Archive for the ‘Wireless’ Category
Ever so often I find myself troubleshooting some type of wireless related issue, and while wireless issue’s vary from
- Slow performance
- Clients can’t connect
- Poor voice performance
- Or even random disconnects, the list is endless.
However one of the common things I hear during the troubleshooting process is without a doubt along the lines of:
“But it says I have an excellent signal with five bars!”
And…. my favorite question in response to that statement is:
“What is your data rate?” (usually with this same expression)
Signal strength is only a small piece to the puzzle what determining whether or not you have a good quality signal strength. The signal strength indicator itself could even be misleading, just because a client is registering ‘5 bars’ with a good RSSI and SNR does not necessarily mean the AP on the other end of the connection is seeing a similar RSSI & SNR to the WLAN Client. Do I hear a transmit power mismatch, or a highly reflected RF environment?
Nowadays WLAN clients comes in all shapes and sizes (Phones, Tablets, wireless scanners, VoIP handsets) long gone are the days of wireless is just for laptops. With this wide array of hardware clients, you can guarantee each of these devices have a wireless transmitter with different specifications, and while it is impossible to take into account every WLAN client, the client audience should be considered when designing a WLAN or deploying AP’s.
Consider the an access point is transmitting at it’s max power rating, you can guarantee the wireless phone or VoIP handset does not have that same power level. It’s like two people trying to communicate with each other that across a football field and only one person has a mega-phone. The other guy without the megaphone will need to probably repeat himself a few times for the other person to understand him (Think of that as Data Retries).
One of the better ways to identify a proper Wireless connection would be to verify the the data rate, and see review the data rate statistics. Many of the different WLAN Client software have this functionality, telling us what percentage of the data was transmitted/received at a specific data rate. Now shifting data rates is common in a WLAN, but seeing 90% of data operating at the 1, 2, or 5.5 Mbps data rate is just poor performance.
A while back I posted about Understanding a wireless connection, and I wanted to dive a bit deeper and expand on the concept (albeit years later, but hey better late then never right?)
If it wasn’t Twitter I probably would not have known there was a CWNP conference going on, but luckily we have Twitter. Looks like it was a 3-day conference about all things wireless, I really will try and go next year.
It also looks like they posted the presentations on their website found here, I’ve looked through almost all of the presentations at this point and there are definitely some good ones in there. I hope they take a page from CiscoLive365 and post videos of the actual presentations in the future. Even if there is a small fee they would be worth it (or maybe be available certified CWNP’s?). While you can read through some of the presentations and get some valuable information from them, there are one or two that would probably offer much more insight if you could see/hear the presentation.
I really like Jerome Henry’s presentation on WLAN ‘range’, because believe it or not there are still quite a few people out there that say ‘This AP can cover 1500 sq. ft’, sure maybe it can but what will data rate be and will clients transmission be able to make it back to the AP when they start approaching the edge of the wireless cell?
Jonathan Linton’s VoWLAN, is another good one going over some best practices and design considerations.
There is also a good one on Transmit Beamforming, now I don’t want to re-type the entire page up so go check it out!
CWNP has posted a new 802.11ac video over on their YouTube Channel (CWNPTV)
This new video covers Planning for 802.11ac, if you still have not had a chance to go through the IEEE document or mess around with any new 802.11ac equipment this video is worth a play through. It covers the common pitfall of coverage vs capacity (Which does not only pertain to 802.11ac). I’ve seen many places and people simply deploy wireless based on signal strength, and if you have done more than a few wireless deployments you know signal strength & coverage is only half the battle. Capacity and throughput is the other half.
The video was posted almost a week ago and still has less than 200 views, so I am just trying to spread the word. As there are a few other useful videos in that Youtube channel, it’s a worthy resource that should not be overlooked for anyone wanting to be more familiar with 802.11 Wireless.
I was catching up on some of my subscribed YouTube channels this weekend when I checked in on the CWNP TV channel, I then found a new video that was posted by Tom Carpenter just last week. It’s a good 45 minute video that goes over some great fundamental details of the new 802.11ac standard.
It’s definitely worth a listen to if you have not done any other research on 802.11ac and you are curious at how 802.11ac is able achieve its new record of wireless speeds. It also explains that we will see 2 different waves of 802.11ac, wave 1 (going on now) and a wave 2 where will see even more improvements.
Now, I don’t want to ruin anything from the video but you can find the video here.
Wireless networks have seen many different trends emerging over the years, probably one of the most prevalent trend is the ability for access points to monitor to the RF spectrum around the access point. Whether you work with Cisco CleanAir, Aerohive, Aruba, or one of the other big wireless vendors they each have their own way of monitoring and reporting the surrounding RF environment. This is definitely a feature that is very useful for viewing the overall health of the wireless spectrum, after all this spectrum is the layer 1 medium for the WLAN and if there is a lot of contention/interference over the RF then the WLAN will see a performance degradation.
One thing I have inadvertently seen this built-in feature do is provide a false sense of security and make people believe they do not have a need to have any other spectrum analysis tool. There is one flaw with relying on this feature, it is the fact that this feature will only provide you a view of the RF surrounding the access point, while your first reaction is going to “well yes it does, that is what it is supposed to do” this feature does not (and will not) provide you with a view of the spectrum from the client’s perspective. Consider a deployment where you do not have access points peppered around an area every 20 feet and instead have access point spread out every 50-80 feet and you hear consistent complaints of wireless issues. The first reaction of any network engineer is going to be check the access point:
- Is the access point servicing clients
- Is there a configuration issue
- How does surrounding RF look
- What do the wireless statistics look like
Now usually one of those four things will point you in the right direction, but what I find that is pretty common trait is that after reviewing the RF statistics surrounding the access point (and assuming it looks good/normal) no further thought will be given to the layer 1 medium simply because they looked to access point. All the while the client wireless device might be a good 30 ft away behind a few obstacles surrounded by or near a set of interfering devices. That is where you would want someone on the ground with a spectrum analyzer.
