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Special Reports, August 1998
Component technology enables high-capacity DWDM systems
By Ronnie Chua and Bo Cai E-TEK Dynamics Inc.
The growth of the Internet, global deregulation of the telecommunications industry, and lowering of trade barriers have created an ever-increasing demand for greater bandwidth in optical networks. A cost-effective way to deliver bandwidth is to send multiple wavelengths through a single fiber and at very high data rates. The high data rates are currently realized through time-division multiplexing (TDM) systems operating at up to 10 Gbit/sec (OC-192). Mulitple wavelengths are sent through a single fiber using dense wavelength-division multiplexing (DWDM) technology that operates in the 1550-nm wavelength window.
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Special Reports, August 1998
Managing DWDM systems in the metropolitan market
By Ron Mackey
The number of Internet users is predicted to top 200 million by 1999, while new applications such as high-definition television, video-on-demand, telemedicine, and videoconferencing loom on the horizon. Even though such demands on the network are growing exponentially, the consumer will continue to demand a high level of service. As the surge of traffic swamps Synchronous Optical Network (SONET) technology, telephone companies must turn to new technology to keep up with this demand for more bandwidth and high service quality.
Metropolitan carriers are turning to dense wavelength-division multiplexing (DWDM) to meet these needs because it promises to increase the power of a single fiber 100-fold. Given the projected universal deployment of DWDM, one of the largest issues users and providers face is how DWDM networks will be managed and monitored.
The melting pot
Typically, metropolitan centers are characterized by a concentration of business, governmental, and educational organizations, each representing a user type that responds differently to new applications and service features. This scenario contrasts with the typical long-haul applications in which DWDM previously has been applied. In today's metropolitan network, if a new application is competitively priced, it can trigger an instant and unpredictable demand for additional bandwidth because a particular demographic takes an interest. Because this shift can happen so quickly, reliability is a critical factor in new high-powered networks. Industry deregulation also has fueled the pressure to maintain a highly reliable network by creating an array of carrier choices for the end user.
Not only has deregulation stepped up industry competition, it also has presented great challenges for maintaining network integrity because of increased network diversity. Under the old Bell system, networks existed under a single carrier, making them homogeneous. The current offerings from equipment suppliers found in different networks use an array of protocols, presenting a challenge for any new network-management technology. Generally, many local/Internet users remain focused on network-management systems based on Bellcore or International Telecommunications Union standards. In the corporate world, as well as in intranet/Internet services, SNMP (simple network management protocol) is clearly the dominant choice. Carriers have opted predominantly for TL-1 (transaction language 1) and may choose the upcoming Q3 (CMIP/CMIS - common management information protocol/common management information services) or in the distant future CORBA (Common Object Request Broker Architecture) protocols. This lack of a single standard makes shared management difficult to achieve.
The financial success of long-haul DWDM vendors testifies to the technology's maturity and effectiveness in multiplying existing fiber activity while keeping infrastructure costs low. Just as the bandwidth squeeze prompted long-distance carriers to turn to DWDM, it is also a viable solution to alleviate network pressure in the metropolitan arena. However, if DWDM is to be successful in the metropolitan market, it's vital that it function with a multiple-access-management scheme that everyone can use - users, craftspeople, and operators alike.
Channel transparency for success
In the past, network operators provided links to customers over several independent fibers. DWDM will mix several customer links onto a common fiber (see Fig. 2). This creates a challenge for a metropolitan network-management scheme, because it must accommodate all those with a need to know management information - capacity providers, wholesalers, and large end users. The mix of broadband applications and services - data, voice, or video over Digital Subscriber Loop; 100-Mbit/sec Fast Ethernet; Gigabit Ethernet; Asynchronous Transfer Mode; frame relay; or SONET - dictates that DWDM systems be channel-transparent to any data format or bit rate and adds to the complexity of network management.
Having many links on a common fiber creates an additional concern: security. Management systems must maintain the same security and isolation between different customers that is provided by transport over different fibers.
Overall, the chosen metropolitan system must be totally channel-transparent and provide standardized network-management systems, as well as a migration path to next-generation management architectures. Because the DWDM system will lie at the heart of the network, metropolitan network-management systems also must be interoperable and have the ability to ultimately manage the network end-to-end.
