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Based on the effective contention access and perfect scheduling transmission, an enhanced multiple access protocol - UPMA++ protocol is proposed, which integrates the advantages of random multiple access and fixed allocation multiple access protocols. It allocates the resource dynamically according to the factual traffic requirement, thus reducing the source waste. This protocol adjusts the length of contention access period and polling period effectively to resolve the conflicts between active stations and transmission stations, which active station expects to access the channel quickly and transmission station wants to transmit packet as soon as possible. By analysis, the optimal contention access period length for the given max polling period length is found, and the performance evaluations show UPMA++ protocol has a high throughput (up to 96%), low average message delay and small average message dropping probability.