Chi-Cheng Lin, Winona State University CS 313 Introduction to Computer Networking & Telecommunication Modulation, Multiplexing, & Public Switched Telephone Network

2 Topics l Modulation l Multiplexing l Telephone System l Switching

3 Modulation l Analog signals are devised to send digital information l Digital modulation  Process of converting between bits and signals that represent them l Schemes  Baseband transmission  Passband transmission (our focus)

4 Passband Transmission l Fact  Baseband signal from 0 to B Hz and be shifted up to a passband of S to S+B Hz for transmission  Amount of information carried not affected  Receiver can shift the signal back down to 0 to B Hz for decoding

5 Passband Transmission l Modulation  Superimpose information signals on to the carrier signal at transmitting end l Basic modulation techniques  Amplitude shift keying (ASK)  Different amplitudes representing different symbols (e.g., 0 and 1)  Problem: vulnerable to noise  Frequency shift keying (FSK)  Different frequencies representing different symbols  Problem: limited by physical capacity of carrier  Phase shift keying (PSK)  Different phases representing different symbols  Problem: Hard to distinguish small phase shift

6 Binary Signal ASK FSK PSK

7 Passband Transmission l Quadratic phase shift keying (QPSK)  4 symbols  2 bits per symbol l Combination of ASK and PSK  QAM-16: 16 symbols  4 bits per symbol  QAM-64: 64 symbols  __ bits per symbol l Constellation patterns  Diagrams showing legal combinations of amplitude and phase

8 More Constellation Diagrams (a) QPSK (4-QAM). (b) QAM-16. (c) QAM-64.

9 Passband Transmission l Gary-code  Adjacent symbols differ in only one bit position  At most 1 bit error in the expected case

10 Multiplexing l Multiplexing  Set of techniques allowing multiple signals to share a single data link  Dividing total available bandwidth over a link into multiple channels l Why multiplexing?

11 Multiplexing l Frequency Division Multiplexing (FDM)  Dividing bandwidth of a link into separate channels l Orthogonal FDM (OFDM)  Problem of FDM: guard bands needed to separate channels well  bandwidth wasted  OFDM uses subcarriers that send data independently without using guard bands l Time Division Multiplexing (TDM)  Combining signals from low speed channels to share time on a high-speed link

Frequency Division Multiplexing (FDM) (a) The original bandwidths. (b) The bandwidths raised in frequency. (c) The multiplexed channel. Computer Networks, Fifth Edition by Andrew Tanenbaum and David Wetherall, © Pearson Education-Prentice Hall, 2011

OFDM Computer Networks, Fifth Edition by Andrew Tanenbaum and David Wetherall, © Pearson Education-Prentice Hall, 2011 l Used in , cable networks, power line networking, (& 4G cellular systems)

Time Division Multiplexing Computer Networks, Fifth Edition by Andrew Tanenbaum and David Wetherall, © Pearson Education-Prentice Hall, 2011 l Round-robin  Each user takes turn  Each one periodically gets the entire bandwidth l Used in telephone and cellular networks

15 Structure of the Telephone System (a) Fully-interconnected network. (b) Centralized switch. (c) Two-level hierarchy.

16 Structure of the Telephone System l A typical circuit route for a medium- distance call.

17 Major Components of the Telephone System l Local loops  Analog twisted pairs going to houses and businesses – “last mile” l Trunks  Digital fiber optics connecting the switching offices l Switching offices  Where calls are moved from one trunk to another

18 Digital Transmission l Why digital?  Low error rate  Mix signals from different sources (multimedia)  Cheaper  Maintenance is easier

19 Modem The use of both analog and digital transmissions for a computer to computer call. Conversion is done by the modems and codecs.

20 Modem l How can we transmit digital data over analog local loop? l Modulator-demodulator  A device that accepts a serial stream of bits as input and produces a modulated (analog) carrier signal as output (or vice versa)  Each high-speed modem standard has its own constellation pattern l Baud rate  Number of signal units transmitted per sec  Number of symbols per second

21 Examples l A modem uses QPSK and sends data at the rate of 2400 baud. What is the data rate? l Compute the bit rate for a 2400-baud 16-QAM signal. l Compute the bit rate for a 2400-baud 64-QAM signal.

22 Telephone Modems l A telephone line has a bandwidth of  3000 Hz (3300 – 300) for voice  2400 Hz (3000 – 600) for data l Modem standards  V.32: 9,600 bps  V.32bis: 14,400 bps  V.34bis: 28,800 ~ 33,600 bps  V.90: download up to 56kbps (56K modem)  V.92: adjustable speed, call waiting, etc.

23 Trellis Coded Modulation l 1 parity per symbol to reduce error l Examples  V.32 standards uses 32 constellation points to transmit data at the rate of 2400 baud and 1 bit per symbol. What is the bit rate? Ans: 2400 x (log ) = 2400 x (5 - 1) = 9600 bps  V.32 bis standards uses 128 constellation points to transmit data at the rate of 2400 baud and 1 bit per symbol. What is the bit rate? Ans: 2400 x (log ) = 2400 x (7 - 1) = bps

24 Trunk and Muliplexing l TDM – Why is T1 line Mbps? l Frame size / duration of frame = 193bits / (125  sec) = Mbps

25 TDM l Multiplexing T1 streams onto higher carriers 24 voice channels___ voice channels Capacity of each digital channel = 64 Kbps

26 Wavelength Division Multiplexing (WDM) l Used over fiber optics, similar to FDM

27 Switching l Switch  Device creating connections between devices linked to it l Switching  Forwarding data from a switch to another device

28 Switching Techniques l Techniques  Circuit switching  Packet switching l Circuit switching  End-to-end path has to be set up BEFORE any data can be sent  Data follow the same path  No danger of congestion (except in path setup phase)

29 Switching Techniques l Packet switching  Store-and-forward  Tight upper limit on block size  allowing packets to be buffered in router main memory  No single block can tie up a link for too long  Shorter delay and higher throughput

30 Circuit Switching Vs. Packet Switching Circuit switching Packet switching

31 Circuit switchingPacket switching # hops=3

32 Circuit Switching Vs. Packet Switching