PRIMER
Using Caller ID With Modems


Caller ID exists in two forms: 1) Standard Caller ID, and 2) Caller ID on Call Waiting, which may be marketed by the various phone companies under many names; "Deluxe Call Waiting", "Extended CID", etc... Note that not all phone companies may offer this service.

1) Standard Caller ID only works when the phones are not in use (ON-HOOK). The telephone company delivers a signal (tone) between the first and second ring of an incoming call. This signal contains the Caller ID information packet.

This packet comes in two forms, the older Single Data Message Format (SDMF) and the Multiple Data Message Format (MDMF). The Single provides only the time, date, and the phone number of the call. Whereas, the Multiple Data Message Format also includes the name of the caller. Both of these messages are displayed on the CID box or modem, if they are so equipped. When using a modem, the associated PC must be powered on and software enabled unless the modem is stand-alone and has storage. Not likely.

2) Caller ID on Call Waiting works the same as Standard Caller ID, but it also works when the phone line is being used (OFF-HOOK).

In this case, the telephone company produces two signals. The first is the audible Call Waiting Tone to alert the person on the phone that someone is calling. This is the same tone that is used with the basic Call Waiting Service. The second is the Caller ID Query otherwise known as the CPE Alerting Signal (CAS). This is a machine tone that only the Caller ID box recognizes.

In response to this query tone, the box sends an Acknowledge Tone back to the telephone company, which in turn responds with the Caller ID MDMF packet, as described above. If the telephone company does not see the Acknowledge Tone from the box, it does not send the Caller ID packet.

Normally, Caller ID on Call Waiting boxes will not operate with modems most of the time. It is a fortunate fluke when they do. The reason is that that the initial Call Waiting Tone and the Query Tone is buried in the overlapping modem signals. The box can’t tell the difference. By the same token, the modem signals may also produce false alerts.

However, the Call Waiting signal combined with the initial query tone may create enough "noise" to knock a modem off-line to, in effect, at least let you know that you have an inbound call. But not normally, especially with the newer modems. They simply try to error correct the noise and may down shift to a lower data rate and hopefully fall forward again when the noise is corrected. They are supposed to anyway.

Observations:
It appears that when Bellcore developed the specifications for Caller ID over Call Waiting, their main concern was accidental voice activation of the Query Tone. Operation with modems was not considered. Another observation is that Caller ID on Call Waiting boxes are designed to only send the Acknowledge Tone, if the device that is "OFF-HOOK" is connected to it. This means that if you are using a phone that is not connected directly to the box (an extension), you may not see the Caller ID associated with the Call Waiting.

The Internet Call Waiting Manager (CPS hardware product) monitors the line and has a powerful filtration algorithm to distinguish the Call Waiting and Caller ID tones from the surrounding modem signals. It operates with both types of Caller ID. Of course, if you don’t subscribe to the Caller ID over Call Waiting service, it will only store the caller IDs that are collected while the modem is off-line or the software is disabled. However, this defeats one of the neatest features of the ICWM. The real time "screen pops" that tell you who is calling.

In fact, the ICWM can also be used with just the basic Call Waiting service. It will give you a screen pop telling you that you have a call waiting when the call waiting tone is detected, but no other information about the call. However, CPS makes less expensive boxes to address this requirement. See the Call Waiting Switch.

Comments are welcome sales@cpscom.com