SPECIAL REPORTS: Passive & Active Components
VCSEL transceivers gain momentum at a 10-Gbit/sec convergence point for Ethernet, SONET, and Fibre Channel.
TRACY EARLES, Picolight Inc.
The optical communications industry is poised to reap the benefits of physical layer data-rate convergence now that Ethernet and Fibre Channel can scale to 10 Gbits/sec. With the development of the 10-Gigabit Ethernet (10-GbE) standard, the cost, implementation, and maintenance benefits of enterprise packaging models can be applied to packet-switching technologies in MANs and WANs. These advances will bring economies of scale to the high-bandwidth optical communications systems market and the opportunity to fundamentally change the face of networking.
One technology helping to make that all possible is multimode 10-Gbit/sec optical modules based on vertical-cavity surface-emitting-laser (VCSEL) transponders. Already deployed in very-short-reach (VSR) enterprise backbone applications, short-wavelength VCSEL modules are now moving into the bandwidth-challenged, intra-point-of-presence switch environment. Long-wavelength VCSEL modules will soon move into the metropolitan-area access space. By providing a common optical platform, these short- and long-wave modules will enable connectivity between enterprise networking boxes and long-haul transport equipment, effectively extending the reach of enterprise networks to 10 km and beyond. Importantly, the stage will be set for manufacturers to slash today's $10,000-$30,000-per-port equipment costs to as little as $1,000, making it economically feasible to solve today's critical metro-access bandwidth bottleneck.
With the successful completion this year of interoperability testing for 10-GbE optical components over multimode fibers, the industry is acknowledging the technical feasibility of 10-Gbit/sec optics. Now, as equipment manufacturers move into the system-development stage, key implementation and test decisions will be made. System designers must maximize the effectiveness of their product roadmaps by planning ahead for long-wave solutions, taking advantage of key optical module packaging capabilities that will enable them to provision bandwidth and protocol compliance on demand, across the full spectrum of short-reach (SR) to 80-km link requirements.
Short-reach optical solution
In 1999, the Institute of Electrical and Electronics Engineers formed a study group to investigate 10-GbE as a higher-speed addition to the IEEE 802.3 Ethernet standard. The IEEE determined that 10-GbE should include a short-wave optical solution. This critical decision ensures the cost and performance necessary to push optics all the way from the Internet's core to its access edge. Due to be approved in March 2002, 10-GbE will leverage the confluence between SONET OC-192 (10 Gbits/sec) and 10-GbE.
The Optical Internetworking Forum (OIF) published an implementation agreement last year that also supports short-wave optical solutions for SONET applications at the OC-192 rate. For the first time, these standards will allow a single piece of equipment to address both SONET and Ethernet applications. This marriage of SONET and Ethernet technology may not end at 10 Gbits/sec either: there is speculation of another convergence at 40 Gbits/sec, or the SONET OC-768 data rate. That would represent yet another opportunity to build a bridge between the optical enterprise and Internet core.
Although distributed-feedback (DFB) lasers and Fabry-Perot (FP) lasers will play a role in 10-GbE market development, VCSELs more readily meet the cost, power, and reliability objectives now formalized by the IEEE 10-GbE task force for high-volume, mainstream 10-GbE applications. Pre-standard 10-GbE VCSEL transceivers have been publicly demonstrated, and volume shipments are expected to commence with the standard's finalization.The next step is to prove the technical feasibility of VCSEL-based 850-nm optical modules interoperating in multivendor 10-GbE applications as defined in the IEEE P802.3ae draft. To support the ratification of this IEEE standard, a working group was formed to show feasibility of 850-nm serial technology. The group included Picolight, another optical-component manufacturer, and fiber and cable suppliers like Cielo Communications. One of the efforts to demonstrate feasibility was to test the interoperability of 10-Gbit/sec 850-nm serial links developed at independent companies. Another effort to show feasibility was to illustrate compliance to key parameters within the current 802.3ae draft 3.2. Both efforts were successful.
Participants in the interoperability tests selected the 10-Gigabit Sixteen Bit Interface (XSBI), which is the dominant optical-module implementation for 10-GbE and is defined as the SFI-4 interface in the OIF OC-192 VSR standard. The first 300-pin multisource agreement (MSA)-based pluggable XSBI module for short-wave 10-GbE and OC-192 VSR applications was introduced last May.
