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Chapter 10      Configuring Radio Settings
Configuring Transmit and Receive Antennas
To disable short radio preambles, follow these steps, beginning in privileged EXEC mode:
Short preambles are enabled by default. Use the preamble-short command to enable short preambles if 
they are disabled. 
Configuring Transmit and Receive Antennas
You can select the antenna the wireless device uses to receive and transmit data. There are three options 
for both the receive and the transmit antenna:
 Gain—Sets the resultant antenna gain in decibels (dB). 
 Diversity—This default setting tells the wireless device to use the antenna that receives the best 
signal. If the wireless device has two fixed (non-removable) antennas, you should use this setting 
for both receive and transmit.
 Right—If the wireless device has removable antennas and you install a high-gain antenna on the 
wireless device’s right connector, you should use this setting for both receive and transmit. When 
you look at the wireless device’s back panel, the right antenna is on the right.
 Left—If the wireless device has removable antennas and you install a high-gain antenna on the 
wireless device’s left connector, you should use this setting for both receive and transmit. When you 
look at the wireless device’s back panel, the left antenna is on the left.
To select the antennas the wireless device uses to receive and transmit data, follow these steps, beginning 
in privileged EXEC mode:
CommandPurpose
Step 1configure terminalEntesr global configuration mode.
Step 2interface dot11radio { 0  }Enters interface configuration mode for the 2.4-GHz radio 
interface.
Step 3no preamble-shortDisables short preambles and enable long preambles.
Step 4endReturns to privileged EXEC mode.
Step 5copy running-config startup-config(Optional) Saves your entries in the configuration file.
CommandPurpose
Step 1configure terminalEnters global configuration mode.
Step 2interface dot11radio { 0 }Enters interface configuration mode for the radio interface.The 
802.11g/n 2.4-GHz radio is radio 0.
Step 3gain dBSpecifies the resultant gain of the antenna attached to the 
device. Enter a value from –128 to 128 dB. If necessary, you 
can use a decimal in the value, such as 1.5.
NoteThe Cisco 860 and Cisco 880 ISRs are shipped with a 
fixed antenna that cannot be removed. Antenna gain 
cannot be configured. 
						

							 
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Chapter 10      Configuring Radio Settings
Disabling and Enabling Aironet Extensions
Disabling and Enabling Aironet Extensions
By default, the wireless device uses Cisco Aironet 802.11 extensions to detect the capabilities of 
Cisco
 Aironet client devices and to support features that require specific interaction between the wireless 
device and associated client devices. Aironet extensions must be enabled to support these features:
 Load balancing—The wireless device uses Aironet extensions to direct client devices to an access 
point that provides the best connection to the network based on factors such as number of users, bit 
error rates, and signal strength.
 Message Integrity Check (MIC)—MIC is an additional WEP security feature that prevents attacks 
on encrypted packets called bit-flip attacks. The MIC, implemented on both the wireless device and 
all associated client devices, adds a few bytes to each packet to make the packets tamper-proof.
 Cisco Key Integrity Protocol (CKIP)—Cisco’s WEP key permutation technique based on an early 
algorithm presented by the IEEE 802.11i security task group. The standards-based algorithm, TKIP, 
does not require Aironet extensions to be enabled.
 World mode (legacy only)—Client devices with legacy world mode enabled receive carrier set 
information from the wireless device and adjust their settings automatically. Aironet extensions are 
not required for 802.11d world mode operation.
 Limiting the power level on associated client devices—When a client device associates to the 
wireless device, the wireless device sends the maximum allowed power level setting to the client.
Disabling Aironet extensions disables the features listed above, but it sometimes improves the ability of 
non-Cisco client devices to associate to the wireless device. 
Aironet extensions are enabled by default. To disable Aironet extensions, follow these steps, beginning 
in privileged EXEC mode:
Step 4antenna receive 
{diversity | left | right}
Sets the receive antenna to diversity, left, or right.
NoteFor best performance with two antennas, leave the 
receive antenna setting at the default setting, diversity. 
For one antenna, attach the antenna on the right and set 
the antenna for right.
Step 5antenna transmit 
{diversity | left | right}
Sets the transmit antenna to diversity, left, or right.
NoteFor best performance with two antennas, leave the 
receive antenna setting at the default setting, diversity. 
For one antenna, attach the antenna on the right and set 
the antenna for right.
Step 6endReturns to privileged EXEC mode.
Step 7copy running-config startup-config(Optional) Saves your entries in the configuration file.
Command Purpose
CommandPurpose
Step 1configure terminalEnters global configuration mode.
Step 2interface dot11radio { 0 }Enters interface configuration mode for the radio interface.
The 802.11g/n 2.4-GHz radio is radio 0. 
						

