Page 372 - The ROV Manual - A User Guide for Remotely Operated Vehicles 2nd edition

364 CHAPTER 13 Communications
“packets” of information, others as “cells” (as used in asynchronous transfer mode—ATM) of information. On the receiving end, the packets or cells are reassembled in the reverse process of segmentation for putting the messages back together for dissemination.
• Encapsulation: This function is the process of adding headers to describe the information packet/cell.
• Connection control: The three basic steps of a connection controlled in this sequence are
(i) connection establishment, (ii) data transfer, and (iii) connection termination. In more sophisticated protocols, various aspects of error control, connection interrupt/recovery, and other session parameters can be controlled.
• Ordered delivery: The packets/cells are sent (much like mailed letters sent from the same address to the same subsequent address through the mail) in sequence, but the packets may not arrive in the same order as transmitted. This function reassembles the data into ordered and sequential segments for processing through use of a numbering plan with a simple numbering sequence.
• Flow control: This function manages the data flow from source to destination assuring the data does not overflow the buffers and memories along the way while maintaining the network components at full capacity.
• Error control: The networking error control function is the “lost and found” service of the computer world. It allows for the recovery of lost data packets identified during the reconstruction process to assure a complete data set on the receiving end. This function is broken down into four possible subfunctions, (i) numbering of packets, (ii) incomplete octets (number of bits—e.g., less than an 8-bit set), (iii) error detection/correction, and (iv) receipt acknowledgment back to the data source.
13.4 Standard protocols
Standards organizations are crucial in data communications platforms. These organizations allow dis- parate manufacturers of sensors and processing equipment to design to the same languages for interop- erability. This benefits the consumer (specifically, the operators of ROV equipment) by allowing all of the technicians to train to the same standards and equip to the common interface. As an example, serial communications devices can exchange information on a common connector speaking in the same lan- guage based upon just a few protocols, such as RS-232, RS-422/485, or Ethernet.
The most common serial protocols in the subsea industry parallel the standards in the computer industry. The most common of these used with ROVs are covered below.
13.4.1 TIA/EIA standards
The Telecommunications Industry Association (TIA) is a Washington, DC (USA)-based standards organization sanctioned by the American National Standards Institute (ANSI) to develop voluntary, consensus-based industry standards for a wide variety of telecommunication products. These pro- ducts include radio equipment, cellular phones/towers, data terminals, satellite equipment, telephone terminal equipment, VoIP devices, mobile device communications, multimedia multicast, and machine-to-machine communications (among others). The Electronics Industries Alliance (EIA)