Today's TDM-based transport networks have been designed to provide a guaranteed level of performance and reliability for major voice and line based services. Proven technologies, such as SDH, have been widely deployed, providing high capacity transport, scalable at gigabit per second rates, for voice and leased line applications. SDH self-healing rings provide service level recovery within tens of milliseconds of network outages. All of these features are supported by well-established global standards allowing a high degree of multi-vendor interoperability.
The current network
Unlike current TDM-based transport networks (and to some extent with ATM networks), “at best” IP networks are generally lacking means to guarantee high reliabilityand predictable performance. The best service provided by most legacy IP networks, with unpredictable delay, jitter and packet loss, is the price paid to achieve maximum link utilization through statistical multiplexing. Link usage (e.g. number of users per unit of bandwidth) has been an important figure of merit for data networks, as links are typically carried on leased circuits through the TDM transport network. .
Given the inherent burst nature of data traffic, fixed bandwidth channels of TDM transport may not be an ideally efficient solution. However, this inefficiency has traditionally been viewed as less important than the network reliability and congestion isolation features of a TDM-based transport network provider.
The growing demand for different data servicesencies and high bandwidth is now a challenge. Dual-architecture model of TDM-based transport and best-effort packet networks. It is not cost effective to extend the utility of best effort networking by over-provisioning network bandwidth and keeping the network lightly loaded. This approach cannot always be realized or guaranteed due to irregular growth in demand and is a particular problem for the area of network access, which is most sensitive to the economic constraints of underutilized facilities. As a result, in general, data service providers today do not have the support of network infrastructure to provide customer specific differentiated service guarantees and service level agreements. correspondents.
Next Generation Network
Next Generation Network Architectures for aCost-effective, reliable and scalable evolution will use both an enhanced transport network and service layers, working together in a complementary and interoperable manner. These next-generation networks will dramatically increase, and share as much as possible, the capacity of the main network infrastructure, and provide sophisticated service differentiation for emerging data applications.
The transport network allows service layers to operate more efficiently, freeing them from the constraints of the physical topology to focus on the sufficiently large challenge of meeting service requirements. Therefore, in addition to the many service layer enhancements, optical transport networking will provide a unified and optimized layer of high capacity and high reliability bandwidth management, and create optical data networking solutions. for data services oflarger capacity with guaranteed quality.
Optical transport network: a practical vision
The visions of optical networking have captured the imaginations of researchers and network planners since the rapid and successful commercialization of the WDM. In the original vision of the optical transport network, a flexible, scalable and robust transport network emerges, responding to an increasing variety of customer signals with equally varied service requirements (flexibility, scalability and survivability associated with bit rate and protocol independence).
The Promise of a Transport Infrastructure Capable of Meeting the Burden Bandwidth demand continues in this new century, where wavelengths replace time slots as the means of providing transfer reliable high bandwidth service across the network, is indeed enticing. But what is the optical network? The responsevaries considerably and has in fact evolved in recent years. Early attempts at optical networking focused on optical transparency and the design of optically transparent networks on a global scale.
In the absence of viable “all-optics” solutions More practical solutions for optical networks meet the need for optoelectronics to support regeneration of the optical signal and optical signal performance monitoring. In what is called an all-optical network, the signals pass through the network entirely in the optical domain, without any form of optoelectronic processing. This implies that all signal processing - including - signal regeneration, routing, and wavelength exchange - takes place entirely in the optical domain.
Due to limitations of analog engineering (e.g. limiting factor in a digital systemthat properly designed is a unique precision of converting the waveform of the original analog message to digital form) and considering the current state of the art of all-optical processing, the notion of global optical networks or even national is hardly achievable.
In particular, optoelectronic conversion may be required in opto network elements to avoid the build-up of transmission degradations - degradations resulting from such factors, areas of fiber chromatic dispersion and non- linearity, non-ideal dish cascade. gain amplifiers, optical signal crosstalk, and narrow transmission spectrum from cascaded non-flat filters. Optoelectronic conversion can also support wavelength exchange, which is currently a difficult feature to achieve in the all-optical field.
In short, in the absence of commercially available devices that performnt signal regeneration to mitigate degradation accumulation and support for wavelength conversion in the all-optical domain, some optoelectronic conversion is to be expected in practical optical network architectures in the short term. The resulting optical network architectures can be characterized by optically transparent (or fully optical) subnets, limited by feature enhanced optoelectronics, as shown in the figure above.
Client Signal Transparency
Beyond analog network engineering, practical considerations will continue to govern the final realization of OTN. The network operator's desire for a high degree of customer signal transparency within the future transport infrastructure is paramount.
What is meant by "customer signal transparency"? Specifically, for the desired set of client signals ciBlocks for transport over the OTN, individual mappings are defined to carry these signals as Optical Channel Server (OCh) signal payloads. Signals expected in OTN include old SDH and PDH signals and packet based traffic such as Internet Protocol (IP), ATM, GbE and Ssimple Ddata L (SDL). Once a client signal has been mapped into its OCh server signal to the OTN input, an operator who deploys such a network does not need to have detailed knowledge of the signal. client (or access to), until it is unmapped when exiting the network.
