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Custom-Designed Ethernet Cable For Deepwater Applications
Falmat Inc.ís New Family of Category5e Cable Enables Ethernet Communications in Deepwater Environments

AUTHOR:

Cal Peters
Director of Engineering
Falmat Inc.
San Marcos, California

Ethernet is a popular communication protocol for local area networks, often using copper cables over distances of less than 100 meters. The protocol provides fast, robust, reliable data transfer with commercial off-the-shelf (COTS) components.

Operating an Ethernet network in an undersea environment, however, requires special material and design considerations, even over short distances. This is especially true in deepwater applications (more than 500 metersí depth).

One such deepwater application is the Monterey Accelerated Research System (MARS), which is managed by the Monterey Bay Aquarium Research Institute (MBARI). The MARS undersea ocean observatory reaches a depth of 900 meters, which equates to pressure of 1,300 pounds per square inch. To meet this and other similar deepwater application requirements, Falmat Inc. developed a new cable family, XtremeNet®, to provide reliable Ethernet communication in high-pressure, deepwater environments.

In testing, XtremeNet Cat5e cable maintains stable impedance even as pressure increases to 6,000 pounds per square inch. COTS Cat5e cable performance degrades quickly at higher pressures.

Cable Applications
Category5e (Cat5e) cable uses balanced-line twisted pairs for transmitting digital data. COTS Cat5e cables in the United States generally use unshielded twisted pairs (UTPs), and the most common Cat5e cable is made up of four UTPs.

Deepwater applications may use a number of network setups. 10BASE-T and 100BASE-TX Ethernet only use two UTPs, one for transmitting and one for receiving, and the remaining UTPs are for plain old telephone service or low-voltage power (less than 400 volts direct current). One exception to this is 1000BASE-T, also called gigabit Ethernet, which requires all four UTPs. Cat5e cable can also support distances of up to 1,200 meters at lower transmission rates using the Electronic Industries Allianceís (EIA) RS-485.


Conductors
Solid conductors in Cat5e UTP cable provide optimum electrical performance, but they limit bending performance. Conversely, conductors made from multiple strands improve bending performance but limit overall signal distance. Solid conductor Cat5e UTP cable supports distances up to 90 meters, while multiple strand conductors normally only support distances between 25 and 75 meters. The challenge is to optimize the multiple strand conductors for both bending and electrical performance.

At the MARS undersea ocean observatory, MBARI deployed 70 meters of the Falmat XtremeNet cable at 890 metersí depth. The researchers measured less than 0.1 percent change in data throughput between testing at the surface and the final 890-meter water depth. This validated that the XtremeNet multiple strand conductors provide both flexibility for bending and the ability to transfer Ethernet data signals over a 70-meter length at 1,300 pounds per square inch.


Insulation
Cellular or foam material is typically used to insulate Cat5e UTP conductors. This insulation provides a low dielectric constant, a high velocity of propagation and a small insulation diameter. In deepwater applications, however, the air in foam or cellular insulation will compress. Solid insulation materials with larger insulation diameters are needed to compensate for the higher dielectric constant and lower velocity of propagation.

In pressure testing, the 100-ohm nominal characteristic impedance in COTS Cat5e samples with foam insulation increased more than 20 percent at 435 pounds per square inch, a pressure equivalent to 300 metersí water depth. Samples of XtremeNet cable, however, maintained stable and uniform characteristic impedance to 6,000 pounds per square inch, which is equivalent to 4.2 kilometers of water depth.


UTP Twist
Cat5e UTP twist is critical. The conductors must form a balanced line with equal twist. Each UTP must also have a different twist rate, or twist per meter, to prevent crosstalk. Purpose-built equipment with extremely tight tolerances on all parameters is critical to optimize the twisting operation.

The twist rate in each UTP must remain constant all the way to the connection point. Any UTP untwisting rapidly degrades signal performance and must be limited.

