Specifications

4-8
Cisco AS5x00 Case Study for Basic IP Modem Services
11/24/1999
Section 4 Verifying Modem Performance
Understanding Modem Modulation Standards
Understanding Modem Modulation Standards
To optimize modem connect speeds, you must understand the basic modem modulation standards.
This section provides the basic rules for achieving maximum V.34 and V.90 modulation speeds:
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V.34 Basic Rules
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V.90 Basic Rules
V.34 Basic Rules
V.34 modulation should work on any land-line voiceband circuit. V.34 supports speeds ranging from
2400 to 33600 bps.
Speed is a function of:
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The amount of usable spectrum across the channel (for example, 2400 to 3429 Hz)
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The signal to noise ratio (SNR)
To achieve 33600 bps, the channel must deliver:
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A response from 244 to 3674 Hz
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A SNR of 38 dB or better
In practice, toll-quality voiceband circuits support V.34 at speeds of 21600 to 33600 bps.
The following six items reduce the achieved V.34 speed:
1.
Robbed-bit signaling links in the circuit, which reduce SNR.
2.
Extra analog-to-digital conversions. For example, non-integrated or universal Subscriber Line
Concentrators (SLCs) reduce bandwidth and SNR.
3.
Load coils on the local loop, which reduce bandwidth.
4.
Long local loops, which reduce bandwidth and SNR.
5.
The following electrical disturbances in the house wiring, which reduce SNR:
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Cross talk from two lines in the same quad cable
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Corroded connectors
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Bridge-tapped lines running parallel to fluorescent lights
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Flat silver-satin cables running parallel to power cables
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Extra electrical equipment sharing the same power jack as the modem
6.
Voiceband circuits that pass through sub-64k coding, such as a cellular or 32k ADPCM link.
With 32k ADMCM, the speed is typically 9600 to 16800 bps.