Fractional T1E1 T1E1

26 SONETSDH AND THE GENERIC FRAME PROCEDURE GFP

2.3.1 The Section, Line, and Path Overheads

Let us consider a simple SONET network consisting of SONET devices A1 to A12, A, B, and B1 to B12 see Figure 2.5. A i , i = 1, 2, . . . , 12 collects user information and transmits it out to A in STS-1 frames. A multiplexes the twelve STS-1 incoming streams to an STS-12 stream, which is then transmitted to B over two regenerators. B demultiplexes the STS-12 stream to 12 STS-1 individual streams and delivers them to the B i , i = 1, 2, . . ., 12 devices so that the STS-1 stream from A i is delivered to B i , i = 1, 2, . . ., 12. The same happens in the opposite direction, but for the sake of simplicity, we only consider the transmission from left to right. The quality of the optical signal deteriorates as it travels through the optical fiber, and so has to be periodically regenerated. The distance that an optical signal can travel without requiring regeneration increases continuously as the optical transmission technol- ogy evolves. Regeneration is done in the electrical domain. That is, the optical signal at a regenerator undergoes conversion from the optical to the electrical domain; then it is processed; and then it is converted back to the optical domain and transmitted out. Electrical regeneration will eventually be replaced by optical regeneration. In SONET, it is assumed that the regeneration of the optical signal is done in the electrical domain. In SONET, a single link with a SONET device or a regenerator on either side of it is known as a section. A link between two SONET devices which might include regenerators is known as a line. The sections and lines are indicated in our example in Figure 2.5. The section overhead in the SONET frame is associated with the transport of STS-1 frames over a section, and the line overhead is associated with the transport of SPEs over a line. SONET is organized into a stack of four layers, all imbedded within the physical layer. The SONET stacks for the SONET network given in Figure 2.5 are shown in Figure 2.6. The lowest layer is the photonic layer, which deals with the optical transmission of the STS frames. The next layer up is the section layer, which manages the transport of STS frames over the photonic layer. It handles section error monitoring, framing, and signal scrambling; it also employs the section overheads. The line layer handles the transport of the SPEs over a line. Its functions include multiplexing and synchronization; it also employs the line overhead. Finally, the path layer processes the end-to-end transmission between the points where the SPE originates and terminates. Recall from the example in Section Section Section Section Section Line A B Regenerator Regenerator STS-1 STS-1 STS-1 STS-1 A 1 A 12 STS-12 ... B 1 B 12 STS-12 ... Line Line Figure 2.5 A simple SONET network.