It was recently shown that IEEE 802.11 MAC has a problem of anomaly. When one user transmits at a lower bit-rate than the others, the performance of all users is considerably degraded. So PackServDiff, packet-based service differentiation, is presented in this paper. In PackServDiff, a user with a higher bit-rate has a higher priority by differentiating contention parameters, namely the minimum contention window, the maximum backoff stage, persistent factor, the maximum contention window and retry limit. A user adapts its contention parameters packet by packet to the current channel conditions. Simulation results show that PackServDiff can resolve the problem of anomaly, and the proposed analytical model is accurate in computing throughput, delay and jitter.