2. Telecommunications n Communications Model n Trasmission Modes n Transmission Direction n Transmission Media n Communication Networks

3. Communications Model SenderReceiver Message Medium

4. Transmission of Digital Data over Analog Links

5. Transmission Modes byte start bit start bit end bit end bit start bit end bit byte Asynchronous transmission: Low-speed transmission of one character at a time, using control bits (i.e., start & stop bits) Synchronous transmission: High-speed simultaneous transmission of large blocks of data byte

6. Transmission Direction n Simplex transmission –Transmission occurs in one direction only –e.g., radio or TV waves n Half-duplex transmission –Transmission occurs in both directions, but only one at a time –e.g., Walkie-Talkie, CB radio, pagers n Full-duplex transmission –Transmission occurs in both directions simultaneously –e.g., telephones

7. Transmission Medium: wire-based n Twisted wire pair –A pair of copper wires are twisted to avoid signal interference –The cost is low, and widely used in local loops of telephone net. –Subject to communication noises n Coaxial cable –Layers of copper conductors covered with shieldings –can transmit data, video, voice signals over one line n Fiber optics cable –Fastest wire-based transmission medium –The cost is too high, and ideal as a corporate back-bone cable to interconnect many LAN’s

8. Transmission Medium: wireless n Terrestrial microwave –microwave dishes needed at certain intervals (appr. 50km) –allows for very high-speed transmissions, and used by phone companies or common carriers n Satellite microwave –involves uplinks & downlinks travelling to and from the satellite –widely used for GPS applications n Cellular communication –An area is divided up into many small cells, and an antenna installed in each cell handles signal transmissions

9. Microwave and Satellite Transmissions - Cont’d Microwaveantenna The satellite and the antenna

10. Cellular phone Mobile telecommunications IMT-2000 video phone

11. Communications Networks n LAN(Local Area Network) –ideal for small area networking needs (e.g., room, floor, building) –widely used to support computing needs in the office environment n MAN(Metropolitan Area Network) –a backbone cable linking small LAN’s throughout an organization –e.g., A network interconnecting agencies of an insurance firm n WAN(Wide Area Network) –networking support covers a large geographical area –used by airlines, banks, etc.