Caller Identification products are becoming a popular alternative to traditional telephone answering and recording machines.
Barring the use of the number withholding option (141 in the UK), caller identification is an excellent way for small businesses to know just who did call them when they were out on a call.
The unit captures the time and telephone number of the caller, who doesn't undergo the ordeal of talking to an answering machine.
At this time, the only country in Europe to offer caller identification is the UK, but other countries such as Sweden and the Netherlands are not far behind.
Other countries such as France and Germany are considering their options, but have yet to implement a standard for the caller identification feature.
The standards currently available are as follows:
 | Australia TP 01627.1 |
| Denmark ????????? |
| Europe ETS300 659-1 |
| Netherlands TS11-12E |
| Singapore TR-NWT-000031 |
| Sweden 8211-A311 UEN |
| UK SIN 227 & 242 (PSTN) TW/P&E/312 (CABLE) |
| USA TR-NWT-000031 |
The facility is sold differently in each country.
For example, in the US, customers pay a monthly charge for the service to a company who sells them the caller identification product at a very low cost.
By contrast, BT in the UK charge a low quarterly fee, but the caller identification units are quite expensive to buy.
It should be noted that caller identification by means of the standalone units or the telephone which has the feature built in has not been well accepted by the UK public, who are prepared to use only the abbreviated functionality offered by the telephone exchanges.
Additionally, very few products other than the BT badged telephone and standalone unit are available and that modems claiming to have the feature usually have only the US protocol which is incompatible with the system used in the UK (and many other countries for that matter).
While we're on the subject of compatibility, it should be remembered that caller identification products designed for the UK PSTN will not work on the cable telephone network and vice versa.
For those interested in the technicalities of caller identification, we have summarised the content of BT SIN 227.
The Protocol operates in the following manner:
| The telephone line voltage polarity is reversed. |
| The telephone exchange remains silent for at least 100ms. |
| The telephone exchange transmits an alert tone 88ms to 110ms in duration. Note: The alert tone signal consists of a dual frequency tone ie 2130Hz and 2750Hz transmitted simultaneously. |
| The CLI device detects the presence of tones over a 20ms period or 30ms of just the 2130Hz tone signal. |
| The telephone exchange remains silent, waiting for a response from the CLI device. |
| In the time period 15ms to 25ms following the termination of the alert tone signal, The
CLI device terminates the telephone line with an AC and DC load. The AC load consists of an 827ohm resistor in series with a parallel network comprising a 1386ohm resistor and a 139nF capacitor. The DC load is a high resistance (approx. 100kohms) intended to make the CLI device draw sufficient current (≤0.5mA) to charge it's internal battery/supercap, while appearing on-hook to the telephone exchange. |
| Commencing 45ms (or later) after completion of the alert tone/guard tone signal transmission, the telephone exchange transmits between 96 and 315 bits of alternating 0's and 1's. |
| The telephone exchange then transmits between 55 and 90 1's, followed by the the CLI data. |
| Following reception of the CLI data, the device is expected to remove the AC and DC loads in a 50ms to 150ms time period. |
| The exchange either commences to transmit ring signal or reverses the telephone line voltage polarity 200ms after it has transmitted the CLI data. |
CLIP Message Timing Diagram
The CLI protocol used in Sweden, Finland and the Netherlands is based around DTMF signalling, which means that only the calling parties number can be displayed.
If this article has whetted your appetite for designing a caller identification product, click here to check out a device which could be the answer to any problems you might be having.
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