Thanks for visiting, The Prof

On the wired side we use the 802.1p standard to provide traffic prioritization at the media access control (MAC) layer. It defines eight priority levels for network traffic and provides classification and tagging of the packets.

In Wi-Fi we have 802.11e, it provides four provides called Access Categories. These include: Background (AC_BK), Best Effort (AC_BE), Video (AC_VI) and Voice (AC_VO), the standard provided a method to map the 8 wired priorities to the 4 Wi-Fi access categories.

Wireless QoS Prioritization (802.11e and WMM)

WMM (Wi-Fi Multimedia) is a subset of the 802.11e standard and provides far more granular QoS mechanisms to prioritize access to the media (air).

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Using these QoS mechanisms APs (or Xirrus Arrays) is able to contend for access to the shared spectrum based on the priority of the traffic to be sent. Note that this contention is performed on a packet by packet basis.

A text book might say something like ‘the AC determines the priority of access and length of access based on the assigned AIFS, contention window and TXOP.  But most of us don’t think that way, so to you I say the priority level of the traffic will determine how long you typically must wait to transmit wireless data.

Here is the process:

First you listen to see if anyone else is ‘talking’, Wi-Fi is designed to be courteous of other devices. If another device is communicating you wait for the channel to become clear.

Once the channel becomes clear you must wait a short period to allow priority traffic, like the ACK from the last transmission to be transmitted.

Next comes the contention window, this is where the real QoS happens. As the name implies, this is a period of time where different stations ‘contend’ for access to the air. Contention is considered fair, but not even, with higher priority traffic, like voice having a distinct advantage over lower priority traffic like data.

This is all based on slot times; these are just a period of time that will vary between the different Wi-Fi technologies. When a station wants to send data it randomly selects a number of slot times to wait before attempting to transmit. The key to Wi-Fi QoS is that high priority traffic typically gets to pick a lower number and gains access to the air quicker.

There are standard slot time values for the different traffic classes; however vendor may alter these values. Using the following example you can see how the Wi-Fi QoS process will work.

In this case we have 3 devices wanting access to the wireless network: a phone, a set-top box, and a laptop with web application.

They all want to send information at the same time

• First there is a wait period called Arbitration Interframe Space (AIFS), the length of this is based on traffic category priority. For traffic categories with higher priority, the wait period is shorter than for those with lower priority. The voice traffic will win this, however there also may be other devices wishing to transmit voice traffic so additional methods are required
• Next comes the contention window, the device with voice traffic will random select a number of slot times to delay before transmitting (between 0 & 3).After waiting the time required the station can now transmit its packets, assuming no other station picked a lower number and started transmitting first.
• Typically voice will use 0-3, video 0-7 and data 0-15 as the slot times they will select from. As you can see the higher priority traffic has the advantage. If a ‘collision’ of communications were to occur due to 2 stations selecting the same delay period this would be detected by both stations not receiving an ACK to their message. In that case the contention window is doubled (from 0-7 to 0-15) and the process repeats. Doubling will continue to occur until the contention window exceeds 0-1023 or the station gives up for another reason.
• This process continues as long as there is traffic to send.

QoS for Voice Communications (Voice over Wi-Fi)

In addition to the previously described standard QoS methods, some vendors have developed their own proprietary QoS extensions; mostly this is done to provide a higher level of services for voice than is currently available with existing standards. My next post will discuss specific QoS requirements of voice traffic (VoWiFi)

Thanks for visiting,

The Prof

I know I started this with the idea of focusing on Wi-Fi tools, however in many discussions at Interop, I learned there are lots of Wi-Fi topics that many are unfamiliar with. This includes the operation of QoS, Antenna options, power settings, site surveys, DFS, roaming, etc. To that end, I am going to start including short tutorials on these topics to my postings.

As with previous posts, my tutorials will be at a common sense and user level, not designed to a level required to build the chipsets themselves.