The interoperability test consisted of data transmitted between the optical-component manufacturers' 10-Gbit/sec 850-nm serial transponders over types of multimode optical fiber found in today's existing cable installations, including high-bandwidth multimode fiber. Link distances ranged up to 450 m, depending on fiber type. By achieving a 450-m link, the interoperability working group exceeded the criteria set by the 802.3ae specification by a margin of 50%, demonstrating that the 850-nm optical link maintains signal integrity without performance degradation.
The 10-GbE transponders used in the demonstration featured integrated transmit and receive functions along with a 300-pin MSA serializer/deserializer device in a small footprint module (see Figure 1). In the transmit direction, an electrical serializer multiplexed the 16 x 644-MHz data channels, laser driver, and 10-Gbit/sec directly modulated 850-nm VCSEL, whose output was directly coupled to a multimode-fiber pigtail. The receive direction comprised a multimode-fiber pigtail coupled to a PIN photodiode and transimpedance amplifier, followed by a post amplifier and a deserializer, which converted the optical signal back into a 16-bit-wide data stream.
P802.3ae specifies five link distances over fiber types representative of the majority of the VSR interconnects within central-office environments. To verify the technology's ability to operate over the existing installed fiber base, all five of the link distances were tested for interoperation over all standard fiber types. For the testing, multimode fibers pre-terminated to the five link distances were provided by the fiber and cable manufacturers. In addition, these manufacturers provided several samples of the 2,000-MHz-km-bandwidth fiber terminated at 350- and 450-m link distances. That was done to demonstrate that 10-Gbit/sec 850-nm transmission technology operates successfully over longer distances under the most stressful conditions. It shows that the actual specified target distances are very conservative.
The 850-nm solutions used in these demonstrations are expected to be one-10th to one-half the cost of 1310- or 1550-nm devices, providing customers with a more affordable option for short-distance 10-Gbit/sec links in high-volume networking applications. Additional demonstrations were conducted at Networld+Interop last September. 10-GbE optical modules were used to interconnect a Layer 3 switch from Foundry Networks-which uses 10GBase-LR and -SR line cards-to a 10-GbE network performance analysis system from Spirent Communications. The optical link enabled by the 850-nm transceivers, both XSBI and 10-Gigabit Attachment Unit Interface (XAUI) modules, operated continuously throughout the conference over a 300-m link.
In addition to the 300-pin MSA package footprint, the industry is evaluating alternative implementations such as XAUI technology. The first 10-Gbit/sec serial-to-XAUI short-wave fiber-optic transceiver using VCSEL technology was introduced last August. By grafting an XAUI transceiver into the popular 300-pin MSA transponder package form factor, this implementation enables switch manufacturers to add system bandwidth with minimal chassis and blade redesign. With these solutions, it will be possible to upgrade a current router or switch to 10-GbE without a mechanical overhaul. The XAUI electrical interface employs only four differential data lines for data communications with the 2x3-inch module, while still using the industry-standard 300-pin multisourced connector footprint.
Meanwhile, a consortium of companies is developing module definitions for the 10-Gigabit Pluggable (XGP) and XENPAK transceiver packages. The availability of widely adopted 10-GbE module definitions will provide manufacturers with a number of options for system design.
Although 10-GbE optical-module technologies can indeed be leveraged across Ethernet, SONET, and Fibre Channel implementations, each application does involve different data rates and therefore requires slightly different testing procedures. It is generally the responsibility of the module component vendors to assure specification compliance, while equipment manufacturers need to implement a set of minimal parameter-verification tests to ensure functional and stable basic operation within a 10-GbE or serial short-wave OC-192 VSR system.
Jitter analysis
One of the more important parameter-verification tests, as specified both for 10-GbE and OIF implementations, is an analysis of the jitter specifications for the link. The 10-Gbit/sec systems push the edge of technology both optically and electronically; noise and signal degradation have greater effect on link stability because of the extremely high data rate. That makes it more difficult-and important-to confirm specification compliance, thereby ensuring the link stability of any high-speed system.
Jitter specification compliance requires jitter testing of the transmitter and sensitivity tolerance testing of the receiver. Transmitter testing involves generating a continuous test pattern from the bit-error-rate (BER) tester into the optical-electrical (OE) converter. Since there is no consistent or repeatable way to create a worst-case fiber link in a short-wave system, it is necessary to use a tightly specified OE and phase locked loop (PLL). With a stressed link, the BER tester (BERT) scans the center of the eye, creating a "bathtub" curve as the measurement points are plotted-any points in the "open eye" region of the curve fail the BER test. A high-quality OE, PLL, and BERT and a 2-5-m optical cable are essential for creating consistent and repeatable measurements.
