DATA COMMUNICATION Lecture-23.

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Presentation transcript:

DATA COMMUNICATION Lecture-23

Recap of Lecture 22 DTE-DCE DTE-DCE Interface EIA-232 EIA-449 EIA-530 X.21 Modems

Overview of Lecture 23 MODEM Standards Bell Modems ITU-T Modems Traditional Modems 56k Modems Cable Modems

Modem Standards Bell modems First commercial modems by Bell Telephone Co. Developed in early 1970s

Modem Standards ITU-T modem standards V-series: Today’s most popular modem standards Bell modem compatible: V.21/22/23/26/27/29

Modem Standards Intelligent modems Hayes (or Hayes-compatible) modems Modem is controlled by instructions (AT commands) Automatic answering, dialing, etc.

BELL Modems

ITU-T Modems

Traditional Modems Data rate of traditional modem is limited by 33.6Kbps Data rate  Signal-to-noise ratio (Shannon formula)

Traditional Modems

56K Modems Asymmetrical transmission 33.6Kbps uploading (user  ISP) 56Kbps downloading (ISP  user)

56K Modems

56K Modems Why 56Kbps? Switching stations of the telephone company use PCM/Inverse PCM for digitizing voice 8000 samples/sec * 7 bits/sample = 56 Kbps

Summary MODEM Standards Bell Modems ITU-T Modems Traditional Modems 56k Modems

Suggested Reading Section 6.4, 6.5, “Data Communications and Networking” 2nd Edition by Behrouz A. Forouzan