Presentation 45 – Modems

1. Presentation 45 – Modems

2. Objectives At the end of this presentation, you will be able to:

3. Explain three problems involved in sending data over ordinary telephone lines. • Explain the purpose of the Modem and the UART and briefly explain how each works. • Match the Modem speed to its “V dot” standard. • Recognize the “V dot” standards for Modem error correction and compression. • Match the Serial Port speed to its UART chip type. • List the de facto standards for the resources assigned to Serial Ports COM1 through COM4.

4. Digital Local Exchange Local Exchange Analog Digital

5. Digital Local Exchange Local Exchange Analog Digital Modem Modem

6. PC-1 Modem-1 Phone System Modem-2 PC-2

7. Digital Signal High Speed Parallel Data Analog Signal Low Speed Serial Data CPU vs. Phone Line

8. Digital Analog 8

9. The Modem • Modulator / Demodulator • Converts the digital signals from the computer into the analog signals required by the telephone line. • Converts the analog signals from the telephone line into the digital signals required by the computer.

10. Pulse Amplitude Modulation (PAM) 1 0 1 1 0 1 10

11. Each Amplitude Level Represents Two Bits 11 10 01 00 11

12. 0º Phase Shift 90º Phase Shift 180º Phase Shift 270º Phase Shift 12

13. Baud • Baud - A single state change of the analog signal. • Baud Rate - The number of times per second that the analog signal changes state. • In early modems, one baud usually represented one bit. • In today’s modems, one baud represents more than one bit.

14. High Speed Low Speed 14

15. 1 0 1 0 1 1 1 0 Parallel Serial 10101110

16. The UART • Universal Asynchronous Receiver/Transmitter • The heart of the Serial (COM) port. • Converts the parallel data required by the CPU into the serial data required by the serial port. • Converts the serial data received at the serial port into the parallel data required by the CPU.

17. Making the CPU and Phone Line Compatible (Transmit). CPU UART Modem Phone Line Buffer Memory 1. High Speed to Low Speed 2. Parallel to Serial Digital to Analog

18. Making the CPU and Phone Line Compatible (Receive). CPU UART Modem Phone Line Buffer Memory Analog to Digital 1. Serial to Parallel 2. Low Speed to High Speed

19. Two Types of Modems • Internal – ISA or PCI board that fits inside the computer. • External – Stand-alone box. Connects to one of the COM Ports.

20. Internal Modem CPU UART Function Modem Function Phone Line Internal Modem

21. CPU UART External Modem Phone Line Motherboard

22. CPU UART External Modem Phone Line I/O Adapter Card Mother- board

23. Modulation Standards • V.22 – 1200bps standard • V.22bis – 2400bps standard • V.32 – 9600bps standard • V.32bis – 14,400 standard • V.FC – 28,800 standard • V.34 – 33,600 standard • V.90 – 56kbps download, 33.6 kbps upload standard • V.92 – 56kbps download/upload standard

24. V.42 Error-Correction Standards • Link Access Procedure for Modems (LAPM) • Microcom Networking Protocol – 4 (MNP-4)

25. V.42bis Compression Standard. • Uses a compression technique to reduce the size of out going data. • Can compress data up to about 4 to 1. • For example, a 33,600-bps Modem could transfer data at a rates up to about 134,400 bps.

26. The COM Port • Also called the Serial Port or RS-232 Port • The link between the computer and the Modem. • The external Modem connects to the COM port on the back of the computer via a serial cable. • The internal Modem has a built-in COM port that duplicates the function of the external COM port.

27. 25-Pin Serial Port 9-Pin Serial Port

28. Standard Modem COM Port Settings

29. CPU IRQ4 I/O Memory UART Modem 03F8 COM1

30. CPU IRQ3 I/O Memory UART Modem 02F8 COM2

31. 33.6 kbps 134.4 kbps 33.6 kbps 134.4 kbps

32. UART Speed • Determines the Serial Port speed. • 16550 UART chip can reach speeds of 256 kbps. • 16440 UART chip can reach speeds of 115.2 kbps. • 8250 UART was supplied with the original IBM XT machine.

33. RJ-11 RJ-45 RJ-11 NIC Internal Modem

34. End