Measuring Telecom Systems Agai

Abstract: Measuring T1/E1 and T3/E3 pulse masks and comparing them against specificaTIons are discussed. A test fixture is uTIlized as an illustraTIon.

In the telecommunicaTIon industry, it is often necessary to design equipment that interfaces with older, existing telecommunication systems. To ensure that a piece of equipment functions properly in a legacy system, several application-dependent specs are defined for manufacturers to follow. These specs pertain to data transfer, signal timing requirements, and functions that must be executed when data-transfer errors are detected.

Transmission signal quality specs are particularly important. The spec for how the transmission signal should appear depends on the type of system. The measured signal should fit within a predefined template called a pulse mask.

This article examines the specifications for T1, T3, E1, and E3 pulse masks and testing transmission signals for pulse mask compliance. It discusses some of the problems that can occur when testing multiport transmission devices.

Maxim has a line of multiport transceivers for T1/E1 systems and T3/E3 systems, as well as support hardware to make testing the pulse mask for multiport devices easier. Interface and pulse-mask specifications for T1/E1/T3/E3 networks follow.

Specifications for the digital networks were taken from the International Telecommunication Union (ITU) document G.703, October 1998, and the American National Standard for Telecommunications (ANSI) document ANSI T1.102-1993.

T1 Pulse-Mask TemplateThe first and most common digital transmission system in North America is a T1 network (1.544Mbps). This system of transmitting digital data was developed in the mid 1960s for public telephone providers. Since then, T1 networks changed their function from the transmission of strictly digital voice conversations to the transmission of the large data packets that are the core technology for applications such as wide area networks (WAN) and the Internet.

For each T1 line, the physical connection that a customer sees is always two twisted-pair lines: one for the transmit data and one for the receive data. Both are differential pairs terminated with a 100Ω resistive load.

To measure the pulse mask, the transmit-data path is selected and measured at the end of the transmission line. Many T1 transceivers provide options to compensate for the resistive and capacitive loading of the transmission line by adjusting the amplitude of the T1 pulse. Maxim has T1 transceivers that can be configured for both short-haul (DSX-1) lines, which can be up to 655ft with 22 AWG cable, and long-haul (CSU) lines, which are rated to a maxim of -36dB of signal loss. This is normally referred to as the line build-out (LBO) of the transmission line. Within the short-haul (DSX-1) lines and long-haul (CSU) lines, Maxim T1 transceivers can be set for the proper LBO. T1 interface specifications for a pulse mask are found in Table 1.

Table 1. T1 Interface Specification for Pulse Mask Interface Parameters