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Tip of the Month

May 2003

More on Wireless Signal Strength – Measurements

Wireless networks’ Receive Signal Strength Indicator (RSSI) is not a definite measure of the RF value associated with a signal. A complete discussion of different wireless signal strength measurements can be found in the white paper, “Converting Signal Strength Percentage to dBm Values” and is available at http://www.wildpackets.com/support/additional_resources/white_papers.

Now that you know you cannot rely on RSSI, then how can you tell that your stations are getting a solid signal on a wireless network?

You may have measured the signal strength at any given time and found that the signal strength was at an acceptable level. You set up stations according to the survey map. Unfortunately, you now discover that the throughput of data on your wireless network is not what you had anticipated. Where do you go from here?

The new version of AiroPeek NX shipping this month (May) supports 26 layer 2 specific wireless events in the Expert Problem Finder! A few of these events specifically relate directly to wireless signal strength. For example, look for “Wireless Data Rate Change” and “Wireless Transmission Retry” as specific events that suggest degraded signal performance.

1. You may still have a strong signal, but if it is conflicting with other bands and ambient noise in the same frequency, the result may not be loss of signal, but rather degraded performance. For instance, Stations capable of higher speeds for data transfer (e.g. 11Mbps for 802.11b) may be found transmitting at substantially lower speeds (e.g. 2Mbps). A conversation that began at 11 Mbps may shift down to 2 or 5.5Mbps dynamically. When this happens, AiroPeek NX will show “Wireless Data Rate Change” and may alert you if configured to do so.
2. If stations continue to have issues, you may see “Wireless Transmission Retry” as a result. Every station sending a data packet in a wireless environment must receive an acknowledgement for the packet or a retransmission at layer 2 will occur. The layer 2 retransmission is separate from retransmissions related to higher layers.

Note: It is also important to understand that some protocols do not use higher data rates. These protocols choose to utilize slower data rates as a conservative measure to decrease retransmissions.

When analyzing a trace file from a wireless network, ensure that nodes are speaking as efficiently as possible by watching data rates, signal strength and retransmissions. Whether you are using an encrypted network or out in the open, AiroPeek NX will assist you in performance capacity planning, security risk analysis and many other aspects of network analysis!