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46 Water & Wastewater Asia March/April 2006

Michael Hogg of Miri Technologies gives us an overview of the GPRS wireless solutions, listing its

advantages and disadvantages

GPRS wireless solutions for

monitoring and control systems

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. Thematurity 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 (for

example, 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

(for example, every hour) for analysis. With suitable event

messaging critical alarms and conditions can be sent between

the regular datalog retrievals.

Usually, GPRS data is billed per kilobytes of informationtransceived 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 approximately 160

kbit/s (using 8 time slots and CS-4). A realistic bit rate is 30 to

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

Michael Hogg, Miri Technologies

A GPRS point-to-multipoint system

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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 maximumdata 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 wastewater industries.

The maximum speed of a GPRS connection is now the

same as modem connection in an analogue wire telephone

network, about 4 to 5 kB/s (depending on the phone used).

Latency is very high; a round-trip ping being typically about

600 to 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 wastewater 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 solutionsGPRS wireless solutions offer communications to

country-wide locations (i.e. across larger distancesin the region of hundreds of kilometres that narrow

band and spread spectrum solutions cannot transmit

across).

It also 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. GPRS is quick and easy

to implement if the GPRS service exists in the area

of installation.

GPRS is suitable 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 successfulconnection. 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 wirelesssolutionsOne disadvantage is the monthly contract costs.This

varies between providers and countries.

Another disadvantage is that service providers

are required 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 areavailable).

GPRS is typically prioritised lower than speech, and thus

the quality of connection varies greatly if voice traffic is high

(for example, 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 to 10 minutes for point to

multi-point systems. WWAEnquiry No: 005

Generally, the GPRS connection speed drops logarithmically with

distance from the base station