The entry and exit points of the optical network must delimit the transparency domain of the client OTN signal. Therefore, the most important factor in achieving customer signal transparency is to eliminate all customer specific equipment and processing between the OTN entry and exit points. Fortunately, he thIt is easier to accept customer dependent equipment on entry / exit, as it is usually dedicated to a ce service.
Optical transport network via digital envelopes
The widespread use of DWDM technology has presented service providers with a new challenge: how to cost-effectively manage the growing number of wavelengths to provide fast and reliable services to their end customers. To effectively manage wavelength or OCh, optical networks must support operations, administration, and maintenance (OAM) functions by wavelength or at the level of the wavelength. 'OCh.
Rec. The G872 defines some functionality for OCh level OAM implemented as overhead without specifying how this overload is to be transported. Until now, the only possible way to support signal regeneration and monitor, analyze and manage OCsh (wavelengths) was to rely on SDH signals and equipment across the network. This requires that the signals on each of the wavelengths of the WDM system be formatted as SDH.
One optical channel (wavelength)
Taking advantage of the optoelectronic regeneration points existing in DWDM systems, the concept of using digital packaging technology will provide functionality and reliability similar to SDH, but for any customer signal, bringing us closer to realizing the original vision of the optical transport network.
The digital packaging technology provides the network management functions described in ITU-T Rec. G.872 to activate OTNs. These include optical layer performance monitoring, Fforward Eerror C (FEC) correction, ring protection, and network restoration on a per wavelength basis, all regardless of the signal format. 'entrance, like mybe the following figure.
The notion of using a digital wrapper (or TDM) by "around" the OCh client to support the OCh overhead associated with the channel has recently been proposed and has in fact been adopted as the basis for the definition of OCh. This scheme will take advantage of the need for an OCh regeneration to add additional capacity to the OCh client. Of course, once we have a way to add some overhead to the OCh client signal digitally, it makes sense to use it to support all OCh level OAM requirements.
In particular, the numerically added overhead makes it almost trivial to solve the major performance monitoring problem of OTN, namely accessing Bbit Eerror Rrate (BER) from a independent of the client. And by optionally using FEC, the digital wrapper method can greatly improve the BER performance of the client signal, further minimizing the need for optoelectrical conversion.onique.
One method to improve the performance of the transport network is to use FEC, which is currently provided in some equipment. Therefore, an additional benefit of the digital wrapper technique is the ability to optionally support FEC for system margin improvement.
OCh frame structure
In functional terms, the OCh and OAM payload should be separable from the FEC mechanism. This allows payload and OAM to be transported end-to-end over the network, while using different FEC schemes on different links. An obvious example of where this could happen is between submarine and land links. In the first, new FEC codes are being explored for the next generation of systems.
The following figure below The figure illustrates the proposed basic frame structure of the OCh, and the types of functions that can be performed.tees in the OCh Frame Structure. While it can be argued that this proposal is incompatible with the long-term goals of the all-optical network, we should not expect the need for regeneration to go away.
The distance between regeneration points will continue to increase; however, the need for regeneration at signal transfer points will remain. Associated with the use of the Ooptical Ssupervisory Cchannel (OSC) to manage the OChs within optically transparent subnets, the digital envelopers will take care of the final management of the OCh (wavelength) on the national OTNs or global.
3R regeneration (remodeling, registration and regeneration) is provided by means of optical-to-electrical conversion and vice versa, and the digital envelope proposal takes advantage of this. Would the image change if all-optical 3R regeneration became available? If all-optical regeneration is capable of aadd an overload, the argument is unchanged; only the regenerator implementation would change.
If the optical regenerators could not add overload, the need for OChs overload will not go away. ; The optical regenerators would then simply increase the potential distance between the optoelectronic regeneration points and the digital packaging would pass through them seamlessly. The implications of the use of digital packaging for the evolution of optical transport networks can be profound, especially in the context of trends in data networks.
Choice of protocol stack
The IP protocol is clearly the convergence layer in today's data communications networks, and it is predictable that 'it will extend this role to multiservice networks in the years to come. IP can be transported over a wide variety of data link layer protocols and infrastructure.Underlying network operations. The following figure below The figure shows some of the possible protocol stacks, or mappings, of IPs in a WDM network infrastructure.
What is IP on WDM?
The protocol stacks labeled a, b, and d in the figure below are the most commonly deployed today. They use the classic IP over ATM over SDH mapping as shown in figure (a);. packet over SDH (POS) as shown in figure (b); or the classic and well-extended IP over Ethernet as shown in figure (d). Cases (e) and (f) use Simple Data Link (SDL), a new data link layer recently offered as a point-of-sale alternative. The protocol stack labeled (c) is an alternative to case (a), where the intermediate SDH layer is removed and a direct mapping of ATM cells in WDM is performed.
These different protocol stacks offer different functionality, in terms of overhead bandwidth , evolvedspeed of rates, traffic management and quality of service. To claim that a particular mapping represents IP on WDM is extremely dishonest.