A transmitting system and a method of transmitting digital broadcast signal are disclosed. The method of transmitting digital broadcast signal includes FEC encoding mobile data to build data frames, dividing the data frames into a plurality of data portions, converting one of the data portions into a plurality of SCCC blocks, performing convolutional coding on mobile data, mapping the SCCC blocks to data blocks and extended data blocks, forming a data group, forming a fixed number of mobile data packets and a first scalable number of mobile data packets, multiplexing the fixed number of mobile data packets, the first scalable number of mobile data packets and a second scalable number of main data packets.