Ten-gigabit Ethernet, also known as 10GBase-T or 10-Gbit/second Ethernet, marks a natural transition in speed, as wider bandwidths are required for moving and storing data. When data transmission consisted of simple e-mails, 10-Mbit/s performance was sufficient. But in today's market--where data centers require 10 Gbits/s to send files back and forth from storage warehouses that contain terabytes of data, and where consumers demand TV program and movie downloads--it is shortsighted to look at anything less than 10GBase-T.
Moving from 1 Gbit/s to 10 Gbits/s requires a wider transmission bandwidth, necessitating the use of components, such as magnetics and silicon, that can perform over the wider bandwidth. Designers can't just replace a 1-Gbit/s module with a 10-Gbit/s module; other design changes have to be made to maximize the performance gains. For instance, as frequency increases with more-complex coding schemes, parasitics (such as leakage inductance) and distributed capacitance will no longer meet the required performance standards. Those factors have to be fine-tuned in a 10-Gbit/s Ethernet solution.
With the approval of the IEEE 802.3an standard, the 10GBase-T specification that establishes standards for running 10-Gbit/s data rates over four pairs of Category 6 copper cabling, designers now have a low-cost alternative to fiber. As a result, designers are reevaluating the platforms they're using as they develop 10-Gbit/s solutions.
There are various modules and system combinations that designers of telecom, datacom and network applications can use. The magnetics, in particular, are a key component of the system.
One difference in designing a system for 10 Gbits/s, as opposed to 1 Gbit/s, is in the traces on the printed-circuit board. A small change in parasitics makes a difference in the system's performance. Traces must be short, and impedance needs to be perfectly matched from beginning to end.
Inductance will also change. In 100Base-TX Ethernet (100-Mbit/s) applications, 350 microhenrys of inductance are required. In 10-Gbit/s Ethernet, there is no minimum inductance requirement. To meet the wider bandwidth, of 500 to 600 MHz, inductance needs to be low. The typical inductance for 10 Gbits/s is around 200 µH. Therefore, there are fewer turns. That results in lower parasitics at high frequencies.
As speed increases, so does the importance of minimizing noise. This can be accomplished by selecting good common-mode chokes in a discrete magnetics package and a shield around an integrated package.
Now that the IEEE 802.3an standard has established guidelines for using copper, that metal will increasingly be substituted for fiber, because it is about one-third the cost and easier to use. This opens many applications. Although there are fewer problems with fiber, copper dominates the 1-Gbit market today. If all interfaces are copper, then it makes sense to have a copper solution.
Copper unshielded twisted-pair cable is built into many systems. It is backward-compatible, meaning that a system designed for 10 Gbits can go back to 10 Mbits. Different crosstalk requirements are needed in the cable for 10 Gbits, however, because it is not the same crosstalk one would get on the cable and connectors for CAT6. A CAT6 channel will work with a minimum of 55 meters, while CAT6A will have to work with the full 100 meters. So, it's important to check the crosstalk on the existing cable. The cables will have to be changed, but it's still less expensive to do that than to use fiber.