Viper SC+™ IP Router for Licensed Spectrum PN 001-5008-000 Rev. C
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2.
NETWORK ARCHITECTURE AND SYSTEM PLANNING
This section discusses network architecture, basic network types, interfacing modems and DTE, data protocols for
efficient channel operation, as well as providing tips for selecting an appropriate site, antenna selection, and reducing
the chance of harmful interference.
2.1.
NETWORK ARCHITECTURE
In a radio system, only one radio should transmit at a time. If two radios transmit at the same time to another radio, RF
collisions occur. Collisions will slow data traffic and may corrupt data. Most SCADA networks have a device that is
configured to be the ‘polling master’. It is the responsibility of this polling master to control RF traffic so RF collisions do
not occur.
Viper has RF collision avoidance technology (checks the air wave for a carrier before transmitting) and Ethernet CSMA
(Carrier Sense Multiple Access). CSMA is an Ethernet collision avoidance mechanism technology built into to all
Ethernet connections. However, these technologies must still be supplemented by the HMI/PLC polling master to
optimize RF data traffic.
Some HMI/PLC Ethernet applications may depend solely on Ethernet CSMA to control the flow of messages to avoid RF
collisions in a Viper data network. This may flood the network with multiple polling messages, making it difficult for the
RTUs to acquire the airwave to transmit their reply messages. This will cause the RTUs to compete for airtime and a
dominant RTU may be created.
While the dominant RTU/radio is transmitting, the other RTUs will send their reply messages to their connected Viper
SC. Viper SCs will buffer reply messages because the dominant RTU/radio is transmitting (carrier is present). A Viper SC
will buffer (while a carrier is present) a reply message until it can capture the airwave (carrier absent) to transmit.
There could be five or six RTU/radios in a small system (or 10 or 20 in a large system), which could be trying to capture
the airwaves to transmit. The RTUs will not respond in the order they were polled but will respond when they are ready
and have captured the airwaves. The dominant RTU is created because it happens to reply at just the right time and be
in the right order in the polling sequence.
A common method for a polling master to manage RF traffic is for the HMI/PLC polling master to poll one remote at a
time. The next polling message is not sent until the current message has been completed (“Done”) or has timed out.
This prevents more than one outstanding polling message. Ladder logic programs typically refer to these parameters as
the message “Done” and “Error” bits. The “Done” and “Error” bits parameter values can be adjusted for longer timeout
values, if required.
Because the Viper SC has the ability to use two completely different and separate SCADA polling protocols, it is
important to have interaction between the two protocols. The Viper SC can send out an Ethernet TCP/IP polling
message and also an RS232 polling message, which may or may not be generated by the same HMI/PLC. CalAmp
recommends the user program the polling sequence in each protocol with logic that interacts with the other’s protocol
“Done” and “Error” bits. The Ethernet polling protocol would not be allowed to send a message until the current
Ethernet message is either “Done” or “Error” and the previous RS232 message are either “Done” or “Error” bits are set.
The RS232 polling protocol would also have a similar logic.