1. Lecture 8:
Modems
(Converts analog signals to digital and vice-versa) 1nd semester
By: Adal ALashban

2. Outline - Modulator “converts digital information to analog”
- Demodulator “converts analog information to digital”
- RS-232 Interface

3. Modems
- A modem: is a hardware device that allows a computer to send and receive data over a telephone lines.
- Computer information is stored digitally, whereas information transmitted over telephone lines is transmitted in the form of analog waves.
- A modem converts between these two forms.

4. Basic Principle
0101

6. Basic Structure

7. Classification of Modems
- Range
- Line Type
- Operation Mode
- Synchronization
- Modulation
- Data Rate

8. Range Line Type - Short Haul - Dial-up - Voice Grade - Leased
- Wide Band
- Dial-up
- Leased
- Private

9. Operation Mode Modulation
- Simplex
- Half-Duplex
- Full-Duplex
- AM
- FM
- PM
- TCM

10. Cable Modem
- Cable Modems make access to TV cable, and let you get  high-speed internet access.
- Availability is still limited in many areas, but is growing rapidly.

11. External Modem
- It usually plug in through a serial port, or a USB port. 
- External modems cost the most, but have the benefits of being easily installed.
- Also plugs into telephone line.
- External power supply is needed.
- Holder case is required.
- Often they have the indicators that tell the status of your connection there is one standard interface for connecting external modems to computers called RS-232.

12. Typical Indicators on External Modems Front Panel
- HS = High speed
- AA = Auto answer
- CD = Carrier detect
- OH = Off hook
- RD = Received data
- SD = Send data
- TR = Terminal ready
- MR = Modem ready

13. Internal Modem - It can insert into a vacant expansion slot.
- No external power supply is needed.
- No holder case is required.
- Least expensive at given speed.
- Incompatible with different kind of PCs.
- These modems go inside the computer, and usually plug directly into the motherboard.

14. ISDN Modem
- ISDN: is an abbreviation for Integrated Services Digital Network.
- There is no such thing as a noisy or fuzzy line. 
- Always runs at 64kbps.
- Ability to bond multiple "B" channels to get speeds of 128kbps.
- Call setup is very fast (dialing, handshake and authentication), usually under 2 seconds (compared with 15 seconds at least for most analog modems).
- Ability to get 2 calls at the same phone number (one line is for voice, and the other line for your internet connection).

15. Wireless Modem - Convert digital data into radio signals and back.
- Support a maximum data transmission rate of 19.2Kbps (kilobits per second).
- Wireless modems can be used up to miles (30-50 KM).
- Operation is in the 900 MHz and 2.4 GHz. 
- Modem names include: BreeseLINK, LATNET-Radio Data Links, AirLink Wireless Modems, and Open Minds.

16. DSL Modem
- DSL Modems: DSL (Digital Subscriber Line) service is usually offered through  the phone the company/ISP.
- You can leave your Internet connection open and still use the phone line for voice calls.
- The speed is much higher than a regular modem (up to Mbps )
- DSL doesn't necessarily require new wiring; it can use the phone line you already have.

17. Modem Speed - The Modulation Speed - The Serial Port Speed Note:
- Modem speed is rated in bps (bits per second). 
- So a 56k modem transmits at 56,000 bits per second in theory.
- While 8 bits make a byte, and 1024 bits equal one kilobits, a 56k modem will transmit a maximum of approximately 7 kilobytes of information every second.  
- A modem will never connect higher than what it is rated for.

18. Modem Installation
- Modem Cable Selection.
- Selection of Port.

19. What is RS232 - RS: means Recommended Standard.
- RS 232 uses both synchronous and asynchronous communication.
- In microcontrollers like ATmel 89C51 asynchronous communication is used and it is called as UART – universal asynchronous receiver and transmitter .
PCI Express x1 card with one RS-232 port

20. Interfacing RS232
- We need a driver to use the RS232 with microcontroller.
- That driver turned out to be MAX232.
- It is a16 PIN Ic which is used to interface RS232 with any other devices.
- The transmission speed in the RS232 is called as baud rate.

21. Setting Serial Port
- In microcontroller there are some special function registers which are allocated to control the serial communication.
- The list of registers which is used to control the serial port are:
SCON
PCON
SBUF

22. Any Questions ?