4003-541-70/4005-741-70 Data Communications and Networks II Module 6. Data Signal Analysis -- Lecture Notes Prof. Alan Kaminsky -- Spring Quarter 2006 Rochester Institute of Technology -- Department of Computer Science Modulation See Data Signal Analysis (PDF -- 1,762,624 bytes)   Historical modulation methods Frequency modulation (FM) Differential phase shift keying (DPSK)   Modern modulation methods Quadrature amplitude modulation (QAM) Wikipedia article on QAM -- http://en.wikipedia.org/wiki/Quadrature_amplitude_modulation Examples of Modem Waveforms Amplitude modulation (AM)   Frequency modulation (FM)   Differential phase shift keying (DPSK)   DPSK, 2 bits per symbol   Quadrature amplitude modulation (QAM), 2 bits per symbol Examples of Modem Frequency Spectra Frequency spectrum of a 300 bps baseband (unmodulated) signal   The horizontal axis shows the frequency in Hz. The vertical axis shows the power (squared magnitude) of each frequency component on a logarithmic scale, normalized so the largest frequency component's power is 1.     Frequency spectrum of a 300 bps FM signal   This is the frequency spectrum of the Bell 103 telephone modem waveform. The telephone line carries two simultaneous communication channels, one for transmitting an outgoing bit stream and one for receiving an incoming bit stream. The modem uses frequency modulation (FM) at a bit rate of 300 bps. For the outgoing waveform, f0 = 1270 Hz and f1 = 1070 Hz. For the incoming waveform, f0 = 2225 Hz and f1 = 2025 Hz.     Frequency spectrum of a 1200 bps multibit DPSK signal   This is the frequency spectrum of the Bell 212A telephone modem waveform. The telephone line carries two simultaneous communication channels, one for transmitting an outgoing bit stream and one for receiving an incoming bit stream. The modem uses DFSK at 1200 bits per second, 600 symbols per second, 2 bits per symbol. The phase shifts are π/2 for 00, π for 01, 0 for 10, and 3π/2 for 11. The outgoing ("originate") channel uses a carrier frequency of 1200 Hz. The incoming ("answer") channel uses a carrier frequency of 2400 Hz.     Frequency spectrum of a 1200 bps multibit QAM signal   This is the frequency spectrum of a hypothetical telephone modem waveform. The telephone line carries two simultaneous communication channels, one for transmitting an outgoing bit stream and one for receiving an incoming bit stream. The modem uses QAM at 1200 bits per second, 600 symbols per second, 2 bits per symbol. The outgoing ("originate") channel uses a carrier frequency of 1200 Hz. The incoming ("answer") channel uses a carrier frequency of 2400 Hz.     Broadband Signaling Cable modem signals Cable modem standard: Data Over Cable Service Interface Specification (DOCSIS) Wikipedia article on DOCSIS -- http://en.wikipedia.org/wiki/DOCSIS Raw data rates Downstream (into the house): 3 Mbps - 15 Mbps Upstream (out of the house): 384 Kbps - 2 Mbps Channel bandwidth is shared by all users in the neighborhood Modulation Downstream: QAM, either 6-bit symbols (64 levels) or 8-bit symbols (256 levels) Upstream: QAM, either 2-bit symbols (4 levels) or 4-bit symbols (16 levels) Channel access Time division multiple access (TDMA)   Asymmetric digital subscriber line (ADSL) signals Wikipedia article on ADSL -- http://en.wikipedia.org/wiki/Asymmetric_Digital_Subscriber_Line Raw data rates Downstream: 256 Kbps - 8 Mbps Upstream: 64 Kpbs - 1 Mbps Data rate depends on distance from telephone central office -- the farther away, the lower the data rate Telephone channel frequency spectrum allocation Divided into 256 bins, 4.3125 KHz wide: center frequency of bin n is n * 4.3125 KHz Unused: Bins 0-6 (voice signal from 0-4 KHz) Upstream data: Bins 7-31 (25 bins) (28-136 KHz) Downstream data: Bins 32-255 (224 bins) (136-1102 KHz) Modulation Within each bin: QAM at a symbol rate of 4,000 symbols per second Bits per symbol can be different in each bin, depending on the bin's measured signal-to-noise ratio (SNR) Typically 2-6, maximum 15 bits per symbol A channel may go unused if the SNR is too low Channel access None needed -- the channel is not shared   Baseband Signaling Wired Ethernet   See Data Signal Analysis (PDF -- 1,762,624 bytes) Wireless Signaling Wireless Ethernet   See Data Signal Analysis (PDF -- 1,762,624 bytes) Data Communications and Networks II 4003-541-70/4005-741-70 Spring Quarter 2006 Course Page Alan Kaminsky • Department of Computer Science • Rochester Institute of Technology • 4486 + 2220 = 6706 Home Page Copyright © 2006 Alan Kaminsky. All rights reserved. Last updated 16-May-2006. Please send comments to ark­@­cs.rit.edu.