To what extent is the network additionally loaded by a serial connection make through a Com-Server?

To ensure that the network is not overloaded by an excessive packet rate, the Com-Server is equipped with the “network delay” system option, which acts as a protective function.

By means of this tool, it is for example possible to define that the Com-Server in the Ethernet handles packets with a minimum interval of 30ms. For a baud rate of 9600 on the serial side, approx. 30 signs can thereby be received within the above time. In the case of a constant serial data flow, a network packet of the following length is generated approximately every 30 ms:

18-byte Ethernet frame + 20-byte IP header + 20-byte TCP header + 30-byte data = 88 bytes (~700 bits)

In the worst case, a TCP acknowledgment from the recipient with a length of approx. 500 bits is generated for each one of these packets. In practice, the number of such packets is, however, generally much less as TCP drivers acknowledge several incoming packets by means of one combined acknowledgment.
In most existing applications, a load of approx. 40.000 bits/s can be projected, which corresponds to a data load of under 0.5% in the case of a 10-Mbit Ethernet. The value of 30 ms for “network delay” is fixed and independent of the set baud rate. If the baud rate is, four example increased to 19,200 baud, the double amount of the data can be transferred per network packet. The network load however only increases to 0.7% as the sizes of the packet headers remain the same.

Note: The calculation shown is based on the assumption of an unloaded Ethernet without considering possible above calculations assume that the Ethernet is not loaded and that there are no collisions or other interruptions of the data flow. In practice, such influences can lead to longer transfer periods and thus larger packets. This also reduces the network load by the Com-Servers as more data is transferred per packet.

Additional information:

• Product Info: Com-Server
• Manual for Com-Server

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