Blocks of input data that include control words and a packet of information words are received (203; 252). The packet has a start preceded by ones of the control words and an end followed by others of the control words. The blocks are smaller than the packet. A determination of whether the block consists exclusively of information words is made (204; 253). When the block consists exclusively of information words, a master transition having a first sense is appended (207; 270) to the block to form a frame. When the block does not consist exclusively of information words, the block is condensed (e.g., 227; e.g., 260) to accommodate a TYPE word, the TYPE word is generated (e.g., 224; 266) and inserted (228; 271) into the block and a master transition is appended (213; 273) to the block to form the frame. The TYPE word has a value that indicates one of the following structural properties of the block: (a) the position of the start of the packet in the block, (b) the position of the end of the packet in the block, and (c) the block being composed exclusively of control words. The master transition appended to the block when the block does not consist exclusively of information words has a second sense, opposite to the first sense.