Comparison between SDH and PDH (1)

The concept of triple play is believed to have been heard by network administrators for several years. But till now, it has not become a reality. One of the important reasons is the transmission system. In the past, the main transmission system adopted on the Internet was PDH (quasi-synchronous digital transmission system ). With the rapid development of the Internet industry, the amount of information transmitted, exchanged, and processed through the network is growing exponentially. The PDH transmission system is no longer able to meet the needs of existing applications. In this case, the SDH Transmission Mechanism is also applied. In this article, I will introduce the comparison of SDH and PDH Internet transmission mechanisms and their respective advantages and disadvantages to help you select an appropriate transmission mechanism in your future work.

I. Main advantages of SDH

The SDH Internet transmission system is actually evolved from PDH. It can also be used as a second-generation product of PDH. However, it has made many improvements based on PDH. SDH is also widely used in practical work. For example, SDH can be seen in integrated business digital networks and broadband integrated business digital networks. Compared with the traditional PDH transmission mechanism, SDH has the following advantages.

Advantage 1: it can provide a higher ratio of Redundancy

The Network redundancy function means that when the communication is interrupted due to a damaged business channel, the network automatically switches the business to the channel of the standby business, enabling the business to resume normal communication within a short period of time. In other Professional Books, This redundancy function is called a self-healing function. However, this title may cause misunderstanding. This is because the communication is only restored, but the faulty device and the faulty channel are still not restored. Manual debugging and troubleshooting are required.

It seems that there are two mountain roads, one of which is truncated due to the slippery wave. At this time, only another backup mountain road is used. This is just a redundant feature, not self-healing. Manual repair is required for damaged mountain roads. For this reason, I still like to refer to this mechanism as a redundancy mechanism. SDH provides DXC functions (this technology is mainly used to switch services from the primary channel to the standby channel) and redundant channels and devices, thus, the SDH network transmission mechanism has a good redundancy function. Obviously, this advantage can improve the availability and Stability of the network.

Advantage 2: more convenient maintenance

In SDH data frames, some bytes are specially arranged to store the required bytes during management. This wastes a certain amount of bandwidth, But it greatly improves the network monitoring performance and facilitates subsequent maintenance work. In the PDH transmission mechanism, there are not many bytes of management overhead, which makes the transmission monitoring and maintenance quite troublesome. If redundant bits need to be added during line encoding. For example, the OAM function cannot be implemented in PDH. This is obviously not conducive to the follow-up work of the network administrator.

In SDH transmission, the number of bytes of management overhead occupies about 1/20 of the length of the entire data frame. It can be seen that the SDH Transmission Mechanism attaches great importance to subsequent maintenance and monitoring. Because of the existence of these bytes, The OAM management function is greatly enhanced, greatly reducing the maintenance cost of the system. According to relevant tests, the management and maintenance costs of the SDH transmission mechanism may be about 50% lower than that of the PDH transmission mechanism, that is, the management cost can be reduced by half.

Advantage 3: excellent compatibility

There are many transmission signals on the Internet. In addition to synchronous transmission signals, there are also asynchronous carrier-profit signals (such as ATM signals) and FDDI signals. In addition, these signals will exist in large quantities in a short period of time and are the key transmission modes of some applications. Therefore, if SDH is to be accepted by everyone, it must provide good compatibility. Otherwise, it is difficult to be adopted on a large scale.

The SDH Transmission Mechanism provides strong compatibility. When SDH is used to build a transmission network, it can be compatible with the traditional PDH transmission mechanism, that is, the two mechanisms can exist at the same time. In addition, SDH transmission networks can transmit multiple types of signals. Signals such as ATM and FDDI can be transmitted over SDH networks.

When training SDH for some network administrators, the author often compares SDH to a container. In this container, signals of different constitution can be transported. These signals are often packed in cartons before being loaded into containers, so they do not interfere with each other. Then the container will be transported on the main road of the Internet. After arriving at the destination, unload the goods. This means that the SDH Transmission Mechanism no matter what type of signal it transmits (in fact, the SDH transmission mechanism does not know what it carries ). As long as various signals are packaged according to SHD requirements, they can be transmitted in SDH networks. Therefore, the SHD Transmission Mechanism provides good compatibility. The SDH mechanism is now widely used in the integrated business digital network. Perhaps in the future era of triple play, it is also one of the main achievements.

Another manifestation of SDH compatibility is the standardization of various interfaces. Currently, interfaces on the Internet mainly include electrical interfaces and optical interfaces. As we all know, the standardization of interfaces directly determines that devices of different manufacturers can be connected, which is also an important embodiment of network compatibility. The SDH Transmission Mechanism provides Unified Specifications for network node interfaces (including electrical interfaces and optical interfaces), that is, global unification. In this case, SDH transmission equipment can be connected with devices of many manufacturers, thus improving the compatibility of SDH transmission mechanism.