The one topic most asked about is how the quality of service can be applied in a Wi-Fi shared environment. That’s a great place to start and will be my next post.

The Prof

iPerf is an easy to use and very popular tool designed to measure communication channel characteristics, I strongly recommend every IT professional have a copy and be comfortable with its use. iPerf operates in a client server manner, generating traffic between two devices and measuring key network criteria. It provides feedback in easy to understand tables and graphs, showing throughput, packet loss, jitter etc. between the client and server. Using this information you are able to tweak (tune) TCP and UDP data settings to optimize connection. iPerf can be run from a command line or a GUI interface (called jPerf).

As mentioned, a primary purpose of iPerf is to allow you to ‘tune’ connections. I am not going to spend time going into depth on the variables of TCP or UDP but there are some basics worth mentioning.

When tweaking a TCP connection the place to start is the TCP window size, the window size determines the amount of traffic a client can transit at any one time.  If too much traffic is sent buffers can be exceeded, too little and the connection is not optimized. iPerf can be used to test transfer rates  across a link while varying the window size. Based on this testing the optimum window size can be determined and the network optimized. A good tutorial for more information on window size is available at http://www.rhyshaden.com/tcp.htm.

UDP traffic does not offer the guarantees of TCP, error or flow control. It is a connectionless protocol and often used for streaming information such as voice or data with no concern (at this layer) for loss. When tweaking a UDP connection the adjustment here is the packet size. UDP testing is useful in determining bandwidth, jitter and packet loss.

As I mentioned iPerf can be used in any IP environment, however of late there is increased interest in it for testing Wi-Fi links. As most are aware, RSSI values identified by many Wi-Fi clients only estimate what physical layer data rates you could expect. They typically do not take into consideration SNR (Signal to Noise Ratio), packet loss, management overhead and other conditions that influence the rating algorithms and define your actual layer 3 throughput numbers. As more and more users are moving to 11n for the increased bandwidth it offers, many want to see what the actual improvements are over legacy 11a/g throughputs.

The below graphic (using the GUI interface) is an example of a Wi-Fi throughput test between an iPerf client (wireless laptop) and an iPerf server (Xirrus Array), as you can see the throughput averaged in the 180Mbps range.  This is actual TCP throughput, not link rate.

To simplify testing and as a value add for their customers, many vendors (including Xirrus) have integrated iPerf in their products.

Getting iPerf:

Iperf is copyrighted by the University of Illinois, except for the gnu_getopt.c, gnu_getopt_long.c, gnu_getopt.h files, and inet_aton.c, which are under the GNU General Public License.

Iperf 2.0.2 installer for Windows – Provided by Ted Fines (fines@macalester.edu) at Macalester College, St. Paul, MN.

To download iPerf, click here: kperf_setup.exe

iPerf tutorials:

http://openmaniak.com/iperf.php

http://pirlwww.lpl.arizona.edu/resources/guide/software/iperf/

Thanks for visiting, in a following post I will discuss some of the other ‘dials’ you can tweak and test with iPerf

The Prof

To get started, if you manage a Wi-Fi network there is some fundamental information you should know about your network, this includes the ability to search for Wi-Fi networks, verify Wi-Fi coverage, detect rogue APs and identify SSIDs policy information. Additional information of value would include the ability to view SSID security settings, locate APs (good or bad) and even perform Wi-Fi performance testing between your laptop and the AP or backend network.

The two components required for a Wi-Fi tool are a Wi-Fi adapter and some form of management software. We all have the first, so what’s most important is the second. Wi-Fi networks generate an enormous amount of useful traffic (as we will see when I discuss packet sniffers), good Wi-Fi tools are able to gather this information and display it in a useful fashion for monitoring, managing and troubleshooting your network.

The first tool I am going to recommend  is the Xirrus Wi-Fi Inspector, at first this might seem a little self-serving however I strongly believe it is worth the 2 minutes it takes to download and the 2 minutes it takes to make operational. I guarantee that within 5 minutes (assuming you’re not on dial-up) of clicking on this link you will be viewing your Wi-Fi environment as you may never have seen before and you might even find a few surprises.

