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Data Transmission

Data Transmission. Data Communications and Networks Mr. Greg Vogl Uganda Martyrs University Lecture 4, 28 March 2003. Overview. Signals, noise and errors Modulation and modems Synchronisation Baseband and broadband. Sources. Hodson Ch. 1, 2, 3, 8.2 Stamper pp. 67-75, Ch. 3

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Data Transmission

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  1. Data Transmission Data Communications and Networks Mr. Greg Vogl Uganda Martyrs University Lecture 4, 28 March 2003

  2. Overview • Signals, noise and errors • Modulation and modems • Synchronisation • Baseband and broadband Data Communications and Networking: Data Transmission

  3. Sources • Hodson Ch. 1, 2, 3, 8.2 • Stamper pp. 67-75, Ch. 3 • BITDCO lectures 3-7 • Recommended, easily accessible at UMU • Go to DCN website, click Links • HTML and GIF files all fit on one floppy disk Data Communications and Networking: Data Transmission

  4. Digitising • Analog  digital conversion uses sampling • Amplitude of each sample converted to bits • Accuracy of digitised info. depends on • Number of bits in sample • How often the samples are taken (period) • Linear or nonlinear digitising of amplitude • Compression/decompression algorithms • Used for voice, music, photos, video, etc. Data Communications and Networking: Data Transmission

  5. Signals • Sinusoidal wave is simplest type of wave • Complex wave is mixture of sinusoidals • Square wave with bit rate B can be converted to infinite Fourier series • with frequencies (2n+1)B/2, decreasing amp. • Bandwidth = highest freq. – lowest freq. • Phone system 3400 Hz – 300 Hz = 3.1 KHz Data Communications and Networking: Data Transmission

  6. Fundamental Limits • Attenuation • amplitude decreases over distance • due to resistance, dispersion, other energy loss • amplifier or repeater restores initial amplitude • Bit rate limited by • Shannon’s law B = H log2 (1 + S/N) • Bandwidth in Hertz H • Noise or Signal-Noise Ratio S/N Data Communications and Networking: Data Transmission

  7. Sources of Noise • Impulse noise: spikes/surges, e.g. lightning • Reduced with power supply/line conditioning • Crosstalk: interference between channels • Reduced with shielding, insulation, separation • Echo: signals reflect at connections/ends • Reduced with terminators, repeaters • White noise: electron thermal energy • Reduced by tuning radio antenna Data Communications and Networking: Data Transmission

  8. Error Detection and Correction • Sender • compute a check value based on the data • append check value to data and sends • Receiver • perform a calculation based on data • compare answer with check value • if they disagree, ask to resend (NAK) • No error detection scheme finds 100% of errors • Usually easier to detect and resend than correct Data Communications and Networking: Data Transmission

  9. Redundancy Checks • Parity Check or Vertical Redundancy Check • Parity bit makes number of 1 bits even/odd • Does not detect even number of bit errors • Longitudinal Redundancy Check • Used to check a whole block of characters • Check character added to end of block • nth bit checks parity of nth bits of other char’s Data Communications and Networking: Data Transmission

  10. Cyclic Redundancy Check • Interpret bits as coefficients of a polynomial • Divide by generator polynomial to get remainder • Subtract remainder from original bits and transmit • Receiver divides received bits by generator • If remainder is 0, no error occurred • Versions (no. of bits R): CRC-12, 16, CCITT, 32 • Detects almost all errors in block transmissions • 1 and 2 bit, odd number of bits, burst errors of <R bits Data Communications and Networking: Data Transmission

  11. Other Error Detection Schemes • Check sums/digits (sum of fields) • Hash totals (sum group of items) • Byte count (message length) • Character echoing to user (asynch. e.g. terminal) Data Communications and Networking: Data Transmission

  12. Error Correction • Usually retransmission of message • Acknowledgment • ACK, NAK, numbered • Timeout, retry limit • Forward Error Correction • Receiver has enough info to reconstruct original • cost of retransmission > cost of reconstruction • E.g. hamming codes (not used in data comm.) Data Communications and Networking: Data Transmission

  13. Sequence Checks • Message sequencing • Messages numbered so none go missing • Messages not received can then be re-sent • Packet sequencing (packet-switched networks) • Sender divides message into numbered packets • Receiver reassembles using packet numbers • Unnecessary in circuit-switched networks • e.g. phone system uses circuit switching • Connection/session is unbroken stream Data Communications and Networking: Data Transmission

  14. Low data rates High error rates Error correction Higher OSI levels Wide Area Networks High data rates Low error rates Just transmit data Lower OSI levels only Local Area Networks Connection vs. Connectionless Data Communications and Networking: Data Transmission

  15. Modulation (Shift Keying) • digital  analog • ac = Ac sin (2fct + ) • vary one or more: • AM/ASK: A=amplitude • FM/FSK: f=frequency • PM/PSK: =phase • Dibits: 0, 90, 180, -90 • Tribits: 0, 45, 90, etc. Data Communications and Networking: Data Transmission

  16. 90º 180º 0º -90º Constellation Diagram • Quadrature (QAM) • Modulation of: • Amplitude (radius) • Phase (angle) • V.32, 32 points • 2400 baud, 9600 bps Data Communications and Networking: Data Transmission

  17. RS232C/V.24 Modem Interface Data Communications and Networking: Data Transmission

  18. RS232C/V.24 Modem Interface • TxD, RxD- Transmit and receive data pins • RTS- Request To Send, from DTE (computer) • CTS- Clear To Send, from DCE (modem)now used for flow control/hardware handshaking • DTR- Data Terminal Ready - computer enabled • DSR- Data Set Ready - modem is turned on • CD- Carrier Detect - this modem is receiving a signal from remote modem Data Communications and Networking: Data Transmission