Migration to TMN carrier management
Large operators worldwide have embraced Telecommunications Management Network (TMN), a standard that defines new protocols and a hierarchy of intercommunicating management systems. While TL-1 remains the current protocol of choice in North America at the network element layer of this hierarchy, TMN is emerging as a new solution (see Lightwave, March 1998, page 44). Moving to TMN is not unlike moving from a basic subcompact car to a loaded sport utility vehicle. It has the capabilities to integrate all of the equipment and systems of networks from the past, present, and as far as we know, the future.
The secret to creating a useful network is to have it function without a single point of failure in the management systems. In a TMN scheme, the end devices and network elements in many cases will be managed by an element management system. This system communicates upward, or "northbound," in the hierarchy to a network-level network-management station. Here, the information is collected and reduced to depict the status of the entire network and its variety of equipment components. It reports upward again to two more tiers of management systems that function as service- and business-related systems to control important functions such as billing and intercommunications with other carriers.
For the purposes of device management, TMN favors a new protocol between the network elements and the first-tier network-management station. While it doesn't preclude using SNMP between the devices and their manager, TMN clearly recommends Q3, an Open System Interconnection-based protocol. This protocol uses a management information base written in asn.1 (abstract syntax notification) format, which is similar to SNMP but much more powerful in the number and type of commands that can be used. For complex products such as large switches and master SONET multiplexers functioning as gateways to their slave systems on a network ring, Q3 is ideal because of its ability to transfer large blocks of information more efficiently and with better assurance of delivery. This ability can be important, as TMN's extensive list of features can prove cumbersome if not handled efficiently.
The need to know
Given the high cost of high-bandwidth transmissions, end users have a vested interest in knowing what is happening on their links. In general, they need to know how many bits are being transmitted, how many are being received, and how long it takes to get from one end of the network to another, prioritizing traffic according to quality-of-service issues. Specifically, the end user is most concerned with what is being transmitted over the network.
In contrast, service providers are more concerned with how the service is performing. For instance, they must control the physical links that compose the backbone of the network and interconnect the various users. It's essential for service providers to know the status of their network to ensure optimum performance, facilitate maintenance procedures, and allocate costs. They also need to know the conditions of any channels leased from other carriers.
The challenge of a workable management system is to grant the information required while securing the equipment from unauthorized access.
Since both service providers and end users have embraced tcp/ip (transmission control protocol/Internet protocol) for Internet-related applications, the development of a maintenance management network based on tcp/ip for shared use would allow common access to "need-to-know" information. A cost-effective means of implementing this approach is to install a small 56K integral router and a digital data service copper line from each end site to the provider's head site (see Fig. 5). This scenario is achievable because IP hubs and routers, as well as digital service units, are now low-cost commodities offering user familiarity with device setup and testing. This simple approach would do much to strengthen relationships between end users and providers. For service providers using DWDM, implementation would proceed as follows.
Customer premise equipment management. At the customer premises, create an isolated Ethernet local area network (LAN) segment with provider-supplied IP addresses for each DWDM channel that has been assigned to the customer. The end user can then attach an existing or separate SNMP workstation to the LAN segment, achieving local management connectivity with the maintenance network. The network provider would supply unique SNMP read and write passwords only for the channels that terminate to the customer.
The single rule to enforce is that the network-management system attached to this LAN must get its address from the provider and be isolated from the customer's network, either by separate LAN card or physical PC. Other equipment at the customer site should have addresses and SNMP community strings assigned for security. To prevent one user from inadvertently disrupting service on a common line, shared items such as optical amplifiers may be monitored by issuing end users a "read-only" community password, while reserving the write password for the provider.
Carrier end-site management. The customer premises equipment can be connected to a carrier-operated LAN via the dedicated 56K router links. The benefit of using low-cost combination router/channel service unit links is their reliability. They function as an independent route to provide two-way maintenance data links that both the end users and carriers can employ regardless of the operational status of the fiber. Both IP subnetting at the remote-management LAN and unnumbered link routing will help to conserve addresses. Routing Internet protocol v2 is sufficient for the routing protocol at these locations.
