Wireless Connectivity

Wireless... The Inlaws of Networking....

Written by Anonymous on 11:52 PM

One the biggest problems with wireless Ethernet networking today is troubleshooting it. The tools are expensive for most network admins for troubleshooting needs. Wireless is tricky, it’s like buying a Yugo, cheap up front cost, long term maintenance drain on the ole’ checking account. Truthfully, most wireless problems can be very easily discovered up front with a solid site survey from a solid wireless expert. But let's face it, the reality is that I MAY get a site survey done when I first deploy wireless, but after that, I am goin' to have at it! I have ran into many wireless problems over the years, but I would say that these are the ones that I either see the most or the ones I've spent the most time troubleshooting.(That' s right Preamble and Hermes chipsets, I'm trash talkin' you!)

Multipathing
This is a big time issue with 802.11b. Multipathing is a form of propagation and it occurs when an RF signal takes different paths when propagating from an AP or WLAN NIC to an AP 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. Metal objects really make a single bounce more then a pinball in a Star Trek NG Pinball Machine. (Awesome Pin by the way…) One clients signal may go directly to the destination, and 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 freaks out the receiver. This is often referred to as intersymbol interference (or the cool way to say it is; ISI. Of course admitting you have it puts you in the networking 101 category). 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 CRC errors and retransmission, wireless performance is slower then Chicago traffic.

How do you fix it?
Bouncing signals around and having reflectors in the network is just all in the days work. Having AP’s that support antenna diversity is the best fix. Antenna diversity means having multiple antennas, inputs, receivers, transmitters, etc to help eliminate Multipathing. There are about five or six different types of antenna diversity but the most common types for 802.11 are Active Antenna Switching and Transmission Diversity. Active Antenna Switching is having multiple antennas on multiple inputs but the signal is received only on one antenna at a time. Transmission Diversity is like poison reverse for L3 routers. It can only transmit out the last antenna it used for reception. It can alternate different antennas for transmissions if needed.


Hidden Node
Hidden Node sounds kinda like a new condo development in Boca. In an ideal world, you fire up your shiny new laptop and thanks to the miracle of 802.11, it immediately scans the airwaves for access points. Drivers have came a long way and wireless cards now quickly evaluates 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. Enter the hidden node. 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. Man does that slow the network down. It makes transmission time as long as the drive to your mother in laws house… For the Collision Detect to work on wireless (CSMA/CA), the client has to hear the other clients because it uses a collision avoidance instead of a collision detect algorithm. If it can not hear then as a result, there can be mega problems in the way the clients share the available bandwidth. Share is the operative word with wireless. Hey it’s like kindergarten without eating paste. Of course some paste actually tasted kinda good… better then the sushi **** I have ate with Robb Boyd. Hidden node 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 time decrease data transmission rates in the wireless network.

How do you fix it?
First thing to do is to find the hidden node. Most likely it’s the user complaining the most, so check the support logs and there you go. Understanding how CSMA/CA works is critical to solving this and the near/far errors. Now a couple things to try:

• Try using RTS/CTS. It does not fix the problem, it will just decrease the effect that the hidden node has on the rest of the users. The rest of the users on that AP will be singing your praises but your hidden node user will really be mad because it will not fix their problem. Check office politics and then proceed.
• Move the user
• Remove any obstacles, if possible
• Give the hidden node users a stronger WLAN card. The Ubiquiti SRC is my fav. That card has so much juice you can synch up with satellites! You could also a add client side antenna, but in my experence those get lost,broken and cause more support calls then less, so I do not recommend that course of action.

Near/Far
I see this one all the time. Near/Far is really just exactly what it states. This is a problem that can happen when multiple wireless users have wireless devices that are very near an access point that is much closer than a user who is right on the edge of the radio signal boundary. Consider that five users can associated with an AP and they each have a laptop, iPaq and wireless phone. Bye bye bandwidth. The user that is farthest away simply cannot be heard over the traffic from the devices closer to the access point. The closer users are hogging all the bandwidth. This seems to happen to me whenever I go see a Star Wars flick, I always sit close to the geeks that want to argue about the most mundane points in the movie and I am sitting closer to them then the speakers. I mean come on man! Everyone knows that the Emperor wore a different cloak clasp in Ep III then in the holo images on EP I and II. Geez man! Be careful with this one, because it looks like a hidden node issue. The only way to know for sure is that a hidden node all has collision errors and near/far will not. Both will have poor bandwidth/response.

How do you fix it?

• Move the client
• Decrease the power of the clients closest to the AP
• Add a stronger WLAN NIC to the remote client

Preamble Scramble
Way too many hours spent troubleshooting this issue. This is by far the auto negotiation of the wireless world in terms of problems. I see this at least three times a month. The original 802.11 specification (which defined only 1 and 2Mbps operation), defined only a long preamble that uses a 128 bit sync field. When the "high rate", i.e. 11Mbps, 802.11b standard was created, an optional short preamble using a 56 bit "sync" field was added. This was intended to improve the efficiency of the wireless network for more "real-time" applications such as streaming video and Voice-over-IP telephony applications. Nice thought. All 802.11 devices in the 2.4 GHz band, including 802.11g devices, must be able to transmit and receive long preamble frames. 802.11g devices are required to be able to transmit and receive both long and short preambles, but support for short preamble in 802.11b devices is optional. And of course if you have ever been up until 3AM troubleshooting a network, it is the word “optional” that tends to be the hidden finance charge of networking…. The problem occurs when an 802.11g AP allows the use of Short Preamble by the stations it communicates with (also known as its BSS — Basic Service Set). The AP may also choose to allow legacy stations that do not support Short Preamble to associate with the BSS. If both these conditions are allowed, the legacy stations that aren't short-preamble-capable will not be able to understand much of the communication in the BSS, and most importantly won't be able to receive the all-important "Protection" frames. This could result in legacy 11b stations transmitting at the same time as 11g stations. Welcome to collision city! Speed Limit…almost zero… I have noticed odd connection problems with preamble mismatch. The short-preamble problem affects only a subset of the 802.11 products in the field. Specifically, ORiNOCO and Symbol cards and those using the Intersil PRISM 2.0 and Agere Systems chipsets.

How do I fix it?
Just one fix here. Like auto negotiation in the 802.3 world, make sure both ends match up. If you are using the auto setting on most AP’s today, test it first to make sure you WLAN NICs actually will auto set the short/long preamble. If not, manually set your clients.

Keep in mind these issues when setting up and/or planning for your wireless network. It will save you tons of time and leave you more time for life’s real rewards, like fishing and smoking Cohiba’s with your friends and discussing Star Wars….

Posted by Jimmy Ray Purser