The long distances made possible by advances in technologies such as optical amplifiers, dispersion compensators, and new fiber types, resulted in the initial deployment of DWDM technology in the long-haul networks. Once these technologies became commercially viable in the long-haul market, it was the next logical step to deploy them in the metro and, eventually, in the access networks. Besides, as we know, the networks are now being asked to carry heavy data loads, deliver streaming video and provide internet access to a rapidly growing numbers of business and private users, therefore an enormous amount of bandwidth capacity is required to satisfy the service demand by customers. As a result, DWDM metro networks emerged at the right moment.
What Is a Metro Network?
The overall network infrastructure can be subdivided in three domains: long-haul network, metro network and access network (see picture below). Long-haul networks are at the core of the global network. Dominated by a small group of large transnational and global carriers, long-haul networks connect the metro networks. At the other end of the spectrum are the access networks. These networks are the closest to the end users, at the edge of the metro network. Between these two large and different networking domains lie the metro networks. The metro network is a network running across the city, or it may span a metropolitan area wherein several cities are connected on close proximity. In a typical scenario this might be in a range of 200-400 kilometers.
There is a natural tendency to regard the metro network as simply a scaled-down version of the long-haul network. But there are some differences between them. Usually, networks serving the metropolitan area encompass shorter distances than in the long-haul transport networks. Besides, network shape is more stable in long-haul, while topologies change frequently in the metro network. Many more types of services and traffic types must be supported in metro networks, from traditional voice and leased line services to new applications, including data storage, distributed applications, and video. The long-haul, by contrast, is about big pipes. In fact, since the metro network lies at a critical juncture, on the one hand, it must meet the needs created by the dynamics of the ever-increasing bandwidth available in long-haul transport networks. On the other hand, it must address the growing connectivity requirements and access technologies that are resulting in demand for high-speed, customized data services.
Why Use DWDM in Metro Networks?
In the past, SDH (Synchronous Digital Hierarchy) which has a North American equivalent named SONET (Synchronous Optical Network) has been the core component in a metro network. The problem with this technology is that it is highly structured, has very specific interfaces, offers limited bandwidth, and does not offer the versatility that’s demanded by enterprises. It is also very difficult and complex to deploy Ethernet over SDH. It requires high capital expenses and also higher operational costs.
DWDM technology has emerged as the alternative to SDH in metro networks. A DWDM environment offers versatility and flexibility and requires enterprises and carriers to deploy more sophisticated technology. A DWDM metro network is typically bit-rate protocol independent and allows you to carry native databases. This lowers capital expenses and operational expenses, and offers greater sophistication over SDH-enabled metro networks. Another important development, which is visible, is the commonality of Ethernet. We have Ethernet in VoIP (Voice over IP), Ethernet for videos etc. Instead of relying on a SONET infrastructure, which is optimized for voice and expensive to deploy, metropolitan Ethernet providers use a combination of fiber, DWDM and Ethernet boxes. The combination of DWDM equipment and simple Ethernet gear is much less expensive than SONET equipment, enabling metropolitan Ethernet providers to offer cut-rate pricing.
DWDM Metro System Components
In this part, we may only analyze the devices used to combine and separate the various wavelength channels. They include multiplexers, demultiplexers, optical add/drop multiplexers and devices based on a new emerging technology called optical cross connects (OXC). As DWDM technology achieves tighter and tighter wavelength spacing, the requirements and performance specification for wavelength selective components become increasingly demanding.
Multiplexer and Demultiplexer
A multiplexer is used to combine different wavelength signals onto a single optical fiber. We could consider a standard broadband coupler as a MUX. Instead DWDM MUX is a device used to combine multiple wavelengths onto a single fiber while keeping the signal loss as small as possible. The demultiplexer is a device that separates a multiple wavelength signal into its individual wavelength components. It is important for the DEMUX to have low insertion loss, but it is much more important for it to reject the unwanted wavelengths so the channel receiver will have a signal with a high S/N ratio. As channel spacing becomes smaller and smaller, the wavelength selectivity of the DEMUX becomes of primary importance.
Optical Add-Drop Multiplexer (OADM)
OADMs are the most critical enablers of metropolitan optical networking and permit selective adding or dropping of a precise number of optical wavelength channels of any rate, format or protocol into the DWDM network. The most straightforward configuration for implementing a fixed OADM is the following. All the N wavelength channels in the aggregate traffic are demultiplexed into their wavelength tributaries. The tributary signals are then multiplexed again to form the outgoing aggregate signal. By using these two devices, one can access the desired tributaries and add or drop any wavelengths.
Optical Cross Connect (OXC)
The OXC is a device used to provide selective routing of DWDM channels. It uses optical switches in combination with other components that can be based on fiber-switching technology or on wavelength-switching technology. The primary purpose for using an OXC is to keep all of the routing at the optical transmission layer without having to convert O/E and regenerate transmission signals. One of the emerging technologies in this field is the arrayed waveguide grating (AWG). An optical signal is routed to a specific output depending on the AWG input and on the wavelength it is using. Adding a control port allows selective wavelength routing.
Optical Amplifier (OA)
The advent of wideband fiber optical amplifiers has played a major role in enabling the practical implementation of metro optical networks. Since most wavelength add/drop elements are passive and present loss, the OA provides additional power gain to meet the losses associated with these devices. In some architectures the OAs must amplify the “add” and “drop” channels as well. Since the OA compromises network transparency, they must be used sparingly in the metro network.
Most part of current networks have been built with G.652 single mode (SM) fibers. G.652 still provides the best balance between non linearities and dispersion accuracy, being in this sense the most promising option for access network to carry DWDM signals. It is optimized to work in the second window and minimizes the non-linear four wave mixing effect. This fiber presents low dispersion and attenuation. It can span over hundreds kilometers carrying signals up to 10 Gbps. Other fibers are the G.653 dispersion shifted (DS) and G.655 Non Zero Dispersion (NZD).
In this article the introduction of DWDM technology into the metro networks was introduced so as to fulfil the increasing bandwidth demand. The definition of the metro network was first discussed, then we explain the advantages of the DWDM metro network solution. Finally, the components needed to build a DWDM metro network system is provided. In the future, DWDM will continue to provide the bandwidth for large amounts of data. In fact, the capacity of systems will grow as technologies advance that allow closer spacing, and therefore higher numbers, of wavelengths.