Measurement scheme of pulse shape in T1/E1 transmission system

T1 (1.544 Mbps) transmission system is widely used in public and private telephone networks in North America. A similar system PCM-30 (2.048 Mbps) is used outside North America. Both of these primary rate transmission systems have pulse shaping line drivers whose output pulses are designed to meet the pulse shape requirements of the specifications.

    Pulse shape requirements

    The T1 equipment designed for the central office must be cross-connected with DSX-1. For most applications, the transmitter is located within 655 feet of the cross-connect. As shown in Figure 1, all T1 pulses arriving at the cross-connect must meet the pulse amplitude and temperature requirements at the cross-connect, regardless of whether the originating transmitter is a few feet away. The line is terminated with a 100Ω load. According to CB-119, the pulse amplitude is measured at the center of the pulse, and the amplitude must be within 20% of 3.0 V. (Other specifications are slightly different; please be sure to refer to the applicable specifications). If the amplitude requirement is met, the pulse can be linearly scaled to fit the template.

    111.png

    ANSI T1.102 – 1993 T1 pulse template

    The PCM-30 pulse shape is specified in Recommendation G.703. In this case, the pulse is only measured at the output of the line driver, and there is no need to meet the pulse template on various cable lengths. The pulse must fit the template without scaling.

    For 2.048 MHz operation, two amplitudes are specified according to the type of cable used. For a 75Ω coaxial cable, the pulse height is 2.37 V±10%. For a 120Ω symmetrical (shielded-twisted pair) pair, the specified pulse amplitude is 3.0 V±10%. Figure 2 shows the CCITT G.703 template for 2.048 MHz operation.

    222.png

    CCITT recommends G.703 pulse template for 2.048 MHz operation

    CCITT also specified a template that works at 1.544 MHz, as shown in Figure 3. The pulse shape is very similar to the pulse shape shown in FIG. 1. In T1 applications, pulses are required to meet the template requirements on the digital distribution frame. . However, in this case, the pulse will encounter the mask without scaling. It is specified that the peak undershoot should not exceed 40% of the peak pulse amplitude.

33.jpg

    CCITT recommends G.703 pulse waveform for 1.544 MHz operation

    The rest of this article discusses the process applied to accurately measure the pulse shape.