Crossconnects for the optical network
As optical networks evolve, optical crossconnects can serve a variety of purposes, from network restoration to improved service provision.
John C. Cox, Philip J. Lauriello,
and N.V. Srinivasan Lucent Technologies
The growing demand for increased network bandwidth brought about by the rapid growth of Internet and data services has led service providers to seek solutions for dramatically increasing the capacity of their existing fiber plant. One such solution has been the application of dense wavelength-division multiplexing (dwdm) systems in point-to-point applications in long-haul networks. Today, such systems can provide 4, 8, 16, or more wavelengths--each capable of carrying signals of at least 2.5 Gbits/sec. Systems are also being introduced to provide flexible routing of those wavelengths.
As more networking is required at the optical layer, service providers will need to manage capacity at that level without costly conversion to the electrical layer. This article addresses the important ingredient for managing at the optical level, i.gif., the optical crossconnect (oxc). It further discusses market drivers and applications for such a function, a possible architecture and the enabling technologies required for its realization, and potential applications in a customer network.
Optical networking evolution
Market trends and economics are influencing the growth of optical networking. There has been an explosive demand for capacity in developing countries such as China. The demands in North America are also significantly fueled by new applications such as the Internet, the popularity of broadband services, and the work-at-home culture. While network demand is increasing severalfold, the use of existing capacity in embedded fiber networks is close to 80%. Optical networking is ideally positioned to address these needs.
In North America, the need for capacity expansion and the concurrent availability of enabling optical technologies ushered in the first application of dwdm systems in the mid-1990s. These systems, through the use of optical-amplifier devices operating in the 1550-nm wavelength range, were readily applicable to existing fiber networks and brought numerous advantages.
Until the advent of the optical amplifier, lightwave-transport time-division multiplexing (tdm) systems of 1.7- to 2.5-Gbit/sec signal rates operated predominately in the 1300-nm wavelength range. As a result, the attenuation characteristics of the embedded fiber plant resulted in repeater spacings for long-range systems up to 40 km. With dwdm systems, repeater spacings are extended by about three times, and the number of terminal systems carried over a single fiber has increased by a factor of 4, 8, 16, or more (see Fig. 1). Consequently, service providers have been able to reduce their outside-plant repeater stations, increase the capacity of their embedded plant (avoiding new fiber installation), and also re-use their already installed tdm systems. In addition, they can add new tdm systems to the dwdm optical pipe to meet increased capacity demands as they arise, thus deferring capital expenses as well.
Currently, dwdm systems are being deployed in point-to-point repeatered applications. As deployment of dwdm systems with extended range and increased number of wavelengths continues, there will an ever-increasing need to add and drop wavelengths along the way. In fact, dwdm systems are available that provide the capability of adding or dropping one or more wavelengths at intermediate sites. As such flexibility extends into networks, the need for management at the optical layer becomes more important. Such a parallel already exists in the digital network, where crossconnects are used for bandwidth management, network survivability, fault tolerance, and restoration. oxcs will meet the same needs in the optical layer.
Market drivers and applications
Optical layer bandwidth management--As more wavelengths and fibers are added to optical-networking nodes, the need for bandwidth management becomes paramount. The oxc provides such management at both the fiber and wavelength level. An initial application is likely to involve manual light-guide crossconnect (lgx) panel replacement. The oxc can dynamically rearrange fiber routes via an operations support system, providing "lease-a-lambda" access to the optical layer, for example. The oxc would replace back-to-back dwdm terminals and lgx panels, saving office space and consolidating operations control to a single network element. When such optical services are provisioned by an oxc, office record-keeping is simplified through the use of a crossconnect database stored in system controller memory.
As oxcs are added to the network, they will ease the traffic rearrangement burden currently handled by electronic digital crossconnects (dxcs). Hence, the size of the dxc can be reduced. This hardware reduction is significant because dxcs are rapidly reaching the limits of implementation feasibility.
As oxcs start to replace dwdm terminals, they can be configured as optical add/drop multiplexers, flexibly adding and dropping wavelengths as required by the network (see Fig. 2). This approach is a natural evolution from current dwdm add/drop architectures, which do not support as many optical ports as an oxc. Such a network architecture can also provide for dynamic fiber fill, moving wavelengths from lightly loaded fibers and consolidating them into full-capacity fibers, freeing up fibers for other use (such as optical layer protection switching).
Network survivability and fault tolerance--With the establishment of the optical layer comes the opportunity to provide optical layer network protection. oxcs can be used as an integral part of this protection architecture. For instance, oxcs can provide a 1+1 protection scheme via a headend bridge/tail-end switch (see Fig. 3). The optical broadcast feature of the oxc is used for the headend bridge, while the tail-end oxc can be provisioned to flexibly switch between any two receive optical ports, based on signal quality. This 1+1 optical layer protection switch guards against fiber cuts at the highest possible level, which is architecturally the most appropriate approach.
