質問：
How do I define a switch system bandwidth for a digital transmission system?
回答：
Bandwidth is defined by the interval between the upper and lower 3dB frequencies, i.e., where the signal falls to -3dB below the passband frequency. This is a fairly straightforward measurement for a typical sinusoidal analog signal. However, it is harder to determine bandwidth for a pulsed, digital signal, where the frequency harmonics of the pulse and their power spectral density must be maintained for signal integrity. A general method for computing bandwidth is
Bandwidth = .35 / Rt
(where Rt, rise time, is the rise of the signal from 10% to 90% amplitude level)
In telecommunications, there are a number of standard digital signals. Common digital signals are:
| Data Rate | Transmission Media | Coding | Bandwidth | |
| DS-0 | 64kbps | Typically twisted pair | PCM (Pulse Code Modulation) |
4 kHz |
| DS-1/T1 | 1.544 Mbps | Typically twisted pair | B8ZS (Bipolar 8 Zeros Substitution) |
2.316MHz |
| DS-3 | 44.763 Mbps | Typically coax | B3ZS (Bipolar 3 Zeros Substitution) |
67.145MHz |
| OC-1 | 51.84 Mbps | Fiber | (Note: BxZS is a derivative of Bipolar Return to Zero Signaling) | |
| OC-3 | 155.52 Mbps | Fiber |
The bandwidth of these signals is a function of the transitions used to represent one bit of data. For example, T1 signals use three transitions to represent two bits of data. Therefore, the bandwidth (in frequency) required to pass this signal is [3/(2-bits)]*1.544 Mbps = 2.316 MHz. (Note: A signal's data rate is commonly referred to as its bandwidth in digital communications.)
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