1. Application of SDH Optical Fiber Communications in Railway Communication Systems
The use of SDH optical fiber communications in railway communication systems has resolved issues present with PDH fiber communications and made further advancements, enhancing the stability and fluidity of the railway communication systems. Through the self-healing protection of ring networks formed by SDH equipment, communications can automatically be restored promptly using the self-healing network when the primary signal in the transmission medium is interrupted. Compared to PDH technology, SDH technology has four significant advantages: stronger network management capabilities; unified bit rates and interface standards that enable interconnectivity between devices from different manufacturers; the innovative concept of “self-healing networks” that allows for quick recovery from major signal interruptions; and the use of byte interleaving technology, which simplifies the signals of various network branches. Given these benefits, the development plans for the railway communication network have explicitly called for the prioritization of transport networks based on Synchronous Digital Hierarchy (SDH). For example, the xx Railway has set up an SDH 2.5Gb/s (1+1) optical synchronous transmission system for long-distance networks based on 4 of the newly laid 20-core optical fibers. At appropriate points along the railway, SDH 2.5Gb/s ADM equipment has been installed and connected to the access layer transmission equipment via a 622Mb/s optical port, serving both uplink and protection roles. Additionally, an SDH 622Mb/s (1+0) optical synchronous transmission system has been established using 2-core fibers as the local relay network, with new SDH 622Mb/s equipment installed at relevant points along the railway and at newly opened intermediate stations and lines.

2. Application of DWDM Optical Fiber Communications in Railway Communication Systems
DWDM optical fiber communication technology utilizes the broadband and low-loss characteristics of single-mode fibers to compose carriers of multiple wavelengths, allowing multiple carrier channels to transmit simultaneously on the same fiber. This capability significantly reduces the total amount of fiber required for a given data transmission capacity. With DWDM technology, a single fiber can transmit data flows up to 400Gb/s. The most notable advantage of DWDM technology is that it is protocol-agnostic regarding transmission speeds; networks based on DWDM technology can use IP, Ethernet, ATM, and other protocols for data transmission, handling data flow rates from 100Mb to 2.5Gb per second. This means that networks based on DWDM technology can transmit various types of data streams at different transmission speeds on the same laser channel. Currently, DWDM technology is widely used in domestic railway communication networks, with the Shanghai-Hangzhou-Zhejiang-Jiangxi railway being the first in China to use a DWDM optical fiber transmission system. Additionally, DWDM optical fiber transmission systems are also being constructed and utilized on railways such as the Beijing-Kowloon and Wuhan-Guangzhou lines. Taking the Beijing-Kowloon Railway as an example, it uses an open DWDM system and equipment compatible with various working wavelengths and manufacturers’ SDH devices. With 16 channels and a basic channel speed of 2.5Gb per second, the system uses 2-core G.652 single-mode fibers from the 20-core cable of the Beijing-Kowloon line for unidirectional single-fiber transmission, meaning the same wavelength can be used multiple times in both directions. The optical interface meets the standards of the ITU-T G.692 protocol.