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LoRaWAN 1.0.3 Specification ©2018 LoRa™ Alliance Page 13 of 72 The authors reserve the right to change specifications without notice. 324 Figure 4: End-device receive slot timing. 325 3.3.1 First receive window channel, data rate, and start 326 The first receive window RX1 uses a frequency that is a function of the uplink frequency and 327 a data rate that is a function of the data rate used for the uplink. RX1 opens 328 RECEIVE_DELAY1 1 seconds (+/- 20 microseconds) after the end of the uplink modulation. 329 The relationship between uplink and RX1 slot downlink data rate is region specific and detailed 330 in the "LoRaWAN regional physical layer specification" document. By default the first receive 331 window datarate is identical to the datarate of the last uplink. 332 3.3.2 Second receive window channel, data rate, and start 333 The second receive window RX2 uses a fixed configurable frequency and data rate and opens 334 RECEIVE_DELAY2 7 seconds (+/- 20 microseconds) after the end of the uplink modulation. 335 The frequency and data rate used can be modified through MAC commands (see Section 336 5).The default frequency and data rate to use are region specific and detailed in the 337 "LoRaWAN regional physical layer specification" document 338 3.3.3 Receive window duration 339 The length of a receive window MUST be at least the time required by the end-device's radio 340 transceiver to effectively detect a downlink preamble. 341 3.3.4 Receiver activity during the receive windows 342 If a preamble is detected during one of the receive windows, the radio receiver stays active 343 until the downlink frame is demodulated. If a frame was detected and subsequently 344 demodulated during the first receive window and the frame was intended for this end-device 345 after address and MIC (message integrity code) checks, the end-device does not open the 346 second receive window. 347 3.3.5 Network sending a message to an end-device 348 If the network intends to transmit a downlink to an end-device, it will always initiate the 349 transmission precisely at the beginning of one of those two receive windows. 350 1 RECEIVE_DELAY1 and RECEIVE_DELAY2 are described in Chapter 6.