As the number of wavelengths in a fiber increases, so does the number of transmission-layer rings associated with that same single fiber. For instance, a single fiber cut could conceivably affect 32 Synchronous Optical Network (sonet) rings, and there would be 32 sonet ring switches (alarming up to 512 nodes). Imagine all the operations support messages when a 32-wavelength fiber is cut without the benefit of an optical layer protection switch! On the other hand, only two oxcs would be alarmed by a fiber cut protected at the optical level. The associated sonet rings would then be free to protect any subsequent faults in the network.
Network restoration--The elements of the 1+1 optical layer protection switch can be expanded to provide more-sophisticated restoration features, such as rings or mesh architectures. These approaches can protect against a wider range of network failures, including oxc node failures and fiber cuts. Given its inherent switching capability, the oxc is an ideal candidate for optical add/drop rings, similar to the sonet add/drop multiplexers used in transmission layer rings today. oxcs could also be used to interconnect optical rings, assuming the role currently played by dxcs in sonet dual-ring architectures (see Fig. 4). As the optical layer matures, it is expected that ring standards will be developed. Such advances will give the optical layer a degree of network robustness on par with that of access, switching, and transmission layers.
Optical test access--The optical bridging feature of the oxc can be used to provide wavelength test access (see Fig. 5), which can help troubleshoot optical-network transmission problems and aid in the initial turn-up of such networks. Any live wavelength could be split and dropped at an oxc for in-service monitoring. Given the multiwavelength nature of optical networks, a free wavelength channel can be used to insert an independent test signal over a fiber, which can be dropped and monitored at a far-end oxc. Service providers could use this approach to proactively check transmission quality of "leased-lambda" services, finding and correcting degradations before they become major problems for their customers.
oxc architecture
The oxc provides crossconnect functionality between four input fibers and four output fibers, each carrying a bundle of up to eight multiplexed single-wavelength signals. It contains 13 circuit packs residing in a single bay and optically interconnected by a singlemode optical backplane. The system has six types of circuit packs: multiwavelength input and output transport interfaces, single-wavelength input and output client interfaces, switch, and monitor/alarm.
The multiwavelength input transport interface provides signals on a given input fiber with enough amplification to compensate for losses incurred in traveling from the upstream network element and within the oxc. This is accomplished by using a two-stage erbium-doped silica fiber amplifier. In addition, it demultiplexes the multiwavelength signal into eight constituent single-wavelength signals. Each single-wavelength signal is then routed to one of eight 4 ¥ 4 optical switches. The multiwavelength output transport interface receives up to eight single-wavelength signals, one from each of the eight 4 ¥ 4 switches, and multiplexes them into a multiwavelength signal bundle. It also provides enough amplification for the multiwavelength signal to compensate for the outside plant loss.
The single-wavelength input client interface allows up to eight single-wavelength signals to be added. Similar to the multiwavelength input transport interface, it provides each signal on a given input fiber with enough amplification to compensate for losses incurred in traveling from the upstream client network element and within the oxc. This is accomplished using eight erbium-doped silica fiber amplifiers, one for each wavelength. The signal wavelengths are then routed to the appropriate 4 ¥ 4 switches. The single wavelength output client interface allows up to eight single-wavelength signals to be dropped.
The oxc includes eight strictly nonblocking 4 ¥ 4 switches (implemented as four dual 4 ¥ 4 switches using lithium niobate guided wavelength switching technology), corresponding to eight wavelengths. Each 4 ¥ 4 switch receives signals of the same wavelength from the multi- and the single wavelength input client interfaces and crossconnects them to any of the four outputs. The signals from the switch outputs are then routed to the appropriate multiwavelength or single-wavelength output client interfaces.
In addition to the crossconnection capability, the 4 ¥ 4 switches provide the ability to connect an input signal to two outputs and the ability to disconnect the inputs. The routing of signals between the four dual 4 ¥ 4 switches and the interfaces is accomplished using a singlemode optical backplane with fiber array connectors, providing 48 fiber interconnects for the switch inputs and 48 fiber interconnects for the switch outputs. The single-wavelength signals multiplexed into a multiwavelength output signal bundle are equalized using variable optical attenuators located at the inputs of the 4 ¥ 4 switches.
