Page 371 - The ROV Manual - A User Guide for Remotely Operated Vehicles 2nd edition

With this table and a little ingenuity, one can refer back to Section 12.1.3 and derive the exact digital stream to send over a line for a GPS receiver. Looking at the first seven characters in a typi- cal NMEA 0183 transmission for a GPS receiver (“$GPGGA,”) in sequence, the result is:
0010010 1110001 0000101 1110001 1110001 1000001 0011010
During troubleshooting of an input device (e.g., a GPS, heading indicator, or other sensor out- putting a digital stream) on a Windowss platform, the ASCII stream can be easily viewed with the HyperTerminal tool by setting the output to the proper data speed and protocol (again, typically 9600, 8-N-1) and reception with the same speed/protocol combination.
13.3.3 Error control
As shown in Section 13.2.2, signaling errors can crop up at unexpected locations. A totally error- free communications channel is impossible as it defies the second law of thermodynamics. All things tend toward the disorganized, making some measure of errors unavoidable. In the early days of telegraph, high error rate lines required the operator to transmit critical words twice, thus halving the transmission speed in order to reduce error rates. This doubled the effective cost of data trans- mission per unit communicated.
As a result of these inherent errors, modern standard data communications protocols have evolved with use of the “parity check” (also referred to as the “vertical redundancy check” or “VRC”). Error detection identifies bit errors that are received while error correction corrects these bit errors received at the far end of the line. Standard data streams transmitting the ASCII character set involve a 7-bit character set with the last bit being the parity bit for a total of an 8-bit byte. This sacrifices 1/8 of the raw data bandwidth to error control as redundancy. For a further explana- tion on error correction techniques, please refer to a basic text on the subject.
For a nice low-noise line with high transmission clarity, a low error rate can be achieved. But for a high-noise environment, the SNR will degrade to such a point that the error rate will slow the transmission due to error correction hogging up the computer processor power. Examples of this phenomenon are (i) a UTP line running along an AC power source in a subsea tether or (more typi- cally) (ii) a data line running along the side of an AC power cord (or thermal noise, generator cir- cuit noise, or any of a host of noise spikes) in the control room.
13.3.4 Protocols
IEEE defines a protocol as “a set of rules that govern functional units to achieve communication.” Certain rules, procedures, and interfaces are established within the data communications framework in order to get the most out of the network. These rules establish the common language of the net- work and are termed “protocol.” These protocols are based upon the network topology/architecture, transmission media, switching, and network hardware/software selected.
Typical basic networking protocol functions (for both connection and connectionless sessions) are:
• Segmentation and reassembly (SAR): the breaking up of messages or files into blocks, packets, or frames into quantized (i.e., nominally measured) packets. Some standards refer to this as
13.3 Communication 363