DC 3 - 2 Basic Concepts l Definitions l Host Computers and Terminals l Bits, Bytes and Codes l Parallel vs Serial l Asynchronous vs Synchronous l Simplex, Half-Duplex and Full-Duplex
DC 3 - 3 Definitions l Exchange of digital information between or among two or more devices using electronic transmission. l Use binary systems for information exchange – on or off – high or low – strong or weak – present or absent – 1 or 0 l So datacomm is art of sending ones and zeroes from one point to another.
DC 3 - 4 Host Computers and Terminals l Host computers – supercomputers --> massively parallel architecture – mainframes – minicomputers – any microcomputer defined as host – server in client/server system l Terminals – CRT = VDT = display station – keyboards, screen – dumb / smart / intelligent
DC 3 - 5 Evolution of Network Architecture l Intelligent terminals
DC 3 - 6 Evolution of Network Architecture l Standalone PCs + terminal emulation
DC 3 - 7 Evolution of Network Architecture l PC: User Interface Server: data I/O Local processing central processing
DC 3 - 8 Evolution of Network Architecture l Servers: many functions
DC 3 - 9 Decline in Mainframes l Intl Data Corp study (1993): – global sales shrinking – 0.4%/yr from peak in 1991 ($28.1B) – US sales shrinking 0.6%/yr from $8.2B l Computer Intelligence study (1992): – 88% of IBM new ES/9000 mainframes to installed base – US # IBM & compatible mainframes dropped 11% in 1991
DC 3 - 10 Bits, Bytes and Codes l Bit: zero or one l Byte: 8 bits l Binary system represents all data as 0s and 1s Decimal to binary01 210 311 4100 5101 etc.
DC 3 - 11 Bits, Bytes and Codes l Keyspace of n bits is 2 n – e.g., using 1 bit, can code ____ values – using 2 bits can code ____ values – using 3 bits can code _____ values l Biggest number in n bits is 2 n -1 – e.g., largest value using 4 bits is _____ – largest number stored in 8 bits is _____ l To find out roughly how many bits are required to store a decimal value D, – divide log 10 D by log 10 2 = 0.30103 – e.g., 10,000,000 = 10 7 = 2 23.2 or about 24 bits
DC 3 - 12 Bits, Bytes and Codes Binary arithmetic l 0 + 0 = 0 l 0 + 1 = 1 + 0 = 1 l 1 + 1 = 10 l Multiplying by 2 is same as left-shift by 1 place l Dividing by 2 is same as right-shift by 1 place l See Appendix: The Binary Number System on p. 233 ff of textbook
DC 3 - 13 Bits, Bytes and Codes l Character codes – Morse (unsuitable for datacomm) – Baudot (5 bits; Telex, punched tape) – Intl Baudot (5 data bits + 1 parity bit) – EBCDIC (IBM: 8 bits) – ASCII (7 bits) – Extended ASCII (8 bits) l Incompatible character codes lead to garbled text upon receipt and display
DC 3 - 14 Parallel vs Serial Parallel transmission l Simultaneous transfer of bits l Multiple wires in parallel; e.g., Ribbon cable l E.G., Parallel cable for pc to printer l Problems of signal leakage across wires l Provides higher speed l Costs more per foot of cable l Used mostly for short-distance high-speed transfers where cost of cable is minor
DC 3 - 15 Parallel vs Serial Serial transmission l Single bit at a time travels along wire l Almost universal today l Generally at least two wires used to connect systems
DC 3 - 16 Asynchronous vs Synchronous Asynchronous l Bytes sent as string of bits – Start bit (0) – 8 data bits (7 + 1 parity bit or 8 data bits) – Stop bit (1) – Thus generally 10 bits per transmitted byte l Speed measured in – bits per second or bps – kilobits per second (Kbps) – megabits per second (Mbps) – gigabits per second (Gbps) … etc.
DC 3 - 17 Asynchronous vs Synchronous Synchronous l No start/stop bits l Large blocks + sync characters l Synchronous clocks l Efficient for high-speed and high-volume xfr l See Figure 3-12, p. 47 for examples of how to calculate efficiency of asynch vs synch as function of block size
DC 3 - 18 Simplex, Half-Duplex and Full-Duplex l Simplex – one way only – e.g., TV transmission to ordinary TV l Half-duplex – one-way at a time – e.g., two-way traffic across one-lane bridge – sometimes called two-wire datacomm l Full-duplex – both ways at once – e.g., two-way street – four-wire datacomm but now OK w/ 2 wires
DC 3 - 19 Homework l Read Chapter 3 of your textbook in detail, adding to your workbook notes as appropriate. l Review and be prepared to define or expand all the terms listed at the end of Chapter 3 of your textbook (no hand-in required) l Answer all the exercises on pages 51-52 of the textbook using a computer word- processing program or absolutely legible handwriting (hand in after quiz tomorrow morning) l Scan Chapters 4 & 5 in preparation for tomorrows class
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