We analyze with NS simulations the aggregated packet arrival process into a bottleneck queue generated by multiplexed TCP flows. We explain qualitativel- y the shape of the packet interarrival time distribution. In particular, we provide conditions under which the distribution of the inter packet arrivals is close to exponential and show how this condition scales when the network capacity becomes large. In addition, we analyze the structure of the autocorrelation function of times between packet arrivals. For the case of a packet arrival process close to Poisson, we develop a Fixed Point based model that allows us to compute the packet loss probability and the utilization of the bottleneck link.