							 
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Chapter 10      Configuring Radio Settings
Configuring the Ethernet Encapsulation Transformation Method
Use the dot11 extension aironet command to enable Aironet extensions if they are disabled. 
Configuring the Ethernet Encapsulation Transformation Method
When the wireless device receives data packets that are not 802.3 packets, the wireless device must 
format the packets to 802.3 using an encapsulation transformation method. These are the two 
transformation methods:
 802.1H—This method provides optimum performance for Cisco wireless products.
 RFC 1042—Use this setting to ensure interoperability with non-Cisco wireless equipment. 
RFC1042 does not provide the interoperability advantages of 802.1H but is used by other 
manufacturers of wireless equipment.
To configure the encapsulation transformation method, follow these steps, beginning in privileged EXEC 
mode:
Enabling and Disabling Public Secure Packet Forwarding
Public Secure Packet Forwarding (PSPF) prevents client devices associated to an access point from 
inadvertently sharing files or communicating with other client devices associated to the access point. It 
provides Internet access to client devices without providing other capabilities of a LAN. This feature is 
useful for public wireless networks like those installed in airports or on college campuses. 
NoteTo prevent communication between clients associated to different access points, you must set up 
protected ports on the switch to which the wireless devices are connected. See the 
“Configuring 
Protected Ports” section on page 10-16 for instructions on setting up protected ports.
Step 3no dot11 extension aironetDisables Aironet extensions.
Step 4endReturns to privileged EXEC mode.
Step 5copy running-config startup-config(Optional) Saves your entries in the configuration file.
Command Purpose
CommandPurpose
Step 1configure terminalEnters global configuration mode.
Step 2interface dot11radio { 0 }Enters interface configuration mode for the radio interface.The 
802.11g/n 2.4-GHz radio is radio 0.
Step 3payload-encapsulation 
snap | dot1h 
Sets the encapsulation transformation method to RFC 1042 
(snap) or 802.1h (dot1h, the default setting).
Step 4endReturns to privileged EXEC mode.
Step 5copy running-config startup-config(Optional) Saves your entries in the configuration file. 
						

							 
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Chapter 10      Configuring Radio Settings
Enabling and Disabling Public Secure Packet Forwarding
To enable and disable PSPF using CLI commands on the wireless device, you use bridge groups. You 
can find a detailed explanation of bridge groups and instructions for implementing them in this 
document:
 Cisco IOS Bridging and IBM Networking Configuration Guide, Release 12.2. Click this link to 
browse to the Configuring Transparent Bridging chapter: 
http://www.cisco.com/en/US/docs/ios/12_2/ibm/configuration/guide/bcftb_ps1835_TSD_Products
_Configuration_Guide_Chapter.html
PSPF is disabled by default. To enable PSPF, follow these steps, beginning in privileged EXEC mode:
Use the no form of the command to disable PSPF.
Configuring Protected Ports
To prevent communication between client devices associated to different access points on your wireless 
LAN, you must set up protected ports on the switch to which the wireless devices are connected. 
To define a port on your switch as a protected port, follow these steps, beginning in privileged EXEC 
mode:
To disable protected port, use the no switchport protected interface configuration command.
CommandPurpose
Step 1configure terminalEnters global configuration mode.
Step 2interface dot11radio { 0 }Enters interface configuration mode for the radio interface. The 
802.11g/n 2.4-GHz radio is radio 0.
Step 3bridge-group group port-protectedEnables PSPF.
Step 4endReturns to privileged EXEC mode.
Step 5copy running-config startup-config(Optional) Saves your entries in the configuration file.
CommandPurpose
Step 1configure terminalEnters global configuration mode.
Step 2interface interface-idEnters interface configuration mode, and enter the type and 
number of the switchport interface to configure, such as 
wlan-gigabitethernet0.
Step 3switchport protectedConfigures the interface to be a protected port.
Step 4endReturns to privileged EXEC mode.
Step 5show interfaces interface-id 
switchportVerifies your entries.
Step 6copy running-config startup-config(Optional) Saves your entries in the configuration file. 
						