The oxc system visual alarms reside on the monitor/alarm pack. The data from all monitoring points within the oxc are routed to the monitor/alarm pack and the system controller. For the purposes of signal monitoring, failure detection, and isolation, optical-power taps are strategically located within the system. The optical signal that is tapped is routed to one of two monitoring mechanisms. The first of these mechanisms, performance monitoring, measures signal power and is used to detect a fiber cut between an upstream network element and the appropriate input interface, or within the oxc. The second monitoring mechanism, enhanced performance monitoring, is used to provide detailed signal characteristics of the signals such as signal-to-noise ratio and wavelength registration.
The data from various monitoring points is used by the system controller to generate alarm and status information and is made available to the network management and control system via a tcp/ip (transmission control protocol/ Internet protocol) link over Asynchronous Transfer Mode on 155-Mbit/sec OC-3c. The system controller also provides a user interface for the purpose of management and control of the oxc.
Enabling technologies
Erbium-doped silica fiber amplifier (edsfa)--edsfas are used at the input and output interfaces of the oxc for the purpose of loss compensation. They accommodate various signal input levels and provide the necessary signal amplification. Signals routed from one network element to another suffer optical-power losses. These power losses and the sensitivity of the optical receivers place an upper limit on the spacing between amplifiers. The flattest possible region of a cascaded edsfa gain spectrum is a key factor in determining the wavelength assignment in multiwavelength networks.
Noise is added to signals as they pass through the oxcs. The noise outside the passband of the demultiplex filters is filtered when the multiwavelength signal is demultiplexed. However, the noise within the passband of the filters accumulates and ultimately affects system performance when the levels yield signal-to-noise ratios that drop below a given threshold. This places an upper limit on the noise that each oxc can contribute at each wavelength. The amplified spontaneous emission noise accumulated comes primarily from the amplifiers. The noise generated within each amplifier is approximately proportional to the linear gain of the amplifier. Thus, for a given number of optical amplifiers in the network and a noise limit per network element, there is an upper bound on each amplifier`s gain.
Dense wavelength-division multiplexers--Multilayer interference filters are used as multiplexers and demultiplexers in the oxc. The dwdm filter`s transmission characteristics are one of the key factors that constrain the allowable wavelength spacing in multiwavelength networks. Where there are multiple filter cascades, the misalignment and bandshape of the filters set a lower bound on the wavelength spacing. There should be very little or no deviation in the center frequency of the transmission passband from its assigned center frequency corresponding to any of the eight wavelengths.
Optical-switch arrays--The optical switches used to implement the oxc exhibit low loss, low crosstalk, broad optical bandwidth, uniform performance, and polarization-independent operation, allowing the system to grow. Optical switches are primary sources of crosstalk introduced because of non-ideal isolation between the ports. Interchannel crosstalk arises from the detection of undesired signals at other channel wavelengths. Common-channel or coherent crosstalk arises due to the detection of other signals at the same wavelengths.
While the effect of interchannel crosstalk could be minimized by the use of narrowband optical filters at the receivers, the common-channel crosstalk is likely to be the performance-limiting factor. For example, a signal crossconnected through an N ¥ N switch encounters N-1 crosstalk signal generators. To estimate the crosstalk performance in a multiwavelength network, it is necessary to know the total number of common-channel crosstalk generators that fall within the receiver bandwidth. Hence, it is critical that optical switches exhibit very low crosstalk performance.
Future directions
An oxc architecture that has been realized as a working system ready for customer evaluation has been presented. The introduction of dwdm systems on a large scale has set the stage for the oxc through the evolution to a fully flexible optical network. As the dxc has arisen to manage capacity at the electrical level, the oxc will serve a similar function at the optical layer. As with dxcs, not only must oxcs operate in a multivendor environment, but they must also serve a function within the continually growing sonet/Synchronous Digital Hierarchy ring and the long-established mesh networks.
As of today, the role of the oxc and its ultimate architecture have yet to be established. Will it be to complement and, hence, reduce the size of the present dxcs by supporting rearrangement at the optical layer at oc-48/stm-16 rates? This would be especially useful where large amounts of traffic that pass through an office are currently routed through a dxc for restoration purposes. Will it be as a hand-off element between dwdm systems deployed in point-to-point or ring topologies? Will it be a restoration vehicle at the optical layer, replacing DS-3 (44.736-Mbit/sec) or equivalent-based restoration approaches currently implemented?
Will it be all or a combination of the above? Today, technology is available to make the oxc a realizable product. Ultimately, deployment of the oxc will be driven by market economics and optical-networking needs. With the increased presence of dwdm systems, that time for deployment is rapidly approaching. u
John C. Cox is technical manager of sonet and optical networking product development, Philip J. Lauriello is technical director of sonet and optical networking product development, and N.V. Srinivasan is technical director of the advanced optical networking department, all at Lucent Technologies, Holmdel, NJ.