Egyptian Computer Science Journal Vol. 43 No.2 May 2019 ISSN-1110-2586 -10- Dynamic Open Spectrum Sharing (DOSS) MAC Protocol with Back-off Algorithm Abeer Al Attal, Saleh Hussin, Mohammed Fouad Electronics and communication department, Faculty of engineering, Zagazig University, Egypt abeer_alattal@yahoo.com , SISaleh@eng.zu.edu.eg , fouadzu@gmail.com Abstract Current independent radio spectrum measurements illustrate that a major portion of the licensed spectrum band (e.g. TV bands) is not used. On contrary, a growth of wireless application devices in the unlicensed spectrum bands is increasing frequently, these bands became overcrowded. Therefore, a new technology to make a better utilization of the available spectrum resource is required with a trade of avoiding overlapping interference. This technology is called cognitive radio (CR). Cognitive radio is a type of radios that is able to be programmed and configured dynamically. The CR network can be classified according to infrastructure into (i) A centralized CR network (CCRN): An infrastructure CR network which consists of CR centralized base-station and cognitive users. (ii) A cognitive radio ad-hoc network (CRAHN): Consists of cognitive users only without centralizing base-station. This research will be focused on CRAHN. One of the main cognitive functions is medium access control (MAC) protocol which is a challenge in the performance of the CR networks. The MAC protocols for cognitive radio networks can be classified into (i) random access protocols like dynamic open spectrum sharing MAC protocol, (ii) time slotted protocols like opportunistic MAC protocol, and (iii) hybrid access protocols like synchronized MAC protocol. In this paper, the dynamic open spectrum sharing (DOSS) MAC protocol is presented, analyzed and simulated. Moreover, an enhancement to DOSS MAC protocol will be executed using an implementation of Back-off algorithm.