Page 369 - The ROV Manual - A User Guide for Remotely Operated Vehicles 2nd edition

further data pipelines over the same fiber. The 8-channel version of the board has communications at 1471, 1491, 1511, 1531, 1551, 1571, 1591, and 1611 nm.
Time division multiplexing (TDM) makes use of assigned time slots to allow information to take its turn down the communications line. This technique, however, has not gained much traction in the subsea industry.
13.2.11 Binary signals
The essence of the binary signal is the concept of discreteness. Whereas the analog signal implies continuity of signal (as in the form of a sine wave of a constantly varying nature), the digital signal is quantized as either on/off, yes/no, up/down, 1/0, or presence/absence of voltage (to name a few). The basic unit of information, termed a “bit”, is encapsulated within this binary information stream and will be covered more extensively in Section 13.3.1.
13.2.12 Directionality
There are three possible directionality terms of data exchange:
1. Transfer in both directions simultaneously (termed “full duplex”)
2. Transfer in both directions with only one direction transmitting at a time (termed “half duplex”) 3. Transfer in one direction only (termed “simplex”)
The synchronous versus asynchronous discussion is often confused with the directionality ques- tion; they are mistakenly viewed as synonymous.
13.3 Communication
As mentioned above, the communication function differs from the transmission function in that communication is the actual information being transferred from transmitter to receiver. In this sec- tion, the elements and symbols of the information being communicated will be examined, thus leading into the final section on typical communications standards used within the subsea industry.
13.3.1 Of bits and bytes
The binary digit (shortened to “bit”) is the most elemental unit of information and in data commu- nications is represented as either a 1 or a 0. In and of itself, the single bit does not carry much information, but it allows for very distinct states of being. For instance, a light is either on or off, a motor running or not and (in Schro¨dinger’s example, for us physics fans) the cat is either alive or dead. The bits come alive as the information is expanded into further combinations of bits. While one bit has only two possible states (1 or 0), two bits have four possible states (00, 01, 10, and 11) and three bits have eight possible states (000, 001, 010, 100, 110, 011, 101, and 111). Therefore, the possible states of bits can be expressed as 2n number of states. The magic number for data transmission in the subsea world for serial communications is 27 number of possibilities (27 is 128 possible states or characters) with one bit as a checksum. This leads us into the worldwide standard
13.3 Communication 361