Multipathing, hidden nodes, data collisions, near/far errors and other common wireless problems can be easily ID'd and resolved. A Cisco networking expert explains how
One the biggest problems with wireless Ethernet networking is troubleshooting it. Troubleshooting tools are expensive for most network administrators. Truthfully, most wireless problems can be easily discovered up front with a site survey from a solid wireless expert. Here are the most common and most preventable wireless problems I see in the field.
Beware of Multipathing
This is a time issue with 802.11b. Multipathing is a form of propagation that occurs when an RF signal takes different paths when propagating from an access point or WLAN NIC to an access point or WLAN NIC. While the signal is en route, walls, chairs, desks, and other items get in the way and cause the signal to bounce in different directions. One client's signal may go directly to the destination; another client may bounce from a desk to the ceiling, and then to the destination. As a result, some of the signal will encounter delay and travel longer paths to the receiver.
In some cases, multipath delay causes the signal to overlap, which confuses the receiver. This is often referred to as intersymbol interference or ISI. Because the shape of the signal conveys the information being transmitted, the receiver will make mistakes when demodulating the signal's information. If the delays are great enough, bit errors in the packet will occur. The receiver won't be able to distinguish the symbols and interpret the corresponding bits correctly. Since this can cause critical redundancy checking (CRC) errors and retransmission, wireless performance is slower than Chicago traffic.
How Do You Fix Multipathing?
Bouncing signals around and having reflectors in the network is all in a day's work. The best fix involves access points that support antenna diversity. Antenna diversity means having multiple antennas, inputs, receivers, transmitters, etc. to help eliminate multipathing. There are five or six different types of antenna diversity, but the most common types for 802.11 are active antenna switching and transmission diversity.
In active antenna switching, there are multiple antennas on multiple inputs but the signal is received on one antenna at a time. Transmission diversity can only transmit out the last antenna it used for reception. It can alternate different antennas for transmissions if needed.
Hidden Node Trouble
In an ideal world, newer laptops, thanks to the miracle of 802.11, immediately scan the airwaves for access points. Drivers have come a long way and wireless cards now quickly evaluate the signal strength of the available access points, and the number of users per access point. Based on this, the device will choose the access point with the strongest RF signal and the fewest users. In a hidden node situation, at least one client (node) is unable to "hear" one or more of the other clients connected to the same access point. Usually this is because of some physical obstruction between it and other users.
"So what," you say? Understand that a hidden node is equal to collisions on a wired 802.3 network. It makes transmission time as long as the drive to your mother-in-law's house.
For the collision detect to work on wireless, the client has to hear the other clients because it uses a collision avoidance instead of a collision detect algorithm. If it cannot hear them, there can be problems in the way the clients share the available bandwidth. Share is the operative word with wireless. Hidden nodes will cause data collisions and/or bit errors. Of course when a bit error occurs, the clients need to re-transmit the data and these collisions can result in a big decrease in data transmission rates in the wireless network.
Next Page: Near/Far Errors, Preamble Mismatches
