1. A computerized method for joint rate control and scalable stream adaptation for multiple clients concurrently competing for one or more access networks, comprising: receiving, by a server, video requests from a plurality of clients; setting up, by the server, connections with the plurality of clients over a plurality of access networks; dividing, by the server, scalable video streams corresponding to the video requests into pluralities of Network Abstraction Layer units (NALUs), each NALU corresponding to a frame of a video stream, and scheduling the NALUs for transmission, wherein the dividing and scheduling comprises: calculating an estimated truncation distortion for each frame for each of the plurality of clients corresponding to distortion caused by dropping NALUs during transmission; calculating an estimated drifting distortion for each frame for each of the plurality of clients corresponding to distortion caused by imperfect reconstruction of frames used for inter-frame prediction; and minimizing a cost function so as to account for random packet losses during delivery of video streams to all of the plurality of clients, wherein the cost function includes a summation taking into account the estimated truncation distortions calculated for each frame for each of the plurality of clients and the estimated drifting distortions calculated for each frame for each of the plurality of clients; and transmitting, by the server, the NALUs over the plurality of access networks to the plurality of clients based on the dividing and scheduling.