×
Siemens Industry Online Support
Siemens AG
Entry type: FAQ Entry ID: 26171811, Entry date: 08/08/2018
(5)
Rate

What properties, advantages and special features are offered by the protocols TCP, ISO-on-TCP, UDP and ISO Transport?

  • Entry
  • Associated product(s)
An overview shows the properties, advantages and special features of the protocols TCP, ISO-on-TCP, UDP and ISO Transport.

Fig. 1 shows how the protocols TCP, ISO-on-TCP, UDP and ISO Transport are ordered in the ISO-OSI reference model.



Fig. 1
   


TCP protocol

If data is transferred with the TCP protocol, the transfer takes the form of a data stream. There is no information transferred about the length nor about the start and end of a message.
The recipient cannot tell where one message ends in the data stream and the next one begins. Therefore, the sender has to specify a message structure which can be interpreted by the recipient. The message structure can, for example, comprise the data followed by a control character, such as "carriage return", signaling the end of a message.
TCP uses the Internet Protocol (IP) which is why people also frequently talk about the "TCP/IP protocol". It is in Layer 4 of the ISO-OSI reference model.
 

ISO-on-TCP protocol

The great advantage of the ISO Transport protocol is the package-oriented transfer of data. However, with the increase in networking, the missing routing functionality became an ever growing disadvantage.
Since the routing-compatible TCP/IP protocol has become predominant due to the internet, an attempt was made to combine the advantages of both protocols. In the extension RFC1006 (RFC = Request for Comments) "ISO on top of TCP", also called "ISO-on-TCP", the image of the properties of the ISO Transport protocol is defined on the TCP protocol. The ISO-on-TCP protocol is also on Layer 4 of the ISO-OSI reference model and defines Port 102 as default port for data transfer.
This protocol can be used in the current modules of SIMATIC S7, SIMATIC PC and, by implementing the CP 1430 TCP, also in SIMATIC S5.
 

UDP protocol

The UDP protocol was introduced to transfer data quickly and simply.
The UDP protocol is on Layer 4 (Transport Layer) of the ISO-OSI reference model and is thus based on the IP layer (Layer 3).
The recipient of data is addressed using IP addresses. Only minimal administration information is added to the data package to be sent, so that the data throughput is higher compared to the TCP protocol and ISO-on-TCP.
Considering the requirement to transfer data rapidly, the UDP protocol only provides basic functions. Thus data can be exchanged between communicating partners with a minimum of effort. The UDP protocol does away with security mechanisms as found with the TCP protocol and ISO-on-TCP protocol. The UDP protocol is connectionless and meets real-time requirements. It is unreliable because there is no guarantee for correct transfer of the data.

Fig. 2 shows the structure of the UDP protocol. Other information is also sent in addition to the user data to be transferred. This information is always at the beginning of a UDP message in the header. The header consists of the following 4 data fields each 16 bits long.

  • Source port
  • Destination port
  • Length
  • Checksum 


Fig. 2

Performance data

Table 1 shows the performance data of the UDP protocol.
 

CriterionPerformance dataNote
DefinitionRFC 768Manufacturer neutral
Transfer mediumCable, fiber-optics, wireless-
Transmission rateUp to 1 Gbit depending on the physical nature of the network-
Connectable devices and access procedures
  • Unicast
  • Multicast
  • Broadcast

Permissible processes:

  • CSMA/CD
  • CSMA/CA

Table 1
 

ISO Transport

From a historical point of view, the ISO transport protocol, as Layer 4 interface of the ISO-OSI reference model, was the first Ethernet protocol in the SIMATIC. The ISO Transport protocol is based on the ISO protocol as defined in ISO 8073 TP0.
The great advantage of this protocol lies in the message-oriented transfer of data, which results in facilitation of the processing in the automation system.
However, since the Layer 3 implementation is not available in the ISO Transport protocol, network addressing and therefore routing is not possible.
Unlike the TCP protocol, the ISO Transport protocol is also suitable for fault-tolerant systems, because data transfer can be easily specified. It is also possible to connect S5 systems with this protocol.
 

Socket und Ports

Several processes can run simultaneously on multitasking operating systems, whereby each process can provide multiple services. They should be able to be addressed separately.
For this reason, special interfaces, or ports, are defined for data communication both in the TCP/IP and the UDP protocol. This expansion of the host-to-host transfer to a process-to-process transfer is termed application multiplexing and demultiplexing.

Together with the IP address for a station, the port numbers (local and remote) form a socket which is defined as the unique address of the user program in the entire network. Therefore, any of a process's services can be addressed to a station within a network using a socket.

In the SIMATIC environment, multiple "simultaneous" processes or user programs cannot be created on one CPU. However, multiple communication jobs may be initiated simultaneously. In order to facilitate unique addressing here as well, assignment must take place via the connection configuration.
   

