Copper versus fibre - the battle continues

Copper conductors have dominated the world of electronic devices since the discovery of electricity - and for good reason. Copper offers relatively low resistance, can be easily drawn into flexible wire, dissipates heat and can be terminated with solder, crimp or compressive connectors.

Many years of experience with copper has given the industry a detailed understanding of the material that permits accurate prediction of its performance.

However, as data transfer rates increased and cables became transmission lines, engineers began to encounter increased signal attenuation as well as distortion.

A series of issues, including skin effect, impedance control, skew and crosstalk, begins to degrade low-voltage signals as speeds move into the gigabit range.

The longer the copper link becomes, the greater the degradation of channel performance.

Replacing copper with fibre-optic links was predicted to be the solution, but engineers have been able to extend the life of passive copper links through improvements in cable design, insulating materials and by using algorithms that can allow the discrimination of low-voltage signals among high levels of noise.

Functions such as pre-emphasis and equalisation have been built into serialiser/deserialiser (SerDes) chips near the I/O port. Widespread adoption of 10 Gbps signalling may represent a transition point on how high-speed signals are transmitted.

Active copper cable assemblies can extend reach and bandwidth by integrating discrete signal conditioning components on a small PCB within the connector strain relief. Power is supplied from the host interface.

The 10 Gbps Thunderbolt cable from Intel and Apple is an example of an active cable assembly. Fabricating active copper cable assemblies adds cost and raises reliability and interoperability issues.

Passive copper conductors have been the preferred media for the vast majority of I/O cables, with the exception of longer links that are measured in kilometres. These are best served by dedicated singlemode fibre-optic links.

The long predicted, yet never quite ready for prime time fibre-optic alternative, has been making steady progress in terms of reduced cost, simplified termination processes, less sensitivity to sharp bends and interface contamination.

Single and multimode cables now offer improved loss characteristics, along with nearly unlimited bandwidth.

Fibre-optic cables are much smaller and lighter than equivalent copper cables, an important feature in large data centres where the mass of interconnecting cables can be overwhelming.

Optical signals still must go through the electro-optic conversion process at both ends of a link, a process that occupies valuable space within the box, consumes power, generates heat and adds cost.

Fibre is becoming more cost-effective in shorter lengths, but typically still costs more than passive copper.

Some users are willing to pay more for fibre to ensure adequate bandwidth headroom to support future product advances, but fibre continues to find resistance in all but longer links.

Active optical cables (AOCs) fill a niche between the two media with advantages borrowed from both technologies.

AOCs consist of a standard electrical connector at both ends but active components within the connector strain relief convert the electrical signals into optical pulses that are coupled into permanently attached optical fibre.

The reverse conversion occurs at the other end of the assembly.

By integrating the electro-optic conversion process within the cable itself, active optical cables can mate with legacy electrical I/O interfaces on equipment while transmitting the signal via low-loss fibre.

The cable installer sees no change, as the interface to the equipment remains the standard copper connector, but with the advantages of greater reliability, longer reach, greater bandwidth and much smaller and lighter cabling.

One of the first protocols to take advantage of active optic cable assemblies was InfiniBand, which features the 4X SFF 8470 (CX4) electrical connector.

The industry has now broadly adopted the QSFP+ form factor, capable of supporting four bidirectional channels each, operating at 10 Gbps. This standard interface offers good port density, with constantly increasing bandwidth in a user-friendly envelope.

Current QSFP+ optical cables are capable of supporting Infiniband SDR, DDR, QDR, and FDR, 10 Gb ethernet, and 8/10 Gb fibre channel applications.

As an alternative, designers can chose to use discrete LC optic cables plugging into small form factor pluggable optical transceivers, but the combination of two transceivers and separate cables may double the total cost and provide less consistent performance.

Active optical cables offer plug-and-play simplicity with fully contained optical links, eliminating potential for eye injury. AOCs also offer reduced power consumption along with high port density.

Small form factor pluggable interconnects allow system designers to chose from a variety of I/O options that matches the bandwidth and length requirements of each I/O to the most cost-effective media.

Using the same connector and cage assembly on the edge of the I/O card, a direct attach copper cable, pluggable optical transceiver with mating fibre-optic cables or an active optical cable can be used.

System designers are constantly pushing for faster links with Infiniband leading the pack. Both SFP and QSFP have experienced continuous evolution to higher speeds and have culminated in the current SFP+ and QSFP+ products.

Single SFP+ links are now rated to 14 Gbps while work is being done to push QSFP+ to four channels of up to 25 Gbps per lane. The long-term objective is to achieve 100 Gbps transfer rates in the smallest possible profile.

The newest entrant is the CXP active optical interface that provides 12 full duplex channels at 10 Gbps each as defined by the Infiniband 12X QDR specification.

Suppliers, including FCI Electronics, Finisar and Molex, see CXP as a viable interface to achieve 100+ Gb links and have introduced compatible active optical cable assemblies.

FCI has already demonstrated 12.5 Gbps CXP channels providing a total bandwidth of 150 Gbps per port. It may be possible for CXP to eventually ramp up to 12 channels of 25 Gbps supporting a 300 Gbps channel.

Active optical cable adapters that breakout three QSFP+ connectors from one CXP connector are now available.

Assuming 10 Gbps data rates, a very general rule of thumb indicates that cables less than 7 m long are typically passive copper as the least costly option. Cables longer than this could utilise active copper technology, but cost has become a limiting factor.

Cables over 8 m but less than about 80 m may best be served by active optical assemblies.

Cables longer than 80 m generally use SFF transceivers and discrete optical cables. Lengths between 5 and 30 m may be the sweet spot for active optical cables, although some are offered at lengths of up to 300 m.

Applications for these high-performance links are concentrated on large data centres and supercomputer installations. The devices tend to be physically arranged in clusters with shorter links between shelves and racks and longer links among clusters and attached storage devices.

Global connector manufacturers have entered this market often by acquiring expertise and products from independent fibre-optic specialty suppliers.

Participants now include such major players as 3M, Amphenol, FCI, Molex, Samtec and TE Connectivity. Additional suppliers include Avago, Finisar, Mellanox and Siemon, which specialise in computer and telecom connectivity and produce a variety of active optical cable assemblies.

Active optical cables may not be limited to high-end network and computing applications. HDMI and DisplayPort connectors are playing an increasing role in the emerging market for digital signage.

Large advertising signs in urban areas as well as digital information kiosks on a campus or airport would be logical applications for active optical cables.

Some market forecasters have predicted that optical links used in high-performance computing applications will be costly and the volume will be relatively low compared to typical consumer applications.

If active optical cables were to be widely adopted in signage, home theatre or as a primary personal computer I/O, the volume would grow logarithmically. This may be where the Intel LightPeak interface with its proposed 100 Gbps channel may be headed.

Bishop & Associates estimates that the market for active optical cables related to QSFP+ InfiniBand in 2011 totaled $78.4 million, with sales growth to reach $1142 million by 2016. During that period InfiniBand 4X QDR will dominate, with volumes rising from approximately 350,000 units in 2011 to 6.3 million units by 2016.

CXP 12X QDR active optical cables will rise from a few thousand units in 2011 to a market value of nearly $11.2 million by 2016.

If digital signage and home theatre applications broadly adopt active optical cables as a primary I/O, by 2016 they could add another $8.19 million to the total annual value of this market.

Robin Pearce, Bishop & Associates
rpearce@bishopinc.com