IS WDM READY FOR LOCAL TWORKS? by Brent Allen and Solomon Wong Nortel Networks, OPTera Metro Solutions KANATA, Canada Wavelength division multiplexing (WDM) technology, in the form of photonic networking, will meet the competitive challenges faced by local network carriers. This paper describes the evolution of WDM technology, the primary values that WDM currently brings to long-haul networks, and the benefits that photonic networking will bring to local networks. THE LOCAL TWORK The telecommunications network is experiencing explosive change. Deregulation, globalization, and ubiquitous access to information, entertainment, and services provided by the Internet have made the local (metropolitan access and interoffice) network the front line of competition. The local network presents diverse challenges to carriers. Competitive pressures drive new services with thin profit margins to meet the disparate demands of residential and business users. Rapid growth in demand and changes in traffic patterns mean that the access network must support many different service interfaces at the same time, must be scalable, must be able to allocate bandwidth on demand, and must be reliable.
The metropolitan interoffice network is no less dynamic. The changes taking place in the access network have a direct impact on the interoffice network. Rapid response to unpredicted changes in demand is crucial. As the capacity of time division multiplexing (TDM) approaches the practical limits of the technology, carriers are looking for more economical, scalable, flexible, and reliable ways to transport more bandwidth. WAVELENGTH DIVISION MULTIPLEXING (WDM) Wavelength division multiplexing (WDM) is an emerging technology that enables carriers to significantly increase transport capacity while leveraging existing fiber-optic equipment. Unlike conventional TDM transport alternatives, WDM gives carriers the flexibility and scalability they need to deploy capacity when and where it is needed. Early deployments of WDM are based on wideband technology. Figure 1 shows how wideband WDM doubles the capacity of fiber plant by optically coupling the outputs of two terminals in a fiber-optic transmission system (FOTS); one terminal operates in the 1310 nm range and the other in the 1550 nm range. Although this is a cost-effective solution for applications with restricted reach, wideband WDM systems, which tend to consist of little more than an optical coupler and splitter, suffer from the absence of maintenance capabilities. Figure 1 Wideband WDM 1310nm FOTS 1310nm FOTS 1550nm FOTS Optical Coupler Optical Splitter/Filter 1550nm FOTS More recently, narrowband or dense WDM (DWDM) is being deployed. With DWDM, the outputs of two or more terminals are optically multiplexed into one fiber as shown in Figure 2. The wavelengths used are all within the 1550 nm range to allow for optical amplification using erbium-doped-fiber-amplifier (EDFA) technology to maximize reach. The wavelength of the optical output from a network element is roughly centered at 1310 nm or 1550 nm with an approximate tolerance of ±20 nm. Such wavelengths would have to be so widely spaced that after optical multiplexing only a few 2 WHITE PAPER: IS WDM READY FOR LOCAL TWORKS?
would be within the EDFA passband. Therefore, DWDM systems must translate the output wavelength from the network element to a specific, stable, and narrowwidth wavelength in the 1550 nm range that can be multiplexed with other similarly constrained wavelengths. The device that does the translation function is sometimes called a transponder. The International Telecommunications Union Telecommunications Standards Sector (ITU-TSS) has recently standardized a grid of wavelengths for use in DWDM systems. The grid defines a minimum channel spacing of 100 GHz in the frequency domain corresponding to approximately 0.8 nm in the wavelength domain. Figure 2 Narrowband (Dense) WDM DWDM DWDM Optical Line Amplifier Transponder Optical Coupler Optical Splitter/ Filter The first DWDM systems were optimized for long-haul interexchange applications; these systems are described later in WDM Values in Long-Haul Networks. As such, these WDM systems are mainly point-to-point or linear configurations, but lack the ability to network at the photonic layer. PHOTONIC TWORKING Photonic networking is the next-generation application of WDM technology (Figure 3). First-generation wideband and DWDM systems focus on the simpler point-to-point applications. Although these systems provide fiber capacity relief, the management of add, drop, and pass-through traffic must be done manually as it was in early back-toback TDM systems. Current second-generation DWDM products can perform linear add/drop multiplexer (ADM) functions in long-haul or interexchange applications, but these lack flexibility. Because the wavelengths that are added and dropped are fixed, there is no way to increase the add/drop capacity without physically reconfiguring the system. WHITE PAPER: IS WDM READY FOR LOCAL TWORKS? 3
Photonic networking has the ability to manage individual non-fixed wavelengths to effectively bring more bandwidth to more places. Ring topologies increase the reliability of equipment and the survivability of traffic transported on WDM systems. Optical ADMs and cross-connects simplify bandwidth management on optical facilities. Optical Channel OXC WDM Ring Optical ADM Optical Cross-Connect WDM Ring Figure 3 Photonic Networking WDM VALUES IN LONG-HAUL TWORKS In long-haul networks, the primary value provided by DWDM in combination with optical line amplifiers is the cost-effective transmission of high aggregate bit rates over large distances on a single fiber. The large distances in long-haul networks make deploying new fiber impractical. Longhaul carriers have traditionally been the first to embrace advances in transmission technologies to gain additional capacity while leveraging their existing equipment. Stateof-the-art TDM technology can transport an OC-192 (10 Gb/s) on one fiber. This is a viable alternative for new fiber builds because the fiber parameters can be controlled through placement of the appropriate fiber type and because new fiber handling 4 WHITE PAPER: IS WDM READY FOR LOCAL TWORKS?