Wi-Fi Inspector allows you to search for Wi-Fi networks, locate APs, verify coverage and even check your Wi-Fi performance.

The following graphic shows the main dashboard, it is configurable to your liking.

The main components of this tool are:

Radar Window

The Radar displays a dynamic view of the Wi-Fi networks (SSIDs) in the local area. The names of the networks and a corresponding dot are displayed in a circular display with their relative distance from the center of the radar based on the strength of their Wi-Fi signal. The orientation of networks around the circle does not indicate actual angle of location, but names are spread for readability. The Radar display can be expanded to large size by selecting “Radar” from the Layout section of the menu.

Connection Window

The Connection window displays detailed information about the current Wi-Fi connection. Wireless information – SSID, BSSID, Channel, Signal level, and Network Mode – is displayed on the left. Address information associated with the connection is displayed to the right. The address information includes the MAC address of the Wi-Fi adapter, the laptop’s IP address, the DNS server address, Gateway address, and External IP address that the system is using to access the Internet.

To the far right of the Connection window, two buttons provide functions to manage your laptop’s Wi-Fi connection. The Connect/Disconnect button opens the Windows Wireless Network Connection window from which a Wi-Fi network can be chosen to connect or disconnect

Networks Window

The Networks window displays a dynamic table of all Wi-Fi networks (SSIDs) that the laptop is detecting in its area. This table updates periodically based on the Polling Interval setting in the Settings window. The row of the currently connected Wi-Fi network (if connected) is highlighted in orange.

Type of information presented  includes: SSID, BSSID, Signal level , Network Mode ( 802.11b, 802.11a, 802.11g, or 802.11n), Channel, Frequency, Default Encryption (None, WEP, TKIP, or AES-CCMP), Default Authentication ( The default authentication type used on the network, whether

Open, WPA/PSK, or WPA2/802.1x.), Vendor/Manufacturer and Network Type (Access Point or Ad Hoc).

Signal History Window

The Signal History window displays a graph of Wi-Fi network signal levels over time, multiple networks can be graphed simultaneously.

Additional features include speed tests, link tests and even the ability to help you locate APs.

The Wi-Fi Inspector is available for downloading at http://www.xirrus.com/library/wifitools.php.

Please note, this tool is 100% free to download, no cost, no account to create and no pages of information to fill in before you can access the tool. Feedback and recommendations for additional features are appreciated, contact me directly at Perry.Correll@Xirrus.Com

My next post will examine another ‘Free’ Wi-Fi tool, iPerf.

Thanks for visiting.

The Prof

The tool is Chanalyzer for Wi-Spy. Wi-Spy is the Spectrum Analyzer component, while the Chanalyzer software takes the Wi-Spy data and displays it in multiple graph formats that are useful for analyzing and troubleshooting RF environments.

The graphic above displays the 2.4 band; with the specific channels indicated across the bottom. Currently I have an AP with channels 1, 6 and 11 enabled. It is quite easy to see the 3 primary Wi-Fi channels (1, 6 & 11) indicated in blue and how they span far beyond just their center frequency. In this screen capture the green indicates interfering noise and is coming from a 2.4 GHz wireless phone I used to demonstrate potential interference sources and to give a sample of what this tool can show. As you can see the phone communication pretty much steps all over channel 1, resulting in the loss of 33% of you available bandwidth (note the green spike on the left 1/3 of the channel one spectrum). Interference from other wireless devices is yet another reason to migrate to 5GHz.

