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Lecture 8: Modems (Converts analog signals to digital and vice-versa) 1nd semester By: Adal ALashban
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Outline - Modulator “converts digital information to analog”
- Demodulator “converts analog information to digital” - RS-232 Interface
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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.
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Basic Principle 0101
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Basic Structure
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Classification of Modems
- Range - Line Type - Operation Mode - Synchronization - Modulation - Data Rate
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Range Line Type - Short Haul - Dial-up - Voice Grade - Leased
- Wide Band - Dial-up - Leased - Private
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Operation Mode Modulation
- Simplex - Half-Duplex - Full-Duplex - AM - FM - PM - TCM
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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.
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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.
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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
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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.
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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).
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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.
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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.
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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.
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Modem Installation - Modem Cable Selection. - Selection of Port.
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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
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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.
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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
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Any Questions ?
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