[riot-notifications] [RIOT] ng_at86rf2xx: Implement CSMA en/disable and setting retries (#3138)

Jonas R. notifications at github.com
Mon Jun 1 19:09:51 CEST 2015

> When enabling CSMA it defaults to 4 retries and sets min BE to 3 and max BE 5. I'm not sure if these are sensible defaults, they are the reset values of the transceiver.

The IEEE 802.15.4 (2011) standard also defines them as standard values. However, if one want to send out data at a precise time, without (unpredictable) delay, BE should be set to 0. Figure 11 (p. 23) in the IEEE 802.15.4 standard shows that the backoff-wait is performed always initially before CCA (and of course later again, if the channel was busy).

Since this behaviour could be of interest for other MAC-protocols and is hidden "hidden" in HW in this Atmel device, we should maybe document that somewhere in the driver. I think e.g. Contiki-MAC would expect a min BE value of 0 to avoid any delays before sending.
The wait-time, after which the packet will be send again, is defined as:
 * ```random(2^(BE–1)) * backoffPeriod```
 *  ```backoffPeriod = 320µs```

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