In the second graphic I show the interference resulting when radios are simultaneously operating on channels 1 & 4. I used Wi-Spy to monitor the RF environment while using a signal generator (AirHORN) to generate traffic on channel 4. The white shaded area on the left shows the frequencies used by channel 1. You can see the energy from the signal generator (solid red bar at top) ‘spreads’ out in both directions and impacts the ‘valid’ traffic on channel 1 (almost 1/3 of channels 4s energy is in the channel 1 space). This clearly shows that simultaneous traffic on channel 1 and 4 will result in interference between the channels, further resulting in reduced network performance

Today I just used Wi-Spy/Chanalyzer to demonstrate interference, in future post I will discuss the product in greater depth.

If you are interested in learning more about this product go to MetaGeek. Wi-Spy offers a 2.4 GHz as well as a 2.4/5 GHz product and comes with the Chanalyzer software and currently there is a special to get the product for under $200.

The Prof

Comment: So why does 2Wire continue to choose all/any available channels in 2.4GHz? I asked them and they seemed to think it was a good idea. I tried to show them it was not but showing retry rates and packet loss and they shrugged.

I can’t really say why you received that answer. Just note that there are many Wi-Fi vendors, consultants and  organizations (vendor neutral) out there and all have their own views on Wi-Fi technology, however the one topic there is general agreement on is limiting the 2.4 GHz band to 1, 6 & 11. My  next blog will use some basic Wi-Fi tools to demonstrate the interference problem

Comment:  Does this mean 11n should be deployed at 5Ghz? Let’s say I want to deploy more than four 11n channels in a confined area — is that possible?

Yes, go with 5 GHz. Even though 802.11n supports both 2.4 GHz & 5 GHz it is strongly recommend to move to 5 GHz when you migrate to 11n. This is for several reasons, cleaner spectrum, more available channels (3 vs. 24)  and the ability to to use channel bonding. Channel bonding is available in the 2.4 GHZ band but with only 3 channels available you won’t be doing much bonding. Also the bottom line is most administrators move to 11n for greater bandwidth, why move to a technology with only 3 usable channels vs one that offers 24.

First – Individual channels in the 2.4 GHz band are only spaced 5 MHz apart (2.412 GHz, 2.417 GHz, 2.422 GHz, 2.427 …. GHz, 2.484 GHz)

Second – 802.11b/g channels are actually 22 MHz wide (spread spectrum)

Third – When we say channels 1, 6 & 11 we are actually identifying the center frequency for each channel. As a result the Wi-Fi signal will ‘spread’ 11 MHz on either side of the center frequency. This means a signal transmitted on channel 6 will actually extend from channel 4 through 8. This is why channels 1, 6 &11 are spaced 25 MHz apart to avoid interfering with each other, see graphic below

The result of deploying overlapping channels is interference between radios, resulting in reduced throughput and poor overall network performance.

A common belief is if you physically separate the APs far enough apart, the overlap (interference) between channels 1, 4, 8 & 11 will not be enough to impact performance. This thinking focuses on the APs and totally overlooks the fact that the majority of interfering radios will be those of the clients, whose physical locations are significantly harder to control.

My next article will discuss some basic RF and Wi-Fi tools (low cost/Free) that you can use to clearly demonstrate the performance impact of deploying APs on overlapping channels. 

Oh, and by the way, all 24 channels are available for simultaneous use in the 5 GHz ,so it’s a good reason to migrate in that direction as you start to prepare for 11n..

The Prof

A final point, a device, a tool or piece of software is only useful if it is actually used, many times tools are so difficult to use and understand, that even if their capabilities are extensive, most will never know because they don’t have the time to learn it .

I strongly solicit your comments, thoughts and recommendations for products that have been or should be reviewed. What I won’t do myself, or allow in other comments is  any competitive marketing, there are enough other sites for those discussions.

Important note: In the following weeks and months I will review many Wi-Fi tools and a couple of these will be available from Xirrus. These applications are freely available from multiple sites and require no contact information to obtain them.

Next week: The first discussion will be “why there are only 3 channels available in the 2.4 band.” Now I know some may think this is topic passed discussion, however time and time again I get questioned or even challenged on the topic