In today’s rapidly developing optical communication, DWDM (dense optical wavelength multiplexing) and OTN (optical transport network) technologies play a vital role in backbone networks. Although they have similarities, there are also many differences. A deep understanding of these differences is important for building efficient and reliable optical communication networks.
1. The concept is different
(1) DWDM: the bandwidth of the optical fiber backbone network “amplifier”
DWDM is a technology that combines multiple optical wavelengths into a single fiber, operating at a wavelength of 1550nm near the window where EDFA can provide gain. To put it simply, the optical fiber is like a highway, and DWDM is like adding more lanes to this highway, allowing more vehicles (optical signals) to travel simultaneously. In a designated fiber, it multiplexes the closely spaced spectral spacing of a single fiber carrier to achieve optimal transmission performance. For example, in inter-provincial trunk networks and provincial backbone networks, DWDM technology significantly boosts network transmission capacity, meeting the demand for long-distance data transmission.
(2) OTN: “intelligent commander” of the next generation backbone transmission network
OTN, based on wavelength division multiplexing (WDM) technology, is a transmission network built on the optical layer, serving as the backbone of the next-generation transmission network. Building upon DWDM technology, it introduces the OTN switching module. While DWDM merely adds lanes to the network, OTN is akin to equipping a highway with an intelligent traffic management system, enabling flexible scheduling of communication services. It is standardized through a series of ITU-T recommendations, addressing issues such as poor service scheduling capabilities, weak networking and protection capabilities in traditional WDM networks. OTN acts like a ‘smart brain’ for optical communication networks, significantly enhancing the network’s service capabilities.
2. The signal format is different
(1) DWDM: simple and direct signal conversion
DWDM technology does not have a corresponding frame structure. During operation, the wavelength of a single signal is converted by a converter into the corresponding wavelength for network communication. Then, through optical technology, these wavelengths are multiplexed and transmitted. This process is akin to transporting goods; DWDM simply loads different signals onto different vehicles (wavelengths) and transports them together on the road (optical fiber), without further sorting or packaging of the goods. It focuses on the efficient transmission of optical signals in the fiber, increasing the transmission capacity by multiplexing different wavelengths.
(2) OTN: fine management of frame structure and scheduling
OTN features a complete frame structure and uses electrical cross-connect technology to manage wavelengths, achieving an electrical cross-connect function similar to SDH. This allows small signals to be combined into larger channels for transmission, much like organizing scattered parcels into larger containers for transportation, thus enhancing transport efficiency. In each wavelength channel of OTN, there are containers of various sizes, similar to those in SDH, which provide the system with the capability to access and process data, significantly improving bandwidth utilization. Moreover, OTN technology can monitor the performance and faults of the network communication regeneration process, demonstrating superior performance in fault detection, akin to a full-process monitoring system that can promptly identify and resolve issues.
3. The networking and protection mechanism are different
(1) DWDM: basic networking and protection mode
DWDM technology primarily uses point-to-point, chain, star, and ring networking configurations. In terms of protection methods, it mainly provides basic protection, primarily through optical multiplexing segment protection, using 1+1 and 1:1 protection on the optical path. This method requires two independent optical cables for routing and does not protect terminal devices. Simply put, it’s like setting up two parallel routes between two points, one for regular use and the other as a backup. When the main route fails, the system switches to the backup route. However, this protection method is relatively simple, while DWDM’s self-healing protection for services is typically implemented in SDH ring networks. In large-scale networks, using only DWDM protection methods can lead to significant service disruptions if a fault occurs. Additionally, due to its relatively simple networking configuration, it has limited adaptability in complex network environments.
(2) OTN: powerful and flexible networking and protection system
OTN networks typically use ring or mesh configurations, offering a variety of protection methods, including line 1+1 protection, wavelength 1+1 protection, customer-side 1+1 protection, ODUk protection, and protection at both the optical and electrical layers. In OTN, data configuration can be used for cross-connections, similar to SDH service configuration, allowing for the simultaneous access of multiple types of services and enabling intelligent service scheduling. It provides hybrid scheduling capabilities at both the optical layer (wavelength level) and the electrical layer (sub-wavelength level). For example, the optical layer supports OADM wavelength scheduling, which allows for the pre-planning of input and output fixed wavelengths or the use of AWG input and output all wavelengths. The required service wavelengths are connected to the service board, eliminating the need for tail fiber jump scheduling. The electrical layer scheduling offers distributed electrical cross-connects at ODU0/ODU1/ODU2/ODU2e/ODU3/ODU4 granularities, with adjacent slot cross-connect capacities reaching 100/200Gbps. OTN is like a complex and intelligent transportation network, featuring multiple types of roads connecting various nodes and equipped with various traffic control and emergency solutions, significantly enhancing the network’s reliability and flexibility. Whether it is the long-distance services carried by the inter-provincial backbone network, such as PSTN, 2G,3G,4G, and Internet, or the large customer dedicated lines between backbone routers within the provincial backbone network, such as NGW, 3G,4G, and IPTV, or the large-granularity services between the local network core router and the aggregation router in the metropolitan core network, OTN can ensure stable service transmission through its robust networking and protection capabilities.
In summary, DWDM and OTN differ significantly in terms of concepts, signal formats, networking, and protection mechanisms. As the demand for communication services grows and network complexity increases, OTN, with its enhanced capabilities and flexibility, is expected to have a broader application in future optical communication backbone networks. However, DWDM still has its value in specific scenarios, such as networks that are cost-sensitive and have relatively simple service requirements. Understanding these differences can help us choose and build efficient optical communication networks based on our actual needs.
