Interview Questions

What are the challenges facing mobile WiFi clients relative to fixed clients?

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What are the challenges facing mobile WiFi clients relative to fixed clients?

Fixed WiFi devices, such as desktop computers, associate with the strongest signal from a nearby AP and establish a connection. Mobile WiFi devices, such as portable inventory devices used in warehouses, tablet PCs used by doctors in hospitals and VoIP handsets, face the constant challenge of moving between APs and utilizing the one providing the best signal at any given time. As the client moves away from the AP and the signal quality degrades, the AP steps down the transmission speed (also known as rate adaptation) to maintain the connection. The client can measure the signal quality from all the APs servicing its area by utilizing the Beacon Frames generated by the APs. At some point the quality of the link will degrade significantly for the client to use a different AP for continued connection. This process is called Roaming. The client will disassociate from the AP and attempt a connection to another AP with a higher quality signal. This process of switching from one AP to another, is rarely instantaneous and an interruption in transmission usually occurs. This interruption can cause frame losses, which may result in increased delay due to errors and re-transmissions. While many protocols can easily recover from such events, other, such as VoIP are more sensitive and this interruption can cause abnormal noises to be heard by the user, or even the call being disconnected.

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