A method and system of transmitting data frames from a sending unit (10) to a receiving unit (12) in a data transmission network comprising at least a backbone (14) wherein the data are transmitted over high speed links enabling long Maximum Transmission Units (MTU) between an ingress node (18) connected to the sending unit by a first access link (16) and an egress node (22) connected to the receiving node by a second access link (20), with at least one of the first and second access links being a low speed access link requiring the data frames to be segmented into short MTUs between the sending unit and the ingress node and between the egress node and the receiving unit. A plurality of consecutive segmented data frames (28) belonging to the same flow of data transmitted from the sending unit to the ingress node are assembled by the ingress node into an assembled data frame (30) corresponding to the long MTU, the assembled data frame is transmitted over the backbone from the ingress node to the egress node at a high speed authorized by the backbone links, and the assembled data frame is de-assembled into consecutive segmented data frames (32) corresponding to the short MTUs by the egress node before being transmitted to the receiving unit.