Allard Van der Horst, director of applications, Phyworks looks at the pros and cons of copper cabling and interconnects

We review here the main advantages and disadvantages of each and propose that emerging active twin-ax interconnects present a radical alternative that’s free of compromise.

10GBASE-SR

When it comes to range, weight and power consumption, multimode optical fibre interconnect ticks all the right boxes.  Table 1 confirms this.
Since 80% of all cables in the data center are less than 30m, clearly optical fiber’s got the range aspect more than covered.  It performs well on power consumption too, with an XFP cable module operating at as little as 1.5W.

It’s the overall link cost however that’s always been the big issue and for data center managers looking for a reduction in capital outlay, fibre optic interconnect provides a useful first port of call. At a link cost of $500, 10G optical interconnects are 2-3 times more than copper alternatives and for the short intra-rack connections, their viability is being questioned.

10GBASE-CX4

CX4 cables are looking increasingly unattractive from the overall link cost perspective too, as the world’s copper prices soar.  They use 8 twin-axial copper pairs, with individual pair and overall EMI screening and a big connector, which sits proud of the rack facia.

The power consumption of the CX4 solution is traditionally considered to be low though, as the previous generation data center equipment had internal 4-lane (XAUI) interfaces which converted relatively to the CX4 interface.

The move in the current generation switches and server NICs however is to enable high port density by using serial interfaces.  These will have to be converted to the 4-lane CX4 interface at significant cost and potentially several Watts of power.

10GBASE-T

10GBASE-T is today presenting an alternative interconnect.  It uses CAT5 copper cable with 8-wire twisted pair operating in a bidirectional communication mode and the ubiquitous RJ45 (or similar) connector.

At $500, link cost is similar to the 10GBASE-SR optical solution which means it’s receiving close scrutiny right now, it is however significantly higher in operating power.  Why?  It’s simply because the signal processing is complex.  Even when implemented in state-of-the-art CMOS, a link needs in excess of 4W per 30m link end.  It’s power hungry.

With the RJ45 jack mounted on the face of the equipment all cable driver and interface electronics again need to be placed on the line card.  This makes it unattractive for high-density switches and routers where up-front investment will have to be made.

Active twin-ax

Phyworks is one semiconductor manufacturer that has now come up with the chip technology to enable active twin-ax interconnects assured of error free transmission at 10G.  Put simply, it has taken its existing equalizer and retimer technology developed for optical fibre cables and repurposed it for copper cables.

Active twin-ax copper cable assemblies, are now being trialled by cable assembly makers and data center equipment manufacturers. Phyworks’ equalisation technology enables active interconnect over thin 24 to 30AWG twin-ax cable of up to 30m.  Importantly, the cable link cost is less than a third that of its 10GBASE-SR optical counterpart, while it’s power consumption is the same.

The solution is also seen in many ways to ‘de-risk’ data center network design.  Since the chip technology fits both XFP and SFP+ modules there’s an element of future proofing, plus it means that the new copper cable interconnects can be hot-plugged into existing optical ports to dramatically increase design flexibility.

Furthermore, unlike 10GBASE-T, the cable driver electronics, the equalizer and retimer technology, is contained in the cable module and not on the line card and so in terms of power consumption, an active twin-ax cable solution is effectively “pay-as-you-grow”.

Conclusion

If intra-rack data center interconnect was being chosen purely by link cost considerations, then only two solutions would stand-up to scrutiny: 10GBASE-T and active twin-ax.  And making a further selection based on link power consumption, then only active twin-ax interconnects can provide a valid alternative to fibre optic interconnects.