The use of optical cables as a data transmission medium has been under consideration since 1950s. The first optical cables were laid in 1970s; however, their massive use has started later thanks to the significant decrease of fiber attenuation along with the development of related technologies, namely transceivers. Nowadays, optical cables and related technologies are commonly affordable, while laying of optical cables is considered a good and promising investment.
Although laying of new cables is considered a good investment, below the ground there are thousands of kilometers of cables already, while majority of these cables is already being used, either fully or to a large extent. As an alternative to laying new cables in parallel to old ones, we can employ Wavelength Division Multiplexing technology – WDM. Until recently, CWDM/DWDM technologies were namely employed for long backbone data transmission routes with prices reaching millions or tens of millions CZK. Nowadays these technologies can also be deployed for access networks, for a price that can compete with that of new cable laying. Furthermore, WDM systems can be built and expanded step by step, based on the growing demand for higher transmission speed. For example, we are witnessing gradual transition from 1G to 10G bandwidth, whereas in real life we are often already using 40G and 100G connections. Technologies for 200G channels are newly available, while future bandwidth increases are expected.
As the simplest example of the so-called passive wavelength multiplex we can simply use multiple “color” transceivers at end-point devices and merge them into a single fiber by means of a multiplexer. However, this solution has a lot of hitches, such as different power levels of individual channels, which makes any amplification impossible, plus monitoring of the optical layer is challenging, there are only limited options for expansion, etc. Therefore, we offer an active solution with each individual signal being processed separately (and actively) with the ability of its merging, separating, amplifying, etc. … Active WDM is a true telecommunications solution, rather than an alternative to local network’s optical fibers. Active WDM technology can be used for virtually any network topology, not only (traditional) point-to-point, but also ring, star, chain or matrix topologies.
Recently we have been increasingly facing a problem caused by the fact that many optical fibers are allocated for relatively slow – and therefore no longer efficient – services, offering only meagre benefits to users. Users are increasingly considering the option to merge these services into a single fiber, while maintaining absolute independence of individual services. The best solution to this challenge is offered by muxponders. At the input there are individual data flows from 2Mbps (E1), 1Gbps (1GbE) up to 4Gbps (FC4); these data flows are transferred via the fiber as a single 10G connection, and then split to individual flows at the output. All of this happens at the first layer, and thus mutual correlation of individual services is out of the question. Muxponder is becoming ever more popular element of the active WDM solution. The arrival of 100G and 200G links is related to the availability of muxponders for 10Gbps and 40Gbps individual data flows.
Deployment of active WDM technologies is not necessarily a task for optical fiber owners, as these technologies may be deployed by tenants of unlit fibers. The payback of these capital investments may certainly be highly dependent on the specific location and time; nonetheless we have reached the point when WDM technology should deserve consideration by all users of optical fibers.
This diagram shows a typical WDM deployment between two sites with one branch along the route.
At the central point there are data storage devices, video servers, data servers and the central data element. Each individual service is associated with a separate transponder (TRP) that performs signal modulation to one optical wavelength. As an alternative, we can use a muxponder (MXP) that can provide not only signal conversion, but also multiplexing of several services, even different ones, into a single channel. Individual wavelengths are merged into a single common fiber by means of a passive Mux (M/D). Most manufacturers employ a modular rack chassis with individual components in form of plug-in cards. On the market there are also compact boxes with identical features available. Using optical fiber, the wavelength multiplex travels to sites located hundreds or even thousands of kilometers away. Depending on the distance, signal can be amplified (even repeatedly) by means of an optical amplifier (OA). At the remote side, the optical signal is demultiplexed and converted, identically to the near side.
Transponders, muxponders and multiplexers/demultiplexers are offered in full-duplex versions, and signal conversion is performed in both directions at the same time, and an optical pair is employed for data transmission. Single-fiber data transmission is also possible.
In case there is a branch along the route, we need to separate individual lambdas from the fiber and then insert them to the fiber at the opposite direction. This can be done by means of add/drop multiplexers (OADM) directly at the optical layer. Besides commonly used passive OADMs, manufactured for specific wave-lengths, there is an increasing interest in reconfigurable ROADMs that allow for remote configuration of individual lambdas. This makes system configuration simpler, should we decide to change or add any services.
blue, the colour of rivers, represents the flow of data communications - Green, the indicator of starboard on ships will help with navigation and improve the quality of communications