FTP transfers are carried over TCP and are subject to TCP flow control. The flow control in one direction is directly regulated by the flow of TCP Receive Acknowledgements in the opposite direction. A slower flow of Acknowledgements back to the sender, will cause the sender to shrink its TCP transmit window and consequently throttle the speed of subsequent TCP Data transmissions to the receiver. Therefore, a FTP downlink transfer requires enough bandwidth not only on the downlink but also on the uplink, in order to accommodate a fast flow of TCP acknowledgements.
With a Bi-Directional FTP transfer, the uplink channel bandwidth will be shared between the TCP ACKs belonging to a downlink FTP transfer and the TCP DATA transmission belonging to an FTP upload transfer. Similarly, the downlink channel bandwidth is shared between TCP ACKs belonging to the uplink FTP transfer and the TCP DATA belonging to the downlink FTP transfer.
The data rates obtained on the uplink transfer will depend heavily on TCP window settings on both Laptop and Server; and on how the UE/Laptop modem driver prioritizes TCP ACK over TCP DATA on the uplink. A higher priority on TCP ACKs will speed up the downlink transfer, while penalizing the uplink transfer. Similarly a higher priority on TCP DATA will result in a faster uplink transmission, while penalizing the downlink transfer.
Since the mechanism of prioritizing TCP ACK vs. TCP DATA varies from UE to UE, some UE's may show higher DL throughput and a low UL throughput. Yet other UE's will show a lower than expected DL throughput while showing a high UL throughput.