Tone signaling over telephone lines_A technician’s primer When planning a system that will rely on telephone circuits for tone signaling, control or access, contact your telephony provider’s engineering office.
If your future plans call for tone signaling or control of remote base stations, polled receivers or other communications equipment, this article is sure to be of interest to you or to your technical and maintenance staff. The technical characteristics of telephone lines, both switched and “nailed up” circuits; the kind of tests frequently used by telephony providers and some simple troubleshooting you can do using imbedded telephone tones will be covered. One word of caution: although the information presented here is referenced to documents from Bellcore (the telephone “standard” setter of the old Bell operating companies), always check with your local provider to see what testing you may perform and what equipment voltage levels are acceptable to their specific equipment_these vary widely in some parts of rural North America.
The telephone lines that you are most likely to use in your control scheme typically are copper wires, twisted together to form “pairs.” The twist in the cable is designed to reduce crosstalk between pairs within a larger cable. If you are going to run tones and digital data over the same provided pair, let the provider know about your plans. Some types of high-speed digital data can cause severe crosstalk. These crosstalk problems can be eliminated by line conditioning. Let’s see how lines are tested and conditioned.
The cable pair(s) you use will, if unterminated, appear to your equipment as a capacitor. Even if correctly terminated, cable runs over longer distances can result in the attenuation of the higher audio or control tone frequencies. This “frequency attenuation distortion” can seriously affect your control circuit. In most cases a simple lumped inductance can balance the overall passband (frequency vs. attenuation) and provide acceptable results. If extremely long distances are used, some type of amplification is required.
This amplification is provided, in analog circuits, by a device that uses a pair of hybrid coils (see Figure 1 at the left) to “split” the signal, at least temporarily, from a two-wire circuit into a four-wire circuit.
A four-wire circuit provides for a separate transmit and receive pair to boost, then the amplified signal is placed back onto the single pair of wires by means of a second hybrid coil. (See Figure 2 below left.) This method of signal conditioning has been in use for years. The only disadvantages are that the amplifier boots both the desired signal along with any noise present on the line, singing and the possibility of echo generation caused by the hybrid coils. Checking for these “by-products” is covered later.
Connectivity to your remote site may be provided as a two-wire or four-wire switched circuit or as a permanent circuit. If the circuit is one or two pairs of wires connected between your sites without the use of switching equipment, it generally is referred to as a “nailed up” circuit. This does not preclude the telephony provider from conditioning or amplifying the line.
The “normal” passband of an unconditioned telephone line is between 300Hz and 4,000Hz_a range of frequencies that should offer flat response (levels) and little distortion. If your line exceeds 9 kilofeet, line conditioning equipment may be required.
Testing and maintenance of the line is generally the responsibility of the provider. Several of the tests performed by telco technicians may be of interest to you. If the circuit is switched_that is, if it runs through a modern digital switch in the local “central office,” then circuit condition testing is mostly automated. The type of tests performed by the switch can also be performed on “nailed up” circuits with technician intervention.
The tests most likely to be of benefit to you are PAR, echo return loss and singing/singing return loss. If your needs are strictly digital data, then the bit-error rate (BER) test is the only one that is needed to test for line quality. The PAR or “peak-to-average ratio” test consists of sending a precise set of pulses, with a known ratio of peak to average full-wave rectified voltage, into the line and measuring the resultant voltage at the far end of the circuit. Although this test will detail bandwidth, poor return loss and its attendant gain and phase distortion, the test will not define noise, jitter, IMD or other factors that can degrade control tones.
Echo on the line is the difference between the original signal and its echo, resulting from the two-to-four wire interfaces found in most central office equipment main distribution frames or transition points. Echo return loss measures loss (distortion) between 560Hz and 1,965Hz, whereas singing return loss (low) defines the distortion between 260Hz and 500Hz; and singing return loss (high) covers 2,700Hz to 3,400Hz. This is especially important if CTCSS tones must be recovered by your control equipment.
For data-only circuits, BER testing requires the use of a protocol analyzer or a purpose-built data test set. First, the line you use is connected back onto itself at the far end. This is called looping, or loopback, and can be performed at either the line or control end. Most modern modems and channel service unit/data service units (CSU/DSU) can be forced into a loopback with a specific tone. Then, once the loopback is established, a data stream is sent through the circuit. Errors caused by noise, jitter, phase drops or excessive distortion are quickly spotted. If the BER exceeds your requirements, call the telephony provider to initiate a trouble ticket. Working with the provider will lead to a better understanding of your needs.
Analog signaling schemes allow you to perform some simple tests prior to contacting your telephony provider. The first step, and an important one, is to record the measurements of line levels measured at your line (equipment side) termination_often called a demark (short for demarcation point). Using the correct load, typically 600V to 900V, or a bridging transformer, measure the voltage delivered to your site with a known or standard source providing a tone at the control end of the circuit. For switched (dial-up) circuits, dial the number for the local office “milliwatt signal.” This is a tone (1,004Hz) produced at a precise level_usually 1mW (0.775Vac @ 600V) and terminated correctly for the equipment in use. Measure and record this (and other tone) signal levels at both the control and line equipment (remote) end of the circuit. Leave a record of measured levels at both the local and remote site to aid in any future troubleshooting that may be required. After all, problems seldom occur during business hours.
With record in hand, you can feed a known signal to the remote site and check for excessive attenuation. In switched circuits, a call to the “milliwatt signal” from either end of your circuit can help to pinpoint potential problems quickly. To test for possible problems with distortion requires the use of a notch filter, signal generator, ac voltmeter and some patience. If you have access to more advanced equipment, such as a sweep generator and oscilloscope, you can perform similar tests much more quickly. By setting your sweep generator to provide a signal between 300Hz and 4,000Hz, you can monitor the results at the opposite end with your oscilloscope. This approach may trigger unwanted responses from central office equipment. Central office equipment is often controlled by tones within the voice frequency (VF) passband. To avoid trouble, check with your local provider before such tests are performed. Equipment used by most telco technicians uses as few as three, and as many as 23, separate (non-sweeping or fixed) tones to perform tests for bandwidth and distortion.
When planning a system that will rely on telephone circuits for tone signaling, control or access, contact your telephony provider engineering office to:
determine the technical characteristics of the telephone circuits available for your use.
discuss the type of equipment and signals you plan to install and use.
ask what support will be available for repair of potential problems.
find out what kind of testing you will be permitted to perform and how to best express this information to the telephony provider’s technical staff.
confirm the costs of any technical assistance required and provided, and the limits of responsibility.
Document these discussions, and provide a copy to your provider. If a problem does arise, records are always better than a memory_and much safer as well.
Acknowledgement The Bellcore “Redbook” of engineering standards was used as a reference in the preparation of this article.