Customers can access and manage any DWDM channels in the end-user zone for which they possess the appropriate addresses and SNMP community strings. Alternatively, the carrier can use SNMP to manage everything in this zone as an additional service. In any case, carriers want a high-level view of the network, including all equipment and facilities, and need to see the status data on their existing network-management systems. In contrast, end users are interested in a view of their circuits that will allow them to be monitored for performance and availability.
Zone-to-carrier network management. A single or dual PC arrangement can provide message conversion for a large number of devices into the carrier's native management protocol. Which protocol to use remains an issue, since either TL-1 or Q3, depending upon whom you talk to, is the desired choice. The PC offers a good migration strategy for carriers who pLAN to move to TMN Q3, because the difference between the protocols is a matter of software.
To fully convert all parameters from SNMP to other protocols, an active database of configuration and status would exist in the PC. Changes in this adapter software would be synchronized with the network provider's main management systems migration. The benefit is that the actual equipment distributed around the city would not have to change at all, and the end users could continue using SNMP regardless of the carrier's choice of management protocols. Additionally, the PC could function as a Web server to provide end users with additional information as a supplemental revenue service. Secure Web-server features also could allow individual customers to receive customized reports. Remote-control software would allow authorized users to access information from the PC.
Zone-to-zone access. Within the provider's network, routers can tie the zones together using isolated addresses. Typically, DS-1 (1.544-Mbit/sec) links would be sufficient between zones. End users could access their remote-end equipment via SNMP using these routes. The only requirements are private addresses, since no traffic traverses the Internet. These networked zones also can be used by the carriers to concentrate more devices into fewer network element managers. The physical isolation of this maintenance network from the Internet simplifies many addressing and security issues.
This zone-to-zone management scheme satisfies the critical information status requirements of service provider and end user alike. It also supports and allows the successful adoption of DWDM in the short-haul metropolitan market.
DWDM's success in the metropolitan area is dependent upon the service providers' ability to identify vendors who can offer DWDM technology and who are skilled at building and deploying management networks. Only in this way can the needs of both end users and network providers be met.
DWDM to the shorter-range metropolitan market promises new high-bandwidth solutions and cost savings for many companies. The burden is on DWDM vendors to offer systems that address the unique demands of metropolitan network operators - systems that are scalable, transparent, flexible, have open architectures, and are equipped to support a flexible management scheme to support the myriad of new customer demands that will surface as consumers take advantage of the emerging technologies.
Ron Mackey is executive vice president of technology at Osicom Technologies Inc. (Santa Monica, CA). He can be contacted via email at: rmackey@osicom.com.
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Research report:
gii.co.jp
Nearly 30 million kilometers of fiber was installed worldwide by the end of 1997, and construction will continue growing at a frantic pace reaching almost 45 million kilometers three years later. By 2002, China is expected to install nearly three times the volume of fiber as the next largest market. Insight provides a comprehensive overview of the overall market trends and the likely impact that WDM will have on the high end of the transmission market. This report studies capacity issues, market demand, as well as the vendors and service providers that are driving network upgrades around the globe.
Enabling DWDM component technology
DWDM could increase data-carrying capacity without having telecommunications companies lay new fiber cables. Unlike TDM, which uses shorter light pulses to achieve higher bandwidths on the same wavelength, DWDM technology combines multiple optical signals onto a single fiber by transporting each signal on a different wavelength or channel. TDM does not fully utilize the intrinsic capacity of fiber compared to DWDM, making the economics of moving beyond OC-192 unattractive compared to the terabit bandwidth potential of DWDM technology.
Today's DWDM is driven by several component technologies, the most prevalent being interference filters, fiber Bragg gratings, and arrayed waveguide gratings. Interference filter technology uses layers of dielectric thin films coated on a glass substrate to combine or separate specific wavelengths in a DWDM system. By controlling the layers of deposit on the substrate, different types of interference effects could be created to produce a diversity of narrowband, wideband, and gain-flattening filters. This technology is flexible because it allows the filters to be cascaded in a variety of ways to produce DWDM modules for add/drop, high-, and low-channel applications.
