Which SFF technology will network equipment suppliers embrace?

Oct. 1, 1999

Test results show two types of small-form-factor connectors support both singlemode- and multimode-fiber networks.

Emerging fiber-connector technologies collectively known as small form factor (SFF) are fueling a major change in the way networking systems are designed and configured. SFF technologies are the driving force behind the recent shift in network design toward smaller form-factor products that incorporate SFF connectors. Other factors influencing this trend include space constraints, higher-bandwidth demands, cost, and a need to "futureproof" the network.

User expectations, the massive migration to enterprise computing, and Internet use have also accelerated the need to pack more bandwidth and performance for more users into systems residing within campus equipment rooms. Faced with these issues, network managers are desperately seeking ways to balance user demands with their own cost/performance objectives. They are also searching for futureproof network solutions as a precursor for fiber-to-the-desk.

Over the past 15 years, network-equipment providers and the cabling industry have scrambled to keep pace with these spiraling demands. As a result, vendors have pushed for adoption of standards-based approaches to evolving local-area and wide-area networks. By ensuring performance interoperability for multivendor solutions across the entire infrastructure of hardware, switches, protocols, applications, and cabling, customers can select from a wide range of choices based on accepted and well-understood industry standards. For vendors, the competitive nature of this evolution has brought about rapid technological advances, which have yielded sustained product sales.

Getting to where we are today has not been a smooth ride, however. When the industry embarked on the standards-based approach to networking, there was a wide range of cabling choices available. These proprietary cabling options finally coalesced into acceptance of Category 3 twisted-pair cabling for a majority of networks. Through an ongoing standardization process, Category 5 twisted-pair cabling is now the norm. History shows that the cabling industry can effectively work with standards organizations to develop common platform solutions that benefit general markets.

Similarly, the development of widely supported SFF-based networking technology is essential to future advances. With last summer's ratification of the IEEE 802.3z Gigabit Ethernet (GbE) standard, network backbone traffic will quickly approach a new baseline performance level of 1 Gbit/sec. A 10-Gbit/sec Ethernet standard is already under consideration by the IEEE's Higher Speed Study Group. The group's aggressive schedule should bring about a new standard in approximately three years. Initial expectations are that a new generation of gigabit switches will be deployed as high-speed backbone connections (see Fig. 1). GbE will also serve as the primary access protocol for storage area networking and other server-based storage technologies.
Fig. 1. After last summer's ratification of the IEEE 802.3z Gigabit Ethernet standard, initial expectations are that a new generation of gigabit switches will be deployed as high-speed backbone connections.

At these higher levels of bandwidth, Category 5 twisted-pair cable technology is eclipsed by fiber. And SFF fiber-connector technology is the logical solution that will allow widespread deployment of more-robust networking products.

Achieving GbE performance requires the use of laser-based light sources. For large-scale multimode applications, the development of vertical-cavity surface-emitting lasers (VCSELs) has been essential and their volume availability for use in GbE systems is timely. VCSEL-based transmitters can operate over both multimode and singlemode fiber plants. With the cost-effective light-source problem solved, the next challenge entails which style of connection is best suited for GbE applications.

Many factors merit consideration in assessing connector suitability. First, the majority of existing local/area network fiber plant is multimode, so the connector solution must cost-effectively support multimode operation. At the same time, however, the convergence of telecommunications and data networking will drive singlemode-fiber demand, so the connector solution must also support singlemode operation. To find a solution for both these constituencies, industry support must be provided by leading OEMs and network solution providers. A range of transceivers, switches, and cabling solutions must be offered, and they must deliver a favorable cost/performance ratio if they are to be accepted in the marketplace.

In this context, a good starting point for SFF connector analysis is to compare the technical performance of today's dominant SFF connector types. High-end system performance and the ability to support both multimode- and singlemode-fiber networks are primary factors to consider.

Computer Crafts Inc. (Hawthorne, NJ), in cooperation with IBM Corp., conducted a study of LC, MT-RJ, SC-DC, and VF-45 SFF connectors. Criteria for testing included mechanical, optical, and environmental performance. The testing objective was to identify a connector type that offered across-the-board balanced performance. The full results of this test program were presented in June at the Electronic Components and Technology Conference in San Diego. These results are available on the Computer Crafts Website (www.computer-crafts.com).

No connector type was determined to be the "best connector." However, tests did reveal that two connector styles--the LC and MT-RJ--at this time could support the ever-growing need for singlemode capability and performance. Commonality across singlemode- and multimode-fiber plants helps drive down component costs and is key to futureproofing any total solution. From the analysis, it appears that the LC (originally designed for singlemode fiber) and the MT-RJ (initially designed for multimode fiber) connectors offered viable across-the-board solutions.
Fig. 2. Computer Crafts' test data on the connection loss for MT-RJ connectors used with singlemode fiber.
A side-by-side comparison of LC and MT-RJ technologies is extremely difficult. Figure 2 shows return losses for the LC. Figure 3 shows connection losses for the MT-RJ.
Fig. 3. Computer Crafts' test data showing the return losses for LC connectors.

The connectors as well as the cables and the transceivers are produced by a number of different companies--with varying degrees of success. In the cable/connector evaluation, jumpers assembled by two different vendors using connectors from the same manufacturer were compared with jumpers assembled by two different vendors using different connector manufacturers. The outcome of this comparison clearly illustrates that connectors of the same style do not always perform comparably.

Even if connectors appear to be equal in performance, a final assembled jumper may not be. This trend holds true throughout the product sets. As a result, end users must carefully select their LC or MT-RJ suppliers to achieve the best jumper performance results. In addition, issues such as system density, current and future costs, availability, and industry acceptance should be factored into an SFF solution.

Ultimately, the switch and hub vendors will strongly influence which SFF connectors will dominate the cabling market. Cisco Systems recently introduced a range of FX modules, based on the MT-RJ interface, for their Catalyst product series. Cisco holds over 43% of the gigabit switch market, according to Dell'Oro Group research. Thus, Cisco's chosen fiber interface could sway future connectivity trends.

In the server and high-end-system market, the decisions made by IBM on connector interfaces often set a benchmark for the industry. IBM's System/390 Enterprise Server Division has expressed strong interest in using the MT-RJ connector for both multimode Escon links and singlemode links at 1-Gbit/sec and 2-Gbit/sec data rates. This decision was based on an extensive evaluation of different SFF-connector interfaces conducted last year by Dr. Casimer DeCusatis of the IBM S/390 network hardware development lab (Poughkeepsie, NY) in cooperation with IBM Research (Yorktown Heights, NY) and Computer Crafts Inc. Other IBM divisions are also interested in adopting the MT-RJ for applications, including Escon storage-control units, Asynchronous Transfer Mode, GbE, and dense wavelength-division multiplexing equipment.

Meanwhile, within the building infrastructure industry, SFF technology suppliers continue to argue over whose connectors are the better choice for the end-user community. Once again, over time, the market will decide. When customers vote their preference, leading OEMs will embrace the technology that best fits the needs of their existing end users.

Sydney Hogg is a managing director at Computer Crafts Inc. in the United Kingdom.