Fiber and copper working together for today’s Ethernet backbones

Ethernet Backbone

In all mobile military and airborne platforms, the transition from mechanical systems to electronically controlled systems is taking place. As the electronics content continues to grow, so do the processing loads that happen on every platform. Embedded computers are rising in sophistication as they need to support sensors, radar, video streams, and remote-control functions. Distributed processing, the interconnection of devices, and communication between devices has led to an exponential jump in bandwidth requirements on the interconnects between these devices. Traditional protocols like IEEE 1394 and USB still have legacy applications on these platforms, but most new platforms and platform retrofits are turning to Ethernet as their de facto communications protocol, supporting 1 Gbps in most platforms and growing to 10 Gbps in certain payloads.

Along with the increase in speed comes the question as to whether to use tradition copper as the physical interconnect or upgrade to fiber to support these higher speeds. In my column I usually cover the technology behind Ethernet. Today, I thought I’d take a step back and talk about the physical mediums and explain the differences and complexities of using both mediums on the same platform.

The obvious: Size and weight: I think everyone that has seen a traditional Cat 5/Cat 6 and a fiber cable can see the obvious differences here: a duplex fiber-optic cable offers approximately 25 percent space savings and 50 percent weight savings over a shielded Cat 5e cable. Optical fiber cables are lighter, thinner and more efficient than their copper counterparts, so they take up much less space in an installation. However, fiber tends to be a bit more fragile and requires special tools to terminate connections.

Supported transmission speeds: Traditional Cat5 twisted-pair copper cables support a maximum data transmission rate of 100 Megabits per second (Mbps) while Cat6 supports a very impressive speed of 10 Gigabits per second (Gbps) up to a distance of 300 feet (see Signal Loss).

For fiber cables, this is a tricky answer. New fibers and new transmission protocols are being tested every day. For the sake of mobile military platforms, let’s say that fiber can support anywhere from 100 Mbps up to 10 Gbps. However, in data centers and service provider interconnects, fiber is supporting transmission speeds of 40, 50, 75, and 100 Gbps – even 400 Gbps speeds are being tested and implemented.

Signal loss: As just mentioned, one of the inherent disadvantages to copper Ethernet cables is that they experience signal loss over long distances. The effective limit of a copper run is about 100 meters. Distances longer than that require Ethernet extender, Ethernet switch or media converter that can basically link two runs of copper together or termination of the copper run and then adding a link to a fiber connection for additional distance.

Fiber optic cables, on the other hand, can run without significant signal loss for distances. New technologies are constantly being developed. Today, a Single Mode Fiber can support up to 100 Gbps up to 10 km without significant signal loss.

EMI and cross talk: By its very nature, copper cabling is susceptible to crosstalk and both radio-frequency and electromagnetic interference (EMI), since it is transferring electrical signals. Cat6 Ethernet cable lessens, but does not eliminate, interference with several measures including an internal spline to separate the twisted pairs.

As fiber optic cable doesn’t carry electricity, and carries light, it is immune to the interference problems that can plague copper cabling
Other environmental concerns: Copper cables can potentially be a fire hazard over the long term, since there is an electrical current passing through a jacket that can wear over time. With no electricity involved in the transmission of fiber optic signals, there is no concern of fire.

There is a widespread belief that fiber’s size and weight make it less able to withstand the strains of installation, but that’s not the case. Fiber is more durable with a much higher tension limit than copper and stands up better to environmental extremes.

So, why haven’t we converted all platforms to use fiber over traditional Cat 5 cable? I guess there are a few reasons.

1) Until recently, fiber was more expensive than cable. The economics of fiber have changed drastically over the last couple of years, so cost is becoming less and less of a mitigation factor.

2) Fiber is sensitive to twisting and kinking and used to be a complex install. That is changing quickly with newer fibers, connectors and installation tools.

3) There is a huge installed base of copper that isn’t going away any time soon, so many feel that the evolution to fiber is still optional.

4) Many of the devices that need to connect still only have copper connector interfaces.

In the meantime, there are several ways to handle this transition. Many Ethernet switches support the connection of both copper and fiber so that devices connected with either physical medium can communicate with each other. Physical media converters can act as a bridge between the two physical media offering a variety of configurations that support fiber in/copper out or copper in/fiber out or other various configurations.

Enterprise data centers and service providers have led the way in replacing copper with fiber in almost every new deployment. Mobile and airborne platforms lag behind this evolution for several reasons–speed requirements aren’t as demanding, field repairs and deployments are often more difficult and, finally, cost. We will continue to see the evolution from copper to fiber, but at a much more controlled pace. In the meantime, we will continue to see both physical connection technologies co-exist for years to come.

This article was originally published on Military Embedded Systems  —