Unlike other DWDM component technologies, interference filters are totally passive and do not require the complexity and added cost of temperature control. Furthermore, they can provide 40-dB or higher isolation on adjacent or nonadjacent channels with very low insertion and polarization-dependent losses. It is perhaps the most cost-effective option for up to 16 channels and the only DWDM component technology suitable for wideband applications. But for DWDMs with narrowband or dense channel spacing requirements of 100 GHz (0.8 nm) or smaller, controlling the appropriate layers of thin-film deposits becomes increasingly difficult. This method requires a lengthy development phase to produce all the individual wavelength channels in the International Telecommunications Union (ITU) grid.
Fiber Bragg gratings are produced from thousands of refractive index modulation sections that are imprinted onto the core of photosensitive fibers using ultraviolet lasers. This process creates interference patterns that reflect specific channels or wavelengths. The technology is capable of producing DWDM filter shapes with very steep slopes and high channel isolation. The drawback is that it requires costly optical circulators or an interferometric Mach-Zehnder setup to pass selective wavelengths. This complication is compounded as channel count increases, often in direct proportion to cost. Another disadvantage is the need for temperature control because of the thermal sensitivity of fiber Bragg gratings.
Arrayed waveguide gratings, however, are fabricated by depositing thin layers of silica glass onto silicon wafers. The fine silica glass layers are processed at high temperatures and patterned into waveguide circuits by using photolithography before the wafers are diced into individual circuit chips. The circuits are designed to direct each wavelength onto an aligned silica glass block containing multiple fiber outputs. Because optical waveguides are patterned onto silicon in much the same way that electronics are integrated on a computer chip, the cost of arrayed waveguide gratings is not proportional to channel count. Thus, this component technology is the most cost-effective for producing DWDM with very high channel counts in a compact package.
The main disadvantage is that performance-arrayed waveguide gratings generally have an inferior filter passband, a higher polarization-dependent loss, and poorer nonadjacent channel isolation compared to other existing component technologies. As a result, moving beyond 16 channels sometimes requires the aid of interference filters. Like fiber Bragg gratings, the thermal sensitivity of arrayed waveguide gratings also necessitates temperature control of its planar substrate.
In addition to the three main technologies, other novel techniques for fabricating DWDM components also are commercially available. A few companies have introduced DWDMs using fused biconic taper (FBT) technology combined with fiber Bragg gratings. A hybrid combination of interference filters with fiber Bragg gratings to achieve narrowly spaced 50-GHz DWDMs has also been demonstrated.
But the types of component technology used depend on the application of the DWDM system. For instance, long-haul terrestrial or undersea systems designed to carry a more or less predictable range of data traffic require DWDM components that are highly reliable and uncompromising in performance. But in metropolitan applications where service providers must offer a whole range of data formats and bit rates, the main requirements of DWDM components are flexibility, channel scalability, and low cost.
Enabling optical amplifier technology
Theoretically, DWDM could effectively transport multiple-wavelength signals over a band as much as 100 nm wide around the 1550-nm window. But today's WDM system could only take advantage of the conventional band (C band) between 1535 and 1565 nm because of the limitations of the erbium-doped fiber amplifier (EDFA).
The EDFA is a critical component for extending the reach of WDM networks because it amplifies signals in the same 1550-nm window. It works by relying on either a 980- or 1480-nm pump laser to excite a rare-earth element known as erbium that is doped into a piece of fiber. When a transmission signal in the 1550-nm window passes through the same fiber, the excited ions will amplify the signal as it exits the amplifier. The advantage of the EDFA is that it performs this amplification in the optical realm, unlike regenerators that require optical-to-electrical and electrical-to-optical conversions before the signal is amplified.
But the EDFA only has sufficient gain in the C band covering a 30-nm bandwidth, limiting the number of channels that a DWDM system could effectively carry. A DWDM system with 100-GHz channel spacings, for instance, could only accommodate about 40 channels on the standard ITU wavelength grid. Narrowing the channel spacings to 50 GHz could potentially double the channel count in the same bandwidth, but this poses technical difficulties if each channel has to be upgraded from carrying OC-48 (2.5-Gbit/sec) data rates to OC-192 data rates. Furthermore, a 50-GHz DWDM requires improvements to enabling components such as filters and laser transmitters.
