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LoRaWAN® Specification v1.0.3

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LoRaWAN 1.0.3 Specification ©2018 LoRa™ Alliance Page 48 of 72 The authors reserve the right to change specifications without notice. 13.2 Slot randomization 1279 To avoid systematic collisions or over-hearing problems the slot index is randomized and 1280 changed at every beacon period. 1281 The following parameters are used: 1282 1283 DevAddr Device 32 bit network unicast or multicast address pingNb Number of ping slots per beacon period. This MUST be a power of 2 integer: pingNb = 2 k where 0 <= k <=7 pingPeriod Period of the device receiver wake-up expressed in number of slots: pingPeriod = 2 12 / pingNb pingOffset Randomized offset computed at each beacon period start. Values can range from 0 to (pingPeriod-1) beaconTime The time carried in the field BCNPayload.Time of the immediately preceding beacon frame slotLen Length of a unit ping slot = 30 ms Table 14 : Class B slot randomization algorithm parameters 1284 At each beacon period the end-device and the server compute a new pseudo-random offset 1285 to align the reception slots. An AES encryption with a fixed key of all zeros is used to 1286 randomize: 1287 Key = 16 x 0x00 1288 Rand = aes128_encrypt(Key, beaconTime | DevAddr | pad16) 1289 pingOffset = (Rand[0] + Rand[1]x 256) modulo pingPeriod 1290 The slots used for this beacon period will be: 1291 pingOffset + N x pingPeriod with N=[0:pingNb-1] 1292 The node therefore opens receive slots starting at : 1293 First slot Beacon_reserved + pingOffset x slotLen Slot 2 Beacon_reserved + (pingOffset + pingPeriod) x slotLen Slot 3 Beacon_reserved + (pingOffset + 2 x pingPeriod) x slotLen … … If the end-device serves simultaneously a unicast and one or more multicast slots this 1294 computation is performed multiple times at the beginning of a new beacon period. Once for 1295 the unicast address (the node network address) and once for each multicast group address. 1296 In the case where a multicast ping slot and a unicast ping slot collide and cannot be served 1297 by the end-device receiver then the end-device should preferentially listen to the multicast 1298 slot. If there is a collision between multicast reception slots the FPending bit of the previous 1299 multicast frame can be used to set a preference. 1300 The randomization scheme prevents a systematic collision between unicast and multicast 1301 slots. If collisions happen during a beacon period then it is unlikely to occur again during the 1302 next beacon period. 1303

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