Impaired Lines: Crash Tests For Modems

The three lines introduced different impairment conditions.

The easiest line, 17d, is quite similar to the throughput line. There is mild white noise (22.0 decibels below 1 milliwatt), second-order nonlinear distortion (40.0 dB compressed), third-order nonlinear distortion (41.0 dB), near echo (40.0 dB), far echo (12.0 dB) ,and a short satellite delay (27.1 milliseconds).

Line 19 introduces a long satellite delay, which is more commonly found outside the U.S. Its parameters include mild white noise (22.0 dBm), second-order nonlinear distortion (43.0 dB compressed), third-order nonlinear distortion (44.0 dB), near echo (40.0 dB), far echo (16.0 dB), and a long satellite delay (337.1 ms).

The last line, 25, removes the satellite delay and introduces phase jitter and frequency shift. Phase jitter occurs when the phase of one part of the frequency tone is shifted relative to an earlier part of the tone; a frequency shift means that the tone that is received is different from the tone that is sent. The conditions for line 25 include mild white noise (22.0 dBm), second-order nonlinear distortion (51.0 dB compressed), third-order nonlinear distortion (53.0 dB), near echo (40.0 dB), far echo (16.0 dB), phase jitter (3.0 degrees, 60.0 Hz, sine wave form), and frequency shift (0.20 Hz).

Two different combinations of loop conditions were used to test modems under the three impaired lines. A "2 2" means that the modem on each end of the line was subject to EIA2 conditions. EIA2 simulates a 4000-foot connection linked to a 3000-foot connection, for a total of 7000 feet. This situation is typical of a telephone connection that is within 7000 feet, or about a mile and a half, of the telephone central office.

A "7 7" is an EIA7 condition on both ends of the connection . This is a much more difficult line for most modems to handle, because it simulates a telephone connection that is approximately 30,000 feet (or over 5 miles) from the telephone office: a 3000-foot connection to a bridged tap to a 6000-foot run with a bridged tap to another 6000-foot link to a bridged tap connected to a third 6000-foot link to a tap, completed by a 9000-foot run.