Data Communication and Media

Transcription

1 Data Communication and Media Concept and Model of Communications Analogy Signal and Digital Signal Signal Frequency, Spectrum and Bandwidth System Frequency Response and Bandwidth Transmission Media and Types Transmission Modes - Parallel & Serial Transmission - Asynchronous & Synchronous Transmissions - Simplex & Duplex Transmission Communication Standards: RS/EIA-232 & Others

2 Concept and Model of Communications Lecture General Communications: face-to-face conversation, write a letter, etc. Electronic Communications: telephone, wireless phone, TV, radar, etc. Our Focus Computer Communication General Communication Model Source S(t) T(t) Transmission Tr(t) Sd(t) Transmitter Receiver System Destination Microphone Telephone Computer Scanner Transformer Encoder Compress Modulator Line/Cable Fiber/Air Satellite Network Transformer Decoder Uncompress Demodulator Speaker Earphone Computer Printer Basic Communication Criteria: Speed, Reliability, Security (SRS)

3 Analogy Signal and Digital Signal Lecture Information must be converted into electrical energy, called signal, before transmission. Text, voice Video, etc Digital Text, voice Video, etc Analog Input Signal s(t) Signal Power: s (t) Signal Energy: ʃ 2 2 s (t)dt Converter Encoder General Communication Component H() Digital-to-Digital Analogy-to-Digital Digital-to-Analogy Analogy-to-Analogy s(t) voltage Digital Signal s(t) voltage Analogy Signal Output Signal o(t) =H[s(t)] 4 classes/types of systems - Input-to-Output Signal デジタル信号 t t

4 Signal Frequency, Spectrum and Bandwidth Signal in time domain Wave s(t) cos2πft T period t Transformation T=/f Lecture Signal in frequency domain Spectrum s(t)=acos2πft + Bcos2πf2t T=LCM(/f, /f2) f s(t) s(t) Analogy Signal Periodic Aperiodic t f: frequency Fourier Transform S(f)=ʃs(t)e -j2πf df S(f) S(f) S(f) S(f) F f f A B = F2 F Bandwidth f2 B F2 f f Digital Signal t Bandwidth f

5 Time-Frequency Relation and Signal Bandwidth General Relations: Time Domain Frequency Domain Signal Bandwidth Change Slow Low Frequency small Change Fast High Frequency large Frequency Unit: Hertz (Hz), Kilohertz (KHz), Megahertz (MHz), Gigahertz (GHz), Terahertz (THz) Earthquake wave:. ~ Hz Nuclear explosion signal:. ~ Hz Electrocardiogram (ECG): ~ Hz Wind noise: ~ Hz Speech: ~ 4 Hz (4 KHz) Audio: 2 ~ 2 Hz (2 KHz) NTSC TV: 6 MHz HDTV: > MHz

6 System Frequency Response & Bandwidth Input Signal x(t) Input Spectrum: X(f) System: H() Output Signal y(t) =H[x(t)] Output Spectrum: Y(f) System Frequency Response: H(f) = Y(f)/X(f) H(f) System Bandwidth B = F2 F F Signal can pass Signal can t pass Transmission Bandwidth F2 f

7 Transmission Media A transmission medium: - a connection between a sender and a receiver - a signal can pass but with attenuation/distortion - a special system with a transmission bandwidth Guided (Wired) Media (lines) - Twisted pair (~MHz) - Coaxial cable (K~5MHz) - Optical fiber (8~37THz) Unguided (Wireless) Media (air, vacuum, water, etc.) - LF (3~3KHz, Navigation) - MF/HF (3~3KHz, AM/SW radio) - VHF (3~3MHz, TV & FM radio) - UHF (.3~3GHz, TV, mobile phone) - SHF (3~3GHz, satellite, microwave) - EHF (3~3GHz, experimental com) - Infrared (no frequency allocation) Transmission Media

8 Frequency and Spectrum ISM band Mhz Ghz Ghz LF MF HF VHF UHF SHF EHF 3kHz 3kHz 3MHz 3MHz 3MHz 3GHz 3GHz 3GHz km km m m m cm cm mm infrared visible MHz khz GHz THz PHz EHz UV X rays Gamma rays Propagation characteristics are different in each frequency band

