An Overview of Analog Dialup Modem Performance, Environments, and Impairments.

REVISED 7/20/98 9
DOC. NUM. PRT/005A/0798
Echo
Echo levels are described in terms of near-end (listener) and far-end (talker) echo levels. This is the
reflection of the signal from the hybrid balances in the network, and must be cancelled out in order
for the modems to work. On a normal voice call, the telephone network will utilize its own echo
cancellers so that the echoes are not distracting to normal voice conversation. On a modem call, the
modems will set up their own echo cancellers and do not utilize the ones on the network.
The network echo cancellers are turned off by the modems by sending a 2100Hz tone of approxi-
mately 1s duration to the network. After this, the modems set up, or ‘train’, their own echo cancel-
lers. This is done so that each modem will be able to cancel out the reflection of its own signal, so
that the only thing it hears is what the remote modem is transmitting.
Delays
A large amount of end-to-end delay in the telephone network can cause problems for modem data
transmission. Large delays are typically encountered on international calls or other calls which are
relayed via satellite. This introduces a delay of up to 200ms (sometimes greater), which makes it
difficult for the modems to properly train and adjust the echo cancellers, among other things.
Digital Impairments
Pads and Attenuation
Pads are commonly used to equalize the volume levels between different lines, so that the volume of
the voice call is consistent from end-to-end and at an acceptable level. With a digital pad, the
digital data is manipulated to adjust the volume. With analog pads, the digital data is converted to
analog where the volume is adjusted and then converted back to digital.
ADPCM
ADPCM (adaptive differential pulse coded modulation) is a digital speech compression technique
that is used to reduce the transmitted bit rate in digital carrier systems for voiceband signals.
ADPCM allows the network provider to accommodate more voiceband channels on a digital carrier
facility than would be present without it, while still maintaining signal and perceptual quality in the
connection. While ADPCM algorithms have no apparent effect on the quality of a voice call, they
have widely differing impact on data modem performance. There are several different ADPCM al-
gorithms, and a description of these can be found ITU-T Recommendations G.721 and G.726.
Robbed-Bit Signaling
Robbed bit signaling (RBS) is a form of in-band signaling which uses the least significant bit from
every 6th and 12th frame of the PCM channel. When this bit is ‘robbed,’ the signal effectively be-
comes a 7 bit quantizer for that sample. Since the signal was originally an 8-bit signal, the resulting
output effectively appears as the original signal and a low level impulse hit. The use of robbed bit
signaling is being phased out by the increased use of common channel signaling in the network.