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được sử dụng để kết nối các mạng tư nhân và thực hiện các dữ liệu nhạy cảm. Nhu cầu của họ đã tạo ra một cơ hội cho các nhà cung cấp dịch vụ Internet để cung cấp các giá trị dịch vụ gia tăng nhấn mạnh khả năng mở rộng và quản lý mạng. Họ có thể cung cấp vận chuyển trên toàn thế giới là một nonissue. Tất nhiên, họ có thể! Nhưng họ có thể cung cấp | 174 Connections Codes Signals and Error Control 10110011 100011 11 00 WWW Waveform Figure A.6 Example of QAM to create a signal in which each symbol represents 2 bits. nique is known as quadrature amplitude modulation QAM . The parameters of the four symbols are shown in the center of Figure A.6. Such a diagram is known as a constellation. Each symbol is a 270 segment of the carrier signal that starts at carrier phase angles of 0 90 180 and 270 . The assignment of codes to the signal points is arbitrary. Once made however they must be preserved for the receiver to interpret the received signal correctly. In the upper half of Figure A.6 the waveform corresponding to the data stream at the top of the figure is shown. A comparison with Figure A.5 reveals that twice as many bits are contained in the signal burst. With each symbol representing 2 bits this was to be expected. Under these circumstances the signal in Figure A.6 achieves a bit rate that is twice the baud rate. In the 1920s Harold Nyquist showed that the maximum signaling rate over a channel with a passband B Hz is 2B baud. This is known as the Nyquist rate. The passband of a given signal is governed by the physical parameters of the transmitter the transmission medium and the receiver. In radio systems filters at the transmitter and receiver establish the passband. They are tightly controlled to prevent one system interfering with another. In the telephone network a passband 4 kHz is established by the digital sampling rate 8 ksamples sec . This gives an upper bound for the signaling rate of 8 kbauds or 8 ksymbols sec. In practice the Nyquist limit cannot be achieved without complex processing of the signal stream. A.4.3.2 Complex Modulation Techniques Implementations of complex modulations may have constellations with as many as 256 or 512 signal points. They correspond to operating at 8 bits baud and 16 bits baud. Great care is taken to arrange the signal points so that they are equidistant from one .
