There are three types of phone connection that are likely to be used for data communications. By far the most common is the ordinary PSTN line. This is the ‘normal’ phone connection that has been around since Mr Bell. PSTN stands for Public Switched Telephone Network, but think of it as POTS (Plain Old Telephone Service). You may hear this service referred to as analogue or analogue lines. For exchanging data with any sort of remote datalogger this service is far more than adequate. Note that conventional Modems will only work on this sort of line.
 If the GSM (mobile phone) network is used to exchange data with the remote site then the data is transmitted over a radio link, from the service provider’s (Vodafone in the U.K.) computers to the GSM (M1 or M20T) modem connected to your Weather Station or remote DataHog. As mentioned above data has to get to and from your PC to the GSM service provider’s computers. The most economic way to do this is usually with a PSTN line, but in some circumstances, a second GSM modem may be used, connected to your PC. Note that this would still route the data via the GSM service provider's computers as for a PSTN line, and would never route directly to another GSM modem however close by it may be.
 The third and most recent service that may be used is ISDN; a service aimed primarily at transmitting huge volumes of data quickly. This is a purely digital service and whilst offering high speed ‘network’ type connections (e.g. to the internet) has no advantage whatsoever for communicating with dataloggers etc that are connected to PSTN or GSM services. This service in the UK may be seen under names such as ‘Home or Business Highway’, ISDN2e and ISDN30 etc. It should be noted that nowadays all telephone calls (other than a few local calls in remote areas) in the UK are digital once they have left your house or office and got to a telephone exchange. Huge trunks of fibre optic cable link towns and cities carrying digitised/packetised voice and data calls. ISDN lines simply bring this digital system to your office or home, instead of converting the signals to analogue and bringing them to you via a PSTN line.
 To make use of ISDN lines you need digital equipment, i.e. a digital telephone or card that will plug into your PC. Note though that Skye has no knowledge of a PC based ISDN card that will reliably implement the communications standards required to communicate with for example a GSM service provider. (See section on ‘soft modems’). To use a conventional modem or ‘normal/analogue’ telephone you need to convert your ISDN connection into an analogue line (usually one ISDN connection gives two ‘normal/analogue’ lines). In the UK, with ‘BTHighway’ services this is done for you, giving both ‘normal/analogue’ sockets and digital sockets. With other ISDN connections you must provide a piece of equipment (often called a terminal adaptor - TA) to do this conversion. Once you have thus given yourself back an analogue phone line simply connect a conventional modem between your PC and the analogue socket. As long as the TA is a standalone, permanently powered unit and not a PC card, Skye knows of no problems with this technique, but our experience is limited, and the possible array of equipment is huge! Be warned that if the TA used is not powered continuously then the analogue phone connection it should provide will be lost as the TA looses its power.

 In summary, for best results in communicating with your remote MiniMet or DataHog from your PC: -
  i) Use a conventional modem connected to a ‘normal/analogue/PSTN’ telephone line.
  ii) A GSM modem connected to your remote MiniMet or DataHog will give you the freedom to locate it wherever you please.
 iii) An ISDN line will give you no advantage whatsoever in communicating with your remote MiniMet or DataHog, but it may give you faster download times from the internet (depending on your Internet Service provider and the WebSite you are dealing with)!

©Skye Instruments Ltd. 2000