Transmission is a fundamental concept in communication networks, referring to the action of moving signals from one point to one or more other points, manifesting as the transfer of information through space. It must rely on signals to convey information.
Transmission systems hold an extremely important position in communication networks and are a basic component of them. Transmission systems are organized in layers, generally including backbone transmission systems (tier-1 trunks), intra-provincial transmission systems (tier-2 trunks), local networks, metropolitan area networks, etc. Each layer forms its own network, with network nodes transmitting information between layers.
The development of photonic devices and Wavelength Division Multiplexing (WDM) and Dense Wavelength Division Multiplexing (DWDM) technologies has not only greatly unlocked the enormous bandwidth capacity of optical fiber but is also driving the formation of all-optical networks.
1. The Emergence and Principles of WDM
1.1 The Emergence of WDM
In recent years, the global communications market has developed rapidly. The rise of new services such as the Internet, high-quality video conferencing systems, and multimedia has led to a surge in demand for high-capacity, high-performance network transmission. Traditional fiber optic transmission systems like SDH and PDH use a “one fiber, one wavelength” approach, and due to limitations in device characteristics, their transmission capacity and expansion methods can no longer meet these demands.
Meanwhile, the waste of fiber optic resources has become a common phenomenon. How to fully utilize these resources to better serve the telecommunications industry? WDM provides an excellent technical direction. The emergence of WDM has brought tremendous changes to the field of transmission.
Dense Wavelength Division Multiplexing (DWDM) is a fiber optic communication technology that enables system capacity expansion by simultaneously transmitting multiple optical carrier signals carrying information (analog or digital) over a single optical fiber. It combines (multiplexes) optical signals of several different wavelengths for transmission, and after transmission, separates (demultiplexes) the combined optical signals in the fiber, sending them to different communication terminals. This effectively provides multiple virtual fiber channels on a single physical fiber, thus saving significant fiber resources.
1.2 Principles of WDM
WDM technology involves transmitting signals using different optical wavelengths within the same optical fiber to achieve multiplexing. At the terminal, a demultiplexer separates the waves of different frequencies, thereby effectively utilizing the optical fiber as a transmission conductor. Below is a diagram illustrating the working principle of optical signal transmission using WDM technology (see Figure 1):
Figure: Working Principle of Optical Signal Transmission Using WDM Technology
A new transmission technology—Dense Wavelength Division Multiplexing (DWDM)—has become the most effective and economical means for fiber optic capacity expansion. With its unique technical advantages, DWDM technology has become a pathway to rapidly, simply, economically, and effectively expand fiber optic transmission capacity, fully meeting the current demands of network broadband service development while also laying a solid foundation for the future all-optical transmission network.
2. Dense Wavelength Division Multiplexing Technology
Dense Wavelength Division Multiplexing is a transmission technology in fiber optic communications. It utilizes the characteristic that a single optical fiber can simultaneously transmit multiple optical waves of different wavelengths, dividing the applicable wavelength range of the fiber into several bands, with each band used as an independent channel to transmit an optical signal of a predetermined wavelength.
DWDM technology aims to fully utilize the enormous bandwidth resources offered by the low-loss region (1550nm) of single-mode fiber. Based on the differing frequencies or wavelengths of each channel’s lightwave, the low-loss window of the fiber is divided into several channels. Lightwaves serve as signal carriers. At the transmitting end, a wavelength division multiplexer (multiplexer) combines signal optical carriers of different specified wavelengths and sends them into a single fiber for transmission. At the receiving end, another wavelength division multiplexer (demultiplexer) separates these optical carriers of different wavelengths, which carry different signals. Since the optical carrier signals of different wavelengths can be considered independent of each other, multiplexed transmission of multiple optical signals can be achieved within a single fiber.
3. Application of DWDM in Backbone Networks
Currently, commercial DWDM systems have data rates of 2.5 Gbps or 10 Gbps per wavelength, with wavelength counts of 4, 8, 16, 32, etc. Products with 40, 80, or even 132 wavelengths in a DWDM system are also available. There are two common configurations.
One type involves placing a Wavelength Transponder (OTU) before the optical multiplexer and after the optical demultiplexer. This configuration is open. It allows the use of any manufacturer’s existing optical transmitter and receiver modules in the 1310nm and 1550nm wavelength regions. The wavelength transponder converts these non-standard optical wavelength signals to standard optical wavelength signals specified within the 1550nm window for transmission in the DWDM system.
Optical component manufacturers like Ciena in the US and Pirelli in Europe use this configuration type. The optical multiplexers and demultiplexers they produce have excellent optical performance parameters. For example, Ciena uses a channel wavelength spacing of 0.8nm, corresponding to a 100GHz bandwidth, providing 16 optical wave channels or paths within the 1545.3~1557.4nm wavelength range. However, they do not manufacture SDH transmission equipment, so unified consideration of system configuration and network management is not possible. The advantage of this configuration is flexible application and strong universality; the disadvantage is the added cost of wavelength transponders.
The other type of configuration does not use wavelength transponders. Instead, it integrates the WDM multiplexing/demultiplexing parts with the transmission system products. This configuration is integrated or converged. This simplifies the system structure, reduces costs, and facilitates the management and operation of both SDH transmission equipment and DWDM equipment on the same network management platform. Manufacturers using this configuration type, such as Lucent, Siemens, and Nortel, are SDH transmission system equipment suppliers. When designing 2.5G 32-channel DWDM systems, they consider compatibility with 10Gbps rates, expansion to 8 wavelengths, 16 wavelengths, or even 40 wavelengths, 80 wavelengths, and hybrid applications of 2.5Gbps and 10Gbps, ensuring continuous online system capacity expansion and smooth transition without affecting communication network services.
4. Testing Requirements for DWDM Systems
The testing requirements for multi-wavelength dense optical WDM systems based on SDH terminal equipment differ significantly from those for single-wavelength SDH systems. Firstly, precise wavelength testing is not critical for single-wavelength optical communication systems. Simply measuring the optical power value with a standard optical power meter can determine if the optical system is functioning normally.
Setting the optical power meter to a specific wavelength value, such as 1310nm or 1550nm, is only used for calibration and correction of the light source emission power test in different wavelength region optical systems. This is because, for a wide-spectrum power meter, the measured optical power value does not vary significantly even if the light source wavelength differs by tens of nm. However, the situation is entirely different for DWDM systems. The system has many wavelengths and many optical paths. The wavelength value and optical power size of each optical path in the system must be measured separately to collectively determine which wavelength and which optical path system has a problem.
Since the wavelength spacing of each optical path is typically 1.6nm (200GHz), 0.8nm (GHz), or even