EET 450 – Advanced Digital

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