GPRS wireless solutions
General Packet Radio Service (GPRS) is a mobile data service available to users of GSM mobile phones. It is often described as "2.5G", that is, a technology between the second (2G) and third (3G) generations of mobile telephony. It provides moderate speed data transfer, by using unused TDMA channels in the GSM network. The maturity of the GPRS system has made it suitable for use as a transport medium for remote monitoring of utility assets such as pipelines, dams, pumping stations and where network flow and pressure measurement is required across a wide area (e.g. district or nationally).
GPRS is different from the older GSM Circuit Switched Data (or CSD) connections that have been popular in the last few years. In CSD, a data connection establishes a circuit, and reserves the full bandwidth of that circuit during the lifetime of the connection. GPRS is packet-switched which means that multiple users share the same transmission channel, only transmitting when they have data to send. This means that the total available bandwidth can be immediately dedicated to those users who are actually sending at any given moment, providing higher utilisation where users only send or receive data intermittently. Web browsing, receiving e-mails as they arrive and instant messaging are examples of uses that require intermittent data transfers, which benefit from sharing the available bandwidth. Point to multi-point remote monitoring systems found in water utility systems also suit this, where historical data can be logged and retrieved periodically (e.g. every hour) for analysis. With suitable event messaging critical alarms and conditions can be sent between the regular datalog retrievals.
This flexibility can only be utilised if the RTU and the associated software package (e.g. HMI/DCS/SCADA) can process such data. The TopKapi HMI software product by AREAL is one known product which works seamlessly with the Miri Technologies AD2000 wireless RTU. Significant time and engineering savings are made by adopting products to suit the application of GPRS technology.
Usually, GPRS data is billed per kilobytes of information transceived while circuit-switched data connections are billed per second. The latter is to reflect the fact that even during times when no data is being transferred, the bandwidth is unavailable to other potential users.
Packet-switched data under GPRS is achieved by allocating unused cell bandwidth to transmit data. As dedicated voice (or data) channels are setup by phones, the bandwidth available for packet switched data shrinks. A consequence of this is that packet switched data has a poor bit rate in busy cells. The theoretical limit for packet switched data is approx. 160.0 kbit/s (using 8 time slots and CS-4). A realistic bit rate is 30–80 kbit/s, because it is possible to use a maximum of 4 time slots for downlink. A change to the radio part of GPRS called EDGE allows higher bit rates of between 160 and 236,8 kbit/s. The maximum data rates are achieved only by allocation of more than one time slot in the TDMA frame. Also, the higher the data rate, the lower the error correction capability. Generally, the connection speed drops logarithmically with distance from the base station. This is not an issue in heavily populated areas with high cell density, but may become an issue in sparsely populated/rural areas.
Telephone operators have priced GPRS relatively cheaply (compared to older GSM data transfer, CSD and HSCSD) in many areas. Mobile phone operators are now offering flat rate access to the Internet or some contracts based on maximum data transferred, usually rounded off per 100 kilobyte. Typical rates vary wildly between countries but these are now becoming attractive enough for use on periodically polled monitoring and control systems found typically in the water and waste water industries.
The maximum speed of a GPRS connection is now the same as modem connection in an analog wire telephone network, about 4–5 kB/s (depending on the phone used). Latency is very high; a round-trip ping being typically about 600–700 ms and often reaching one second round trip time. GPRS is typically prioritised lower than speech, and thus the quality of connection varies greatly so system design must consider that critical conditions be handled via a local controller.
In order to set up a GPRS connection, a user needs to specify an Access Point Name (APN), user name and password, and very often an IP address, all provided by the network operator. For the purposes of monitoring and control systems found typically in the water and waste water industries GPRS systems a static IP number must be assigned as this simplifies the networking software required to identify each device in the system.
Advantages of “GPRS” Wireless solutions
- Offers communications to country-wide locations (i.e. across larger distances in the region of hundreds of kilometers that narrow band and spread spectrum solutions cannot transmit across).
- Offers greater portability and mobility where the user can move terminals around his plant or plants.
- The user does not have to fund the infrastructure costs of putting up significant masts and installing antennae.
- “Quick” and “easy” to implement if the GPRS service exists in the area of installation.
- Ideal for historical datalog data retrieval based systems where regardless of the actual connection reliability the data since last connection is always gathered on the next successful connection. Suitable software is required to handle this type of historical time stamped data and products exist on the market to integrate this data seamlessly into data trends.
Disadvantages of “GPRS” Wireless solutions
- Monthly contract costs are required. This varies between providers and countries.
- Requires service providers to have static IP networks set up on their GSM system network.
- Data “used” costs are incurred and these can be significant with increased activity which can be difficult to predict and control (unless some other contract can be negotiated by the end user or fixed priced “unlimited” Mbyte contracts are available).
- GPRS is typically prioritised lower than speech, and thus the quality of connection varies greatly if voice traffic is high (e.g. holiday periods).
- When the location is far from the nearest GSM cell tower, the connection speed drops logarithmically with distance from the base station.
- Experience suggests GPRS is not suitable for “real-time” data faster than 5-10 minutes for point to multi-point systems.
Michael Hogg B.Sc., CPEng., AMIEE, MIEAust
International Marketing & Technical Support
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