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Chapter 4 Deployment of the CPU 21x-2BT10 with TCP/IP Manual VIPA CPU 21x
4-4 HB103E - Rev. 05/45
Layer 1 Bit communication layer (physical layer)
The bit communication layer (physical layer) is concerned with the transfer
of data bits via the communication channel. This layer is therefore
responsible for the mechanical, electrical and the procedural interfaces and
the physical communication medium located below the bit communication
layer:
•
Which voltage represents a logical 0 or a 1?
•
The minimum time that the voltage be present to be recognized as a bit.
•
The pin assignment of the respective interface.
Layer 2 Security layer (data link layer)
This layer performs error-checking functions for bit strings transferred
between two communicating partners. This includes the recognition and
correction or flagging of communication errors and flow control functions.
The security layer (data link layer) converts raw communication data into a
sequence of frames. This is where frame limits are inserted on the
transmitting side and where the receiving side detects them. These limits
consist of special bit patterns that are inserted at the beginning and at the
end of every frame. The security layer often also incorporates flow control
and error detection functions.
The data security layer is devided into two sub-levels, the LLC and the
MAC level.
The MAC ( M edia A ccess C ontrol) is the lower level and controls how
senders are sharing a single transmit channel.
The LLC ( L ogical L ink C ontrol) is the upper level that establishes the
connection for transferring the data frames from one device into the other.
Layer 3 Network layer
The network layer is an agency layer.
Business of this layer is to control the exchange of binary data between
stations that are not directly connected. It is responsible for the logical
connections of layer 2 communication. Layer 3 supports the identification of
the single network addresses and the establishing and disconnecting of
logical communication channels.
Additionally, layer 3 manages the prior transfer of data and the error
processing of data packets. IP (
I nternet P rotocol) is based on Layer 3.
Layer 4 Transport layer
Layer 4 connects the network structures with the structures of the higher
levels by dividing the messages of higher layers into segments and pass
them on to the network layer. Hereby, the transport layer converts the
transport addresses into network addresses.
Common transport protocols are: TCP, SPX, NWLink and NetBEUI.
Layers