Filed under: Uncategorized | Tags: 802.11h, DFS, extended UNII, UNII, WiFi Radar
Dynamic Frequency Selection (DFS) When additional channels were added to the available 5GHz spectrum there was much concern on how Wi-Fi’s use of these channels may impact devices already operating in these ranges. To address this the 802.11h spec, commonly referred to as Dynamic Frequency Selection (DFS) was created to define a set of procedures to detect and avoid interference with Radar systems operating in the 5 GHz range (UNII channels – 52-64 & 100-140). Note that many people believe that DFS is only required for the extended UNII band, when in fact it also includes the upper 4 channels in UNII (52-64). There are several parts of the specification, however the one that is most visible to users is the ability of an AP to detect and move from a channel that interferes with radar systems. Basically AP’s supporting the standard will designate a ‘Quiet’ period using information in the Beacon frame. This information will tell the stations to set their Network Allocation Vector (NAV) to allow for a quiet period when the AP can listen for transmitting Radar. If radar is detected the AP must alter the channel it is operating on and most have the ability to tell associated stations what channel they will be moving to. This allows stations to re-associate with minimum interruption. APs that do not support DFS are not allowed to operate on the channels where interference occurs; this significantly limits the number of channels available in the 5GHZ spectrum.
Thanks for visiting, The Prof
Filed under: Uncategorized | Tags: 802.11e, AIFS, CWmax, CWmin, EDCF, TXOP, VoWiFi, WiFi Qos
Ok, we all know that real-time, delay sensitive applications such as voice and video require a higher level of service than traditional data communications. Data traffic can take forever to arrive, it may annoy you but it doesn’t impact the communication. Voice and data however are different; if real-time traffic of this type is significantly delayed your communication could become unusable.
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
Filed under: Uncategorized | Tags: 802.11 antennas, 802.11e, DFS, EIRP, site Surveys, Wi-Fi, WiFi roaming, wifi tutorial, WMM
Well all my additional activities associated with Interop have come and gone and now it’s time to get back to posting, however there will be some changes.
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
