Technical White Paper
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Unidata’s Neon Internet Protocol Telemetry Dataloggers and associated Neon Server Applications Software facilitate transport of data from the measuring instruments in the field to a central office. Telemetry systems have been used extensively for successfully transporting data from field measurement devices to central computer systems for many years. With the growth of coverage of internet and telecommunications networks, especially cellular and satellite networks, there are now more options available for telemetry.
Pull and Push Telemetry – A Short History
In past years, data was recorded from field instruments with chart recorders and the charts were collected from the field and brought into the office for analysis. In the 1970’s data loggers came into wide usage allowing the transfer of data either in the field or by taking the logger back to the office. Fixed phone lines and modems at the site allowed data transfer without a physical visit to the site. With this development, “pull telemetry” was born and soon expanded with the advent of mobile and satellite phones. However, the transaction still required modem and a connection to be initiated to transfer data.
With the arrival and expansion of the internet and TCP/IP data a new method was possible, whereby a shared packet networks (public and private) could be used to transfer data from the field to the office by sending the data in packets across shared TCP/IP networks. With a packet network the sending end initiates the transfer, hence this type of telemetry is called push telemetry or connectionless telemetry, removing the need for a dedicated connection. Another method of push telemetry is cellular short messaging, but this is not a guaranteed delivery method, which is an important consideration for telemetry applications. With TCP/IP each packet sent should receive an acknowledgement or the packet will be resent, ensuring data delivery. With SMS type messages there is generally no such acknowledgement method built into the system and messages may be lost.
Mobile / Cellular Phone Networks
Telecommunications providers continue to expand their cellular networks to provide more coverage, services and speed.
Whilst mobile / cellular networks are an ideal fit for push telemetry, which only require GPRS (2G) speeds, there however is one design aspect which is very relevant to telemetry applications. Network growth is generally related to population growth, with most providers advertising coverage by percentage of population. However most of the population is in the large cities where infrastructure shared across many users make expansion economically viable. Generally, telemetry is needed in remote, less populated areas where extra base stations are not economically viable. Hence while 90% of the population may be covered by such networks, perhaps only 50% of the country area is covered by the network.
Regardless of this, mobile / cellular phone networks will always offer the most economic method of communication where there is coverage.
Whilst there has been a rapid expansion of cellular / mobile networks, the global coverage of satellite communications still offer the most effective means of communications with remote sites outside of cellular coverage. There are two main types of satellite services, equatorial orbit satellites and low earth orbit satellites. The diagram below shows in diagrammatic for such services.
Equatorial satellites orbit the earth at around 25,000km at the same speed as the earth rotates; and so are stationary with respect to the earth’s surface. This means that they can act as a stationary radio repeater, receiving microwave signals from one point, amplifying and changing their frequency and re transmitting them back to earth, usually using focused antennas which point to the required area of coverage.
Low earth orbit (LEO) satellites orbit the earth within an hour or two and cover a smaller area which is only visible during the time when they are overhead, within view of the user on the earth. They are at a height of around 1000 km and there are generally 20 or more satellites in any LEO system and on average there will be at least one to 4 satellites in view at any one time.
For communications to occur one of these satellites needs to be acquired, communications established and the data transfer completed in 10 to 20 minutes while the satellite is in view.
Some systems can effectively transfer the call or data transfer to another satellite automatically as indicated in the diagram below.
As a general rule equatorial satellites require more power and are more expensive per call than a LEO satellite system. Also as the equatorial satellite does not move in relation to a user on the earth, they are always available immediately.
Unidata offer a range of products that allow communication using cellular or satellite networks and these are detailed further in the Products section.
For more information on these communications technologies, please refer to the Product Catalog.