Protocol Operation
Crescendo VHF Half-Duplex User Manual
Page 39 of 74
6.8 Datagram Protocol
When using the datagram protocol, packets are passed to the Crescendo to be transmitted over the air. The
packet format is given in Figure 10.
Start
0xC1
Destination
(16)
Source
(16)
DP
(2)
CRC
(32)
Data
(1…Slot Size)
SP
(2)
Res.
(4)
Length
(16)
Figure 10: Datagram protocol packet format
Note that the number of data bytes in a datagram packet must be no greater than the max packet size
configured. The fields of a datagram packet are:
Start (8-bit):
A fixed start of packet marker, always
0xC1
.
Length (16-bit):
The length of the packet, including all fields except
Start
,
Length
, and
CRC
.
Destination (16-bit):
The 16-bit destination address. This can be any singlecast address, or the
broadcast address. The destination cannot be the same as the radio‟s source address.
Source (16-bit):
The 16-bit source address. When sending a packet to a radio, this field does not
need to be set. The source will automatically be set to the radio‟s local address.
DP (2-bit):
The destination serial port. This can be either 00 for the main port or 01 for the auxiliary
port.
SP (2-bit)
: The source serial port. When sending a packet to a radio, this field does not need to be
set. The value will automatically be set to the serial port the packet was inserted on.
Res (4-bit):
Reserved for future use. This field should be set to all zeros.
Data:
The data to be transmitted. The amount of data in each datagram must be equal to or less than
the slot size.
CRC (32-bit):
A 32-bit CRC value calculated over the entire packet, excluding the
Start
and
CRC
fields.
There are two options for the CRC field.
1.
Fixed:
The value is fixed to
0xEDB88320
.
2.
CRC:
The CRC value is calculated using a 32-bit CRC with
Initial Value: 0xFFFFFFFF
Polynomial: 0xEDB88320
Final XOR: 0xFFFFFFFF
When a radio modem receives a datagram packet over the air, the data will be output by the datagram
protocol in the same packet format, as described in Figure 10.
An example Datagram packet with a calculated CRC-32 is shown in Figure 11.