2. Learning Outcomes  Know the basic components of a network  Know data transmission methods, including types of signals, modulation, demodulation, and choices among transmission modes  Differentiate between various kinds of communication links and appreciate the need for protocols

3. Data Communication  Traditional ways  Traditional ways for sending and receiving information are : Mail, telephone, TV and radio, books, newspapers and periodicals  :  Data Communications Systems : Computer systems that transmit data over communications lines such as telephone lines or cables --- mid 1960s  All processing, hardware, and software in one central location  Centralized Data Processing --- All processing, hardware, and software in one central location  Distributed Data Processing --- A llows both remote access and remote processing. Processing can be done by the central computer and the other computers that are hooked up to it.

4. Data Communication   Network --- A computer system that uses communications equipment to connect two or more computers and their resources   The basic components of a data communications system are --- sending device, receiving device and a communication link  Data may travel to a large computer through a communications control unit called a front-end processor, which is actually a computer in itself. Its purpose is to relieve the central computer of some communications tasks  Modem --- needed to convert computer data to signals that can be carried by the communication channel and vice versa

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6. Data Transmission  Digital transmission sends data as distinct on or off pulses  Analog  Analog transmission uses a continuous electric signal in a carrier wave having a particular amplitude and frequency  Digital signals are converted to analog signals by modulation (change) of a characteristic, such as the amplitude or frequency of the carrier wave  Demodulation is the reverse process; both processes are performed by a device called a modem

7. Data Communication   Modems –A direct-connect modem is connected directly to the telephone line by means of a telephone jack –An external modem is not built into the computer and can therefore be used with a variety of computers –An internal modem is on a board that fits inside a personal computer –Notebook and laptop computers often use a PC card modem or PCMCIA that slides into a slot in the computer   Modem speeds can be 33.6 K, 56 K   ISDN --- integrated Services Digital Network --- uses another telephone line for digital transmission and can move date at 128000 bps

8. Data Communication  Asynchronous transmission --- start/stop, method keeps the units in step by including special signals at the beginning and end of each group of message bits--a group is usually a character  Synchronous transmission --- the internal clocks of the units are put in time with each other at the beginning of the transmission, and the characters are transmitted in a continuous stream  Simplex transmission --- data in one direction only, TV  Half-duplex transmission --- transmission in either direction but only one at a time, e.g. ATM  Full-duplex transmission --- transmission in both direction at once, e.g. telephone conversation