Chapter 2. Description of a Repeatered LineA T1 repeatered line provides a physical 4-wire transmission path for cable carrier systems that transmit bipolar pulse streams at bit rates of 1.544 Mbps. At each central office, cable pairs connect to an office repeater. Between offices, line repeaters are located at nominal spacing of 32 dB at 772 kHz. Automatic line buildout (ALBO) equalizers in each repeater can compensate for a range of losses in the preceding cable section. Note that the following overview diagram (Figure 3) is simplified by the use of one line in each direction to symbolize one twisted pair of wires. Other diagrams use one line to indicate both directions (two twisted pairs), and still other diagrams use four lines to indicate both pairs. The nature of the diagram dictates which symbol convention is used. The triangular symbol indicates a digital regenerator, which is somewhat related to an amplifier. Note that office repeaters use a regenerator in only one direction, and line repeaters use regenerators in two directions. With this view of DS-1 signals, let's examine other network elements that may connect.
Figure 3 A T1 Channel Service Unit (CSU) interfaces a typical piece of customer equipment (such as a channel bank) with a public T1 transmission facility. There are several specific types of CSUs and the most common type has a DS-1 signal interface on each side of the unit (the Data Terminal side and the Network side). In some respects, this CSU resembles an office repeater, however, additional diagnostic features are present on most CSU products. One of the most common functions is that of Ones Density Enforcement (also referred to as Ones Stuffing). Customer DS-1 signals are not allowed onto the public network containing very long strings of zeros (since there may be network elements that require occasional ones for timing purposes). As a result, most CSU devices change a zero to one to suppress the 16th consecutive zero, depending on specific requirements for a public facility. If the traffic has simply voice circuits, an occasional forced error of this sort is a negligible problem. If, on the other hand, the traffic is high-priority unrestricted data, then an occasional error is not acceptable. If this is the case, then the Line Code must be selected as B8ZS and not standard AMI. B8ZS uses selected bipolar violations (BPVs) as a means of signaling the far end that strings of zeros are in the customer data. Therefore, no errors are introduced and the system works fine. If B8ZS Line Code is used, then each and every network element in the path must be provisioned for B8ZS instead of AMI (or else the element must completely ignore Line Codes). Note that some T1 office repeaters are aware of B8ZS and others are not aware. Note that these repeaters will not be disrupted by one line code or the other, but many have BPV monitors that will detect the unintentional BPVs but ignore the intentional BPVs that are part of B8ZS coding.
DS-1 Signal Precompensation and Signal Levels In most central offices, there is a DSX-1 cross-connect jackfield located between the channel bank and the office repeater, or between any two dissimilar elements in a whole end-to-end system. The jackfield serves as a maintenance test point for craft people and as a wirewrap interconnection point between the two elements. See Figure 4. If a transmit port must send a DS-1 signal only 50 to 100 feet to the DSX-1 panel (typical in small equipment rooms), then it can send a "normal" DS-1 signal waveform. That waveform won't be affected much by only 50 to 100 feet of cable capacitance if the cable is good, so the waveform looks perfect at the DSX-1 panel when viewed on an oscilloscope. That is the objective, to get a perfect signal waveform presented to the DSX-1 panel. This standard signal is referred to as having a level of 0 dBdsx. A standard signal that has been attenuated by several hundred feet of cable might have a level of -3 dBdsx.
Figure 4 Looking at the outbound DS-1 signal going from the terminal equipment toward the span line, if it is perfect at the DSX-1 panel, then it gets minimally affected by the 50 to 100 feet of cable capacitance, so the signal still looks fairly normal at the receiver. Often the assumption is made that the transmit cable distance to the DSX-1 is the same as the receive cable distance from the DSX-1 (on the same side of the DSX-1). If the cable distance to the DSX-1 is much longer, say 655 feet, then a normal signal is going to be attenuated and "rounded off" due to cable capacitance. In many pieces of equipment, a pre-equalizer is used in the DS-1 interface on the transmit port. This pre-equalizer "sharpens up" the pulse edges to exaggerated amplitudes. This is launched down through the 655 feet of cable and the sharp edges become smoothed down ("rolled off") due to cable capacitance. When it arrives at the DSX-1 panel, it should be exactly a perfect waveform (see Figure 5). When the signal comes into an office repeater port after passing 655 feet of cable from the DSX-1, it can be successfully received without errors. It might be possible to attenuate the signal even more without degradation, say from 700 or 800 feet, but errors in transmission might also appear (which would be unacceptable). It depends on the specific equipment and cable in use. Some types of Inside Plant equipment are only capable of transmitting and receiving a signal through 0 to 133 feet to the DSX-1 panel. It is relatively important to have each and every network element meeting the "perfect waveform" mask, which is an industry standard.
Figure 5 The idea is to consider each direction of signal flow. Know what type of cable is in use and roughly what its attenuation characteristics are (22 gauge ABAM cable has a nominal capacitance of 14 to 16 pF/foot @ 772 kHz). Also consider that 24 gauge cable has approximately 25% higher attenuation than standard 22 gauge. Know what signal levels should be present at each point and what the receive sensitivity is for each device. Many problems are traced to the use of cable that is not intended for T1 use. Office repeaters are somewhat more complex, since they have one set of capabilities on the Equipment side (inside) and one set on the Facility side (outside). A typical office repeater has a digital regenerator only on the receive side (receive from facility), and has enough sensitivity to regenerate a signal from a line repeater that is about 3000 to 4000 feet away. It has a typical inside cable specification for 0 to 655 feet, depending on the exact pre-equalization. See Figure 6.
Figure 6
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