Modems, Modems, and more Modems Oct 24, 2002. Announcements Test #2 – Oct 31 –It will cover Chapters 8, 11, 12, 13, and 17.

Modems, Modems, and more Modems

Oct 24, 2002

Announcements

• Test #2 – Oct 31– It will cover Chapters 8, 11, 12, 13, and 17

Class Objectives

• Modems-Past– Bell Family

• Modems-Present– ITU Recommendations– Cable Modem– ADSL

FIGURE 13-2 Acoustically coupled modem. (Reprinted with permission from Racal-Vadic.)

Warren HiokiTelecommunications, Fourth Edition

Copyright ©2001 by Prentice-Hall, Inc.Upper Saddle River, New Jersey 07458

All rights reserved.

FIGURE 13-3 Common modem voice and data jacks. (Reprinted with permission from Racal-Vadic.)




FIGURE 13-4 Typical direct-connection installation. (Reprinted with permission from Racal-Vadic.)




FIGURE 13-5 Modulation techniques employed by modems.




Transmission Modes

• Simplex– Data is sent in only one direction

• Half-Duplex– Data sent in both directions, but only one at a

time

• Full-Duplex– Data sent both directions simultaneously

FIGURE 13-6 Bell 103/113 frequency assignment. (Courtesy of Racal-Vadic.)




FIGURE 13-7 Bell 202 frequency assignment. (Courtesy of Racal-Vadic.)




FIGURE 13-9 Bell 212A frequency assignment for the high-speed mode. Frequency assignment for the low-speed mode is identical to the Bell 103 specification (See Figure 13-6).




M-ary

• Applies for PSK or Quadrature Amplitude Modulation (QAM)

• M = log2(n)

where M is the number of encoded bits

and n is the number of state changes

• See Example 13.1 in textbook

Phasor Diagram

• Based on geometry and the unit circle

• We use it to give us a visual representation of how the modulated signal behaves

• Used mainly to represent phase and amplitude relationship

• Commonly called a “Constellation” Diagram

Unit Circle

45

90

180

270

0

FIGURE 13-10 Phasor diagram for the Bell 212A modem.




FIGURE 13-12 Phasor diagram for the Bell 208A modem.




FIGURE 13-13 Phasor diagram for the Bell 209A modem depicting 9600-bps QAM.




ITU Recommendations

• ITU sets standards for the international community.

• As a practical matter, most of the standards are compatible with the US Bell standards.

• Present modems can reconfigure themselves to be compatible with many V. recommendations through software options and upgrades.

See Table 15-1 of textbook

Internal PCI Modem Card

Features:•Internal Modem •PCI Slot required •56K v.90 Voice Capable

FIGURE 13-14 V.22bis 16-point signal constellation.




FIGURE 13-15 V.32 trellis 32-point signal constellation.




FIGURE 13-16 V.33 128-point signal constellation.




ITU-TS V.90

• Is the basis of our “advertised” 56 kbps modems

• Has different data rate for transmit and receive– Transmits: 33.6 kbps– Receives: 40-53 kbps– Uses the characteristics of telephone company

infrastructure to optimize the receiver’s specs. – Look at example in textbook.

FIGURE 13-17 Comparison of a V.34 and V.90 modem connection: (a) V.34 connection showing analog-to-digital conversions in both directions; (b) V.90 connection to an ISP showing only one analog-to-digital conversion.




Cable Modem

• Shares coax cable with television programming

• Uses “DOCSIS”: Data Over Cable Service Interface Specification

• Bandwidths are shared with other users– Downstream rates are the most affected by high

use– Effective if cable infrastructure is already in place

FIGURE 13-19 Cable modem setup in relation to CATV system.




www.cable-modems.org




Asymmetric Digital Subscriber Line (ADSL)

• Uses the subscriber loop

• Uses equalizer-based design to improve bandwidth of subscriber loop cable

• Dedicated service– Data rates between Central Office and user

does not depend on the other users vs the Cable Modem

FIGURE 13-20 ADSL (digital subscriber line) modem connection.




FIGURE 13-21 The ADSL modem uses frequency-division multiplexing (FDM) in the subscriber loop.




FIGURE 13-22 Analog loopback test setup. (Reprinted with permission from Racal-Vadic.)




FIGURE 13-23 (a) Local DLB test setup; (b) remote DLB test setup. (Reprinted with permission from Racal-Vadic.)




Read the following

• 13.12

• 13.14-13.14.7

• 13.15-13.18

Modems, Modems, and more Modems Oct 24, 2002. Announcements Test #2 – Oct 31 –It will cover Chapters 8, 11, 12, 13, and 17.

Documents

new jersey

upper saddle river

warren hiokitelecommunications

us bell standards

frequency assignment

courtesy of racal

present modems

common modem voice