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= Tutorial: Building a Custom Frequency Hopping MAC with the 802.11 Reference Design =

In many communication systems, it is advantageous to "hop" between many different frequencies. These systems, which fall under a broad class of techniques called [https://en.wikipedia.org/wiki/Spread_spectrum spread spectrum], can provide robustness against interference and jamming.

In this tutorial, we will modify the Mango 802.11 Reference Design to enable frequency hopping and we will use the [wiki:802.11/wlan_exp WLAN Experiments Framework] to evaluate the changes. Specifically, we will implement the frequency hopping behavior in two different ways. First, we will use [wiki:./slow_hopping Slow Hopping Approach] that remains interoperable with standard Wi-Fi clients. This design uses existing 802.11 management frame parameters to instruct client stations to hop to a new center frequency. However, these management frame parameters were never intended for fast frequency hopping; they were intended for an AP to shift its network to a different frequency over long time scales (e.g., after the detection of a radar system when using [https://en.wikipedia.org/wiki/IEEE_802.11h-2003 Dynamic Frequency Selection]).

We can improve the frequency hopping rate significantly by breaking compatibility and interoperability with Wi-Fi devices. In the [wiki:./fast_hopping Fast Hopping Approach], we will employ an 'a priori' known frequency hopping schedule at each device in the network rather than explicitly communicate the intention to hop in a management frame. The key challenge here is ensuring that each device remains synchronized despite sharing no absolute time reference.