Priority mechanisms are used to optimize the network utilization, while meeting the requirements of each type of traffic. The user may generate different priority traffic flows by using the loss priority bit capability and when buffer overflow occurs, packets from the low priority flow can be selectively discarded by network elements. Priority mechanisms can be classified into two categories: time priority and space priority. Time priority mechanisms control the transmission sequences of buffered packets while space priority mechanisms control the access to buffer studied the performance of time priority mechanisms including Minimum Laxity Threshold (MLT) and Queue Length Threshold (QLT) under mixed traffic of realtime and non-real-time packets. Their results show that the First In First Out (no special priority) policy causes relatively high losses for real-time traffic while providing low delays for non-real-time traffic. The converse holds true when priority is given to real-time traffic unconditionally. Space (or loss) priorities propose to provide several grades of services through the selectively discarding low priority packets. This type of priority mechanisms exploit the fact that low priority packets may be discarded in case of congestion, without significantly compromising the source‟s QoS requirements.