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Transmission of Digital Signals
The previous discussion asserts that a digital signal, periodic or nonperiodic, is a composite analog signal with frequencies between zero and infinity For the remainder of the discussion, let us consider the case of a nonperiodic digital signal, similar to the ones we encounter in data communications The fundamental question is, How can we send a digital signal from point A to point B We can transmit a digital signal by using one of two different approaches: baseband transmission or broadband transmission (using modulation)
SECTION 33
DIGITAL SIGNALS
Baseband Transmission
Baseband transmission means sending a digital signal over a channel without changing the digital signal to an analog signal Figure 318 shows baseband transmission
Figure 318 Baseband transmission
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Channel
Digital signal
A digital signal is a composite analog signal with an infinite bandwidth
Baseband transmission requires that we have a low-pass channel, a channel with a bandwidth that starts from zero This is the case if we have a dedicated medium with a bandwidth constituting only one channel For example, the entire bandwidth of a cable connecting two computers is one single channel As another example, we may connect several computers to a bus, but not allow more than two stations to communicate at a time Again we have a low-pass channel, and we can use it for baseband communication Figure 319 shows two low-pass channels: one with a narrow bandwidth and the other with a wide bandwidth We need to remember that a low-pass channel with infinite bandwidth is ideal, but we cannot have such a channel in real life However, we can get close
Figure 319 Bandwidths of two low-pass channels
a Low-pass channel, wide bandwidth
b Low-pass channel, narrow bandwidth
Let us study two cases of a baseband communication: a low-pass channel with a wide bandwidth and one with a limited bandwidth
DATA AND SIGNALS
Case 1: Low-Pass Channel with Wide Bandwidth If we want to preserve the exact form of a nonperiodic digital signal with vertical segments vertical and horizontal segments horizontal, we need to send the entire spectrum, the continuous range of frequencies between zero and infinity This is possible if we have a dedicated medium with an infinite bandwidth between the sender and receiver that preserves the exact amplitude of each component of the composite signal Although this may be possible inside a computer (eg, between CPU and memory), it is not possible between two devices Fortunately, the amplitudes of the frequencies at the border of the bandwidth are so small that they can be ignored This means that if we have a medium, such as a coaxial cable or fiber optic, with a very wide bandwidth, two stations can communicate by using digital signals with very good accuracy, as shown in Figure 320 Note that!i is close to zero, andh is very high Figure 320
Baseband transmission using a dedicated medium
Bandwidth supported by medium
Input signal bandwidth
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Although the output signal is not an exact replica of the original signal, the data can still be deduced from the received signal Note that although some of the frequencies are blocked by the medium, they are not critical
Baseband transmission of a digital signal that preserves the shape of the digital signal is possible only if we have a low-pass channel with an infinite or very wide bandwidth
Example 321
An example of a dedicated channel where the entire bandwidth of the medium is used as one single channel is a LAN Almost every wired LAN today uses a dedicated channel for two stations communicating with each other In a bus topology LAN with multipoint connections, only two stations can communicate with each other at each moment in time (timesharing); the other stations need to refrain from sending data In a star topology LAN, the entire channel between each station and the hub is used for communication between these two entities We study LANs in 14
Case 2: Low-Pass Channel with Limited Bandwidth In a low-pass channel with limited bandwidth, we approximate the digital signal with an analog signal The level of approximation depends on the bandwidth available Rough Approximation Let us assume that we have a digital signal of bit rate N If we want to send analog signals to roughly simulate this signal, we need to consider the worst case, a maximum number of changes in the digital signal This happens when the signal
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