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QoS Overview

Wireless has somehow made it to the human needs pyramid and has become mission-critical for most of the business around the world. Proper RF and QoS design is the only way to ensure real-time apps have acceptable QoE (Quality of Experience).

The wireless environments are half duplex shared medium they are quite susceptible to collisions. One of the biggest challenges for 802.11 networks is that there is no way to detect that the collision even occured.

802.11e was introduced to bring QoS to Wi-Fi

• EDCA was introduced by IEEE 802.11e in 2005, and has been adopted by the Wi-Fi Alliance as Wireless Multimedia (WMM)
• WMM is now a mandatory part of modern Wi-Fi
• 802.11a/b/g are based on DCF (no QoS) • 802.11n/ac are based on EDCA (QoS is supported)

NOTE: The post describes about QoS in general and can be applied to any networking realm.

Latency, Jitter, and Loss

The quality of a network transmission is a result of three things:

■ Latency
■ Jitter
■ Loss

Latency is how long it takes for a packet to be received by the endpoint after it is sent from the source. Latency is also referred to as delay. Asymmetrical tunneling after a Layer 3 roaming event between controllers can introduce delay. Again, symmetrical mobility tunneling is the recommended configuration.

Delay can be broken into two parts:

■ Fixed delay: The time it takes to encode and decode the packets and the time it takes for the packet to traverse the network.

■ Variable delay: Caused by network conditions. If the network is highly utilized at certain times of the day, the variable delay would be higher at those times than others.

Jitter is the value that results from the difference in end-to-end latency between packets. If a packet takes 50 ms to traverse the network and the next packet takes 100 ms, you have a jitter value of 50 ms.

Loss is simply the ratio of packets that are successfully received by the endpoint to those that were sent by the transmitter.

Correct Packet Marking

Depending on the traffic flow of a packet, traffic can be classified or tagged. This can be used to prioritise the packet thereby impacting the traffic flow. Efforts should be made to ensure that QoS policies are applied end to end which means from WLAN Controller > Core Switch Ports > Access Switch Ports > AP Ports.

Upstream and Downstream QoS

As discussed above, it is important to understand the terminology and direction of the traffic flow to and from the AP and the controller. You have both upstream and downstream QoS:

Radio downstream: Traffic leaving the AP and traveling to the WLAN clients.

Radio upstream: Traffic leaving the WLAN clients and travelling to the AP. Enhanced Distributed Channel Access (EDCA) rules provide upstream QoS settings for WLAN clients

Network downstream: Traffic leaving the controller travelling to the AP. QoS can be applied at this point to prioritize and rate-limit LWAPP/CAPWAP traffic to the AP.

Wi-Fi Multimedia

WMM is a certification that applies to both clients and APs. The features are taken from the 802.11e draft.

Each of the four WMM queues competes for the wireless bandwidth available on the channel. Four queues namely – Background, Best Effort, Video, Voice. WMM uses Enhanced Distributed Coordination Function (EDCF) for handling the queue traffic. If more than one frame from different access categories collides internally, the frame with the higher priority is sent. The lower-priority frame adjusts its backoff parameters as though it had collided with a frame external to the queuing mechanism.

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