Page 352 - The ROV Manual - A User Guide for Remotely Operated Vehicles 2nd edition

344 CHAPTER 13 Communications
balanced specifically to reduce the crosstalk phenomenon from galvanic, capacitive, and inductive coupling. Further, wire pairs in close proximity induce current in adjacent lines, causing ghosting (or echoing) in other lines. This was a real problem in the early days of the telephone industry with other conversations echoing through to a telephone conversation and rendering the line useless. To curb this inductance impairment, twists in the amount of 6
36 turns per meter are induced into the line to inhibit the inductance field (hence the name “twisted pair,” or TP). Also, shielding was introduced in the form of either foil or metallic braided mesh weaved around the TP to further miti- gate inductance noise as well as the problem of crosstalk. What has come to be known as shielded twisted pair (STP) and unshielded twisted pair (UTP) are standard conductors in the tethers of all complex ROVs.
UTPs predominate in the telecommunications industry where conductors can be routed to avoid high inductance noise areas. Electronics Industries Alliance/Telecommunications Industry Association (EIA/TIA) standards have evolved for transmission of data over UTP cables for use in telecommunications and have categorized UTP cables in six categories (the first two categories are for voice transmission and low speed data while the remainder is suitable for data transmission). This standard is widely used in ROV cabling and has broad applicability for defining data through- put capabilities of twisted pair wires—the most common of which is the Category 5e (simply named “Cat 5e”) and Category 6 (“Cat 6”). Cat 5e and Cat 6 cabling have a nominal maximum operating length of 100 m. However, in spite of the nominal certified cabling length, some OCROV manufacturers still make use of Cat 5e and Cat 6 cabling for telemetry and sensor data transmission over longer (.300 m) lengths of tether.
The useful bandwidth (Hertz) of a twisted pair varies with the type/quality of wire as well as its length. Typical UTP wiring used in the telephony application can support a 2 MHz bandwidth over about 1.5 km of length. Cat 5e twisted pair displays a 67 dB loss at 100 MHz over a length of 300 m (the typical maximum tether length of a small OCROV).
Coaxial cable: The other wire pair used in data communication is the coaxial cable, named for its arrangement with a single centralized conductor held in place by insulating material and sur- rounded by a cylindrical tube. This arrangement is advantageous when high frequencies are involved. The nominal (characteristic) impedance of the coax is 75Ω with specialized cabling avail- able with 50Ω impedance. From the mid-1950s to the mid-1980s, coax cabling was widely used for long-distance telephony. These days, coax is mostly used for the last 30 m for cable television from the termination point at the curb to the television set. Frequency response for coax is exponential, making its loss increase drastically as frequency is increased—thus equalization is required in order to level out the frequency response. With all of these various issues with copper-based data trans- mission, it is a wonder that fiber is not exclusively used for all data transmission applications (espe- cially in ROV applications) as it is superior in practically all parameters. However, coax is still used in some ROV tether applications for telemetry.
13.2.5 Optical fiber
Guiding light by refraction, the principle upon which all optical fiber transmission is based, has its roots in nineteenth century physics. Optical refraction was first introduced by Daniel Colladon and Jacques Babinet in France during the early 1840s through their demonstration of their “light fountain” or “light pipe” (Figure 13.17).