The evolution of computer systems over the past two decades shows a tremendous increase in processing ability per square inch, as postulated by Moore's Law. However, a closer look at the progression of interconnection technologies shows growth trends that, while impressive, have not kept pace with processing capabilities. As a result, the movement of acres of data generated by high-speed processors has been hampered, making the interconnect a major bottleneck. To relieve it, systems and network designers are moving away from copper-based interconnects, with their inherently high capacitance, noise susceptibility, and relatively poor signal-loss/distance ratios, to more advanced, high-speed optical connections.
While copper links still carry the bulk of the load for workgroup-level networks, optical links are emerging as the preferred media for campus backbones, central-office networks, and WANs. These systems demand the low noise, faster transmission rates, zero crosstalk, and wide bandwidth that only optical systems can provide. Despite inherent advantages, the proliferation of optical connector technology has been slow, due to its relatively high cost of implementation. Recent advances in laser, packaging, and testing, however, are lowering this technology's cost premium, such that it is now penetrating both WANs and LANs.
As costs come down, technologies such as Gigabit Ethernet (GbE), which leverage existing Ethernet standards, are ideally positioned to provide the bridge between existing, copper-based, 10/100Base-T local networks and new, higher-bandwidth optical infrastructures. At the same time, fiber-based, high-speed data-storage networks, such as Fibre Channel, are rapidly being deployed to support enterprise-wide data warehousing strategies.
Implementation Challenges
A major challenge to the effective implementation of these new, high-speed optical links will be the creation of cost-effective, robust, standards-based optical transceiver components that can support Gbit/s bandwidths. As the demand for gigabit optical links accelerates, system designers need access to readily available, high-volume supplies of reliable, electro-optical, physical-layer components.
These optical transceivers will spark the gigabit revolution in much the same way the availability of standard Ethernet physical-layer components (PHYs) helped fuel the ubiquitous deployments and falling cost curves of 10Base-T and 100Base-T over the past decade. However, the design and production of optical interconnects presents a totally new and unique set of problems. These are associated with the cost-effective production of gigabit-speed lasers using standard semiconductor processes, and the effective packaging and alignment of these devices to optimally launch light into standard optical fiber.
The evolution of higher data rates and the migration toward optical links has created a need for greater flexibility in interconnect form factors. For instance, link distances and data rates in 10BaseT or 100Base-T links could typically be handled by a single type of physical-layer component, which could cost-effectively be soldered directly onto a system-level pc board, whether it's a network interface or a switch card.
But, the greater data rates required by optical gigabit links present network-configuration challenges that cannot always be cost-effectively resolved by a common interconnect or transmission methodology. For example, GbE links may be implemented either as 1000Base-SX links using less expensive, short-wavelength (850-nm) laser technology over multimode fiber, or as 1000Base-LX links using more expensive, long-wavelength (1300-nm) laser technology over multimode or singlemode fiber. Depending upon the fiber diameter used, 1000Base-SX can support distances up to 550 m on 50-µm multimode fiber, and up to 275 m on 62.5-µm multimode fiber. On the other hand, 1000Base-LX can support distances of 550 m on either 50- or 62.5-µm multimode fiber, and up to 5000 m on 9-µm single-mode fiber (see the table).
Because long-wavelength transceivers for 1000Base-LX interconnects are inherently more expensive than short-wavelength, 1000Base-SX components, it's important to instill a higher degree of configuration flexibility into the deployment of optical transceivers. Along with the traditional 1 by 9 (1 row by 9 pins) solderable component form factor used throughout copper-based Ethernet and Fiber Distributed Data Interface (FDDI) implementations, GbE links can also be implemented using complete module-level pluggable transceivers based on the widely-adopted industry-standard Gigabit Interface Converter (GBIC) form factor.
Not only do interchangeable GBICs give network administrators the flexibility to tailor network topologies and link distances and costs to specific requirements, they also allow for subsequent network reconfiguration as needs change—without wholesale replacement or system-level investments.
Design Issues
The design and manufacture of cost-effective, optical interconnect solutions for use in Gbit/s GBICs, hinges on successfully meeting reliability and distance requirements of the GbE specifications. At the same time, companies need to drive down packaging, alignment, and test costs of these devices to support high-volume deployments.
Reliability and distance: The most critical factor impacting the reliable transport of optical signals over practical distances is differential mode delay (DMD), important because light traveling through multimode optical fibers tends to bounce through the fiber at different angles, depending upon its distance from the center of the fiber. This results in different propagation paths (modes) for different parts of the fiber. As a result, a signal launched across the entire fiber tends to "smear" as it goes through the fiber. In essence, the same signal is being simultaneously propagated down the fiber following different path lengths in the center than at the edges. Thus, a digital pulse of a given output power will tend to move toward a bell-shaped curve as the signal is smeared over a given distance.
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