Chapter 14: Repeater Power Feed

Simplex Power Design

Office repeaters are located at the central office where noise-free -48 VDC power is plentiful. Line repeaters seldom have any local source of AC or DC power. Instead, the power for line repeater electronics is fed down the copper pairs from the office repeater. Inside a line repeater, there are two digital regenerators designated side 1 and side 2. Line repeaters are powered by DC current flow through a loop formed from the simplexes of the two cable pairs associated with side 1 and side. The line repeater must have current flowing though it in the right polarity for proper operation. In this case, the repeater represents an equivalent resistance of 100 to 120 ohms on a common 60 mA simplex loop. Note that in most cases the loop must be completed at the office repeaters for this to be a valid loop, and office repeaters must have some type of switch or jumper specifically for this purpose. In a few cases, the simplex loop is not made this way. Sometimes the power is fed all the way into some CPE terminal equipment and the loop is made there, but this is not the most common case.

After this simplex power loop is engineered and installed correctly, a voltage drop can be measured across side 1 of a line repeater (about 7 VDC). Similarly measure from the span receive side of the office repeater to the span transmit side (about 7 to 12 VDC). In many far end office repeaters, T1 Channel service units, "Smart Jacks" and Network Interface Units (NIUs) that receive power from the span line, this simplex arrangement presents a voltage drop of 11 to 12 VDC. This is one method of verifying that the current has the correct polarity. If it is wrong, then the voltage measurement is only 0 to 1 VDC. Here is the way to remember it: The positive current flows in the same direction as the PCM signal direction. Note that there are alternative schemes of powering each element in a T1 transmission facility, but the most common one is illustrated here. See Figure 10.

Figure 10

In the classic T1 repeatered line, the simplex power planning must account for the equivalent DC resistances of office repeaters, line repeaters, attenuation pads and equalizers, and the copper conductors themselves. The longer the span line is, the more repeaters must be in series, hence, the more voltage must be applied at one end to feed the current loop. In a short length repeatered line, this voltage might be only 20 to 30 VDC, but as the length is stretched out to 10 to 15 miles, this might become ±130 VDC. As it gets extremely long, simplex current might be fed from both ends with the simplex looping back both ways in the middle. If these DC calculations must be made and there are no further engineering guidelines, assume that 22 gauge cable has an equivalent DC resistance of about 18 ohms per 1000 feet. Of course, 24 gauge and 26 gauge have much higher resistance. Assume that each line repeater is 120 ohms and one unpowered office repeater is 170 ohms.

There are small additional resistances for LBO networks and other pads in the circuit, and these must be added into the calculation. If you know this total equivalent resistance in the loop and you know the current is 60 mA, then apply Ohm's Law to solve for the minimum necessary voltage. Note that many of the most modern Automatic Span Powering Repeaters simply need local -48 VDC and they will develop the necessary voltage to regulate 60 mA into the simplex loop up to a maximum loop of 4000 to 4200 ohms.

However, in many high-rise building installations, the T1 span line is rather short, perhaps from the basement equipment room to the tenth floor. So short, in fact, that no line repeaters are needed. We can name it the Un-repeatered T1 Span Line. In these, we might see an office repeater at the near end and a "Smart Jack" or Network Interface Unit (NIU) at the far end, only 3000 cable feet away. In this short span line case, the voltage necessary to drive the simplex current loop is only 20 VDC, but the current loop must still be connected at the ends for the current flow to be correct. One small aggravation is that experienced T1 transmission people will refer to simplex power, loop current, simplex voltage, plus a few other terms, and they use these terms interchangeably to mean about the same thing. Obviously, the voltage applied to a loop resistance yields a loop current. Just keep in mind that equivalent resistance through cable is based on one twisted pair acting as parallel resistance (resistance of one wire ÷2). However, the simplex loop current must pass out the cable length then pass back the same length (×2). This, effectively, makes equivalent resistance the same as one wire for the one-way length.

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