
System Design Considerations 175
November, 2008
The MOTOTRBO data applications, both in the radio and externally on the PC, perform this
conversion to an IP address when sending and transmitting. Understanding this conversion is
important, because it is possible to send traffic directly to the IP address of the radio, though in
most cases this happens transparently to the user. For example, if a user creates a text message,
and selects a user from the address book with an Individual Radio ID of 12045 (which can be
aliased), the text message is sent over the air to radio 12045, and is addressed to IP Address
12.0.47.13. When radio 12045 receives the over-the-air data message, it opens the data message
and looks at the target IP address. Because the target IP address matches its own IP, the
message is sent to the internal radio application. The target application is dependent on the UDP
message and the destination address used at the source.
If the target of a data message is an external PC connected to the MOTOTRBO radio, the sending
device will use an IP address with the CAI Network address plus 1. For example, if a MOTOTRBO
radio receives a data message for its Radio ID (12045), and the data message inside is targeted
towards the address 13.0.47.13, it will forward that message to the connected PC.
For ease of use, the MOTOTRBO radio has the option to be configured with a “Forward to PC”
option, which is available in the Network settings of the radio CPS. With this option enabled, all
messages targeted to both the 12.x.x.x and 13.x.x.x addresses are routed to the PC. It is
recommended that this option be chosen whenever a MOTOTRBO radio is connected to the
Application Server. The “Forward to PC” option also applies to a MOTOTRBO radio (portable or
mobile) installed in a mobile environment, i.e. a vehicle, or in a fixed location (a mobile in a tray
located on someone’s desk). If a radio is not connected to an external PC, the “Forward to PC”
option should be disabled.
It is recommended that the default value of the CAI Network address is used. If this value is
changed, all MOTOTRBO radios in the system must be updated with the same CAI Network
address. Also available for configuration is the Group CAI Network address. This is used for
broadcast data messages. Again, it is recommended that this value remain at its default value.
Figure 4-15 “Air Interface Network Connectivity” displays the IP connectivity with the radio
network. Also included is a simplified Network Address Table (NAT) that shows how the Over-the-
Air traffic is routed to either the Radio or the Mobile Client. The NAT is a translation table within the
MOTOTRBO radio that allows packets to be routed from the PC through the radio and over the air
to the destination address. As previously mentioned, when the “Forward to PC” option is selected,
traffic for both the 12.x.x.x and 13.x.x.x addresses is forwarded to the PC. If disabled, that NAT
Unit ID = 05000032
Convert to Hexadecimal = 4C4B60
Separate into 8 bit sections = 4C, 4B, 60
Each 8 bit section represents 1 octet of the IP address
Convert each section into decimal = 76, 75, 96
Assemble IP address from conversion above = 12.A.B.C where
A = The first 8 bit section in decimal format. In this example, A = 76
B = The second 8 bit section in decimal format. In this example B = 75
C = The third 8 bit section in decimal format. In this example C = 96
The IP address for Unit ID 05000032 is: 12.76.75.96
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