Description:
The following information only applies to the Industrial Ethernet CPs 443-1 Advanced:

• 6GK7443-1EX40-0XE0
• 6GK7443-1EX41-0XE0
• 6GK7443-1GX20-0XE0
• 6GK7443-1EX20-0XE0

These modules support open TCP/IP communication via Industrial Ethernet using the following communication blocks (T blocks):
 

FB	Block name	Function
FB63	TSEND	For transmitting data packets via an established communication connection
FB64	TRCV	For receiving data packets via an established communication connection
FB65	TCON	For establishing a communication connection
FB66	TDISCON	For ending an existing communication connection

Here, the communication processors only support the ISO-on-TCP connection type. Consequently, the RFC 1006 protocol extension of the TCP/IP protocol is used. Thus a maximum of 1452 bytes of user data are transferred.

With Industrial Ethernet the maximum frame size is 1460 bytes (maxSegmentSize). Consequently, the 1452 bytes of data fit in one Ethernet message, i.e. the above-mentioned communication processors can send the data in one message.

The CPUs with integrated IE interface also support open TCP/IP communication via Industrial Ethernet using T blocks.

The Industrial Ethernet CP sends the 1452 bytes of data in one message. The S7 CPU sends the same volume of data in two messages.

Explanation:
To understand this difference, you must consider two LAN (Local Area Network) recordings.

When a connection is established, the two communication partners negotiate the connection parameters.
They include the maxSegmentSize, among others. This is proposed by the active partner and acknowledge or negotiated by the communication partner. This returned value then counts for the communication connection.
Another parameter is the MaxTPDUSize. This is the maximum size of each protocol element. If the value of this parameter is greater than the maxSegmentSize, a corresponding protocol element has to be transferred segmented.

Below, we first consider a connection of the TCP/IP communication between two IE CPs using T blocks.
Then we consider a connection of the TCP/IP communication between CPU and IE CP using T blocks. This makes clear why the data has to be sent in two messages here.

1. Communication connection between two IE CPs
   
   Fig. 01 "LAN recording of communication between two IE CPs" shows that the IE CP with IP address 140.90.36.1 actively establishes the connection and proposes a MAXTPDUSize of 2048 bytes. In the total recording you see that the communication partner (IE CP) acknowledges the MAXTPDUSize of 2048 bytes. This then counts for the communication connection.
   
   
   Fig. 01: LAN recording communication between two IE CPs
   
   This means that only one message has to be sent. The maximum user data length of 1452 bytes is not limited by the maxSegmentSize and the maxTPDUSize. Both values are greater than 1452 bytes.
   The IE CP only has to send one message!
   
   Here are the corresponding network recordings of the communication between two IE CPs. You can open these with Ethereal or Wireshark, for example.
   
   CP-CP_IoT_1452Byte.zip ( 14 KB )
    
2. Communication connection between CPU and IE CP

   
   Fig. 02 "LAN recording of communication between CPU and IE CP" shows that the CPU with IP address 140.90.37.10 actively establishes the connection and proposes a MAXTPDUSize of 1024 bytes. In the total recording you see that the communication partner (IE CP) acknowledges the MAXTPDUSize of 1024 bytes although it supports a greater maxTPDUSize (see Fig. 01). The maxTPDUSize for the communication connection between CPU and IE CP is 1024 bytes.
   
   
   Fig. 02: LAN recording communication between CPU and IE CP
   
   This means that the maximum user data length of 1452 bytes is sent in two messages. The maximum user data length is limited by the maxSegmentSize. This is value is less than 1452 bytes.
   The CPU has to send two messages.
   
   Here are the corresponding network recordings of the communication between a CPU and an IE CP. You can open these with Ethereal or Wireshark, for example.
   
   CP-CPU_IoT.zip ( 11 KB )

Effects for users:
Initially, the user does not see any of the behavior at the application level. The mechanisms are coordinated independently by the protocol levels of the two communication partners. However, if you compare the number of messages on the LAN, you see that more messages are visible in the second constellation using a CPU.