  19. Modem Features and Types • Speed (56Kbps is most common) • Auto/manual dial/redial/answer/disconnect • Fax, digitised voice (for Internet phone calls) • Speaker, indicator lights (external), self-testing • MNP 1-4 error correction, 5 compression • Standards (ITU V.90, EIA, USR, Hayes AT cmds) • Mobile (PCMCIA, cellular, RF) • HDSL, Cable, ISDN, DSL, Fibre optic Data Communications and Networking: Data Transmission

  20. The Synchronisation Problem • How to synchronise Sender and Receiver? • Asynchronous • Bit (clock) level • Byte (character) level • Frame level • Synchronous - Character and Bit Oriented • Bit level • Clock encoding and extraction • Data encoding and clock synchronisation • Byte level • Frame (block) level Data Communications and Networking: Data Transmission

  21. Asynchronous Bit Synch. Data Communications and Networking: Data Transmission

  22. Asynchronous Bit Synch. • Resynchronise for each byte (character) • Isolated characters received at random times • Transition between voltage levels is a bit • 1 start bit, 8 bits for ASCII char., 1 stop bit • Efficiency only 80% (2 bits of overhead) • Used in low bit rate, low volume links • Used by RS232 ports on PCs Data Communications and Networking: Data Transmission

  23. Character   ASCIIRepresentation   EBCDICRepresentation 0 00110000 11110000 1 00110001 11110001 2 00110010 11110010 3 00110011 11110011 4 00110100 11110100 5 00110101 11110101 6 00110110 11110110 7 00110111 11110111 8 00111000 11111000 9 00111001 11111001 A 01000001 11000001 B 01000010 11000010 C 01000011 11000011 D 01000100 11000100 E 01000101 11000101 F 01000110 11000110 G 01000111 11000111 H 01001000 11001000 I 01001001 11001001 J 01001010 11010001 K 01001011 11010011 L 01001100 11010011 M 01001101 11010100 N 01001110 11010101 O 01001111 11010110 P 01010000 11010111 Q 01010001 11011000 R 01010010 11011001 S 01010011 11100010 T 01010100 11100011 U 01010101 11100100 V 01010110 11100101 W 01010111 11100110 X 01011000 11100111 Y 01011001 11101000 Z 01011010 11101001 ASCII vs. EBCDIC Codes Data Communications and Networking: Data Transmission

  24. Synchronous Bit Synch. • A frame (block of bits) is sent • Preamble uses unique bit sequence Data Communications and Networking: Data Transmission

  25. HDLC Synchronisation • High-level Data Link Control Protocol • Use 01111110 as start/stop flags • If 1111111 appears anywhere in data, • Sender inserts a 0 after fifth 1 (“bit stuffing”) • Receiver removes the 0 Data Communications and Networking: Data Transmission

  26. Baseband • Data and clocking signal combined • Single channel (takes whole bandwidth) • Usually bi-directional (two way) • Uses an encoding scheme such as: • Manchester • Diff. Manchester • Phase modulation • 4/5 bit • 5/6 bit Data Communications and Networking: Data Transmission

  27. Manchester Encoding • Transmitted signal is Data XOR Clock • Transition in middle of each cell • Lohi = 1, hilo = 0 • Often used in Ethernet • Preamble for synchronising and start of data • If polarity reversal, data misrepresented Data Communications and Networking: Data Transmission

  28. Data Communications and Networking: Data Transmission

  29. Differential Manchester Encoding • Transitions still occur at centre of each bit • 1=no transition at data start boundary • 0=transition at data start boundary • Typically used in token ring systems • No need for long preamble • J and K signals have no midpoint transition • used to signal start and end of data Data Communications and Networking: Data Transmission

  30. Phase Modulation Encoding • Twisted pair provides two channels • Phase difference used to signal data values • 0 if a transition in one channel • 1 if a transition in both channels Data Communications and Networking: Data Transmission

  31. 4 bit/5 bit Encoding • Represent each 4 bits as 5 bits • Ensure each 5 bit pattern has signal change • Not use all the same bit (00000 or 11111) • Use fixed translation/mapping table • Avoids problem of many repeated 1’s or 0’s • 100 Mbit/s data rate requires 125 Mbit/s signal rate • Used in high-speed fibre optic LANs Data Communications and Networking: Data Transmission

  32. 5 bit/6 bit Encoding • High speed LANs on UTP cable • Copper wire must use low frequencies • US/int’l interference and emission regulations • Two steps used • Quintet scrambler • 5B/6B two-level non-return-to-zero encoder • result is 3 1’s and 3 0’s; allows error check Data Communications and Networking: Data Transmission

  33. Broadband • Modulated using modem (digital  analog) • Phase coherent so unidirectional • Multiple channels divide bandwidth: FDM • Channels often 6MHz wide, 10 Mbit/s • The cable uses low freq. 0-50 Hz for power • Used for cable TV or data (cable modems) • Carrierband: one channel=whole bandwidth Data Communications and Networking: Data Transmission

  34. Single vs. Dual Cable Broadband • Single cable • traffic to/from head end (amp/freq translator) • high=outbound 216-300 MHz • low=inbound 5-110 MHz • Dual cable • Each cable has 5-300 MHz bandwidth • Simpler headend (no amp/freq translation) • Higher cost for cabling Data Communications and Networking: Data Transmission

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