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After either of the RB and RO conditions are met, the modem then initializes a communications
channel. Serial data in the DI buffer is grouped into RF packets (up to 2048 bytes in each packet
- refer to PK (Maximum RF Packet Size) Command), converted to RF data and then is transmitted
over-the-air until the DI buffer is empty.
Channel initialization is the process of sending an RF initializer that synchronizes receiving
modems with the transmitting modem. During channel initialization, incoming serial data
accumulates in the DI buffer.
RF data, which includes the payload data, follows the RF initializer. The payload includes up to
the maximum packet size (PK Command) bytes. As the TX modem nears the end of the
transmission, it inspects the DI buffer to see if more data exists to be transmitted. This could be
the case if more than PK bytes were originally pending in the DI buffer or if more bytes arrived
from the host during transmission. If more data is pending, the transmitting modem instructs all
listening modems that a subsequent packet is coming. Receiving modems move to the new
frequency and listen for the subsequent packet.
Refer to the RF Communication Options [p24] section for information and state diagrams that
illustrate channel initialization and the sequence of events that follow.
RF Packet
Figure 3.5. RF Packet Components
* When streaming multiple RF packets, the RF Initializer is only sent in front of the first packet.
RF Initializer
An RF initializer is sent each time a new connection sequence begins. The RF initializer contains
channel information that notifies receiving modems of information such as the hopping pattern
used by the transmitting modem. Channel initialization takes 5 ms at the 115k RF data rate and
54 ms at the 9600 RF data rate. The first transmission always sends an RF initializer.
An RF initializer can be of various lengths depending on the amount of time determined to be
required to prepare a receiving modem. For example, a wake-up initializer is a type of RF
initializer used to wake remote modems from Sleep Mode (Refer to the FH, LH, HT and SM
Commands for more information). The length of the wake-up initializer should be longer than the
length of time remote modems are in cyclic sleep.
Header
The header contains network addressing information that filters incoming RF data. The receiving
modem checks for matching a VID, Hopping Channel and Destination Address. Data that does not
pass through all three network security layers is discarded.
Figure 3.6. Network Layers Contained in the Header
RS‐232/485 RF Modem Product Manual – MAN0782‐02
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