monitoring and alarm applications, such as traditional river
monitoring systems, construction is difficult because river
environments are not well suited for using wire or fiber
transmission lines. In comparison, GPRS technology gives
system integrators an excellent alternative. In addition to
being easy to set up and configure, the cost of operating a
GPRS monitoring system can be substantially lower, since the
cost is directly proportional to the volume of the data and the
frequency with which it is transmitted.
Although GPRS technology makes it easier and more
convenient to set up a river monitoring system, 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.
Since most carriers do not offer basic service packages for
enterprise clients, many enterprise clients turn to mobile
virtual network operators (MVNO). MVNOs are companies
that provide mobile phone services, but they do not have their
own licensed frequency allocation of the radio spectrum, and
may not have the infrastructure needed to provide cellular
telephone services. In fact, an MVNO acquires numerous GPRS
services and then rents them out to customers who are looking
for a small number of IP addresses. In general, MVNOs will also
set up a VPN server to divide their clients into different groups.
Unfortunately, some countries do not have MVNOs, and some
carriers do not provide VPN services. For this reason, this
solution may be unfeasible for some users.
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. DDNS is one type of DNS server. The difference between
DDNS and DNS is that DDNS takes care of the Dynamic IP
address of a device, and DNS the static IP address. With most
remote GPRS devices, you need to apply for a hostname for
each of the devices handled by the DDNS server. 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.
The host can find a device’s IP address from the DDNS’s
mapping table by looking up the device’s hostname. For this
solution there are two concerns: (1) A majority of DDNS
servers do not have standard protocols to implement IP
address updates, which makes it difficult for GPRS devices to
provide client APs to the DDNS. (2) The quality of the service;
as DDNS service is usually provided by a third party service
provider, the system may crash when the DDNS loses
connection or is being maintained. In addition, it may be
necessary to pay a premium to the DDNS service provider for
better quality of service.
The Pros and Cons of Polling
The advantage of polling architecture is that it operates in the
same manner as wired Ethernet environments. In order to
implement these solutions, we need a third party, such as a
carrier or MVNO/DDNS service provider. For the most part, the
solutions will require time and money, and collaborating with
third parties to find a suitable means of implementing your
GPRS remote monitoring and alarming system.
New Push Architecture for GPRS Networks
Push Architecture is a mobile centric solution. Service
providers such as web portals and e-mail servers use a fixed
domain name. Clients such as mobile phones get information
from these service providers by “pushing” the connection
request to the Web and e-mail servers, and when a connection
is established, the communication is bi-directional.
Unlike the so-called polling architecture, push technology
makes bi-directional communication possible for GPRS
networks that are using either a dynamic or a static IP address.
A remote device with front-end intelligence can report its I/O
status to the host and connect to the GPRS network when it
needs to.
Since Moxa’s Active OPC Server supports push technology, our
GPRS I/O family of products creates a software-based gateway
that makes communications easier. By using a static IP address
on the Active OPC Server, the GPRS I/O device can connect to
the GPRS network and Active OPC Server without needing to
worry about the IP address issues.
Moxa’s ioLogik W5340 Active GPRS I/Os takes full advantage of
all the benefits of the push technology and Active OPC Server.
What Active GPRS I/O and Active OPC Server provide are:
1. SCADA Data Acquisition by OPC protocol.
2. SCADA Data Acquisition by Modbus/TCP protocol.
3. ioAdmin.exe: active GPRS I/O’s configuration software.
Alarm messages, such as e-mail and SNMP trap or user
definable TCP/UDP raw packets, can all be actively pushed to
e-mail servers, SNMP trap servers, or TCP/UDP servers. SMS
can be pushed from the Active GPRS I/O to an engineer’s
cellular phone.