Page 341 - The ROV Manual - A User Guide for Remotely Operated Vehicles 2nd edition

13.1 Overview 333
FIGURE 13.3
Application layer
Presentation layer
Session layer
Transport layer
Network layer
Data link layer
Message
Application layer
Presentation layer
Session layer
Transport layer
Network layer
Data link layer
Physical layer
Physical layer
System A
System B
The OSI reference model.
This model is not per se a “standard” but a model by which other standards organizations may concentrate within the various layers and compartments.
ISO uses the term “peers” to define communications among related layers of various systems. Figure 13.3 shows that communications between peers (in this case, Systems A and B) are only made directly through the physical layer and are indirect at all other layers. Communications, instead, travel from the original “talker” down the various layers to the physical layer. The signal is then transmitted between devices and back up through the hierarchy of the second “receiver” device of the corresponding peer.
Communications between two devices can be either as a “connection service” or a “connection- less service.” In a connection service, the message is being relayed from one peer (in Figure 13.3, System A) of the system down the various layers within that system to the physical layer, on to the physical layer of the corresponding system (in this case System B), and then up the layers to the corresponding peer in a direct communication session. As shown in the figure, communications are represented by dashed lines through all other layers except the physical layer due to this indirect communication between peers. An example of this method is a Telnet connection whereby a “ses- sion” is opened so that data terminal equipment (DTE—in this case a personal computer emulating a standard VT-100 terminal) “talks” directly to a host computer. On an ROV, the vehicle’s sonar,
Message
Message