procedures can be accommodated. However, for existing fiber applications, the ability to install 10Gb/s TDM systems depends on fiber properties such as chromatic and polarization mode dispersion, which may differ from fiber span to fiber span. In addition, high-rate TDM systems require specialized fiber handling and termination procedures when compared to lower rate OC-48 systems. In contrast, current narrowband or DWDM systems can transport up to 16 wavelengths at OC-48 (2.5 Gb/s) each, giving an aggregate capacity of 40 Gb/s on one fiber. This means WDM technology surpasses TDM in terms of the aggregate capacity offered on a single fiber, while maintaining the same fiber handling procedures developed for OC-48 TDM systems. Figure 4 shows a baseline transmission route that consists of four parallel systems. WDM and TDM technology could be considered to reduce the number of fibers used in this application. WDM technology has traditionally been considered more expensive than TDM. One reason is that a WDM solution requires a separate terminal for each channel in addition to the WDM terminal, as shown in Figure 5. Figure 4 A Baseline Long-Haul Application Electronic Regenerators WHITE PAPER: IS WDM READY FOR LOCAL TWORKS? 5
Figure 5 Long-Haul Application WDM Solution without Optical Line Amplifiers Electronic Regenerators WDM WDM WDM WDM The equivalent TDM solution has only one terminal. For example, a WDM system that transports an aggregate capacity of 10 Gb/s requires four 2.5 Gb/s terminals plus a WDM terminal per end. On the other hand, the equivalent TDM solution has only one 10 Gb/s terminal. Because TDM technology has typically quadrupled its capacity for a cost multiplier of 2.5, the 10 Gb/s solution appears to be more cost-effective. However, if the TDM system also requires four 2.5 Gb/s terminals to provide the first stage of multiplexing, as shown in Figure 6, the 10 Gb/s solution might actually be more costly. In addition, if the 2.5 Gb/s terminals are already in the network, they represent a sunk cost and might not be included in the cost analysis. Figure 6 Long Haul Application TDM Solution First Multiplex Stage (if required) Electronic Regenerator 6 WHITE PAPER: IS WDM READY FOR LOCAL TWORKS?
Long-haul applications require periodic regeneration to ensure sufficient signal strength and quality at the receiving end. Though the first cost and life-cycle cost of one regenerator seems insignificant, these costs can dominate when multiplied over the total length of a route, especially for heavy routes where there might be several systems contained within the same fiber sheath. Before optical line amplifiers were introduced, higher speed TDM systems were more attractive than WDM because the TDM systems allowed multiple lower speed electronic regenerators at a site to be replaced with one higher speed regenerator, as shown in Figure 6. The introduction of optical line amplifiers with the ability to amplify the entire EDFA passband at once allows multiple lower speed electronic regenerators at a site to be replaced with one optical amplifier, as shown in Figure 7. This tips the scales in favor of WDM. Figure 7 Long-Haul Application WDM Solution with Optical Line Amplifiers WDM Optical Line Amplifier OLA WDM WDM VALUES IN LOCAL TWORKS Although capacity per fiber is important in the local networks, the value equation is different because the topologies and traffic patterns are more meshed, resulting in shorter spans with less capacity per span. The capacity demand is still within the realm of TDM technology and the cost advantages of optical amplification cannot be fully exploited because of the shorter distances. As such, WDM technology must bring a richer set of values to meet the diverse challenges of the local network. WHITE PAPER: IS WDM READY FOR LOCAL TWORKS? 7
COST EFFECTIVESS Transport networking in a TDM network is provided by multiplexers, such as linear and ring add/drop multiplexers (ADMs), and cross-connects. These network elements must convert the optical signal to an electrical signal before performing their primary function, then convert the result back to an optical signal for transmission on the next span. This back-to-back electro-optic conversion is a significant cost component of these network elements. In long-haul networks, the bulk of the network transport cost is due to the regenerators, which are numerous compared to the multiplexers and cross-connects. Using optical line amplifiers reduces this cost by eliminating the back-to-back electro-optic conversion. As the cost of optical amplifier technology drops, WDM will be increasingly favored in long-haul networks. However, in local networks, where distances are shorter, the bulk of the network transport cost is due to the multiplexers and cross-connects. Photonic networking does multiplexing and crossconnecting purely in the optical domain; this significantly reduces the number of back-to-back electro-optic conversions and the cost of the network. SERVICE OPPORTUNITIES Photonic networks are protocol and bit-rate independent. This enables these networks to carry many different types of traffic over an optical channel regardless of the protocol (Ethernet, ATM,, etc.) or bit rate (10Mb/s, 2.5Gb/s, etc.). Figure 8 shows an access network with protocol and bit-rate independence. Figure 8 Local Network Application Access Multi-Tenant Office Tower 100Mb/s Ethernet, Gigabit Ethernet, ATM,, etc. Carrier Central Office Customer Network Interface Devices WDM Other Building Access Ring WDM Carrier Service Devices Interoffice Network Other Building 8 WHITE PAPER: IS WDM READY FOR LOCAL TWORKS?
