Connecting the Internet of Things by Anil Kumar & Jafer Hussain & Anthony Chun

Author:Anil Kumar & Jafer Hussain & Anthony Chun
Language: eng
Format: epub
ISBN: 9781484288979
Publisher: Apress

Best Effort Wi-Fi

Wi-Fi is spoken of as a “best effort” protocol. What does this mean and how does this affect the user experience for Wi-Fi?

Earlier, we described the advantages of the original Aloha net: a simple approach for sending data from geographically dispersed terminals without the need for a complex and potentially expensive network management capability. If a device has a packet to send, it sends the packet over the shared radio channel and waits for an acknowledgment from the destination that the packet was received correctly; if the acknowledgment is not received over a specified period of time, the device assumes that its packet was affected by a collision with the packet from another device and resends the original packet.

Wi-Fi versions up to Wi-Fi 5 used this “best effort” approach called Carrier Sense Multiple Access with Collision Avoidance (CSMA-CA): a device with data to send listens to the shared radio channel to see if any other devices were sending packets and if the channel is clear, it sends its message; if another device is sending data, then wait a random time after the channel clears before sending a message.48

Note that Ethernet LAN uses a similar approach called Carrier Sense Multiple Access with Collision Detection (CSMA-CD). In the case of Ethernet, the device is able to monitor the shared medium and determine if its transmitted packet was affected by the packet from another device in the network.

It is apparent that the “best effort” approach, while simple and easy to implement, has limitations compared to a managed network: latency (the time the packet is sent to the time the packet is received correctly at the destination) is variable. This lack of determinism is an issue for time-sensitive applications such as controlling robots and production machinery in factories or augmented reality where data are overlaid on a real-time image. Later we will discuss techniques for mitigating these latency and determinism issues.

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