CS335 Networking &

Network Administration

Tuesday, April 6

Local Asynchronous Communication (RS-332)

Binary digits (bits) represent data Short distances ex. Keyboard to computer http://www2.rad.com/networks/1995/rs232/his

t.htm#hist

Electric current to send bits

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

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.

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

Start bit and Stop bit

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

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?

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.

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.

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

ASCII standard

7 bit representation What character is represented here?

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

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

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

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 asynchronous communication, 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.

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

RS-232 full-duplex

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/rs

232.htm

Hardware limitations

Can’t change voltage instantly Electric conductivity not perfect RS-232 standard allows for these

imperfections

Hardware limitations

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)

Long-distance communication

Carrier wave Modify or modulate Amplitude and Frequency modulation

Amplitude modulation

Phase Shift modulation

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

Modulator/Demodulator

Modem

Dialup modems

Send bits a long distance

Multiplexing

Two or more signals that use different carrier frequencies can be transmitted over a single medium simultaneously without interference

Cable television for instance

Multiplexing

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