Developments are matching needs on today's communications equipment backplanes as manufacturers move towards optical technology and fibre replaces copper. This move is being driven principally by the need to fit more traffic onto communications systems. Eric Russell reports.
Backplanes are often the greatest bottleneck in a communication system. Increasing speed is one area in need of improvement. Fibre optic cable can carry much more data than copper and is now permeating through to the higher volume marketplace of local loops.
The enabling developments behind optical backplanes include vertical cavity surface emitting lasers (VCSEL). These lower cost devices enable a cost effective laser on the backplane, boosting light levels locally and eliminating the attenuation when light paths from a central laser change direction or are split.
In addition, today's connector designs, from companies such as Molex and US-based Stratos Lightwave, feature much lower levels of light loss than early designs. Back reflections have been cut, reducing the potential for interference. Multi-way connectors can now accept 16 fibre optic cables either individually or in a ribbon format.
This has led to the use of parallel optical transmission where a popular format is an array of 12 transceivers of 2.5Gbps capability giving an aggregate bandwidth of 30Gbps.
Economics and market forces are also driving change. As fibre optics move from long haul operation and are being used more in local, or metro, systems, the volume of optical equipment is rising rapidly. This is placing economic pressure on manufacturers to drop prices and produce more standardised units in the face of greater sales volumes.
Metro optical systems have significantly different operational and economic requirements from the long distance market. Shorter network spans, tougher system price points and more interconnectivity require new approaches.
Michelle Rae McLean, Director of Strategic Marketing at Pluris, the leading US manufacturer of routers, says an array of four to twelve VCSELs can create a 15Gbps link at about one tenth the cost of a single high power laser solution.
At the gigabit speeds demanded by today's internet and telecoms traffic volumes, optical solutions offer a number of benefits over electronic options. In an electrical backplane at these speeds, performance is limited by signal propagation delay, skew, crosstalk and ground loops. Optical transmission avoids these problems.
Fibre optic backplanes also mean greater deployment flexibility, not only within racks but between them. As internet hotels and server farms grow, optical transmission maintains high transmission speeds as the distance between racks of equipment increases.
A copper interconnect forces a service provider either to move existing equipment to accommodate the proximity requirement or to leave contiguous space empty in anticipation of future growth. But, with optic technology, carriers can locate parts of the same router on different floors, even in different buildings, providing some added disaster recovery advantages as well.
Improved future-proofing
Improved future-proofing is another important benefit. Rarely can one product support copper technologies running at different speeds, so when a vendor increases the capacity of the system, old and new generation components are not compatible.
The fibre does not generate any electrical interference to create problems with neighbouring components at different speeds, and the fibre medium itself can accommodate a range of speeds with no changes.
McLean says that replicating the Pluris TeraPlex 20 router backplane in copper would have required approximately 36864 tracks, an unsolvable troubleshooting problem from an interference standpoint. It also would have added as much as 35per cent more weight to the system.
Pluris has just demonstrated a 40Gbps routed connection between its TeraPlex core IP routers to show their capability, scalability and fault tolerance. The router's TeraConnect fibre optic backplane and chassis interconnect enables service providers to locate chassis as far as 100 metres apart with standard units.
Joseph Kennedy, president and CEO of Pluris, says that while the ability to scale capacity is one key element, simply increasing port counts and fabric is not enough. New levels of system availability and reliability such as transparent control card fail-over and IP-Bond, Pluris' port-aggregation technique, are also needed.
IP-Bond enables service providers to group multiple physical ports into a single, logical, fat pipe, simplifying interface management and scaling bandwidth. Next-generation architectures such as TeraPlex are designed to grow system capacity with multiple chassis acting as a single router.
Internal switch capacity is used instead of line cards to interconnect boxes. The target is to meet the five 9s reliability that service providers expect from voice telecom equipment, with no single point of failure in the architecture.
