Procedures for Optimizing Wireless Networks
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each port independently and at the highest rate that the connected device and switch support The vast majority of devices use wire speed nonblocking ASICs, which means that each connection can function at the highest speed supported by both the port and the connecting device
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There are many tools that can report the throughput ratings for a network connection (whether wireless or wired) One example of such a tool is the free bandwidth monitor included in the Axence NetTools suite available at wwwAxenceSoftwarecom This kind of software can report on network bandwidth, ping histories, traceroutes, and DNS lookups all of which can be very beneficial in any network troubleshooting scenario, not just WLANs Figure 8-1 shows the bandwidth monitoring component of this suite
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Axence NetTools bandwidth monitoring
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Administering and Optimizing a Wireless LAN
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The solutions to throughput problems include the following:
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Install more APs in the coverage area using different channels and possibly
lower output power settings
Install 80211a APs and clients that use nonoverlapping channels and can
accommodate many more APs in a coverage area
Install faster switches between the APs and the wired network infrastructure Remove all 80211b client devices from your WLAN coverage space and
Ban, through both enforced and documented policies, bandwidth-consuming
applications such as streaming radio, legal and illegal movie and music downloads, and other large data downloads
EXERCISE 8-1 Analyzing Network Bandwidth
ON THE CD CertCam
In order to perform this exercise, you will need a laptop with WLAN client capabilities and an FTP server from which you can download files An internal FTP server will work best, as it should provide greater consistency on the wired side of the AP Using a Windows XP client, you can test the speed of your WLAN by downloading FTP files Before starting this exercise, make sure that a large file (more than 1 megabyte should suffice) is on the FTP server and that an account exists that you can use to access the file
Testing the Speed in Close Proximity
1 Boot your laptop and connect to the WLAN within 10 20 feet of the AP 2 Launch the Windows command line: click Start and select Run, and then type CMD and press ENTER 3 Connect to your FTP server (For example, if the FTP server is named ftpthecompanyserverinfo, type ftpthecompanyserverinfo) 4 Enter the proper authentication credentials 5 Download a large file with the command get filename 6 When the download completes, note the speed in the download report The speed is listed as 21793 Kbytes/sec in the lower right in Figure 8-2
Procedures for Optimizing Wireless Networks
FTP download screen
Testing the Speed at a Distance
1 Now that you have a baseline, move the laptop approximately 100 feet from the AP and download the same file What is the stated speed 2 Move approximately 150 feet away and test again 3 Continue until you have lost the connection
Resolving Multipath and Hidden Node Problems
Two common problems in WLANs are multipath and hidden node problems Multipath is the result of normal WLAN operations Hidden node problems are often the result of poor planning or changes within the environment
Multipath and Performance
Since WLANs have RF LOS instead of just visual LOS, the RF receivers can receive signals that travel directly from the transmitter to the receiver as well as signals that reflect and diffract off of or around other objects and then travel to the receiver Multipath is the term for signals traveling multiple paths and still arriving at the receiver Multipath can be good for the communication link, and it can be bad for the communication Some newer wireless technologies take advantage of multipath in order to increase the data rate and throughput of wireless communications
Administering and Optimizing a Wireless LAN
An example of this is the MIMO technology on which the HT PHY is based in the IEEE 80211n draft document As I stated, multipath can provide good and bad results In most cases, the results are negative unless specific technologies are implemented to deal with them The results include
Increased signal amplitude at the receiver Decreased signal amplitude at the receiver Data corruption Signal nullification
Increased signal amplitude at the receiver can result from multiple signal paths arriving at the receiving antenna in phase This is known as upfade Of course, the signal is not stronger than when it was transmitted and, in fact, will always be weaker than the originally transmitted signal; however, the signal may be stronger than it would have been at the point of reception had the upfading not occurred As you learned in preceding chapters, free space path loss ensures that the received signal will be weaker than the transmitted signal As the wave travels, the wavefront broadens and the signal strength at a given far point will be less than at a point nearer the origination Multipath may also cause signal reduction or a decrease in the signal amplitude When this occurs, it is known as downfade, which should be considered during the selection of antennas at the time of the site survey Downfade occurs when two copies of the same signal arrive at the receiver out of phase In addition, out-of-phase signals may also cause corruption of the main signal This is because the amplitude of the received signal is reduced to such a point that the receiver can understand only part of the frame being transmitted, not the complete frame This usually happens when the signal-to-noise ratio is very low In other words, the RF signal is very close to the noise floor This result of multipath usually causes a retransmission of the corrupted frame from the transmitter, and there may need to be multiple retransmissions before the frame actually makes it through The last possible result of multipath, nulling, occurs when one or more reflected waves arrive at the receiver out-of-phase with the main wave In this case, instead of weakening the signal, the main wave s amplitude is canceled and the signal cannot be received by the receiver In these cases, retransmission of the frame will not likely resolve the problem unless the multipath occurred because of a moving vehicle in the area, which is not the likely case You will most likely have to reposition one or both of the ends of the link