Telcos fight crime with derived channel technology

Telcos fight crime with derived channel technology

James E. Poon

Telcos fight crime with derived channel technology

Telcos are joining alarm system companies in the battle against crime. A home is burglarized every eight seconds in the United States, and consequently the alarm security business is thriving. Nine percent of all residential homes and 40% of all businesses have an alarm system, and the annual growth rate for these services is 30%.

Alarm companies are presented with the task of providing secure supervised alarm service to subscribers at an affordable price. Many of the available alarm technologies fall short of this objective. Digital dialers, for instance, are a cost-effective solution but are rendered useless if the telephone line is cut. Private lines and long range radio provide secure and supervised alarm services but are not economically attractive.

Derived channel technology makes security affordable. It uses unused bandwidth in existing telephone lines to provide alarm services to subscribers. In this way, derived channel services can be offered without affecting normal telephone operation. This technology uses in-band signals (400-Hz to 2800-Hz range) while the subscriber is on-hook and out-of-band low-tone signals (10 Hz to 50 Hz) while the subscriber is off-hook. A low-tone signal is used because it is transparent to subscribers.

Sending Alarms

One type of derived channel alarm service uses a subscriber terminal unit, or STU, to monitor alarm input points, and outputs a signal on the customer’s existing phone line. A receiver at the telco’s CO monitors the alarm status signals from subscriber terminals. The subscriber terminal relays alarm information to the CO receiver, which in turn notifies the appropriate alarm agency. To maintain line integrity, the STU transmits the low-tone signal to the CO scanner.

Derived channel technology was originally developed for analog loops, which provide a direct metallic connection between the subscriber’s telephone and the CO switching system. However, telcos frequently provide phone service to subscribers by using universal digital loop carriers, or UDLCs. This is a cost-effective, efficient way of providing phone service to a large number of subscribers within a specific geographical area.

UDLC systems concentrate many subscribers onto a few digital links. They differ from analog loop circuits in that two additional elements are introduced–the remote terminal, or RT, and the central office terminal, or COT (Figure 1).

The function of the RT in the transmit direction is to filter analog signals from subscribers, convert them to digital signals and multiplex these digital signals onto T-1 carrier lines. At the COT the digital signals are demultiplexed, converted back to analog signals and sent to the CO switch.

Typically, UDLC systems are designed to pass voice frequencies (400-Hz to 2800-Hz range) and to attenuate frequencies that fall outside of that range (Figure 2). Although attenuation of frequencies below 400 Hz is not a requirement in UDLC applications, most UDLCs provide this attenuation in order to filter out 60-Hz noise induced from power lines. This attenuation, however, has the unwanted effect of blocking the derived channel low tone.

Different Types

The transmission of the derived channel low tone through the UDLC requires modifications (Figure 3). Several methods are available. Channel units located at the RT and COT filter and amplify analog signals.

They also relay status information to each other by using digital signaling bits such as on/off hook, ringing and channel test. One method of providing derived channel services in a UDLC is to equip the RT channel units with a low-tone detector and the COT channel units with a low-tone generator. When the RT channel unit detects that a low tone is present, it can activate the COT generator by the use of signaling bits.

The advantage of this implementation is that the frequency response of the UDLC is basically left unmodified so that the 60-Hz requirement is easily met. The disadvantage is that the telcos must purchase a matched pair of channel units (RT and COT) from a specific vendor.

A second derived channel implementation method is to modify the existing RT’s frequency response to accommodate low-tone signals. To compensate for transformer losses at both the RT and COT, amplification and filtering must be added to the RT’s transmit section.

Filtering is necessary to guarantee that the UDLC’s 60-Hz requirement is met. For example, if a low-tone frequency of 40 Hz is used and the UDLC requirement for 60 Hz is –20 dB, then at a minimum, a sixth order low-pass filter would be required (Figure 4).

This implementation scheme is advantageous because it provides a single-ended solution in which only the RT channel unit is changed. It will, however, increase the noise floor of the UDLC. Most analog-to-digital converters used in UDLCs are non-linear devices that are designed to maintain a constant signal-to-noise ratio. Because a signal is always present in the derived channel application, the noise in the system will increase.

Another disadvantage of the RT implementation scheme is that telcos must dispatch craft personnel to the RT in order to replace existing channel units. If the RT is in a remotely located unmanned underground vault or aboveground hut, changes involving the RT are time-consuming and expensive.

A final implementation method is to perform amplification and filtering on the COT receive circuitry. As with the RT implementation, it is advantageous because it is a single-ended solution. In addition, the noise floor will not be increased since the amplification will be performed after the derived signals have passed through the analog to digital converters. Unlike the RT implementation, changes in the CO do not require dispatch of personnel.

UDLC is the predominant method of providing new phone service to areas where there is concentrated subscriber growth. These systems are growing 6% annually in the United States.

Until recently, derived channel and UDLCs were incompatible.

Of the three different implementation schemes for providing derived channel service in a UDLC, the third method, which involves changing only the COT channel unit, is the least expensive and most logical.

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