RF WIRELESS FOUNDATION COURSE BOOK ONE
12 Rev. 0.1
LESSON 3: SENDING PACKETS
There are few rules which are needed when sending data through an RF link; however, there are some differences
compared to a typical wired link. After enabling a receiver, data is received even if there is nothing in the air. The
receiver receives noise (random data). In a wireless environment, the real data has to be found somehow in the
noise. To help a receiver, the transmitted packet should start with a preamble and a synchron pattern:
The preamble is a continuous 0101 sequence. It helps the receiver synchronize with the transmitter. The
preamble is a known sequence with continuous edge changes in the data so during that period the demodulator
and clock recovery circuit of the receiver node can be settle easily, which means that the receiver finds out
where the sample from the data has to be taken.
After the transmitter and receiver node synchronize, the receiver has to find out where the real data in the
packet starts. For that purpose the transmitter sends some data with a known bit pattern. This is called the
synchron word.
The receiver also needs to know how long the transmitted packet is. Remember: if there is no transmitted signal,
the receiver receives noise which is seen as 1s and 0s, so even if the transmitter node finishes sending a packet,
the receiver does not realize this without knowing how long the packet is! There are a few possibilities of how to
solve this issue:
Wait for a special end character.
Always send a fixed length message.
Include the length information into the packet.
In the RF physical layer, the transmitted packet can be easily disturbed by another node, or another RF system
also of noise. To solve this issue it is highly recommended to use a form error checking. Error checking will confirm
whether the received data is valid or contains bit error(s).
Unfortunately, if the communication is disturbed it causes not only a bit error, but usually causes burst bit errors
which can usually be identified by using a CRC check only (a simple checksum can recognize bit errors and but not
burst errors).
Figure 15. Typical RF Packet with Addressing
Tips & Tricks
"What do EZRadio Universal ISM band FSK receivers and transceivers do to help in realizing packet reception?"
By using FIFO mode in the EZRadio receiver or transceiver ICs, the RF chip can be configured in such way that it
will automatically recognize the preambles and synchron words (16bits: 0x2DD4 is used by Silicon Labs) and only
the real data (after synchron word) will be filled into the FIFO.
AA AA 2D D4
Length Payload CRC16Source ID Dest. ID
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