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Comparison of PDH and SDH

PDH (Quasi-Synchronous Digital Transmission System) is the main transmission system used in the Internet. However, with the rapid development of the Internet industry, the amount of information transmitted, exchanged, and processed through the network has risen at a geometric level. The PDH transmission system can not meet the existing applications, so the SDH transmission system came into being.


I. the advantages of SDH

SDH evolved from PDH. As the second generation of PDH, SDH has made many improvements and is more widely used in practical work. Compared with the traditional PDH, the advantages of SDH are mainly the following:

Advantage 1: provide redundancy function

The redundancy function of the network means that when the traffic channel is damaged and the communication is interrupted, the network will automatically switch the service to the channel of the standby service, so that the service can resume normal communication in a relatively short time. In some professional books, this redundant function is called self-healing. However, it must be clear that here only the communication is restored, the failed device and the failed channel are still not recovered, and manual debugging and troubleshooting are required.

Advantage 2: Maintenance is more convenient

In the data frame of the SDH protocol, some bytes are specially arranged to store the bytes required in the management process. It greatly improves the monitoring performance of the network and facilitates subsequent maintenance work. The number of bytes of management overhead in the PDH is not very large, which makes the monitoring and maintenance of the transmission more troublesome. The SDH transmission mechanism attaches great importance to subsequent maintenance and monitoring, greatly enhancing the OAM management function, and greatly reducing the maintenance cost of the system. According to the relevant tests, the management and maintenance cost of the SDH transmission mechanism is about 50% lower than that of the PDH transmission mechanism, that is half of the management cost can be saved.

Advantage 3: Provides good compatibility

There are many transmission signals used in the Internet, such as synchronous transmission signals, asynchronous ship mode signals, FDDI signals etc. Moreover, these signals will still exist in a short period of time and are the key transmission modes for some applications. Therefore, if SDH is to be accepted by everyone, it must provide good compatibility, otherwise it will be difficult to adopt it on a large scale.

SDH compatibility is reflected in network transmission. When SDH is used to construct a transmission network, it can be compatible with the original traditional PDH transmission mechanism, that is the two mechanisms can exist at the same time. Regardless of the type of signal it transmits, as long as the various signals are packaged as required by the SHD, they can be transmitted over the SDH network,such as ATM and FDDI can be transmitted on the SDH network.

Another manifestation of SDH compatibility is the specification of various interfaces. The SDH transmission mechanism uniformly regulates the node interfaces (including electrical interfaces and optical interfaces) of the network, that is global unification. In this case, the SDH transmission device can be interconnected with devices of many manufacturers, thereby improving the compatibility of the SDH transmission mechanism.

Advantage 4: Reusable structure with wide adaptability

The PDH only supports point-to-point connections and forms a segmental network. It can only perform segment protection. It cannot implement ring protection of multiple routes for unified work, and lacks flexibility of network topology. SDH adopts synchronous multiplexing mode and flexible multiplexing mapping structure. It can use software to directly separate high-speed signals from low-speed tributary signals at one time, which is called one-step multiplexing. Simplify the process of going up and down and improve network transparency.

II the disadvantages of SDH

Disadvantage 1: Reduced bandwidth utilization

In the SDH transmission mechanism, about one-twentieth of the data frames are used as management signals, and the number of signal frames used to transmit user data is significantly reduced in fixed-size data frames. Therefore, while improving maintainability, the use efficiency of bandwidth is reduced. In the PDH transmission mechanism, there is no extra overhead byte of the OAM function, although its management and maintenance advantages are reduced, but the bandwidth usage efficiency is improved.

Disadvantage 2: Lack of security

There are some vulnerabilities in the design and development of software, and these vulnerabilities will become targets of Trojans and virus attacks. OAM is also a software. OAM is adopted in the SDH transmission mechanism as a tool for subsequent performance monitoring and maintenance, which brings great hidden dangers to the security of the transport layer. Therefore, when deploying the SDH mechanism, network administrators may need to take additional security measures to ensure the security of the OAM software.

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