Chapter 4 Deployment of the CPU 21x-2BT10 with TCP/IP Manual VIPA CPU 21x
4-8 HB103E - Rev. 05/45
The internet protocol covers the network layer (Layer 3) of the ISO/OSI
layer model.
The purpose of IP is to send data packages from on PC to another passing
several other PCs. These data packages are referred to as datagrams.
The IP doesn't guarantee the correct sequence of the datagrams nor the
delivery at the receiver.
For the unambiguous identification between sender and receiver 32Bit
addresses (IP addresses) are used that are normally written as four octets
(exactly 8Bit), e.g. 172.16.192.11.
These internet addresses are defined and assigned worldwide from the
DDN network (Defense Department Network), thus every user may
communicate with all other TCP/IP users.
One part of the address specifies the network, the rest serves the
identification of the participants inside the network. The boarder between
the network and the host part is variable and depends on the size of the
network.
To save IP addresses, so called NAT router are used that have one official
IP address and cover the network. Then the network can use any IP
address.
The TCP (Transmission Control Protocol) bases directly on the IP and
thus covers the transport layer (layer 4) of the OSI layer model. TCP is a
connection orientated end-to-end protocol and serves the logic connection
between two partners.
TCP guarantees the correct sequence and reliability of the data transfer.
Therefore you need a relatively large protocol overhead that slows down
the transfer speed.
Every datagram gets a header of at least 20Byte. This header also
contains a sequence number identifying the series. This has the
consequence that the single datagrams may reach the destination on
different ways through the network.
Using TCP connections, the telegram length is not transmitted. This means
that the recipient has to know how many bytes belong to a message. To
transfer data with variable length you may begin the user data with the
length information and evaluate this at the counter station.
• Besides of the IP address ports are used for the addressing. A port
address should be within the range of 2000...65535. Partner and local
ports may only be identical at one connection.
• Not depending on the used protocol, the PLC needs the VIPA handling
blocks AG_SEND (FC5) and AG_RECV (FC6) for data transfer.
IP
TCP
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