In this paper we propose an analysis of a slot-based protocol designed for devices of the Internet of Things (IoT). In contrast to other TDMA-based protocols, this scheme uses a random technique to access shared slots, similarly to CSMA protocols. In practice, the transmissions are scheduled in a given back-off window of slots whose duration allows the transmission of a packet and its acknowledgment. Therefore this protocol can be analyzed according to the methodology introduced by Bianchi for the IEEE 802.11 protocol even if the protocol studied differs in many aspects. The model we use is also particular because we succeed in obtaining a Markov model even though the scheme used to send a packet (in a node) may depend on the transmission of the previous packet. We distinguish two protocols; in the first one, at the initial stage or after a successful transmission, the packets are transmitted without any back-off, whereas in the second protocol each transmission is always preceded by the count down of a random back-off. Extensive simulations validate the both protocols models. In addition, the performances of these protocols are compared with those of slotted Aloha and a protocol using a constant backoff window. Index Terms Wireless networks, Model, Collision Avoidance, Shared slots, Medium Access, Markov Model, Slotted Aloha.