The probability A of having an erroneous CoAP packet at the application layer including the retransmission at MAC layer is derived as A. Here, r is the maximum number of retransmission allowed for the MAC layer (default value is 5). However, the entire IEEE 802.15.4 MTU is 127bytes. Hence, the CoAP packet should be fragmented. If a CoAP message is comprised of f fragments, the probability   of having an erroneous fragment of a CoAP message is given by: