The study focuses on packet aggregation mechanisms on the edge router of an optical network. The device works as an interface between the electronic and optical domains : it takes IP packets coming from client layers and converts them into optical packets to be sent into the optical network. An efficient aggregation mechanism supporting QoS (Quality of Service) requirements of IP flows is presented. A timer is implemented to limit the aggregation delay. Analytical models based on Markov chains are presented in order to study the packetisation efficiency (filling ratio) and the mean time of packetisation for each class. Numerical results show the effect of IP packet lengths distribution on aggregation delay and efficiency. Also, we show the importance of the timer in bounding the delay of transmitted packets without much altering the aggregation efficiency.