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In the High-Speed Downlink Packet Access (HS-DPA) Iub Transport Network (TN) which connects the Radio Network Controller (RNC) with base stations (Node Bs), congestion control is needed. This paper reviews existing congestion control solutions and describes a novel, non-standardized, cross-layer, window-based HSDPA Transport Network Congestion Control which uses the standardized congestion detection and signalling framework. This solution is based on the idea of extending the Radio Link Control (RLC) protocol with congestion control functionality. This idea originates from the interesting duality between the TCP and the RLC protocols. TCP was originally designed for wired networks, therefore many extensions have been proposed to make TCP to be efficiently used over wireless networks. RLC was originally designed exclusively for handling radio link failures and no RLC extensions have been proposed to make RLC to be efficiently used over congested wired links. This paper proposes such an RLC extension where the RLC transmission window size is used to control the bitrate of the flow. Simulation analysis of this window-based solution is performed and it is shown that the proposed method is a possible solution for and efficient Iub Transport Network congestion control.