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Transmission Technology Most transmission—at least most transmission in the local exchange plant—is analog in nature. That is, the signal being transmitted varies continuously, both in frequency and in amplitude. A high-pitched voice mostly contains high frequencies; a low-pitched voice contains low frequencies. A loud voice contains a high-amplitude signal; a soft voice contains a low-amplitude signal. In the long-distance network, and more and more in the local exchange plant, digital transmission is being used. A digital signal is comprised of a stream of 1s and 0s that portray the analog voice signal by means of a code. Analog signals can be combined (i.e., multiplexed) by combining them with a carrier frequency. When there is more than one channel, this is called frequency division multiplexing (FDM). FDM was used extensively in the past but now has generally been replaced with the digital equivalent: time division multiplexing (TDM). The most popular TDM system is known as tier 1 (T1). In a T1 system, an analog voice channel is sampled 8,000 times per second, and each sample is encoded into a 7-bit byte. Twenty-four such channels are mixed on these two copper pairs and transmitted at a bit rate of 1.544 megabits per second. T1 remains an important method of transmitting voice and data in the PSTN (see Figure 5).
Figure 5. Pulse Code Modulation (PCM) Sampling, Quantizing, and
Encoding Process: In This Example, a 3-Bit Encoding Scheme Is Used for
Quantizing the Total Amplitude
Such a digital transmission scheme (and certainly there are modifications of it that improve efficiency, capacity, or quality, etc.) works hand in glove with the digital-switching schemes we talked about previously. Those 1s and 0s need not be transmitted through an actual circuit in that switch; rather, one can simply turn on and off the various electronic devices that make up that switch. Thus a talking path (i.e., a switched circuit) in the PSTN can be either analog or digital or a combination thereof. In fact, a digital signal can be transmitted over a packet-switched network as easily as a circuit-switched network. Now if we consider the next step, we see that digitized voice is not very different from data, and if data can be transmitted over a packet network, then so can digitized voice. This, of course, is now known as voice over the Internet. The challenge, of course, is to get the transmitted signal to the destination fast enough. After all, this may well be a time-sensitive voice conversation. A second challenge is to get each packet, which is a small piece of a voice conversation, to the destination in the proper order. Progress is being made, and we can well believe that packet switching will play an important role in the PSTN of tomorrow. |
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