Troubleshooting

 

windows connection process

 

1. Scan for wireless networks.

The wireless client network adapter performs a scan (about once every 60 seconds) for the available wireless networks within range. When scanning, the wireless network adapter sends a series of Probe Request frames and the APs that receive the frames respond with a Beacon frame that contains the capabilities of the wireless AP, such as the supported speeds, SSID, and security options.

2. Choose an AP. After receiving the frames from the AP, the wireless device chooses a wireless AP with which it will attempt to authenticate and associate. The decision of which AP to choose is based on the following factors:

● Wireless AP capabilities. Based on the AP capabilities in the Beacon frame, the wireless device must be able to support those capabilities. If the device’s abilities do not match those of the AP, then the device cannot choose that AP.

● Preferred networks. Users can create a list of preferred wireless networks based on SSID. If the device receives Beacon frames from multiple APs that are in the preferred list, then the most preferred wireless network (the highest one in the list) is chosen.

● Signal strength. The wireless network adapter of the wireless client chooses the AP with the highest signal strength for the wireless network name that is highest in the preferred list.

3. Authenticate. After choosing the AP with which to connect, the device and AP perform authentication.

4. Associate. After authentication has successfully completed, the wireless network adapter and the wireless AP exchange a series of messages to create an association.

5. Obtain an IP address. The final step is for the wireless device to obtain an IP address.
Depending upon the configuration this can be accomplished in several ways:
● Manual addressing. The device can be configured manually with a static IP address that allows the device to communicate with other computers on the network and to reach the Internet.
● DHCP addressing. The device may be configured to use the Dynamic Host Configuration Protocol (DHCP) to request an IP address.
● APIPA addressing. If the wireless device is configured for DHCP yet no DHCP server is active. If an APIPA address is assigned to a device,  Windows displays “Limited or no connectivity” for the status of the wireless connection.

Tools in windows:

  • Network and sharing center
  • Wireless network connection status
  • Wireless network connection properties
  • Event viewer
Near far transmission problem

 

near_far1 The stronger signal from Laptop A “drowns out” the weaker signal from Laptop B. This is known as the near/far transmission problem.

Solutions:

● Move the device with the stronger transmission power farther away from the AP
● Reduce the transmission power of devices that are closer to the AP
● Increase the transmission power of devices that are farther away from the AP

Near/far transmission problems are not common due to the nature of CSMA/CA.

Hidden node problem:

 

wireless Devices 1 and 2 may be within range of the AP but not within range of each other. If Device 2 “listens” and hears no traffic, it might assume no transmissions are taking place, while actually Device 1 is already transmitting, resulting in a collision.

Resolve a hidden node problem:
● Move the hidden node device
● Remove any physical obstacles that may be interfering with devices communicating with each other
● Increase the client device’s power level
● Add an additional AP to the WLAN

Optimization

A wireless device can only roam from the coverage area of one AP to another AP if the SSID and security settings are identical, although the channel number will be different to avoid cochannel and adjacent-channel interference. Generally, when the signal strength drops to 75 dB or packet error rates exceed 8 percent, a wireless device will begin looking for another AP.

Ensuring smooth roaming is primarily based on cell overlap. Cell overlap is the area between two APs, in which a wireless device begins to search for a new AP with which to associate. Recommended settings for cell overlap vary.

Some vendors  recommend an overlap of 20 percent when using 802.11b/g and 15 percent when using 802.11a/n is desirable.

Trough put for different standards:

 

Advertised/Actual Data rate(Mbps) Number of users /average throughput
802.11b 11  7.2 10  720 Kbps
802.11b 11  7.2  30  240 Kbps
802.11g 54  13  10  1.3 Mbps
802.11g 54  13  30  430 Kbps
802.11a 54  25  10  2.5 Mbps
802.11a 54  25  30  833 Kbps
802.11n (MCS7) 72  35  10  3.5 Mbps

AP optimization

● Never accept the default configuration of an AP. Always configure it for a unique SSID, a channel to minimize cochannel and adjacent-channel interference, and the highest level of security authentication and encryption settings.
● Install lightweight APs to minimize the total cost of ownership (TCO).
● Configure wireless VLANs for additional security.
● Take advantage of wireless network management systems (WNMS).
● For guest accounts use a captive portal AP.
● Use picocells when necessary. A picocell is a WLAN that uses a reduced power output from the AP that results in a smaller coverage area but can allow for increased performance due to channel reuse.
● Size the coverage area to the corresponding standard.  Figure bellow compares the coverage area of a 5 GHz (802.11a) with a 2 GHz (802.11b) with no obstacles to create interference.

coverage-area
Note that higher data rates do not extend as far from the AP.

When possible a wireless device should be set to “disable upon wired connect.” This turns off the wireless connection whenever the device connects to a standard wired network by using a cable connection. This provides a higher degree of security so that the device cannot be used as a rogue AP to circumvent security.