CS335 Networking & Network Administration

1. CS335 Networking &Network Administration Tuesday, April 6

2. Local Asynchronous Communication (RS-332) • Binary digits (bits) represent data • Short distances ex. Keyboard to computer • http://www2.rad.com/networks/1995/rs232/hist.htm#hist

3. Electric current to send bits

4. Parallel vs. Serial • Parallel • Multiple parallel data paths • Transmit bits simultaneously • Travel in 8,16, 32 or 64 bit paths • Need physical circuit for each channel • Serial • Single lane • Used in modems, older terminal connections, some serial printers

5. Asynchronous • Sender and receiver do not need to coordinate before each transmission. • The electrical signal the transmitter sends does not contain info that the user can use to determine where individual bits begin and end. • Receiving hardware must be built to accept and interpret the signal the sending hardware generates.

6. Asynchronous • Telegraph • Morse code HE is 4 dots for H and 1 dot for E • Numeral 5 is 5 dots • Operators need to know there is a timed pause • Digital transmissions need a timing mechanism or coding mechanism between bytes of data

7. Start bit and Stop bit

8. Synchronous transmissions • Send multiple bytes of data as one transmission without a start and stop bit for each byte • Instead sends preceding sync bits with info about transmission rate to alert receiving device that it is about to receive data • Other types use a separate channel instead of sync bits

9. Standards again! • How long to hold voltage for a single bit? • What is the maximum rate at which hardware can change the voltage? • Will hardware be interchangeable with other vendors?

10. EIA Standard RS-232 • Defines serial, asynchronous communication • Serial – bits travel on the wire one after another • Parallel – multiple wires allow one bit on each wire • Connection less than 50 ft • Voltages range between -15 and +15 volts • Can send 8 bit characters but often configured to send 7 data bits.

11. RS-232 • Can send a character any time • Delay arbitrarily long to send another • Asynchronous because sender and receiver do not coordinate before transmission • Once starts sends all bits one after another with no delay between • Never leaves 0 volts on the wire, when there is nothing to send it leaves the wire with a negative voltage that corresponds to bit value 1.

12. Bits, Bytes, Data Encoding • Each 1 or 0 is a bit • 8 bits is a byte • Bits and bytes are encoded to represent characters • ASCII, EBCDIC, and Unicode

13. ASCII standard • 7 bit representation • What character is represented here?

14. EBCDIC • Extended binary coded decimal interchange code • IBM proprietary encoding scheme • Used in legacy IBM mainframes • Eight bits to represent letters, numerals, and special characters • 256 characters can be represented

15. Unicode • http://www.unicode.org • First 128 characters are same as ASCII • Unicode uses 16 bits instead of the 7 bits of ascii • Allows for 65,536 different characters to include Chinese, Greek, Hebrew, Russian, etc. • Unicode supported by modern browsers as well as OS’s like Windows, Netware, Linux, Unix

16. Baud Rate • Baud - Number of changes in signal per second that the hardware generates • Sending and receiving hardware must agree on the length of time voltage will be held on the line • Instead of time per bit, bits per second • Baud rates are configured by hardware or software • Early connections operated at 300 baud • Currently 19,200 or 33,600 bits per second more common

17. Baud • At slow speeds, only one bit of information (signaling element) is encoded in each electrical change. The baud, therefore, indicates the number of bits per second that are transmitted. For example, 300 baud means that 300 bits are transmitted each second (abbreviated 300 bps ). Assuming asynchronouscommunication, which requires 10 bits per character, this translates to 30 characters per second (cps). For slow rates (below 1,200 baud), you can divide the baud by 10 to see how many characters per second are sent. • At higher speeds, it is possible to encode more than one bit in each electrical change. 4,800 baud may allow 9,600 bits to be sent each second. At high data transfer speeds, therefore, data transmission rates are usually expressed in bits per second (bps) rather than baud. For example, a 9,600 bps modem may operate at only 2,400 baud.

18. Simplex, half and full duplex • Simplex – one way data flow • Half-duplex – one way at a time • Full-duplex – bidirectional on a single channel or one channel for each direction

19. RS-232 full-duplex

20. RS-232 resources • http://www.camiresearch.com/Data_Com_Basics/RS232_standard.html • http://www.arcelect.com/rs232.htm • http://www2.rad.com/networks/1995/rs232/rs232.htm

21. Hardware limitations • Can’t change voltage instantly • Electric conductivity not perfect • RS-232 standard allows for these imperfections

22. Hardware limitations

23. Hardware bandwidth • Measured in cycles per second or Hertz (Hz) • Nyquist theorem states maximum data rate • Shannon theorem gives a limit to data rate because of noise (background interference)

24. Long-distance communication • Carrier wave • Modify or modulate • Amplitude and Frequency modulation

25. Amplitude modulation

26. Phase Shift modulation

27. Phase shift modulation • If the system can shift the phase by 8 possible amounts (23)the transmitter uses 3 bits of data to select the shift • Receiver determines how much the carrier shifted and uses the shift to recreate the bits that caused it • Thus bits per second is a multiple of the baud rate

28. Modulator/Demodulator • Modem

29. Dialup modems • Send bits a long distance

30. Multiplexing • Two or more signals that use different carrier frequencies can be transmitted over a single medium simultaneously without interference • Cable television for instance

31. Multiplexing • Wavelength division (FDM) – optical fiber • Spread spectrum • Time division (TDM) – sources “take turns”