9 Parallel Transmission and Serial Transmission Segment the / stream into N bits groups Sender N N N N? Receiver Parallel Transmission Sender Receiver 7 (N) bits are sent together 7 (N) lines are needed Serial Transmission Sender P/S converter Receiver 7 (N) bits are sent one after another Only line is needed S/P converter

10 Asynchronous and Synchronous Transmission Timing or synchronization between a sender and a receiver is very important for data transmission Asynchronous transmission: ) A bit stream is segmented into small groups characters (5~8 bits) 2) Add a start bit () and a stop bit () at the beginning and end of each character 3) Frame = start_bit + character + stop_bit (7~ bits), but 2/9~2/ no real data 4) Arbitrary long gap/interval/idle between two characters or frames Frame4 Frame3 Frame2 Frame idle Sender Receiver Independent clocks Synchronous transmission: ) A bit stream is segmented into relative large groups/blocks many characters or bytes 2) Add control bits at the beginning and end of each block 3) Frame = H_control_bits + characters (data_bits) + T_control_bits 4) No gap/interval/idle between two characters in a data block/frame Sender... Trailer Frame Header Con_bits Con_bits Synchronized clocks Receiver

11 Simplex Transmission and Duplex Transmission Simplex Transmission Device A Direction of data One can send and the other can receive Device B Half Duplex Transmission Direction of data at time Device A Device B Direction of data at time 2 Both can send and receive but in different time Full Duplex Transmission Device A Direction of data all the time Both can send and receive simultaneously Device B 複信 - Wikipedia

12 Communication Standards and Related Organizations Communications need standards for inter-operations of different devices Standard Organizations: - ISO (International Standards Organization): ISO number - ITU (International Telecommunication Union): V.num & X.num - EIA (Electronic Industries Association): EIA-num - IEEE (Institute of Electronics Engineers): IEEE.num - ANSI (American National Standards Institute): ASCII, etc. - ATM Forum and ATM Consortium - IETF (Internet Society and Internet Engineering Task Force): RFC num - W3C (World Wide Web Consortium): HTTP, HTML, XML, - WAP Forum (Wireless Application Protocol): WAP-num

13 Serial & Asynchronous Transmission Standards Standards of transmission in short distance: - EIA-232 or RS V.24 - ISO 2 - EIA-449/RS-422/RS EIA-53 - X.2 Their common features - Serial & asynchronous transmission - Transmissions of ASCII code, byte, char - Use twisted copper lines - Low speed: several Kbits ~ Mbits per second - Short distance: < several tens of meters

14 EIA/RS-232 Standard bit Wave form of +, 2B or Device A Sender Device B Receiver Transmit characters (7 or 8 bits) Sender: +5v and -5v Start bit () and stop bit () for every character 9/ bits in total A sender never leaves wire at v; when idle, puts 5v, i.e., Receiver: (+3v, +5v) and (-3v, -5v), otherwise error RS-232, Wikipedia

15 EIA/RS-232 Standard (cont.) Agreement of transmission timing or rate/speed bps bits per second, bit rate or transmission speed - 3bps, 2.4Kbps, 4.8Kbps,, 9.2Kbps, 33.6Kbps, 56Kbps Setting bit rate (transmission speed) of devices/hardware - switch (manually), software, auto-detection Either simplex or duplex T: Transmitter R: Receiver G: Ground

16 EIA/RS-232 and Other Standards EIA-232: rate<64kbps; connection length< 5 meters; 25 pin connector - pin 2: receive (RxD); pin 3: transmit (TxD); pin 7: groud - other pins for transmission control EIA-449: rate<mbps; connection length< 2 meters; 37/9 pin connector EIA-53: same as the above; 25 pin connector X.2: 64/92 Kbps (N-ISDN rate); 5/8 pin connector RS ウィキペディア

17 Exercise. Two signals are given in the following figures. Whose bandwidth is large? Why? s(t) s(t) (a) t (b) t 2. Draw the RS-232 waveform diagrams of ASCII letters of R () and S (). 3. Give at least one example for each of the following transmission/communication modes: parallel transmission, serial transmission, simplex transmission and duplex transmission. 4. Suppose one sent 7bit characters across an EIA-232 or RS-232 connection that operated at 96 bps (9.6Kbps). How long will the minimum transmission time be required? (Hint: remember to add a start bit and a stop bit on each character.)