if access points alone managed these requests;

• Long DTIMs: Long DTIM intervals are enabled by a battery boost feature, set by SSID, that permits the conversion of multicast / broadcast frames to unicast frames without having to buffer every DTIM period. Client devices can define their own DTIM periods thereby extending battery life without negatively affecting network performance;

• Multicast suppression: Mobility Controllers employ real-time packet inspection to identify and block network chatter (multicast traffic) that would negatively affect mobile devices. As a result, mobile devices able to remain in sleep mode longer and conserve additional power.

This three-pronged approach to power saving allows for longer sleep times on mobile devices such as scanners and voice handsets. Longer operating service from a single charge can have significant logistics and cost benefits, requiring fewer mobile devices, battery packs, and /or charging stations. Additionally, battery service life will be extended since service time is inversely related to the number of charge cycles.

Aruba’s standards-based approach also frees customers to use any Wi-Fi certified mobile device on the market, with the assurance that its battery life will be maximized regardless of make, model, form-factor or application. Eliminating sole-sourced products in favor of a procurement process based on price and/or performance can yield significant cost savings.