As mentioned earlier, SDH is compatible, meaning that signals from all rate levels of the PDH series can be incorporated into SDH transmission modules, allowing existing PDH equipment to continue to be used without waste. Simultaneously, SDH is also compatible with various new services incorporated into its transmission modules. This process of loading PDH signals and various new services into the SDH signal space to form SDH frames is called mapping and multiplexing.
Mapping refers to a rate conversion and adaptation. In SDH, mapping means placing PDH signal bytes into precise positions within SDH containers according to certain correspondences. Its essence is to synchronize the rates of various tributary signals with the rates of the corresponding virtual containers, enabling the virtual containers to become entities capable of independent transmission, multiplexing, and cross-connection. For example, code rate adjustment, adding channel overhead, constitutes a virtual container. Mapping is divided into two main categories: synchronous mapping and asynchronous mapping. Asynchronous mapping uses code rate adjustment for rate adaptation; SDH uses both positive/zero/negative code rate adjustment and positive code rate adjustment. Synchronization mapping does not require rate adaptation. Synchronization is divided into bit synchronization and byte synchronization. SDH uses byte synchronization, which can be further subdivided into floating mode and locked mode.
Multiplexing refers to the process of combining several signals byte-by-byte or bit-by-bit into a single signal. SDH uses byte-by-byte multiplexing.
Multiplexing has different implementation methods. For example, in the European standard PDH system, it is specified that 30 voice channels are multiplexed into a 2048 kbit/s primary group signal, 4 2048 kbit/s tributary signals are multiplexed into an 8448 kbit/s signal, and 4 8448 kbit/s signals are multiplexed into a 34368 kbit/s signal, etc. This is the so-called PDH multiplexing structure or multiplexing route. The original ITD-T made strict provisions for the multiplexing mapping structure or multiplexing route of SDH, as shown in the figure. PDH signals of various rates can be mapped to SDH transmission modules according to the multiplexing route.
As shown in the figure, in the multiplexing structure recommended by G.709, the multiplexing path from each PDH rate signal to STM-N is not unique; however, for a country or region, the multiplexing path must be unique. my country's optical synchronous transmission network technology system stipulates that the 2 Mbit/s-based PDH series is used as the SDH payload, and the AU-4 multiplexing path is selected. Its basic multiplexing mapping structure is shown in the figure. As can be seen from the figure, my country's SDH multiplexing mapping structure has three PDH tributary signal input interfaces: 139,264 kbit/s, 34,368 kbit/s,
2,048 kbit/s. Because one STM-1 can only map into three 34 Mbit/s tributary signals, the channel utilization is too low. Therefore, 34 Mbits tributary interfaces are generally not used. In the future, for certain applications, such as international leased services, a 1.544 Mbit/s transparent tributary may be required, which can be mapped in the manner of C-11 to VC-12 to TU-12. For image services and LAN services, the compression encoding of images has not yet been finalized. SDH can provide transmission methods such as VC-2 and VC-2 concatenation.
