Seeing the light F
or approximately ten years, there has been a trend towards developing universal digital I/O interfaces for data
centres and telecommunication applications, whixh have been primarily driven by the desire to have general- purpose ports that are able to accept plug- in modules for various types of media, depending on the data rate and the application. The advantage of this is a uniform, general-purpose I/O assembly that can be configured with modules according to local circumstances only at commissioning time. To fulfil electromagnetic compatibility
(EMC) requirements, designers have developed “cages,” which are usually mounted on the PCB of the module and have an EMC seal at the front that mates with the front panel and a host connector at the back.
The “grandfather” of all plug-in cages, called the GigaBit Interface Converter (usually abbreviated as GBIC), is only a distant memory today. It was the first cage with gigabit bandwidth and a 9-pin subminiature D connector as the host connector. Since then the bandwidth demands of the
Herbert Endres looks at the advantages of using active optical cable assemblies
Along with the proven Small Form factor Pluggable (SFP) variant, which has now been boosted from 5 to 10 Gbps, there is Quad SFP (QSFP), which allows a bandwidth of 40 Gbps to be transmitted over four lanes (with each lane consisting of a pair of RX and TX links). However, the largest bandwith can be obtained with the CXP form factor, which, in the InfiniBand standard, enables twelve lanes at 10 Gbps each, for a total of 120 Gbps. This form factor (with different mechanical coding) also supports the 100 Gigabit Ethernet standard, with only ten of the twelve lanes being used. The latest standard comes from the Storage Working Group, and has been dubbed iPass+ HD (High Density). In the SAS 3.0 variant, it is designed to support four lanes at 12 Gbps each for storage media such as hard disks and solid-state disks, with a front panel opening measuring just 12.07 x 11.68 mm.
Leaving the cage Convertible I/O cages were originally developed for modules used with a diverse signal transmission media, including copper cable as well as RJ45 Gigabit Ethernet, coaxial cable and fibre optic cables, either single- mode or multi-mode, with various core diameters. Due to increased data rates, only three configurations have actually achieved a persistent market presence:
twinax copper cable, active twinax copper cable with equalisers and fibre optic
industry have advanced rapidly and now 1 Gbps is just about DC, and the InfiniBand Trade Association has adopted the Fourteen Gigabit Data Rate (FDR) standard, which supports a data rate of 14 Gbps. The Extended Data Rate (EDR) standard, with a speed of 25 Gbps, will be introduced in the coming years and as a result current cage standards are being refined to enable these new data rates to be managed at the front panel.
34 December 2011/January 2012
transceivers. Unfortunately, all three versions have their drawbacks: passive twinax copper cable has a maximum transmission distance of approximately 10 metres at 10 Gbps, and thick twinax conductors must be used to keep attenuation low, this limits the bending radius of the cable; active twinax copper cable manages with thinner twinax conductors, but due to the electronics in the module it has a power dissipation of around one watt per lane. The transmission distance is limited to approximately 25 metres at 10 Gbps and if using fibre optic
Components in Electronics
transceivers they will require additional depth due to the fibre optic connector, and the SFP module has a power dissipation of over 1 watt. The commonly used LC connectors require careful handling to avoid contamination and achieve proper alignment of the fibres. It is preferable to have a data cable that avoids all of these drawbacks.
Non Plus Ultra: active optical cables
Luxtera has developed a new technology in the domain of silicon photonics. It transmits data using light with a wavelength of 1490 nm, which can also be carried in silicon waveguides. This technology has many advantages including:
1. Since the light is guided through the silicon, integrated light modulators can be used à control electronics, light modulators and receivers can be integrated in a single CMOS chip.
2. Multiple lanes can be powered by a
shared laser operating in CW mode à very low system power dissipation and long lifetime thanks to using a CW DFB laser. 3. The fibre optics
are coupled directly into and out of the CMOS chip. This allows the use of monomode fibres, which enables a significantly longer transmission distance à transmission distance up to 4000 metres at 10 Gbps thanks to very low dispersion. There are products on the market that use VCSEL lasers but these have a limited transmission rate (maximum 16 Gbps) and restricted transmission distance due to the need to use multimode fibres, which are limited to 100 metres due to dispersion. In March this year, Molex took over the fabrication of Luxtera active optical cables (AOCs); the cable consists of a single photonic chip equipped with a single continuous wave distributed feedback (CWDFB) laser, which is shared across four externally modulated channels, and only a few additional components. Active optical cables are presently available in the form of QSFP AOC devices with 40 Gbps data rate. Their advantages in active operation are many and include: • Higher reliability thanks to lower
component count and long-life DFB lasers. The average useful life of a DFB laser is 12 years, compared to 5 years for a typical VCSEL laser (four of which are contained in conventional AOCs).
• Power dissipation is only 780 mW per cable end, versus 1.1 to 1.6 W with comparable products. This impacts direct energy costs as well as indirect energy costs for cooling, • Multi-protocol capable for InfiniBand,
Ethernet, Fibre Channel and PCI e. • Bit error rate (BER) better than 10-18 – much lower than the industry standard expectation of 10-15.
• Higher level of integration allows
extremely low package density. • Reliability more than twice the estimated 5 million hours MTBF. • Interoperability proven by InfiniBand
Trade Association (IBTA) 2010 plugfest: 40G-IB-QDR Certified InfiniBand Cables • The same concept can be used for data rates of 25 Gbps and above.
This AOC needs only a few components; by contrast, competitive products tend to be stuffed full with components. A QSFP FDR version with 4x 14 Gbps will be available shortly; prototypes are currently available and prototypes of the 100 Gbps version (4x 25 Gbps) are scheduled to become available in mid-2012.
Molex and Luxtera are collaborating on the development of transceivers based on the OptoPHY concept that can be mounted directly on the module, for data rates of 10, 14 and 25 Gbps.
Molex Incorporated | www.molex.com
Herbert Endres is Director of Technology Marketing at Molex Incorporated
| Page 2
| Page 3
| Page 4
| Page 5
| Page 6
| Page 7
| Page 8
| Page 9
| Page 10
| Page 11
| Page 12
| Page 13
| Page 14
| Page 15
| Page 16
| Page 17
| Page 18
| Page 19
| Page 20
| Page 21
| Page 22
| Page 23
| Page 24
| Page 25
| Page 26
| Page 27
| Page 28
| Page 29
| Page 30
| Page 31
| Page 32
| Page 33
| Page 34
| Page 35
| Page 36
| Page 37
| Page 38
| Page 39
| Page 40
| Page 41
| Page 42
| Page 43
| Page 44
| Page 45
| Page 46
| Page 47
| Page 48