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Understanding Modems: Types, Standards, and Digital Communication Technologies

This chapter delves into modems, crucial devices that convert digital information into audible tones for transmission over switched telephone networks. It explores various types of modems, including external and internal configurations, their connection methods, data speeds, and standards established by organizations like CCITT and ITU. Additionally, it discusses modulation techniques such as FSK, PSK, and QAM, and outlines fax standards and broadband technologies like DSL and ISDN. It is essential for understanding both historical and modern data communication.

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Understanding Modems: Types, Standards, and Digital Communication Technologies

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  1. EET 450 – Advanced Digital Chapter 17 Modems

  2. Modem • Modulator – Demodulator • Converts digital information into audible tones • Uses Switched Telephone Network • Telephone optimized for human voice – centered on 3kHz bandwidth • Very narrow dynamic range

  3. Modems • Physical device • External Modem • Connects using RS-232 serial interface (typically) • Separate power supply • Limited to speed of serial port

  4. Modems • Internal Modem • Connects using expansion slot • ISA or PCI typical • AMR (Audio/Modem riser) or CNR (Communication and Networking Riser) • May be integrated into chipset design • Retains use of PCI connection

  5. Modems • Connection to Telco network established using modular connectors – RJ-11 • Settings • Full duplex • Send and receive simultaneously • Half duplex • Send OR receive alternately

  6. Ratings • Establishing link • Handshaking or Training • Exchange information that allows settings to be adjusted on Modem to accommodate server modem

  7. Modem Speed • Speed ratings • Bits per second – bps • Baud Rate – antiquated speed rating based on changes in level • 56K is rated speed of most modems currently in use • Actually adjusts speed based on connection • 28.8kbps or 33.6 kbps typical

  8. Modem Speed • Telcom limitations • 8000 samples per second limits speed to 56k • Actual limit in US is lower than 56k due to maximum levels allowed under FCC regulation. • Roughly 53kbps FCC reg. Part 68 • Actual connect speed may be limited by noise and level of connection

  9. Modem Standards • Standards organizations establish protocols, levels and software specifications for international compatiblity • CCITT – Comite Consultatif International Telegraphique et Telephonique • ITU – International Telephone Union

  10. Modem Standards

  11. Modem Standards

  12. MNP Classes

  13. Modulation standards • Frequency-shift keying FSK • aka FM – centered around 2kHz • Phase-shift keying PSK • Form of phase modulation – timing is altered but frequency held constant • Quadrature amplitude modulation QAM • Combines phase and signal amplitude variations

  14. Fax Modem Standards • Group III Fax • 9600 baud w/ 2 levels of resolution 203x98 & 303x196 • Class 1 – software generates image and handles session • Class 2 - software generates image, but modem handles session • Group IV Fax protocol • Digital standard • Requires ISDN or other digital connection • Resolution to 400dpi

  15. ISDN • Integrated Services Digital Network • Designed to provide both data and voice to home/business • Basic Rate Interface (BRI) • Two ‘B’ channels (64K each) • One ‘D’ channel (16K) • Primary Rate Interface • 23 ‘B’ Channels • One ‘D’ channel (64Kbps) • Total of 1536 Kbps

  16. Leased Lines • T-1 • Digital connection at 1.55 Mbps • Can be split (fractioned) into 64Kbps lines • T – 3 • Roughly 28 T-1 lines • 45Mbps

  17. Broadband • Cable Modems • Bandwidth depends on ‘neighborhood’ loading • May be as high as 4Mbps, but probably at 1.1Mbps or slower • Always on • DirecPC • Modem uplink (56K) – satellite down (400Kbps)

  18. Broadband • DSL – Digital Subscriber Line • ADSL – asymmetrical • Up to 1.6Mbps down 640Kbps up • CDSL – consumer • 1 Mbps upstream • G.Lite – Universal DSL • 1.544 to 6.0 Mbps up, 128 to 384 Kbps down • HDSL – high bit rate • SDSL – symmetric DSL • Speed is limited by distance from telco central office

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