Dtim Period; Session Timeouts; Cisco Client Extensions - Cisco Catalyst 3650 Series Configuration Manual

Configuring WLANs

DTIM Period

In the 802.11 networks, lightweight access points broadcast a beacon at regular intervals, which coincides
with the Delivery Traffic Indication Map (DTIM). After the access point broadcasts the beacon, it transmits
any buffered broadcast and multicast frames based on the value set for the DTIM period. This feature allows
power-saving clients to wake up at the appropriate time if they are expecting broadcast or multicast data.
Typically, the DTIM value is set to 1 (to transmit broadcast and multicast frames after every beacon) or 2 (to
transmit after every other beacon). For instance, if the beacon period of the 802.11 network is 100 ms and the
DTIM value is set to 1, the access point transmits buffered broadcast and multicast frames 10 times per second.
If the beacon period is 100 ms and the DTIM value is set to 2, the access point transmits buffered broadcast
and multicast frames 5 times per second. Either of these settings are suitable for applications, including Voice
Over IP (VoIP), that expect frequent broadcast and multicast frames.
However, the DTIM value can be set as high as 255 (to transmit broadcast and multicast frames after every
255th beacon) if all 802.11 clients have power save enabled. Because the clients have to listen only when the
DTIM period is reached, they can be set to listen for broadcasts and multicasts less frequently which results
in a longer battery life. For example, if the beacon period is 100 ms and you set the DTIM value to 100, the
access point transmits buffered broadcast and multicast frames once every 10 seconds. This rate allows the
power-saving clients to sleep longer before they have to wake up and listen for broadcasts and multicasts,
which results in a longer battery life.
A beacon period, which is specified in milliseconds on the switch, is converted internally by the software
Note
to 802.11 Time Units (TUs), where 1 TU = 1.024 milliseconds. On Cisco's 802.11n access points, this
value is rounded to the nearest multiple of 17 TUs. For example, a configured beacon period of 100 ms
results in an actual beacon period of 104 ms.
Many applications cannot tolerate a long time between broadcast and multicast messages, which results in
poor protocol and application performance. We recommend that you set a low DTIM value for 802.11 networks
that support such clients.

Session Timeouts

You can configure a WLAN with a session timeout. The session timeout is the maximum time for a client
session to remain active before requiring reauthorization.

Cisco Client Extensions

The Cisco Client Extensions (CCX) software is licensed to manufacturers and vendors of third-party client
devices. The CCX code resident on these clients enables them to communicate wirelessly with Cisco access
points and to support Cisco features that other client devices do not, including those features that are related
to increased security, enhanced performance, fast roaming, and power management.
• The software supports CCX versions 1 through 5, which enables switches and their access points to
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communicate wirelessly with third-party client devices that support CCX. CCX support is enabled
automatically for every WLAN on the switch and cannot be disabled. However, you can configure
Aironet information elements (IEs).
WLAN Configuration Guide, Cisco IOS XE Release 3E (Catalyst 3650 Switches)
DTIM Period
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