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Designing for other wireless features



Quality of Service (QoS)
As mentioned previously in other blogs with voice and video communication, QoS is an essential technology to implement. QoS needs to be end-to-end, so that means you also need to implement QoS at the wired network. There are four types of QoS:
First-in-First-out (FIFO) which means packets leave the device in the order they arrive.
Fair Queuing means an equal amount of time for each packet.
Class-based queuing means that packages get a priority class—the highest get transferred first.
Reservation means that packets that belong to a reservation queue grant some specific percentage of the throughput.

802.11 QoS is defined in the 802.11e standards. On the wired network, you have terms as Class of Services (CoS), Type of Service (ToS) and Differentiated Services Code Point (DSCP) markings where CoS (802.1p) is Layer 2 and ToS and DSCP Layer 3. The CoS values are 0 to 7.
0 – Best Effort data
1 – Medium Priority data
2 – High-priority data
3 – Call signalling
4 – Video conferencing
5 – Voice
6 – Reserved
7 – Reserved

CoS uses a 3-bits field, so voice will be 101 since that is the binary notation of 5. DSCP uses a 6-bit value. For voice, this is 101000 which is 40 in binary. Sometimes vendors use their own number; however, they always should map to a CoS value. 802.11e is a Layer 2 solution and also needs to be converted to 802.1p or DSCP values. The basic enhancements for 802.11 frames are:
AC_BK – Background (lowest priority)
AC_BE – Best Effort (lowest priority)
AC_VI – Video (middle priority)
AC_VO – Voice (highest priority)
The higher the priority, the lower the contention value is that is used for who is allowed to talk first on the medium. For example, AC_VO has a contention window between 3 and 7, while AC_BE and AC_BK have a window between 15 and 1023. Wi-Fi Alliance created Wireless Multimedia (WMM) as certification to validate the Enhanced Distributed Channel Access (EDCA) that is introduced in 802.11e. The wireless LAN controller converts the DSCP values to 802.1Q user priority (UP) tags or to 802.1p tags.

Load Balancing
In the most businesses, we do not want any downtime of the wireless network. When upgrades of the firmware are needed, it is hard to come to an agreement on the change window. With load balancing, it is possible that the clients pick a least loaded access point, but this needs to still be still access point with good signal strength. In other words, we need to design the network so that there is always a back-up access point available.

Airtime Fairness
With moving to newer PHY technology we have higher data rate STAs. Those need less time in the air/medium (airtime). The slower data rate clients take longer amounts of time to transmit the same amount of data than the higher data rate clients (5.5 Mbps for 802.11b vs 300 Mbps for 802.11n). Airtime Fairness is a proprietary feature that seeks the balance in airtime instead of the number of frames. This will improve the performance.

Band Steering
For high density networks, it is possible to use the band steering technique. As the name says, you steer the clients to another band. When clients support 2.4 GHz and 5 GHz bands and the 2.4 GHz band has reached its limit, the access point can ignore the 2.4 GHz requests and respond only to the 5 GHz requests. It is also possible that the technique will be used when the 2.4 GHz is not full. You can configure that all 2.4 GHz requests get ignored and respond only to the 5 GHz where more channels are available and higher densities can be achieved. The 2.4 GHz and the 5 GHz radios of the clients have the same MAC address, so the access point knows that it is the same client. When the client does not connect to the 5 GHz, the access point may respond to future attempts for the 2.4 GHz, so the client can still join the network.