Controller Based WLANs

Product and Software: This article applies to all Aruba controllers and ArubaOS versions.

With wireless, clients sometimes go into a powersaving mode where they "sleep" for a while. The client waits for the AP to wake up each time there is any unicast packets for it. This time is measured in beacon intervals and is called the "listen interval." The AP flags whether or not there are unicast packets in a bitmap inside the beacon called a Traffic Indication Message (TIM).

Note that you can see a client's listen interval with "show stm association" and it is totally defined by the client (for example, power save mode- high/medium/low/none).

However, things are treated differently for broadcast traffic because, by definition, all clients must hear a broadcast packet. What happens is this: after a certain number of beacons that include TIMs, a special beacon is sent called the Delivery Traffic Indication Message (DTIM). For this beacon, all clients are expected to wake up and listen to broadcast traffic. The number of beacons between DTIMs is configurable per SSID. For a normal system, if you set a small DTIM (for example, 1), then all broadcast traffic (mainly ARPs) gets delivered quickly to all clients with no delay. This means that all clients are waking up every other beacon even if there is no traffic for them. With a large DTIM (for example, 10) the clients can sleep for 10 times longer (10x ~100ms ~= 1 second). This longer sleep time has a significant effect on battery life for some small devices, but the delay in traffic flow is also increased because clients need to wait for an ARP response before transmitting.

Two Aruba features, "Proxy-ARP" and Client "Battery Boost," may benefit some networks by significantly improving battery life. They do this by automatically responding to ARPs and by translating multicast into unicasts respectively. These features are most useful to increase the standby time for WLAN devices such as VoWLAN phones. To use these new features, you should set a large DTIM for the SSID and then check the listen interval for your devices. These features need customers to have deep understanding of the network environment. For example, as DTIM value is large, they may add delay for voice devices.

If the manufacturer of the single-purpose highly mobile device (HMD) recommends a specific Delivery Traffic Indication Message (DTIM) value, you must implement this value on the controller and the device.

For more information, see the "Configuring Incremental Settings for Single-Purpose HMDs" chapter in the Virtual Optimizing Aruba WLANs for Roaming Devices document.