By deploying a WDM-based photonic network, the service provider gains an access or interoffice transport infrastructure that is flexible and scalable. Native data interfaces, such as Ethernet, can be connected directly to the transport platform without costly adaptation. User requests for increased bandwidth or different protocols can be filled quickly, so the network provider can realize increased revenue sooner. New services such as optical-channel leased lines that provide end-to-end protocol and bit-rate independent connections can be offered, attracting new revenue for the network provider. In WDM-based photonic networks, transport interfaces that are specific to protocol and bit rates are no longer be required, minimizing the network provider s inventory and operating costs. NATIVE DATA INTERFACES Data is consuming more network capacity due to increasing numbers of users and escalating capacity demands from each user; this is the result of bandwidth-hungry applications such as real-time video, image retrieval, and large file transfers. The protocols, bit rates, and interfaces used by the networks and the devices that provide these applications are different from those used in the carrier networks. Typically, there has to be a device at or near the customer location to do conversion or adaptation. For example, consider asynchronous transfer mode (ATM). One of the driving factors behind the deployment of ATM in carrier networks is its multiservice networking capability. However, to exploit this capability the service must first be mapped into ATM cells. Ethernet is a relatively inexpensive interface used in enterprise data networks. The 10Mb/s version is the most prevalent desktop data interface today, and at higher speeds such as 100Mb/s and 1Gb/s Ethernet is also a key enterprise backbone interface. A fullduplex, non-shared Ethernet could be used as a network access interface, terminating on a router, ATM switch, or Ethernet switch in the carrier s network, if carrier transport equipment could handle the interface without having to convert it first, for example, to ATM. Because Ethernet interfaces are very cost-effective compared to traditional WAN interfaces, this would be an attractive offering to users. It also saves costs for the network provider by eliminating the adaptation function. Native data interfaces for other protocols, such as ESCON or HPPI, would provide similar values depending on userspecific requirements. RAPID RESPONSE TO CHANGE More and more businesses are expanding and decentralizing to be closer to their customers and supply lines or to take advantage of lower real estate costs. At the same time, corporate intranets and extranets are driving the need for increased connectivity and increased bandwidth between the various locations of a business, its customers, and its WHITE PAPER: IS WDM READY FOR LOCAL TWORKS? 9
suppliers. The next generation of enterprise networking technology, based on new protocols or bit rates, seems to arrive daily. In a competitive environment, rapid, unpredictable growth and change cause users to migrate to the network provider that can provide the fastest response. W SERVICES Many high-volume enterprise users prefer to own and operate their own networks. What enterprise users need from the carrier is reliable, error-free, end-to-end connections that do not restrict the choice of technology for their network and do not require costly adaptation. WDM-based photonic networks offer a new service opportunity and new revenue opportunity to network providers so they can provide the ultimate leased line: a protocol and bit-rate independent optical channel. By subscribing to this service, users have the flexibility to use whatever protocol is the right choice for their network; they can even add encryption mechanisms to satisfy their concerns about data security. REDUCED INVENTORY AND OPERATING COSTS For the network provider, service flexibility often comes at a significant cost, either in the form of operating complexity or a myriad of interfaces with their accompanying inventory and operating costs. However, the protocol and bit-rate independence of photonic networks means a minimum of interface types, each handling a wide range of services without requiring complex configuration for each type. This means lower inventory costs and easier operation. TWORK SURVIVABILITY The growth in the amount and type of information being carried across the network has triggered an increase in the importance of survivability the ability to restore traffic in the event of facility and equipment failures. Users routinely transfer mission-critical information from one location to another; network failure could endanger lives or result in substantial financial loss. Network providers offer network survivability as a competitive differentiator or a premium service offering. Rings are the preferred method of providing survivability. A ring inherently provides two paths between any two points on the ring. Any failure that affects one of those paths can be survived by rerouting the traffic on the other path. Photonic networking provides the ability to route wavelengths, and therefore has the same survivability capabilities as current TDM rings when deployed in a WDM ring topology (Figures 8 and 9). In fact, photonic networks can significantly enhance survivability by reducing the number of electro-optic devices in the network a major source of equipment failures. 10 WHITE PAPER: IS WDM READY FOR LOCAL TWORKS?
