Copper and Radio Links

Copper

Quadrature Amplitude Modulation (QAM)
Group signal into symbols each corresponding to b bits
Quadrature Phase Shift Key (QPSK) is QAM with b=3 and 2^3 = 8 symbols
Assign a phase theta to each symbol
For each symbol, send a sine wave:
- for fixed time T
- with fixed frequency f
- with fixed amplitude A
- with phase theta
allows us to send multiple bits/Hz
noise makes it harder to distinguish phases

Figures 7.11, 7.12

9.6Kb/s to 33.6Kb/s use QAM with 32 to 1664 points
compression: Lempel-Ziv compression (V.42 bis), run-length encoding (MNP5)
56Kb/s: ISDN data to central office, 8K samples/s, converted to analog. Decoding assumes noise is Gaussian, does not use QAM

Multiplex signals from sources A, B, C, D
Time-domain multiplexing:
Ai, Bi, Ci, Di, A[i+1], B[i+1], ...
bit rate of wire must be N * bit rate of sources
Frequency-domain multiplexing:
use a separate channel for each of A, B, C, D
frequency of wire must be N * frequency of sources
Code-Division Multiple Access:
Multiply each signal by a different pseudo-random sequence of (+1, -1), RA, RB, RC, RD, transmit all together
Average received data to remove "noise" of other transmissions

antenna length about 1/2 of wavelength
low-bandwidth signal is modulated by high-frequency signal
different frequencies can be used for different channels
attenuation difficult to predict, power at least inversely proportional to the square of the distance from the sender

Serial line connection
Data Terminal Equipment (DTE, i.e. computer) talking to Data Circuit-Terminating Equipment (DCE, i.e.. modem)
Asynchronous Baseband Transmission with Bipolar Modulation
Two-way transmission
Receiver can always stop transmission
Universal Asynchronous Receiver/Transmitter (UART) adds start and stop/parity bits, interprets received data