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communication (3)

PHY physical layer, frame structure, parameters, data, energy, modulation, frame, format

1. What are the main functions of the PHY layer?

Activation and shutdown of radio transceiver

Energy detection (ED) in the current channel

Link quality indication (LQI) of received data packets

Idle channel evaluation (CCA) for carrier listening multiple access/conflict avoidance (CSMA-CA)

Channel frequency selection

Data transmission and reception

2. Physical parameters

868/915 MHz DSSS (direct sequence spread spectrum) adopts BPSK (binary phase shift keying) modulation

868/915 MHz DSSS (direct sequence spread spectrum) adopts O-QPSK modulation

868/915 MHz PSSS (parallel sequence spread spectrum) adopts BPSK and ASK (amplitude shift keying) modulation

2450 MHz DSSS (direct sequence spread spectrum) adopts O-QPSK modulation


3. PHY frame format


SHR: Synchronization header (including Preamble and SFD), allowing a receiving device to synchronize and lock to each stream at the same time.

PHR: PHY frame header (including Frame length and Reserved), the length information of the frame.

PHY Payload: A variable-length payload that carries the data frame of the MAC sublayer.

Among them, the SHR field: Preamble, which is used by the radio transceiver to obtain the chip and synchronization identification from the received data information. The length of the Preamble field is determined by the physical parameters as shown in the figure below.:


SFD, indicating the end of the SHR and the beginning of the packet, different physical parameters have different lengths as shown in the figure below:



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Shared massage chairs are not a rare thing anymore. We often see them when we go shopping. Do you know why it can start working immediately after scanning the QR code for payment? What principle is this based on? Let'sl take a look at the "story behind" the shared massage chair.

In addition to the basic massage function, the shared massage chair also integrates a wireless module for data transmission and control. On this large-scale shared device, due to the number of access and real-time reasons, 4G and GPRS are generally used. But let's also take a look at using NB-IoT modules and look into which of these is more suitable for use on shared massagers. 

Shared products need to be promoted and distributed in large quantities to cultivate users' usage and consumption habits. Therefore, it is necessary to choose a communication solution with relatively cheap tariffs, chips, and modules.
Among 4G, GPRS, and NB-IoT modules, 4G has the highest cost, but it has a high transmission rate and a large infrastructure coverage. Relatively speaking, the Cat1 module is relatively cost-effective. Secondly, the price of GPRS is moderate, but GPRS faces the risk of withdrawing from the network; the last is The NB-IoT module has the lowest cost, but the transmission rate is small, but it is enough to be used on a shared massage chair.

Remote monitoring and sharing of product data, visual presentation of product energy consumption, location, battery, operating data, etc. This is why wireless radio frequency modules such as LoRa, ZigBee, and Sub-G are not applicable, and NB-IoT modules are relatively more suitable.

Cellular data conforms to the usage habits of users and has a wide coverage area. It can be covered as long as there is an operator's network. At the same time, it can provide products with a standby time of more than several years. By the end of 2020, NB has covered major cities and towns. Covered, you can also apply for coverage if necessary.

Through analysis, we found that the NB-IoT module is really more suitable for shared massage chairs!
Ebyte's NB-IoT modules are mainly represented by the EA01 series, especially the EA01-SG, which integrates a high-precision, high-performance positioning chip, which is more convenient for sharing devices. Let's take a look at the application of EA01-SG in shared massage chairs.


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