EET 450 – Advanced Digital Chapter 17 Modems

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

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

Modems –Internal Modem Connects using expansion slot 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

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

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

Modem Speed Speed ratings 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 56K is rated speed of most modems currently in use –Actually adjusts speed based on connection 28.8kbps or 33.6 kbps typical 28.8kbps or 33.6 kbps typical

Modem Speed Telcom limitations 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

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

Modem Standards StandardAffectsDescription Bell 212A Speed up to 1200 Older standard remnant of Bell system CCITT V.32 Speed up to 9600 Error checking HST Speed up to 14.4 Proprietary USR CCITT V.32bis Speed up to 14.4 Improved V.32 CCITT V.34 Up to 33.6 Transmits at 28.8kbps MNP Class 4 Adds error correction Microcom Networking Protocol (MNP) CCITT V.42 Error correction Adopted MNP 4

Modem Standards StandardAffectsDescription MNP 5 Data compression Can double transmission speeds CCITT V.52bis Data compression Improved V.42 V.44 Data compression Improved V.42bis K56flex Speed to 56k Lucent standard x2 Speed to 56k USR standard V.90 Speed to 56k International standard replacing both proprietaries V.92 Speed to 56k Improved V.90

MNP Classes StandardNotes MNP Class 1 Block Mode – 70% efficiency – obsolete MNP Class 2 Stream mode – 84% efficient MNP Class 3 108% overall eff. Incorporates class 2 MNP Class % overall efficiency (may be as high as %) MNP Class 5 Data compression; can increase to 150% for text – may be slower for compressed files MNP Class10 Error correction for bad lines – adjusts rate MNP Class10EC Enhanced Cellular version of Class 10

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

Fax Modem Standards Group III Fax 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 Group IV Fax protocol –Digital standard –Requires ISDN or other digital connection –Resolution to 400dpi

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

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

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

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