I’m not saying you need to have a spectrum analyzer everywhere you have wireless because we all know in large networks wireless can grow drastically and certain large networks can see thousands of access points across many different offices, but I merely want to point out a flaw in completely relying on a technology that provides RF Spectrum visibility from the access points perspective, because that is all it is.
Probably the most common 802.11 management frame is the beacon frame. This frame is broadcasted from the AP listing the capabilities of the WLAN, but what exactly is listed? Let’s take a quick look.
Here we can easily make out some of the capabilities of the WLAN, for example the SSID is probably going to be the first thing you notice TestSSID in this case, this is what provides the name of the WLAN. As you know SSID broadcasting can be disabled (Also called Closed System) at which point the SSID field will be blank.
Next we will take a look at the what data rates are supported by this WLAN:
Here you can see the a few supported data rates and the extended supported data rates. This tells us the WLAN is capable of supporting both 802.11b and 802.11g WLAN clients. You’ll notice the Data rate of 5.5 has a (B) next to it, it just so happens 5.5 is set a mandatory data for this WLAN, so if a WLAN client can not support a data rate of 5.5 it will not be able to successfully authenticate and associate to the WLAN.
Next up we are going to at the RSN (Robust Security Network) information for this WLAN.
As you can see from the RSN Information element (IE), this WLAN is 802.11i compliant, showing it’s capable of AES/CCMP which means your WLAN clients will need to support 802.11i/WPA2 with AES in order to successfully communicate with this WLAN.
A few other noticeable features we can find in this beacon frame is the WLAN supports High-Throughput (HT), which is 802.11n (Draft 1 in this case). So this is an 802.11b/b/n WLAN in the 2.4 GHz RF range.
This access point did not have multiple antennas attached to it, which is why the TxBF and ASEL capabilities are currently at 0x0000 and 0x00.
You will also notice in the 6th line down that the WLAN is 802.11e compliant meaning some QoS will be performed, and the line after that states: no non-ERP STA’s, do not use protection this states no 802.11b clients are currently connected only 802.11g WLAN clients are currently connected to this WLAN so protection mechanism’s are not in use.
Now, at the end of the beacon frame you will notice all this Unknown information, as you have probably guessed (or read) this information is vendor specific, which is common for every vendor to have put their own proprietary information within the Beacon frames. The main thing is make sure this extra information does not create its own incompatible issues with older WLAN client devices.
So in conclusion there’s a basic Cisco WLAN beacon frame, it should be a mirror image of the configuration for your WLAN. Looking at the beacon is just a simple way to make sure the WLAN is doing what’s it’s configured to do.
Just some more details on how drastically different wireless networks differ from the traditional wired network is understanding the client connection. Surely we all understand how the wired connection works, we plug in a cable two of the four pairs carry data then speed and duplex setting are auto-negotatiated. However when you look at a wireless client you see an antenna, signal strength, data rate, RSSI, power level, and SNR values definitely a little more to think about.
I’ll start with RSSI, which is the Received Signal Strength Indicator this value is typically shown as a negative dBm value (dB and watt values are a topic for another post). RSSI is the measurement of power in an RF signal, the more power in an RF signal the better the connection quality is. So the closer this value is to 0 the stronger the signal is. So a value of -61 is stronger then a value of -74. Now different vendors do have different scales some vendors will have a max value of -100 while others go higher or lower, of course signals that weak should be avoided (and probably won’t work anyway). So it’s best to get some documentation from the vendor of your client WLAN cards to see the RSSI value range. The value of the RSSI will also play a role in the connection speed, and once again vendor documentation will provide the RSSI value to link speed ratio (and do keep in mind many other factors play a role in the connection speed as well).
SNR is the Signal to Noise ratio, this is how much stronger the wireless signal is compared to the noise floor surrounding the WLAN client. This is shown in a positive dB value. Too much RF noise around the WLAN client will cause collisions resulting in frames being retransmitted thus lowering the throughput of the connection. Try connecting a cordless phone that works in the 2.4 GHz range right next to a b/g access point, the phone can generate enough RF noise to cancel out the wireless signal completely. It’s typically best practice to have the SNR value 20 to 25 dB’s away from the RSSI value. So to go back to our previous example if our RSSI is -61 we would want our SNR value to be around -86, or if our RSSI is -74 we would want the SNR to be -99.
The data rate can be one of many values depending on which wireless standard you are connecting with. Be aware though that wireless is a shared medium so it’s half duplex it can not transmit and receive at the same time. So your actual throughput will be about half of what your client is connecting at. A WLAN device showing a connection of 54 Mbps will really have throughput of maybe 30 Mbps. Throughput can be tested using nice little utility called iperf which is available on both Windows and Linux platforms for free.
The power level is measured in mW and depicts how much power a WLAN device is using to maintain the connection. Its typically best practice to design your WLAN infrastructure so your devices operate at half their max output power. This way if an AP goes down neighboring AP’s can double their output power and maintain the availability of the WLAN.
So the overall signal strength/quality registered by client will be a mixture of all those variables.
Below is a screen shot from the Cisco Aironet Site Survey Utility
Here you will see the RSSI at -50 dBm and noise level of -96 dBm, resulting in an SNR value of 46 dB. This utility will also provide you with the BSSID (MAC Address) of the AP you are connecting to along with the RF Channel, 64 in this case utilizing 802.11a.