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How much data flows through an Ethernet?
Customers have asked us the following: "Does high speed acquisition of hundreds of data put a strain on the LAN?" To transfer various data efficiently across a network, the original data is divided up and tagged with "headers" and other information. Certainly, the mechanics of this and other processes is complicated relative to RS232, RS485, and other forms of serial communication. When trying to describe the actual amount of data traveling on a network, strictly speaking it must be calculated while taking various conditions into account. However for our purposes, we offer an example of a straightforward data acquisition task using a Yokogawa instrument, and narrow the focus to the approximate amount of data that flows on a consistent basis.
In this example we consider the use of a Yokogawa data acquisition station called the DX2000 DAQSTATION DXAdvanced, together with the DAQWORX DAQLOGGER data acquisition software to acquire data at 1-second intervals on all of the DX2000's 348 (maximum) channels.
First, let us look at the exchange of communications during acquisition on the DX2000's communication log screen.
As you can see from the figure, DAQLOGGER makes three exchanges of FD commands per interval for acquisition in the order: 48 measurement channels (no. 1-48), 60 MATH channels (no. 101-160), and 240 external input channels (no. 201-440). The amount of data in this 1 interval of commands is as follows (owners of DXAdvanced can also refer to the DX1000/DX1000N/DX2000 Communication Interface User's Manual (IM 04L41B01-17E)).
Now we turn to how these data flow on the Ethernet. The figure shows the structure of the final Ethernet signal as it appears according to the various communication standards for user data.
Taking the first output request of FD1,01,48CRLF as an example, for 11 bytes of user data, an 8-byte preamble signal and 14+20+20+4=58 bytes of data is added, resulting in a signal with a total length of 77 bytes. Basically this calculation is sufficient, however:
- There is an upper limit to the amount of data that can be sent at one time. DAQSTATION can only send 1024 bytes of user data at once, therefore data of 1025 bytes or more must be divided before sending.
- With the TCP protocol, the sender is notified as to how many of the data were received. If other data is sent subsequently it will get mixed in with the response, but if not, only the notification is sent.
Based on this, one interval's worth of commands ultimately results in a signal on the Ethernet of the following length:
Since 1 byte = 8 bits, a signal of 2863 × 8 = 22904 bits flows during a 1-second interval. By comparison, a signal of ten million bits per second (10 Mbps) can be sent on a 10Base-T Ethernet (the DX2000 uses 10Base-T), and a signal of one hundred million bits per second (100 Mbps) can be sent across an Ethernet of 100Base-TX (common in offices). Therefore, you can see that the size of the data above will not place a significant strain on the LAN.
Note that this data quantity is typical of a favorable communication environment. The actual quantity of data may vary due to irregularities that can occur on TCP other networking devices. Consider the numbers as approximations based on acquisition of 348 channels worth of data from the DX2000 at 1-second intervals in a favorable communication environment with signals flowing steadily at 22904 bps.