DATA SECURITY Although rings are the preferred method of providing network survivability, users who are transporting sensitive or mission-critical information across the network might be reluctant to be on the same ring as a user in another building especially if that user could be a competitor. This is of particular concern with TDM solutions such as because the entire optical signal terminates in every building on the ring, potentially making one user s data accessible by another. For this reason, many premium service users demand dedicated rings. Other users might have the same security concerns but cannot afford a dedicated ring. WDM rings are the solution. Because each user s traffic is carried on a separate wavelength, it is not accessible by other users on the same ring. Furthermore, protocol independence allows each user to encode data using secure encryption mechanisms, making it virtually impossible for any other user to access the data. TWORK UPGRADES New technology often arrives before the old technology has completed its useful life. Too often, the deployment of the new technology requires a forklift upgrade of the old system or network. Protocol and bit-rate independence in photonic networks offers a better upgrade path. When the new photonic network is installed, the legacy fiber-optic system can be retained doing exactly what it was doing before, but now it occupies only one wavelength of the new photonic network (Figure 9). The additional wavelengths are now in place for service growth or new services. This in-service upgrade means network providers can tap the inherent bandwidth of existing fiber without stranding the capital associated with old systems that are currently underutilizing the fiber. Figure 9 Local Network Application Interoffice Carrier Central Office Legacy FOTS WDM CO Interoffice Ring CO CO CO WHITE PAPER: IS WDM READY FOR LOCAL TWORKS? 11
OPERATIONS, ADMINISTRATION, MAINTENANCE, AND PROVISIONING Long-haul applications do not require flexible management of individual wavelengths; therefore, one optical surveillance channel carried on a dedicated wavelength is sufficient to control and monitor all remote WDM network elements such as optical line amplifiers. Photonic networks in metropolitan access and interoffice applications manage individual wavelengths. Because individual wavelengths have different endpoints and take different paths across the network, one optical surveillance channel carried on a separate wavelength is not sufficient. Fault information is required for, and must accompany, each wavelength. Per-wavelength fault information is needed for fault detection and isolation, which are necessary to support survivability mechanisms and trigger fast maintenance and repair actions. In photonic networks, service providers can also verify the connectivity and monitor the performance of each connection; this is especially important if the connection is a leased line between user locations. CONCLUSION Existing WDM solutions successfully address the needs of long-haul carriers by providing cost-effective transmission of high aggregate bit rates over large distances. These systems increase the bandwidth of the fiber plant while the subtending systems provide survivability and networking. When deployed with optical line amplifiers, these systems reduce costs by displacing regenerators. Photonic networking realizes the potential of WDM in metropolitan access and interoffice networks by providing cost-effective, survivable networking of protocol and bit-rate independent connections. WDM is ready for local networks. OPTera Metro optical networking platform is optimized for metropolitan access and interoffice applications. OPTera Metro addresses capacity problems where fiber is scarce. But more importantly, it provides the service flexibility required for carriers to offer a variety of new services and service interfaces and to rapidly respond to unpredicted changes in user demand in an environment where demand is changing quickly and competition is stiff. 1997 by OPTera Solutions. All rights reserved. Nortel Networks, OPTera Solutions the Nortel logo, and OPTera are registered trademarks of Nortel Networks Corporation. www.nortelnetworks.com Rev 1.0 12 WHITE PAPER: IS WDM READY FOR LOCAL TWORKS?