SDH (Synchronous Digital Hierarchy) - is a data transmission system based on time synchronization of the transmitting and receiving device. SDH standards define the characteristics of digital signals, including the structure of frames (cycle) method of multiplexing hierarchy speed digital interfaces and code patterns, etc. 
All information is transmitted to the SDH system containers. The container is a structured data transmitted in the system. If the system generates PDH traffic to be transmitted by the system SDH, PDH data is so structured in a first SDH containers and then is added to a container header and pointers, the result is a synchronous transport module STM-1. Over the network containers STM-1 SDH system transferred to different levels (STM-n), but in all cases once formed STM-1 can only emerge with another vehicle unit, ie holds multiplexing transport modules. 
Since the low-speed signals are multiplexed in PDH frame structure of high-speed SDH signals by byte multiplexing method, their location in the frame of high-speed signal is fixed and determined, or, say, predictable. Therefore, the low-speed signal SDH, e.g. 155 Mbit / s (STM-1) can be directly added or extracted from the high-speed signal, for example 2.5 Gbit / s (STM-16). This simplifies the process of multiplexing and demultiplexing the signal and makes the SDH hierarchy especially suitable for high-speed fiber-optic transmission systems that have high productivity. 
Since the received synchronous multiplexing method and the structure of the flexible display, low-speed signals PDH (e.g., 2 Mbit / s) may also be multiplexed into the signal SDH (STM-N). Their location in the frame STM-N is also predictable. Therefore, the low-speed tributary signal (until the DS-0 signals, that is, one time slot PDH, 64 kbps) could be directly added to or removed from the signal STM-N. Note that this is not the same with the above process add / drop low-speed SDH signal to / from the high-speed signal SDH. Here it refers to the direct addition / release low-speed tributary signals such as 2 Mbit / s, 34Mbit / s and 140Mbit / s to / from signal SDH. This eliminates the need to use a large amount of equipment multiplexing / demultiplexing (interconnected), increases reliability, and reduces the likelihood that the signal quality, reduces the cost, power consumption and hardware complexity. Adding / selection services further simplified. 
For functions operating, administration and maintenance (OAM) in the SDH signal frame structure organized numerous bits. This feature greatly facilitates network monitoring, that is, automatic maintenance. Multiple redundant bits have to be added during the linear coding line performance monitoring, as very few bytes arranged in the signal PDH. For example, the signal frame structure, only the bits in PCM30/32 TS0 and TS16 are used for the OAM. 
Multiple headers SDH signals are 1/20 of the total number of bytes in the frame. This greatly facilitates the OAM function and reduces the cost of maintenance, which is very important because it is a significant part of the total cost of the equipment. 
Basic Transport Module (STM-1) can locate and three types of signals PDH, and signals ATM, FDDI, DQDB. This causes bilateral interoperability and ensures a seamless transition from network to network SDH PDH and SDH from ATM to. To organize these hierarchies SDH signal multiplexes low-speed signals from the various hierarchies in the frame structure of STM-1 signal at the edge of the network (the starting point - the insertion point), and then demultiplexes them to the edge of the network (the end point - the point O). Thus, digital signals of different hierarchies can be transmitted over communication network SDH.