Crestron e-Control, Справочное руководство

Страница: 59 / 74

Crestron e-Control
®
Software
Appendix B: The OSI Reference Model
Virtually all networks in use today are based in some fashion on the Open Systems
Interconnection (OSI) standard. OSI was developed in 1984 by the International
Organization for Standardization (ISO), a global federation representing approximately
130 countries.
The core of this standard is the OSI Reference Model , a set of seven layers that define
the different stages that data must go through to travel from one device to another over
a network. At each layer, certain things happen to the data that prepare it for the next
layer.
The seven layers, which separate into two sets , are:
Application Set
Layer 7: Application - This is the layer that actually interacts with the operating
system or application whenever the user chooses to transfer files, read messages or
perform other network-related activities.
Layer 6: Presentation - Layer 6 takes the data provided by the Application layer and
converts it into a standard format that the other layers can understand.
Layer 5: Session - Layer 5 establishes, maintains and ends communication with the
receiving device.
Transport Set
Layer 4: Transport - This layer maintains flow control of data and provides for error
checking and recovery of data between the devices. Flow control means that the
Transport layer looks to see if data is coming from more than one application and
integrates each application's data into a single stream for the physical network.
Layer 3: Network - The way that the data will be sent to the recipient device is
determined in this layer. Logical protocols, routing, and addressing are handled here.
Layer 2: Data - In this layer, the appropriate physical protocol is assigned to the data.
In addition, the type of network and the packet sequencing is defined.
Layer 1: Physical - This is the level of the actual hardware. It defines the physical
characteristics of the network such as connections, voltage levels and timing.
The OSI Reference Model is simply a guideline. Actual protocol stacks often combine
one or more of the layers.
A protocol stack is a group of protocols that all work together to allow software or
hardware to perform a function. For example, the
TCP/IP (and UDP/IP) protocol stack
uses four layers that map to the OSI model as follows:
Layer 1: Network Interface - This layer combines the Physical and Data layers and
routes the data between devices on the same network. It also manages the exchange of
data between the network and other devices.
Layer 2: Internet - This layer corresponds to the Network layer. The Internet
Protocol (IP) uses the IP address, consisting of a network identifier and host identifier,
to determine the address of the device with which it is communicating.
Reference Guide – DOC. 6052 Crestron e-Control®
•
55