Page 342 - The ROV Manual - A User Guide for Remotely Operated Vehicles 2nd edition

334 CHAPTER 13 Communications
communicating directly (via an RS-422 combination physical/data-link layer) with a surface com- puter (for display) through a separate twisted pair conductor within the tether (direct continuity of the conductor can be checked between the sonar head and the serial connector at the surface) would actually be communication within a single system. In a connectionless service, all aspects are the same except that the data being transmitted is wrapped into a data “packet” and addressed to a node within the network. The data is then received through the physical layer, transmitted to the peer and then received, reassembled, and disseminated (much like a letter that is written, put into an envelope addressed to the recipient, and mailed through the postal service). A simplified exam- ple of this is an e-mail routed through the Internet. Network protocols are further explained at Section 13.3.4.
13.2 Transmission
Data transmission is the physical/electrical transfer of a signal, message, or other form of intelli- gence from one location to another. Just as fluids flow through pipelines, data flows through a series of conductors, ports, connections, and links. In this section, the issues involved with the physical conduction of data flow will be addressed.
13.2.1 Basic data transmission model
Data transmission is the basic exchange of information between two “agents” linked together through a series of data pipelines (see Figure 12.13 for a graphical depiction of this concept). This exchange can be between a machine and a human sitting at a terminal or it can be between a machine (program) exchanging data with another machine (program). Reliability, integrity, and intelligibility of the message are achieved through the processing of this information from transmis- sion to receipt as an integral part of this process. The source must produce a time varying signal of binary data couched in a format suitable for the medium being used. The signal received at the far end of the cycle must be similar to the signal transmitted at the source; therefore, the essence of this model has a time varying signal generated that emulates the initial source input (say, a sonar reflection off a gold-laden Spanish galleon protruding off the deep seafloor [to make this interest- ing]). The sonar reflection is transferred to time varying electrical signals through the sonar’s trans- ducer, which in turn is modulated to a carrier signal that is converted (through the interface) to binary data and sent down the line. On the far end, the process is reversed to deliver the informa- tion to the capture device for final processing. The end sonar rendering of a beautiful square-rigged sailing ship (filled with valuable loot) is generated on the computer display as the dollar signs light up in the sonar operator’s eyes (with dreams of fast cars, lavish houses, and expensive vacations dancing in his/her head). But I digress. . . .
13.2.2 Electrical signal transmission
The two basic methods of data transmission relate to the data sequencing. This sequencing comes in two broad categories, parallel and serial data flows.