encoded using this technique, there would be no way for a decoder to synchronize to a video stream because it would be impossible to obtain the reference images. Because of these drawbacks, a reliable and time periodic reference frame must be used for this technique to be efficient and useful. That reference frame is known as Intra-frame, which is strictly intra coded, so it can always be decoded without additional information. In most designs, there are two types of inter frames: P-frames and B-frames. These two kinds of frames and the I-frames (Intra-coded pictures) usually join in a GOP (Group Of Pictures). The I-frame doesn't need additional information to be decoded and it can be used as a reliable reference. This structure also allows to achieve an I-frame periodicity, which is needed for decoder synchronization. Frame types The difference between P-frames and B-frames is the reference frame they are allowed to use. P-frame P-frame is the term used to define the forward Predicted pictures. The prediction is made from an earlier picture, mainly an I-frame or P-frame, so that require less coding data (≈50% when compared to I-frame size). The amount of data needed for doing this prediction consist of motion vectors and transform coefficients describing prediction correction. It involves the use of motion compensation. B-frame B-frame is the term for bidirectionally predicted pictures. This kind of prediction method occupies less coding data than P-frames generally (≈25% when compared to I-frame size) because the prediction is made from either an earlier frame or a later frame or both them. (B-frames can also be less efficient than P-frames under certain cases, e.g.: lossless encoding) Similar to P-frames, B-frames are expressed as motion vectors and transform coefficients. In order to avoid a growing propagation error, B-frames are not used as a reference to make further predictions in most encoding standards. However, in newer encoding methods (such as H.264/MPEG-4 AVC and HEVC), B-frames may be used as reference for better exploitation of temporal redundancy. Typical Group Of Pictures (GOP) structure The typical Group of pictures (GOP) structure is IBBPBBP... The I-frame is used to predict the first P-frame and these two frames are also used to predict the first and the second B-frames. The second P-frame is predicted also using the first I-frame. Both P-frames join to predict the third and fourth B-frames. The scheme is shown in the next picture: This structure