Services of the TCP, ISO-on-TCP and ISO Transport protocols

  • Send/Receive
    The SEND/RECEIVE interface is used for actively transferring data between two client stations. Data is exchanged here between the stations using send and receive blocks.
  • Fetch/Write
    Fetch/Write is a service which is used to read data from a server station or to write data to it. Access takes place via the operating system for the passive server station without any further function calls within the server station. In SIMATIC S7, the fetch/write service is only implemented in server functionality. Therefore, the SIMATIC S7 is unable to actively read or write data. Clients can actively read data from the SIMATIC S7 and actively write data to the SIMATIC S7.

Quantity framework

Entry 18909487 provides you with information about the maximum volume of user data per job and the number of possible connections that are supported by the CPUs with integrated PROFINET interface, the S7 software controller and the WinAC RTX.

Entry 16767769 provides you with information about the maximum volume of user data per job and the number of possible connections that are supported by the Industrial Ethernet CPs of the SIMATIC S7-300.

Entry 15368142 provides you with information about the maximum volume of user data per job and the number of possible connections that are supported by the Industrial Ethernet CPs of the SIMATIC S7-400.

Properties of the TCP protocol
The following table shows the properties of the TCP, ISO-on-TCP, UDP and ISO Transport protocols.
 

TCP protocolISO-on-TCP protocol UDP protocol  ISO Transport protocol
Rapid data transferRapid data transferVery rapid data transferVery rapid data transfer

Suitable for transferring medium-to-large data volumes

  • Up to 64 Kbytes per job with S7-1500
  • Up to 8 Kbytes per job with S7-1200
  • Up to 32 Kbytes per job with S7-300 and S7-400
  • Up to 8 Kbytes per job with the Industrial Ethernet CPs of S7-300 and S7-400

Suitable for transferring medium-to-large data volumes

  • Up to 64 Kbytes per job with S7-1500
  • Up to 8 Kbytes per job with S7-1200
  • Up to 32 Kbytes per job with S7-300 and S7-400
  • Up to 8 Kbytes per job with the Industrial Ethernet CPs of S7-300 and S7-400

Suitable for transferring small-to-medium data volumes:

  • The following modules support up to 1472 bytes per job:
    - S7-1500 CPUs firmware version < V2.5
    - S7-1500 CPUs firmware
    version V2.5 and higher (Broadcast)
    - S7-1200 CPU
    - S7-300 CPUs
    - S7-400 CPUs 
  • The following modules support up to 2 Kbytes per job
    - Industrial Ethernet CPs of the S7-300 and S7-400
    - S7-1500 CPUs firmware
    version V2.5 and higher (Unicast and Multicast)

Suitable for transferring medium-to-large data volumes

  • Up to 8 Kbytes per job with Industrial Ethernet CPs of S7-300 and S7-400
Based on the Internet Protocol (IP)Based on the Internet Protocol (IP)Based on the Internet Protocol (IP)Based on MAC addresses
Connection-orientedConnection-orientedConnectionlessConnection-oriented
Routing-compatibleRouting-compatibleRouting-compatibleNot routing-compatible

Data is transferred as a data stream.

Package-oriented data transfer

Package-oriented data transferPackage-oriented data transfer
The protocol acknowledges the sending and receiving of the data.The protocol acknowledges the sending and receiving of the data.The protocol only acknowledges successful sending of the data into the network and not the arrival of the data at the target station.The protocol acknowledges the sending and receiving of the data.
Data losses are recognized and automatically remedied.

Data losses are recognized and automatically remedied.

  • There is no guarantee that a sent data package arrives at the receiver or that a sent data package reaches the receiver only once.
  • The order of arrival of the packages at the receiver's end cannot be predicted.
  • Data packages with checksum errors are discarded and not requested again.

Data losses are recognized.

When transferring dynamic data volumes the sender has to define a message structure that can be interpreted by the receiver, because the length and information about beginning and end of a message are not transferred.

Transfer of dynamic data volumes is possible, because the length and information about the beginning and end of a message are transferred as well. Transfer of dynamic data volumes is possible, because the length and information about the beginning and end of a message are transferred as well. Transfer of dynamic data volumes is possible, because the length and information about the beginning and end of a message are transferred as well. 

Flexible, can be implemented with third-party systems 

Can be used mainly in homogeneous SIMATIC structuresFlexible, can be implemented with third-party systems Can be used mainly in homogeneous SIMATIC structures
Separate router configuration/settings required (router must be entered in the hardware configuration of the CPU or the CP)Separate router configuration/settings required (router must be entered in the hardware configuration of the CPU or the CP)Separate router configuration/settings required (router must be entered in the hardware configuration of the CPU or the CP) -

Table 2

Notes

  • Entry 109747710 provides Basic examples of Open User Communication (OUC).
  • You can find general information about communication via SIMATIC S7 in Entry ID: 20982954.
Security information
In order to protect technical infrastructures, systems, machines and networks against cyber threats, it is necessary to implement – and continuously maintain – a holistic, state-of-the-art IT security concept. Siemens’ products and solutions constitute one element of such a concept. For more information about cyber security, please visit
https://www.siemens.com/cybersecurity#Ouraspiration.
Support to the statistics
With this function the IDs found are listed according to number (format .txt).

Generate list
Copy URL
Display page in new design
mySupport Cockpit
Related links