User Manual

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2009 Industrial Wireless Guidebook
Cellular Networks
3
We encourage you to complete your DCOM setup with this step. Integrators frequently establish OPC
communication and don’t spend the necessary time to secure the computers again. This can lead to
catastrophic results if network security is compromised due to a virus, worm, malicious intent, or simply
unauthorized “experimentation” by well-meaning coworkers.
For more detail information, please refer to the OPC Training Institute: http://www.opcti.com
Enhance OPC Capability for Cellular Communications
GPRS is a communication technology that allows data acquisition systems to overcome the difficulty of
cabling for wide area remote sites. GPRS applications are becoming more and more prevalent due to the ease
with which they can be implemented, but the dynamic IP address issues associated with GPRS networking
continue to frustrate system integrators. The fact that most GPRS devices use dynamic IP addresses can be
somewhat frustrating. What this means is that telecom service providers (commonly referred to as carriers)
often assign temporary IP addresses to their clients to access the Internet. Compared with static IP addresses,
using dynamic IP addresses make it difficult for the control centers to keep in constant contact with remote
devices.
The Traditional Polling Architecture of GPRS Networks
Traditional monitoring and alarm systems use a polling architecture that will only work properly if the host
knows the IP addresses of the I/O devices used by the system. The trouble with I/O devices with GPRS
capability is that the devices receive a different IP address every time they connect to the GPRS network.
Three distinct solutions have been developed to tackle this obstacle:
Solution 1: Public Static IP Address
The first choice is to get a public static IP address; some carriers can assign a static IP address to a specific
SIM card. This way, all the I/O devices will have their own static IP address and the entire system will
operate in the same manner as a traditional monitoring system that uses physical wiring. Perhaps the main
benefit of this solution is that it behaves the same as a wired solution. However, not all carriers offer this kind
of service, and when they do the cost is relatively high.
Solution 2: VPN Service Provided by Carrier/MVNO
A VPN (Virtual Private Network) is a secure LAN solution that groups specific devices together. VPN has
two major functions—security and grouping—and for the GPRS world the VPN grouping concept solves
the dynamic IP address issues. The grouping of the devices into one private network prevents unauthorized
persons from accessing the data. For this VPN solution, customers are required to buy a number of different
GPRS on-line services, and to apply for access to a Virtual Private Network (VPN). When the GPRS device
dials up, the carrier will assign a private IP address to it and because the private IP address is on the same
network segment as the host, the host and devices can maintain bi-directional communication using a
polling architecture.
Solution 3: DDNS
Using dynamic IP addresses is often necessary since many ISPs do not provide static IP addresses, or
because the cost of obtaining a static IP address is too expensive. The Dynamic Domain Name System
(DDNS) is used to convert a device’s name into a dynamic IP address so that remote devices can
communicate with the control center using a fixed domain name. When GPRS devices get an IP from the
carrier, they will automatically connect to the GPRS network. Each time a GPRS device’s built-in DDNS
client gets a new IP address, it will send the IP address to the DDNS sever. The mapping table in the DDNS
server is refreshed each time the DDNS receives a new IP address from the devices.