							 
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Chapter 10      Configuring Radio Settings
Configuring the Beacon Period and the DTIM
For detailed information on protected ports and port blocking, refer to the “Configuring Port-Based 
Traffic Control” chapter in the Catalyst 3550 Multilayer Switch Software Configuration Guide, 
12.1(12c)EA1. Click this link to browse to that guide:
http://www.cisco.com/en/US/docs/switches/lan/catalyst3550/software/release/12.1_12c_ea1/configurat
ion/guide/3550scg.html
Configuring the Beacon Period and the DTIM
The beacon period is the amount of time between access point beacons in kilomicroseconds (Kµsecs). 
One Kµsec equals 1,024 microseconds. The Data Beacon Rate, always a multiple of the beacon period, 
determines how often the beacon contains a delivery traffic indication message (DTIM). The DTIM tells 
power-save client devices that a packet is waiting for them. 
For example, if the beacon period is set at 100, its default setting, and the data beacon rate is set at 2, its 
default setting, then the wireless device sends a beacon containing a DTIM every 200 Kµsecs. 
The default beacon period is 100, and the default DTIM is 2. To configure the beacon period and the 
DTIM, follow these steps, beginning in privileged EXEC mode:
Configure RTS Threshold and Retries
The RTS threshold determines the packet size at which the wireless device issues a request to send (RTS) 
before sending the packet. A low RTS threshold setting can be useful in areas where many client devices 
are associating with the wireless device, or in areas where the clients are far apart and can detect only 
the wireless device and not each other. You can enter a setting ranging from 0 to 2347 bytes.
Maximum RTS retries is the maximum number of times the wireless device issues an RTS before 
stopping the attempt to send the packet over the radio. Enter a value from 1 to 128.
The default RTS threshold is 2347 for all access points and bridges, and the default maximum RTS 
retries setting is 32. 
To configure the RTS threshold and maximum RTS retries, follow these steps, beginning in privileged 
EXEC mode:
CommandPurpose
Step 1configure terminalEnters global configuration mode.
Step 2interface dot11radio { 0 }Enters interface configuration mode for the radio interface. The 
802.11g/n 2.4-GHz radio is radio 0.
Step 3beacon period value Sets the beacon period. Enter a value in Kilomicroseconds.
Step 4beacon dtim-period valueSets the DTIM. Enter a value in Kilomicroseconds.
Step 5endReturns to privileged EXEC mode.
Step 6copy running-config startup-config(Optional) Saves your entries in the configuration file. 
						

							 
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Chapter 10      Configuring Radio Settings
Configuring the Maximum Data Retries
Use the no form of the command to reset the RTS settings to defaults.
Configuring the Maximum Data Retries
The maximum data retries setting determines the number of attempts the wireless device makes to send 
a packet before dropping the packet. The default setting is 32. 
To configure the maximum data retries, follow these steps, beginning in privileged EXEC mode:
Use the no form of the command to reset the setting to defaults.
CommandPurpose
Step 1configure terminalEnters global configuration mode.
Step 2interface dot11radio { 0 }Enters interface configuration mode for the radio interface. The 
2.4-GHz and the 802.11g/n 2.4-GHz radios are radio 0. 
Step 3rts threshold valueSets the RTS threshold. Enter an RTS threshold from 0 to 2347.
Step 4rts retries valueSets the maximum RTS retries. Enter a setting from 1 to 128.
Step 5endReturns to privileged EXEC mode.
Step 6copy running-config startup-config(Optional) Saves your entries in the configuration file.
CommandPurpose
Step 1configure terminalEnters global configuration mode.
Step 2interface dot11radio { 0 }Enters interface configuration mode for the radio interface.
The 802.11g/n 2.4-GHz radio is radio 0.
Step 3packet retries value Sets the maximum data retries. Enter a setting from 1 to 128.
Step 4endReturns to privileged EXEC mode.
Step 5copy running-config startup-config(Optional) Saves your entries in the configuration file. 
						