Perhaps even more critical than the transmitter test is the receiver jitter tolerance test. The purpose of the receiver test is to gauge the ability of the module's receiver OE to perform over worst-case link conditions and over a specified range of ambient environmental conditions. Link stability and low BER (typically 10-12 without forward error correction) are both important. The procedure is implemented by controlled signal degradation to a specified degree of "eye closure" while measuring BER.
The test is typically performed over the module's specified operating-temperature range. Receiver sensitivity is measured by intrinsic- or stressed-sensitivity methods, with the latter representing a real working link and therefore considered more indicative of the performance of the module.
Another important optical parameter is optical modulation amplitude (OMA), which specifies the range, or optical envelope, in which the receiver will adequately detect an incoming signal. The current IEEE 802.3ae D3.2 draft document specifies the stressed received sensitivity for OMA, and in the case of 10-GbE optical transmission systems, it is used instead of extinction ratio to define the receiver's input-signal swing acceptance range. Assuming a minimum extinction ratio, the OMA confines the incoming transmitter signal swing to within the receiver's dynamic range, optimizing transmitter and receiver compatibility and relaxing extinction-ratio constraints without compromising link quality.
The other important components of optical parameter verification include an analysis of the eye-mask of the transmit-optical and receive-electrical signals, optical link budget (in terms of total insertion loss), wavelength, and spectral width. As module and equipment manufacturers finalize their test procedures, these companies will need to make decisions early in the process to lower the economic and resource demands of compliance testing. Due to the diligent effort of the IEEE, much of the specification drafting for physical media-dependents (PMDs) operating in the 10-Gbit/sec window is finished. The OIF, via the Serial Shortwave OC-192 VSR standard (OIF-VSR4-04.0), and T11.2, via 10-Gigabit Fibre Channel, are leveraging this work wherever possible.
Extending reach, versatilityUntil now, high-bandwidth connections in the enterprise were not easily extended, creating a chasm between the short-distance optical enterprise network and the optical Internet environment. As 10-GbE reaches the enterprise backbone, the speed gap will become even more apparent. A 1310-nm VCSEL coupled into singlemode fiber is considered the most cost-effective solution to this dilemma. It can provide a 10,000-m link using the same materials, packaging, and fabrication approaches, already proven with high-volume, SR VCSEL transceivers. That is where VCSELs have the opportunity to fill a critical bandwidth gap that exists between enterprise backbones and the access network (see Figure 2).
Long-wave VCSEL-based transceivers will also enable optical service provisioning on demand by using the same hot-pluggable, compact package configurations as those proven in the enterprise and SAN space. Hot-pluggability revolutionized the way enterprise LANs and SANs are designed and deployed by making it possible to purchase a 32- or 64-socket concentrator or switch, then populate ports as needed without bringing down the network. In the rapidly emerging access equipment market, which is expected to dwarf the network core in terms of unit shipments, the combination of hot-pluggability, longer reach, and 10-Gbit/sec physical layer data-rate convergence will have an enormous impact. It will enable equipment de signers to provision bandwidth on demand-so valuable in the enterprise LAN and SAN market-while also provisioning SONET or Ethernet connectivity on demand, as required.
Ultimately, the concept of provisioning bandwidth and protocol compliance on demand will extend across the full spectrum of SR to 80-km link requirements. Some vendors will offer integrated families of transceivers that span all three types of modules. At the low end, that will include cost-effective VSR 850-nm VCSEL solutions. For medium- and long-reach applications, that will include 1310-nm VCSELs with DFB-like performance, FP lasers, and DFB solutions and will extend all the way to 1550-nm DFB transceiver products. True convergence will be achieved when the choice to support Ethernet or SONET-in either bandwidth increment, across any link distance-is as easy as dropping in the appropriate optical module. System designers who plan for this capability can maximize their investments across significantly broader product portfolios.
High reliablity, superior power
Meanwhile, VCSEL transceivers offer an ideal first step. These devices promise high reliability with superior power and thermal efficiency for space-constrained equipment racks. VCSEL transceivers also provide manufacturing advantages and can support the full range of OC-192 SONET and 10-GbE requirements, including the gray area between classic Ethernet and SONET applications. The 850-nm multimode solution offers many advantages for SR high-volume networking applications, which represent the vast majority of ports.
Proof of interoperability and technical feasibility for serial short-wave 10-GbE VCSEL modules last year was a key milestone on the road to final 802.3ae ratification this year. With close attention to implementation and test issues, equipment manufacturers can count on the cost-effective, high-speed data links needed to meet next-generation networking requirements.
Tracy Earles is a product-line manager at Picolight Inc. (Boulder, CO).