Telephone-based RMR using line sharing modems

Telephone as a means of remote meter reading (RMR) is a well proven technology be it via analogue landline, ISDN landline, or by cellular telephone. Costs vary according to network, interface type and network connection problems at each site. Analogue landline installation/operating cost is however, still generally cheapest overall where there are no special costs to bring a telephone line up to the meter. A key advantage of the analogue landline network is that it is almost universal in penetration of premises of consequence in C&I metering worldwide. Now line sharing modems fitted to meters enable existing telephone lines to be shared between an existing user and a meter. This eliminates the cost of a separate telephone line to the meter. This major saving with high, existing availability of, and ease of, network connection can make RMR logistically and economically viable for many C&I metering operations.

The key desirable, if not essential, aspects of line-sharing are: ‘No-ring calling’, that is people are not disturbed by their phone ringing when a parallel modem/meter is called by the collection computer; and that the modem should release the line immediately a person picks up a phone handset. The technology and its implementation need some discussion.

Caller Line Identification (CLI) decoding can be employed on a modem for detection of calls from the data collection computer. In those countries where the CLI signal arrives at the modem before the first ring signal, there is effectively no-ring calling since a suitable modem can answer a data collection call before the parallel phone rings audibly. There are disadvantages for CLI operation in some circumstances. While no-ring calling can be applied in the UK, the Netherlands, Sweden and France as examples, there are countries where the ring signal arrives before the CLI signal, meaning that no-ring calling is not possible. CLI is not automatically available on most national networks. In some countries the CLI service is free, while in others a small subscription charge is made. 

It should not be assumed that CLI can be applied to every line where it is generally available on a national network. Some countries have no CLI operation on their analogue landline telephone networks. CLI signals do not propagate through PBXs (internal telephone exchanges), meaning that only direct lines can be shared. A single technical strategy for international use of CLI for line sharing is therefore not possible. Although excellent as a solution in some countries, it is not so for all.

‘Dial-inbound’ operation offers an alternative line sharing method to CLI operation. In dial-inbound operation the telephone call is generated by an outstation/meter to the central data collection computer. Some, but not all, modern electronic electricity meters are capable of issuing dialling instructions to a connected modem. For meters not capable of this function, there is the useful alternative of modems that are designed to do this autonomously and which can be programmed with an appropriate call-in strategy and the telephone number(s) of the distant computer. This method of line sharing has many advantages over dial-outbound CLI operation. The key advantages are that it is potentially usable on all national telephone networks, there are no CLI subscription fees to pay, the modem can work from extension lines on a PBX system and, given the call direction, there is no parallel phone ringing to disturb people at the meter location.