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  1. Introduction
  2. Network Model
  3. Topology
  4. Physical Media
  5. Wireless Media
  6. Network Card
  7. Modems
  8. Outside Connections
  9. Wide Area Network Connections
  10. Repeaters, Bridges, Routers
  11. Network Types
  12. Ethernet
  13. Token Ring
  14. ARCnet
  15. AppleTalk
  16. FDDI
  17. Architecture Comparisons
  18. Categories
  19. TCP/IP
  20. IPX/SPX
  21. NetBEUI
  22. AppleTalk
  23. SNA
  24. Others
  25. Suites and Network Layers
  26. Installing Drivers
  27. DNS
  28. Network Operating Systems
  29. Applications, mail, groupware, DBMS
  30. Backing up the network
  31. Troubleshooting
  32. Web, SNMP, admin, firewalls
  33. Networking Terms and Definitions
  34. Credits

Modems

A modem is a modulator/demodulator. It converts digital signals from a computer to analog signals so they can be sent over a telephone line. The receiving computer will normally have a modem which will convert the analog signals back to digital signals. A modem may be an internal modem which is a card inside your computer or it may be external with a connection to a serial RS-232 line on your computer. The telephone jack that will plug into the modem is called an RJ-11 jack. Speed of the modem is measured in bits per second (bps). There are a set of V-series standards developed by the International Telecommunications Union which indicate the speed of the modem.

  • V.22bis - 2400bps
  • V.32 - 9600bps
  • V.32bis -14,400bps
  • V.32terbo - 19,200bps
  • V.FastClass (V.FC) - 28,800bps
  • V.34 - 28,800bps
  • V.42 - 57,600bps

These are data compression standards which enable the transmission to operate at a higher speed. Due to data compression used on modems in recent years baud rates and bps when referring to modem speed are no longer the same. Now, more than one bit can be sent with each sound wave oscillation (baud). In the past, only one bit could be sent per sound wave oscillation.

Modem Types

  • Asynchronous - The common modem used today. Each byte is placed between a stop and a start bit. Each modem must operate with the same start and stop bit sequence, operate at the same baud rate and have the same parity settings for the data checking in order to communicate correctly. Define parity checking.
  • Synchronous - Synchronous modems can be faster than asynchronous. They depend on timing to communicate. Data is transmitted in frames with synchronization bits which are used to be sure the timing of the transmission and reception of data is accurate. Synchronous modems are normally used on dedicated leased lines. Synchronous modems are one of binary synchronous communications protocol (bisync), high level data link control (HDLC), or synchronous data link control (SLDC). Three methods can be used to control synchronization:
    • Additional clock signal
    • Guaranteed state change - Clocking is part of the data signal.
    • Oversampling - The reciever samples the signal much faster than the data is sent. The extra samples can be used to be sure the clock is synchronized.
  • Digital Modems - These are used with ISDN services and are not actually modems, although they are called modems. They can provide connection speeds of 128Kbps.

Terms

  • BRI - Basic rate interface
  • CSU - Channel Service Unit are used to translate network signal strengths to signal levels for a leased line
  • DSU - Digital Service Unit
  • MNP - Microcom Network Protocol is a modem standard used for modem error detection and correction. classes 2,3, and 4 are used for error correction.
  • MNP Class 5 - A data compression standard by Microcom.
  • LAPM - Link access protocol for modems is the V.42 standards developed by the International Telecommunications Union.