Unfortunately, deepwater applications require circular connectors to withstand the high-pressure environment underwater. These connectors are not impedance matched and introduce impedance mismatches that further complicate the issue. Falmat followed special construction techniques to maintain the UTP twist rate and uniformity that would allow the cable to tolerate both the deep-sea pressure environment and attaching the connector. The XtremeNet cable family has a special thermoplastic layer over the UTP to maintain both uniform twist and dielectric properties at pressure in deep water.


UTP Cable Bundle
Cabling the UTPs together allows the cable to bend, but it requires maintaining the UTP symmetry during both the initial winding process and the actual field operations.

Spline construction, which uses an x-shaped plastic spacer to maintain UTP geometry, is one technique to maintain the symmetry in COTS Cat5e cable. However, this construction is unusual in deepwater applications because water-blocking compounds are compulsory to eliminate air voids in the cable. Therefore, other techniques similar to a spline are used to maintain UTP spacing and symmetry and to provide proper signal performance.

Geometry changes directly affect signal transmission rates through impedance changes, reflections and return loss, so Falmat incorporated proprietary materials and construction techniques in the XtremeNet family to ensure proper mechanical and electrical performance in deep water.


Strength Member
Standard Cat5e cable is unsuitable for mechanical loads, but deepwater applications may require the cable to provide both a mechanical and an electrical link. For this purpose, deepwater cable frequently includes a strength member to protect the electrical conductors from damage.

XtremeNet cables can be built with metallic or nonmetallic strength-member materials, depending on application and weight requirements. Metallic strength members can be used to increase the weight, while nonmetallic material can be used to decrease the weight. In either case, the strength member must limit the overall cable elongation or strain to protect UTP signal integrity.

XtremeNet validation test data with Fluke DTX-1800.
Cable Jacket
Jackets on underwater cables are inherently different than those on standard Cat5e cable. Most Cat5e cable has an outlet jacket made from polyvinyl chloride, which is usually inappropriate for deep water. Instead, most cables for deepwater applications use jackets made from either thermoplastic elastomer, polyurethane or a combination of both, depending on the application.

Determining which jacket to use can be difficult to precisely predict in advance of each scenario. MBARIís initial deployment of XtremeNet at the MARS underwater observatory indicated the outer jacket material required more flexibility than initially thought. Therefore, Falmat provided another XtremeNet cable with a more flexible outer jacket, and MBARI deployed this sample to validate that the jacket improved the mechanical performance.


Verification and Validation
Factory verification tests using a cable analyzer, such as the Fluke (Everett, Washington) DTX-1800, can prove that the cable meets the design criteria. Validation in the actual environment is equally important in deepwater applications because the cable may degrade under pressure. Falmat performed both verification and validation testing on its XtremeNet cables to ensure they met the requirements for deepwater Ethernet.


Conclusions
Deepwater Ethernet cable requires different materials and construction techniques than COTS Cat5e in order to withstand the harsh application environment. Falmat developed the XtremeNet cable family to balance performance with the tradeoffs required for reliable performance at extreme pressures.

XtremeNet cable uses multiple strands to improve bending performance and solid insulation to achieve pressure resistance at depth.

UTP twisting of the cable was designed to remain uniform and intact all the way to the connector. Circular connectors used in deepwater Cats5e cable create impedance mismatches that can cause problems, but these were minimized by following special construction techniques. The XtremeNet cable bundle includes water blocking and maintains symmetry under pressure.

If the cable will support mechanical tension loads, XtremeNet cables can be built with metallic or nonmetallic strength-member materials. Depending on the application, the cables can use different types of jackets, such as thermoplastic polyurethane. Finally, all deepwater cables should be field tested, as the XtremeNet has been tested for the MARS observatory.



Cal Peters is the director of engineering at Falmat Inc. He has been involved in designing, manufacturing and testing custom electrical, optical and mechanical cable for harsh environments—including aerial, terrestrial and underwater applications—for 36 years.


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