							 
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Chapter 10      Configuring Radio Settings
Configuring the Fragmentation Threshold
Configuring the Fragmentation Threshold
The fragmentation threshold determines the size at which packets are fragmented (sent as several pieces 
instead of as one block). Use a low setting in areas where communication is poor or where there is a great 
deal of radio interference.The default setting is 2346 bytes. 
To configure the fragmentation threshold, follow these steps, beginning in privileged EXEC mode:
Use the no form of the command to reset the setting to defaults.
Enabling Short Slot Time for 802.11g Radios
You can increase throughput on the 802.11g 2.4-GHz radio by enabling short slot time. Reducing the 
slot time from the standard 20 microseconds to the 9-microsecond short slot time decreases the overall 
backoff, which increases throughput. Backoff, which is a multiple of the slot time, is the random length 
of time that a station waits before sending a packet on the LAN.
Many 802.11g radios support short slot time, but some do not. When you enable short slot time, the 
wireless device uses the short slot time only when all clients associated to the 802.11g 2.4-GHz radio 
support short slot time.
Short slot time is supported only on the 802.11g 2.4-GHz radio. Short slot time is disabled by default.
In radio interface mode, enter this command to enable short slot time:
ap(config-if)# short-slot-time 
Enter no short- slot-time to disable short slot time.
Performing a Carrier Busy Test
You can perform a carrier busy test to check the radio activity on wireless channels. During the carrier 
busy test, the wireless device drops all associations with wireless networking devices for 4 seconds while 
it conducts the carrier test and then displays the test results.
In privileged EXEC mode, enter this command to perform a carrier busy test:
dot11 interface-number carrier busy 
CommandPurpose
Step 1configure terminalEnters global configuration mode.
Step 2interface dot11radio { 0 }Enters interface configuration mode for the radio interface. The 
802.11g/n 2.4-GHz radio is radio 0.
Step 3fragment-threshold value Sets the fragmentation threshold. Enter a setting from 256 to 
2346 bytes for the 2.4-GHz radio. Enter a setting from 256 to 
2346 bytes for the 5-GHz radio. 
Step 4endReturns to privileged EXEC mode.
Step 5copy running-config startup-config(Optional) Saves your entries in the configuration file. 
						

							 
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Chapter 10      Configuring Radio Settings
Configuring VoIP Packet Handling
For interface-number, enter dot11radio 0 to run the test on the 2.4-GHz radio, or enter dot11radio 1 to 
run the test on the 5-GHz radio.
Use the show dot11 carrier busy command to re-display the carrier busy test results.
Configuring VoIP Packet Handling
You can improve the quality of VoIP packet handling per radio on access points by enhancing 802.11 
MAC behavior for lower latency for the CoS 5 (Video) and CoS 6 (Voice) user priorities.
To configure VoIP packet handling on an access point, follow these steps:
Step 1Using a browser, log in to the access point.
Step 2Click Services in the task menu on the left side of the web-browser interface. 
Step 3When the list of Services expands, click Stream. 
The Stream page appears.
Step 4Click the tab for the radio to configure.
Step 5For both CoS 5 (Video) and CoS 6 (Voice) user priorities, choose Low Latency from the Packet Handling 
drop-down menu, and enter a value for maximum retries for packet discard in the corresponding field.
The default value for maximum retries is 3 for the Low Latency setting (Figure 10-1). This value 
indicates how many times the access point will try to retrieve a lost packet before discarding it. 
NoteYou may also configure the CoS 4 (Controlled Load) user priority and its maximum retries value.
Step 6Click Apply.
Figure 10-1 Packet Handling Configuration 
						

							 
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Chapter 10      Configuring Radio Settings
Configuring VoIP Packet Handling
You can also configure VoIP packet handling using the CLI. For a list of Cisco IOS commands for 
configuring VoIP packet handling using the CLI, consult the Cisco IOS Command Reference for Cisco 
Aironet Access Points and Bridges.  
						

							 
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Chapter 10      Configuring Radio Settings
Configuring VoIP Packet Handling