This paper presents a new end-to-end available band- width estimation tool called IGMPS (Improved Gap Model using Packet Size parameter). IGMPS is a lightweight tool based on a new deterministic model of packet delays de- rived from the Probe Gap Model which uses packet pair technique for actively assessing bottleneck link characteris- tics. In a previous work that presented a detailed analysis of the capabilities and limitations of packet dispersion tech- niques, it is showed that probing packet size parameter has a considerable effect on the measurement accuracy. Based on this insight, the model implemented in IGMPS takes into account this parameter and introduce it in available band- width estimation formula. Through measurements on sev- eral network testbed configurations, we evaluated IGMPS in terms of accuracy, convergence time and intrusiveness and found that IGMPS provides available bandwidth esti- mates with high accuracy. We investigated the effect of sev- eral deciding factors on IGMPS measurement accuracy and found that when the probing packet size is equal or close enough to the cross traffic packet size, IGMPS estimates available bandwidth with a very high accuracy.