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STAR H GATE™
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4.5 NOTES FOR THE PROGRAMMER
STARGATE™ does not carry out continuous polling, but it is only enabled each time
an RF device sends it a sequence of data. This means the radio channel is always
free when the RF devices are not transmitting.
The media access procedure used to manage the shared RF channel is a Carrier
Sense Multiple Access Collision Avoidance (CSMA/CA) protocol that aims to
minimize the number of collisions of packets sent by different RF devices.
In terms of global throughput of the entire system a single RF device is not able to
exploit the whole capability of the cell (bytes of data per second). By adding RF
devices there is an increase of the global throughput and then a saturation (the total
throughput remains constant and the throughput of the single RF device is the total
number divided by the number of RF devices).
Therefore, the useful number of RF devices per cell depends on the traffic generated
by the specific application (the higher the number of packets per second for an RF
device, the lower the number of RF devices for a given latency time).
The system functioning is based upon a client-server architecture, where each
operation is driven by the RF device (client). In this way the data cannot be sent to
the device from the PC application. This is only possible when it is the RF device
asking for it.
The protocol allows transferring data packets up to 240 bytes. Data transfer is always
requested by the RF device and it may occur according to the following modes:
• 1 way mode: the RF device requests to send a data packet to the host and waits
for an acknowledgement from the STARGATE™ RF base station only;
• 2
way mode: the RF device requests to send a data packet to the host and waits
for a data packet in response (this is the only way to receive data from the host
PC). If this transaction ends successfully, the application program can retrieve
the response packet from the protocol library.
NOTE
For best performance do not exceed 240 bytes for each transaction.
If working in 2 way mode, this value corresponds to the sum of the
sent data and the received data.
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