Sercel Inc 0104A ユーザーズマニュアル
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Description of radio protocol
:
The LRC-508 uses a half-duplex transmission protocol with a Frequency Hopping Spread Spectrum (FHSS) method. A
mechanism of Listen Before Talk (LBT) is implemented to avoid radio collisions inside the LRC-508 network or with an
external radio network. Moreover, two security keys are configured to secure the LRC-508 network.
Frequency Hopping Spread Spectrum (FHSS)
The FHSS operates on a set of frequencies. It uses one frequency for a fixed period of time and then switches to
another channel. The next frequency is given by a pseudo-random sequence. In order to communicate, the
transmitter and the receiver have to us the same set of frequencies, the same frequency sequence defined by the
Frequency key. Moreover the LRC-508 has to be synchronized either by internal Gps or by IEEE1588 Ethernet
protocol.
Example of FHSS based on a set of 6 frequencies
Listen Before Talk (LBT)
The LBT is based on a Channel Control Access mechanism: the LRC measures the Received Signal Strength Indication
(RSSI) before beginning packet transmission. If the RSSI is too high, the media is said “busy” and the LRC-508
postpones the transmission for a random back off time.