Wireless sensor networks have long been characterized by short range communications. Lately, long range wireless sensor networks such as LoRaWAN (long range wide area network) have emerged, increasing the communication range at the cost of reduced bit rate. This paper focuses on the MAC layer of LoRaWAN and studies communications from a gateway to an end-device. We propose a Markov chain model to estimate the delivery delay of an acknowledged communication (with retransmissions) and study the impact of the number of channels, the data-rate and the number of nodes.