COTS Journal

Cable Technology Advances Meet Shipboard Network Needs

By: Robert Moore, Global Specialist High Speed Cable TE Connectivity

The open architectures of today’s shipboard naval systems offer huge benefits. But cable technologies need to keep pace with new performance and bandwidth demands.

 

It's perhaps stating the obvious, but today's naval vessels are electronic creatures: heavily networked and relying on these networks for every aspect of the ship's operation. As with any network, there is a never-ending thirst for higher data rates and more connectivity to allow more sophisticated capabilities. Connectivity ranges from individual subsystems to fleet operations.

The U.S. military favors using off-the-shelf technology as a lower risk path than developing things from the ground up. Such technologies enjoy widespread use so that benefits and drawbacks are well understood. They likewise reap the rewards of high-volume production that translates into lower costs as well as a wide supplier base to foster competition and prevent vendor lock. Even so, off-the-shelf technology often needs to be ruggedized to meet the more demanding environments of military applications.

The traditional closed-architecture that long characterized naval systems suffered from not being able to provide a suitable upgrade path to keep up with progress in state-of-the-art computers. One example of the moves to COTS-based systems is the CANES (Consolidated Afloat Networks and Enterprise Services) program to create a single integrated software-based computing platform to replace five separate legacy systems. CANES unifies command, control, communication, computer, and intelligence systems, but not combat systems or machinery control (Figure 1). The result is a flexible, robust system that can evolve in capabilities in much the same way as commercial systems.

Ethernet and More

As with any high-performance computer system, the network is critical in connecting users to each other and to the information from sensors such as radar. As the de facto protocol for high-speed networks, Ethernet reigns supreme. It has been shown to be robust, scalable, and capable of remarkable evolution in data rates. Success breeds success as the dominance of Ethernet continues to grow. For "industrial" applications, such as the ship's mechanical systems, Profibus protocols also find use. Profibus signals, through EtherNet/IP, easily integrate in the Ethernet ecosystem.

Shipboard cabling must not only meet the electrical requirements of the protocols carried, it must also meet the special needs of marine applications. Resistance to seawater is an obvious requirement. Depending on the application, resistance to oils, solvents, and other fluids may also be required.

Safety Is Paramount

In the enclosed environment of a ship, fire is a major concern. Cables must be selected with careful attention to their flammability and smoke properties. Traditional PVC cables, for example, contain halogens that present a health hazard when burned. A halogen-containing plastic can release hydrogen chloride, hydrogen fluoride, and other dangerous gases when burned. When hydrogen chloride comes into contact with water, it forms hydrochloric acid, which is also dangerous. Beyond being toxic to humans and animals, these gases are also highly corrosive to metal.

High levels of smoke, whether containing toxic gases or not, are another hazard. Smoke inhalation is the major source of death in fires. In addition, smoke generation can hamper safe evacuation and fire-fighting efforts by reducing visibility. As a result, low-smoke, zero-halogen (LSZH) cable jackets and insulations are preferred to increase safety in the enclosed ship environment. Zero-halogen materials contain only trace amount of halogens-less than 0.2 percent-meaning they are essentially halogen free. Figure 2 shows the relative smoke generation of different plastics over time. It shows that smoke density of most common jacket materials can cause loss of visibility quickly. LSZH materials allow visibility to be maintained.

LSZH materials do not offer the same moisture resistance as some alternative materials. LSZH high-speed data cables (Ethernet cabling) that transit through watertight compartments not only need to be waterblocked to prevent water moving from compartment to compartment in the case of damage, but also need to prevent moisture ingression into the cable through the jacket that would degrade the data transmission properties of the cable over time.

Cable size and weight are also issues. Reduced-diameter cables make more efficient use of valuable shipboard space. Cross-linked materials, such as ETFE, allow thin-wall constructions that can significantly reduce the overall diameter and weight of the cable on the order of 30 to 40 percent. Fluorinated polymers such as ETFE are not a zero-halogen materials, but do find use in insulations or where LSZH cables may not be needed.

Electromagnetic Compatibility

Electromagnetic compatibility (EMC) is also critical to allow the cable to operate properly in the electromagnetic environment of a ship. Of special interest is the ability to withstand electromagnetic pulses (EMP)-noise spikes generated external to the cable. About 120 dB of EMP protection is required. Of that, half-about 60 dB-is provided by the ship's structure. The remaining 60 dB must be provided by the cabling system. Traditionally, this additional protection was mainly achieved by running cable in metal conduit. The conduit provides most of the protection; the cable itself offered only minimum protection. The trend today is to move away from metal conduits and place the burden of EMP protection onto the cable. This requires a well-shielded cable, typically one with a double braid and single foil.

Most shipboard cables meet the requirements of either MIL-DTL-24640 or MIL-DTL-24643. M24640 cables use LSZH jackets, but allow non-LSZH insulation for conductors. These cables are not generally used for high-speed signals, but for power and control applications. M24643 cables, on the other hand, use LSZH materials throughout the cable. The specification includes several constructions of Cat 5e Ethernet cables, differing in waterblocking components, shielding, and other details. The release of spec sheets for Cat 6a is imminent, giving a path to high 10 Gb/s data rates.

Tradeoffs in Cable Design

The requirements of water resistance, low smoke and fire hazards, size and weight, and EMP protection compete with one another and therefore represent tradeoffs in cable design. For example, shielding added for EMP protection increases the size and weight of the cable. In addition, the cable must meet the electrical requirements for carrying high-speed signals.

Material choices involve balancing the tradeoffs in performance characteristics and, of course, cost. Because the primary function of the cable jacket is to insulate and protect, it can be made of different material than the dielectrics of the internal components. As an example, foamed dielectrics may be used in an Ethernet cable to save weight, but the LSZH jacket necessitates the use of a more rugged solid material.

Jacket materials can be modified to highlight certain characteristics, which is especially helpful if you have a particular challenge. TE Connectivity, for example, uses special formulations to achieve specific characteristics rather than relying on generic, off-the-shelf polymers. Our expertise in material science allows us to create proprietary polymers that can be further modified to enhance mechanical, environmental, and electrical performance.

Differential Twisted Pairs

Ethernet cable are constructed as differential twisted pairs with a characteristic impedance of 100 ohms. Four-pair cables are the standard configuration, although two-pair cables also exist. The characteristic impedance is determined by the geometry of the cable and the dielectric properties of the insulation and jacket. Changes in materials, insulation thickness, conductor size, and the spatial relationship of the pairs all affect the characteristic impedance and other electrical parameters such a crosstalk. Cable design becomes an act of balancing the various parameters. Figure 3 shows an example of a size and weight-reduced Ethernet cable, compared to a standard commercial cable.

Since the U.S. military tends to be cautious about adopting new technology, 1 Gbit/s Ethernet and Category 5e is still the prevalent cable in naval applications. While widely used in commercial applications, Cat 6a cable, with the ability to carry 10 Gbits/s, has not been as widely adopted by the military as it has by the commercial world. Mil specs exist for Cat 6a cables for aerospace applications, for example, but specifications for marine cables are still in process.

Safer, Higher Performance

Today's LSZH cables create a safer shipboard environment by reducing the cable's flammability, its ability to generate toxic gases, and its low-smoke generation. At the same time, they continue to evolve electrically to carry higher data rates and mechanically to meet requirements of size and weight reduction while maintaining robust characteristics.

TE Connectivity
Berwyn, PA
(610) 893-9800
www.te.com

 

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