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Clients cannot associate with the Access point (AP), the data rates and radio preamble are configured incorrectly.

Introduction

 

The problem occurs if the data rates and radio preamble are configured incorrectly

Resolution

You use the data rate settings to choose the data rates the wireless device uses for data transmission. The rates are expressed in megabits per second. The wireless device always attempts to transmit at the highest data rate set to Basic, also called Require on the browser-based interface. If there are obstacles or interference, the wireless device steps down to the highest rate that allows data transmission. You can set each data rate to one of three states:

 

Basic (the GUI labels Basic rates as Required)—Allows transmission at this rate for all packets, both unicast and multicast. At least one of the wireless device's data rates must be set to Basic.

Enabled—The wireless device transmits only unicast packets at this rate; multicast packets are sent at one of the data rates set to Basic.

Disabled—The wireless device does not transmit data at this rate.

Note At least one data rate must be set to basic.

 

You can use the Data Rate settings to set an access point to serve client devices operating at specific data rates. For example, to set the 2.4-GHz radio for 11 megabits per second (Mbps) service only, set the 11-Mbps rate to Basic and set the other data rates to Disabled. To set the wireless device to serve only client devices operating at 1 and 2 Mbps, set 1 and 2 to Basic and set the rest of the data rates to Disabled. To set the 2.4-GHz, 802.11g radio to serve only 802.11g client devices, set any Orthogonal Frequency Division Multiplexing (OFDM) data rate (6, 9, 12, 18, 24, 36, 48, 54) to Basic. To set the 5-GHz radio for 54 Mbps service only, set the 54-Mbps rate to Basic and set the other data rates to Disabled.

 

You can configure the wireless device to set the data rates automatically to optimize either the range or the throughput. When you enter range for the data rate setting, the wireless device sets the 1 Mbps rate to basic and the other rates to enabled. The range setting allows the access point to extend the coverage area by compromising on the data rate. Therefore, if you have a client that is not able to connect to the access point while other clients can, one reason may be because the client is not within the coverage area of the access point. In such a case using the range option will help in extending the coverage area and the client may be able to connect to the access point.

 

Typically the trade-off is between throughput and range. When the signal degrades (possibly due to distance from the access point,) the rates will renegotiate down in order to maintain the link (but at a lower data rate). Contrast that against a link configured for a higher throughput that will simply drop when the signal degrades enough to no longer sustain a configured high data rate, or roam to another access point with sufficient coverage, if

one is available. The balance between the two (throughput vs. range) is one of those design decisions that has to be made based on resources available  to the wireless project, type of traffic the users will be passing, service level desired, and as always, the quality of the RF environment.When you enter throughput for the data rate setting, the wireless device sets all four data rates to basic.

 

Note When a wireless network has a mixed environment of 802.11b clients and 802.11g clients, make sure that data rates 1, 2, 5.5, and 11 Mbps are set to required (basic) and that all other data rates are set to enable. The 802.11b adapters do not recognize the 54 Mbps data rate and do not operate if data rates higher than 11Mbps are set to require on the connecting access point.

For more information on how to configure the data rates on the AP radio, refer to the Configuring Radio Data Rates section of Configuring Radio Settings

.

The radio preamble is a section of data at the head of a packet that contains information that the wireless devices (which include wireless clients)

need when they send and receive packets. Radio preambles can be either short preambles or long preambles. If someone configures the radio preambles incorrectly, the client is not able to associate with the wireless AP. The radio preamble configuration is dependent on the client cards that are used in the wireless network. Aironet WLAN Client Adapters support short preambles. Early models of the Aironet WLAN Adapter (PC4800 and PC4800A) require long preambles. If these client devices do not associate to the wireless devices, then you should not use short preambles.

For more information on how to configure radio preamble, refer to the Disabling and Enabling Short Radio Preambles section of Configuring Radio Settings.

Problem Type

Device cannot associate

Products

Access point

ACU (Aironet Client Utility)

Reference

Configuring Radio Data Rates

Configuring Radio Settings

Version history
Revision #:
1 of 1
Last update:
‎06-22-2009 05:40 PM
Updated by: