Chapter 1. Introduction

General Modern telecommunications user companies often purchase one product from vendor A, another product from vendor B, and so forth. No single vendor necessarily integrates the entire end-to-end transmission project. The telecommunications user company must design its own facilities, purchase the correct products from the many vendors, install and test the products properly, and document what it has for future reference. Unfortunately, the first step (engineering design) sometimes is loosely considered until the installation fails to test correctly. This document addresses engineering planning and design of a T1 repeatered line so service can be delivered on a more predictable schedule.

Signals

The 1.544 Mbps bipolar Pulse Code Modulation (PCM) line signals of T1-type terminal equipment (such as channel banks, data terminals, and repeatered lines) are designated DS-1, meaning "digital signal first level." At the standard cross-connect point for DS-1 signals, DSX-1, the voltage level of pulses is about +3 volts and -3 volts. DS-1 means that the signal meets the interface specification for DS-1 signals (1.544 Mbps, bipolar +3 and -3 volt pulses, 50% duty cycle, etc.). The data pattern means the 192 bits of payload data contains live traffic (that's 24 time slots and 8 bits per time slot) or some test pattern. After each 192 bits, there is a framing bit for #193. That bit is either one or zero depending on whether it must follow the special pattern for Superframe Format (SF, also referred to as D4) or Extended Superframe Format (at this point, the use of SF or ESF is purely at customer discretion based on the types of channel banks and other network elements present in the network). See Figure 1. This 193-bit frame is repeated 8000 times per second. This is also called a frame rate of 8 kHz. Each of 24 channels is sampled and transported 8000 times per second. If a terminal receiver cannot determine the start and finish of a frame, then receivers will always be "out of frame sync" with respect to transmitters and very poor performance is noted. There is one more qualifier on the signal, and that is Line Code. The two line codes are Alternate Mark Inversion (the old standard) and B8ZS (bipolar eight zero substitution). For network elements that carry live traffic, this is very important.
 

What does Alternate Mark Inversion (AMI) mean? In the data communications world, all digital data is either One or Zero, nicknamed either Mark or Space. If the data bore a pattern of All Ones, then the first One is transmitted on the AMI transmission link as +3 volt pulse, then the next One as -3 volt pulse, then the next One as +3 volt pulse. In a live traffic signal with mixed Ones and Zeros, there will be Zeros between the positive and negative pulses as depicted in the following figure. See Figure 2. Internally to the electronics of equipment, the 1.544 Mbps signal might be unipolar, but everywhere accessible to the user, the signal is bipolar. DS-1 signals may fall into one of three categories: SF, ESF, and possibly Unframed. Sometimes test equipment will use an unframed DS-1 test signal to check a transmission facility, but most terminal equipment must be selected for SF or ESF only. If terminal equipment expects to see SF and it receives ESF instead, it will probably not be able to lock its framer circuit onto the input signal and everything will stay in alarm.