where Ts(i) is the smoothed through

where Ts(i) is the smoothed throughput measure of segment i. However, the disadvantage of smoothed throughput is that it causes late reaction of the client to large throughput decrease,
which in turn must be handled by having a large buffer size. In [5], we propose a method that has the advantage of both instant throughput and smoothed throughput based methods. Further,
throughput estimate can be computed from sampled throughput values and RTTs as in [6]. More detailed discussion about these throughput estimates can be found in [5]. In addition, a
throughput estimate can be obtained by some other different manners, e.g. based on probing or based on stored data (lookup table) [11]. The buffer-based methods decide the bitrate chiefly based on the buffer characteristics. Usually, these methods take into account the throughput as well. In [8][9][12], the buffer is divided into multiple ranges with B1, B2, B3,Bmax (0 < B1 < B2 < B3 6 Bmax) being the buffer thresholds. In our preliminary evaluation, [8] and [12] are similar and can maintain a smooth bitrate in on-demand streaming. The method of [12] seems more stable because the criterion to maintain the bitrate is dependent on the buffer range, rather
than a delay time of [8]. With the method of [9], if the buffer level is about 35% ∼ 50% of the maximum level, the throughput estimate is the same as the previous throughput. If the buffer is reduced into lower ranges, the throughput estimate is equal to the previous throughput multiplied with a downscaling factor. So, this method is actually more aggressive than
the instant throughput based method, where the throughput estimate is simply equal to the previous segment throughput.
In [13], the buffer level deviation and instant throughput are employed as inputs of a proportional-integral (PI) controller for bitrate adaptation. This method is somewhat similar to [9] in that a deviation-derived factor is multiplied with the throughput to decide the bitrate. The smoothness of this method is enabled by delaying the switching-up operation. However, the problem of this method is that the PI controller has some tuning parameters which are not easy to set.