Delta RMC101 User Manual
Page 312
RMC100 and RMCWin User Manual
5-62
This diagram shows the four conceptual layers of TCP/IP: application, transport, internet, and
framing. A fifth layer—the hardware layer—is often added below these four layers, but is left out
of this diagram because it is more of a specification of how the data is sent rather than another
protocol header. When a device is sending a packet the packet is assembled from the top layer
down, but when receiving a packet, it must be processed from the bottom layer up.
Here is how the RMC might look at an incoming packet with this structure:
1. Hardware Layer: A full packet is received and passed to the Framing Layer.
2. Framing Layer: The CRC (cyclic redundancy check) is verified. If this fails, the packet is
discarded. Next, the destination MAC address in the framing header is compared with the RMC's
MAC address. If the addresses do not match and the destination address was not a special
broadcast address, the packet is discarded. Otherwise it is passed to the Internet Layer.
3. Internet Layer: The IP address in the IP header is compared with the RMC's user-selectable IP
address. If it does not match, the packet is discarded. Otherwise, it is passed to the Transport
Layer.
4. Transport Layer: The transport layer provides a number of services, but minimally must specify
the port that the data should be sent to. A port is an abstract connection point on a device that
allows for multiple connections to exist on a single device. It also helps determine which
application protocol will follow. The packet may be discarded here too if the destination port is not
one that the RMC supports.
5. Application Layer: In our example, the application protocol is Modbus/TCP, so the Modbus/TCP
header contains data such as the RMC register address to begin reading or writing from, the
number of registers to access, and whether the operation is a read or write. The Modbus/TCP
data area holds the actual words to be written.
Here is a diagram demonstrating all protocols supported by the RMC and the layers to which they
belong: