s 1 byte. This representation is shown in Figure A.1 besides the serial frame structure adopted for serial bytes transmission. In accordance with this representation, the framing byte appears in the top left-hand box (the byte located in row 1, column 1), which by convention is referred to as byte 1 of the SDH frame. The frame bytes are transmitted bit by bit, sequentially, starting with those in the first row. After the transmission of a row is completed, the bits in the next lower row are trans- mitted. The order of transmission within each row is from left to right. After transmission of the last byte in the frame (the byte located in row N, column M), the whole sequence repeats-starting with the framing byte of the next frame. As shown in Figure A.2, the SDH frame is organized in two distinct parts: the section overhead that collects all the overhead information for the layers involved in the active SDH section and the virtual container (VC) wherein the tributaries are stored. As explained in Chapter 3, each layer of the SDH architecture has its own overhead pro- viding transport capacity for all the signaling needs both among entities of the same layer and with entities of the adjacent layers through the relative access points. All the overhead information is carried out in the overhead area of the frame located in the first H columns, while H-M are devoted to the VC. From now on, to be specific, we will take into consideration the STM-1 frame and the sec- tion overhead, whose structure is shown in Figure A.3. All the other elements of the SDH/ SONET hierarchy have similar section overheads. Appendix A: SDH/SONET Signaling N x M bytes N x M bytes N rows M columns FIGURE A.1 Rectangle representation of the SDH frame. F indicates the framing byte and B the generic signal byte. Section Virtual container N rows overhead M columns- FIGURE A.2 Section overhead and VC in an SDH frame. The STM-1 section overhead is composed of Section overhead-carried in the first nine columns of the STM 1 frame. Multiplexer section