Recent studies have demonstrated a potential solution to the problem by extending the amplification bandwidth of the optical amplifier to incorporate the longer wavelengths (L band) around 1570 to 1610 nm. At OFC '98, Lucent Technologies presented an ultra-broadband amplifier solution that uses silica-based erbium-doped fibers in a dual-stage amplification architecture. It works by amplifying all wavelengths as a group before they are separated into the C and L bands. The L band signals undergo two further amplifications before they are recombined with the C-band signals. The dual-stage amplifier is pumped by higher-power 980- and 1480-nm lasers to achieve a 25-dBm output and 6-dB noise figure.
By broadening the EDFA's amplification bandwidth, the DWDM system capacity could be increased to achieve terabit-per-second transmission rates, as demonstrated by Pirelli's high-capacity 128-channel transceiver system at the recent supercomm '98. To achieve these rates, optical components used in the EDFA must be developed for broadband performance.
Enabling EDFA component technology
An EDFA system consists of inline fiber isolators and pump wavelength-division multiplexers. The isolator prevents backreflection of light that might degrade a transmission signal that passes through the EDFA. The WDM is necessary to combine the 980-nm pump energy with the 1550-nm signal in the amplifier. These critical components are mostly available for conventional EDFA systems operating in the C band. But recent research by E-TEK Dynamics has produced isolators and pump WDMs that cover both the C and L bands for EDFAs in high-capacity DWDM systems.
The ultra-wideband isolator consists of a pair of collimators, wedge polarizers, Faraday rotators, and magnets. As the first collimated beam enters the birefringent wedge, it is separated into two polarization states at an angle normal to the wedge's incident plane. The Faraday rotator then turns the optical axis of both polarization states by 45° before they are recombined through a second birefringent wedge that has an axis oriented at 45° to the first wedge. This arrangement enables light to pass through with minimal losses before it is refocused into the output fiber collimator. Light propagating in the opposite direction passes through the second wedge and is similarly separated into two polarization states, but at a refracted angle not normal to the wedge's incident plane. The Faraday rotator then causes both states to rotate by another 45°, thus displacing the beams' angular propagation, which prevents light from being refocused into the input fiber.
In this architecture, a polarization-insensitive isolator assembled with specially chosen materials could operate with at least 45 dB of isolation across a wide 1530- to 1620-nm wavelength range. Other critical parameters of an isolator in an EDFA include insertion loss, polarization-dependent loss (PDL), and polarization-mode dispersion (PMD). Insertion loss directly affects the gain and gain saturation of the amplifier; PDL defines the change in insertion loss as a result of a change in the polarization of light; and PMD measures the transmission delay of the orthogonal propagation of light in the two polarization states.
The ultra-wideband WDM is produced by using an FBT process to combine light at 980 and 1570 nm. Until now, wideband pump WDMs were available only in a costly filter-based solution that required a pair of collimators and a thin-film interference filter to separate or combine the wavelengths. The FBT process is an attractive low-cost alternative that only requires two fibers to be fused to create a taper that allows light at the two wavelengths to transfer between both fibers. This transfer is possible because the fiber core has a higher index of refraction than air. The length and diameter of the fused region directly affect the optimal wavelength and performance of the pump WDM.
Because of the sinusoidal nature of the FBT process, the insertion loss generally is not uniform across the signal passband. Thus, commercially available WDMs made using the FBT process typically will have a high 0.5-dB insertion loss at the wavelength band edges of 1520 and 1620 nm. In order to overcome this problem, an advanced FBT process that controls the wavelengths at the length and duration of the tapered region has been developed. The result is a pump WDM with low insertion loss and high signal channel isolation across a 1520- to 1620-nm bandwidth.
As DWDM systems move toward higher channels to accommodate the growing demand for greater bandwidth, the wavelength window of opportunity also must open up possibilities for transmitting a wider range of channel signals. While this is limited by conventional optical components, recent research on ultra-wideband EDFA and amplifier components has shown great promise in enabling the reality of a wider wavelength window in high-capacity DWDM systems. Such component technology eventually will enable DWDMs to fully utilize the intrinsic capacity of the optical fiber.
Ronnie Chua is assistant marketing manager and Bo Cai is a senior engineer in the Technical Service Group of E-TEK (San Jose, CA).
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