Redundant control cards, internal links and system modules coupled with multiple forwarding paths are all important. But ensuring that redundancy enables non-stop operation, without disrupting traffic, is what really distinguishes a fault tolerant router. But Kennedy admits that optical fibre busses do increase power requirements compared to traditional copper backplane, by about 10 to 12per cent. He is also finding that telecoms and internet service providers prefer to upgrade equipment rather than buy in new to keep up with market growth. New equipment means extensive testing, high capital cost and potential service disruption.
But using scalable equipment requires highly flexible backplane design and the development of standardised optical transceiver modules so backplanes can be expanded and modified using readily available units
To make the most of the potential of optical backplanes, several companies are developing multi-source agreements to ensure availability of compatible transceiver modules and connectors from different manufacturers. Backplane manufacturers can then postpone final board assembly until the customer orders, knowing components are readily available and ensuring a custom product can be delivered very quickly.
One group of companies has recently announced a new initiative for next-generation 10 Gigabit Pluggable (XGP) fibre-optic transceivers that will offer their customers maximum configuration flexibility, resulting in quicker time to market.
The group comprises Blaze Network Products, E2O Communications, Finisar Corporation, Ignis Optics, Infineon Technologies, Intel, Molex, Optical Communication Products, Picolight, Samsung, Sigma-Links (a joint venture of Fujikura and OKI) and Tyco Electronics. Members of this XGP group will independently develop 10-gigabit transceivers based on a multiple source agreement.
The specification covers five 10-gigabit transceiver types: 850nm serial, 850nm coarse wavelength division multiplexing (CWDM), 1310nm serial, 1310nm CWDM and 1550nm serial. The companies intend to work on a common design interface for multiple protocols in 10-gigabit networking and storage applications.
Bill Wiedemann, chairman of the XGP MSA, says the Z-axis pluggable XGP is targeting a size that is approximately the same as the standard GBIC to maximise the potential for system designers to pack more ports on a line card.
Fibre-optic system designers using the XGP transceivers can take advantage of postponement manufacturing, configuring the host just prior to shipment, allowing flexibility and just-in-time cost and inventory advantages.
Around the suppliers
For low cost, small form factor, low power applications, Applied Micro Circuits, through UK sole distributor Amega Group, has recently announced enhanced chip sets for its range of high bandwidth silicon solutions for optical networking.
The S7022 is a 3.2Gbps quad VCSEL driver and the S7025 is a matching transimpedance amplifier. The set can be used on a backplane or rack-to-rack connectivity over 300m. A number of on-board features reduces the number of external components needed.
Infineon produces the components for optical backplanes, which it says are mostly designed and produced by telecoms equipment manufacturers at the moment. It adds that although a purely optical backplane does not yet exist, developments in optical switches will make it possible in the future.
Molex says that optical transmission equipment is being pushed deeper into the metropolitan network and local loops, ramping up demand for optical backplane solutions.
To meet the demand, the company has developed high density, multi-way connectors up to 16 ways to suit all popular types of rack and housing. Its high density backplane MT connector, for example, provides edge connector capability between system PCBs and backplane. Connector features include a patented dual shutter to keep out dust and to protect eyes from laser light.
Molex has also developed gigabit interface converter (GBIC) modules to help in optical backplane design. To counteract the problems of noise when a number of optical transceivers are used on a board, the company offers superior shielding and a fully grounded metal housing and rail system.
Stratos Lightwave offers the MPX optical backplane connectivity system that holds up to 12 fibres in a single housing. Cable assemblies are also available.
Zarlink, previously Mitel, develops and manufactures parallel fibre optic modules. Latest product is a 12 channel 2.5Gbps transceiver module. This adds to a range of receiver and transmitter modules based on VCSEL technology.
While most developers are still talking gigabits, Roke Manor Research has moved onto petabits with Ripcore, which it claims is the fastest internet traffic router in the world. It uses a large array of transceivers and patented multiplexing and compression routines to achieve 5.12 terabits per second at the moment, scalable to 1.3 petabits per second.
It is obvious that the potential of optical backplanes is just being realised and is the key to ever higher data transmission speeds.
