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Chapter 4 - UDP/IP Implementation
This section describes how the radio firmware composes RS232 data into UDP
packets and sends them out, as well as how it receives the UDP packets and
performs necessary checks. If all checks are passed, it retrieves the RS232 data
from the packets, and sends the data out to RS232 cable.
When composing the UDP/IP packet header, the firmware only sets the
important fields in the packet header, other less important fields are set with
correct, fixed values and these values are unchanged during the operation time
of the radio.
Upon receiving UDP/IP packets, the firmware only checks for validity of some
important fields in the packet header. Others may be ignored.
Learning the destination MAC address from ARP packets.
After powered-up or its configuration is reset by the utility, the radio needs to
learn the MAC address associated with the IP address of the destination device.
It does this by composing an ARP packet and sends it out every second until it
receives an ARP response from the intended device. This radio unit will not send
and receive any thing until it receives the correct ARP response.
This part is very critical. So, if the radio seems not to communicate, please
check the correctness of the “Dest unit’s IP address” using the utility.
Outgoing UDP/IP packet:
a. IP Header:
i. Version: 4. (Fixed)
ii. Header length: 5 (Fixed)
iii. TOS: 8 (Fixed)
iv. Total length = IP header length + UDP header length + serial data length
v. ID = some timing value. Not important.
vi. Flags and fragment offset = 0x4000. (Don’t fragment, Last fragment,
fragment offset is 0)
vii. TTL: 128 (Fixed)
viii. Protocol: UDP (Fixed)
ix. IP Header check sum = this IP header’s checksum.
x. Source IP address = “IP Addr” in the “This Unit” portion in the
configuration panel.
xi. Destination IP address = “IP Addr” in the “Dest. Unit” portion in the
configuration panel.
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xii. No option.