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Computer Network

Computer Network. An interconnected collection of autonomous computers. Networks. LAN MAN WAN Internetwork. Classification by type. Broadcast network Point-to-Point network Client-Server . Local Area Network (LAN). Restricted in size

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Computer Network

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  1. Computer Network • An interconnected collection of autonomous computers.

  2. Networks • LAN • MAN • WAN • Internetwork

  3. Classification by type • Broadcast network • Point-to-Point network • Client-Server

  4. Local Area Network (LAN) • Restricted in size • Worst-case transmission time is bounded and known in advance. • Uses a single cable to which all machines are attached. • Low delay and very few errors. • IEEE 802.3 (Ethernet) ..bus-based broadcast network.

  5. Metropolitan Area Network (MAN) • A bigger version of LAN • Uses similar technology • IEEE 802.6

  6. Wide Area Network (WAN) • Spans a large geographical area • The hosts are connected by a subnet • The subnet carries messages from host or host • The hosts do the applications. • The subnet consists of transmission lines (circuits, channels, trunks) and switching elements (routers).

  7. Hosts - Subnet Relationship subnet router

  8. Classification by scale • Local Area Network (~ A few km) • Room, building, campus • Metropolitan Area Network (~10 km) • City • Wide Area Network (100 km - 1,000 km) • Country, Continent • Internet • planet

  9. Parts needed for data communication • Transmitter or Source of information • Receiver of transmitted information • Message • Transmission channel or carrier (data link)

  10. Communication Process Transmitter Person/Machine Message Ideas, Feelings, Facts, Sounds, Pictures Carrier Electric Cable, Sound Wave, Microwave, Lightwave Receiver Person/ Machine Receiver Person/Machine Message Feedback/ Confirmation Transmitter Person/ Machine Carrier

  11. Channel (Communication Link) • A path for electrical transmission between two or more stations or terminals to carry information from one location to another. • All channels have limitations on their information handling abilities, depending upon their electrical and physical characteristics.

  12. Simplex : A to B only Half Duplex: A to B and then B to A, but not simultaneously Full Duplex: A to B and from B to A simultaneously.

  13. Bandwidth • Bandwidth: A term used to describe the maximum speed at which a given device can transfer data. • In general the greater the bandwidth, the higher the possible speed of transmission.

  14. Broad Band Channel Voice Grade Channel Sub-voice Grade Channel (150 to 600 bps) Telegraph Channel (45 to 75 bps)

  15. Backbone A series of very high-speed phone lines, in the order of several hundred MBPS.

  16. Communications coding structure • Information is transmitted in sequence bit by bit over an information channel (timed bit train) State ON OFF 1 0 1 1 0

  17. Transmission Modes • Asynchronous • Synchronous

  18. Synchronous • Data are transmitted at regular intervals • Both the sender and receiver agree upon specific protocols and specified “time”, ergo, the term “synchronous.” • Less overhead for blocks of large data.

  19. Synchronous transmission • A synchronous system is a "clocked" or "fixed rate" system, meaning the line is sampled at regular intervals to receive and record information bits. Synchronous transmission permits more information to be passed over a circuit per unit time because no transmission time is required for the insertion of start-stop signals.

  20. Asynchronous • Data are transmitted at irregular intervals rather than in steady stream. • Examples are telephone conversation and data entry from KB. • Need a way of distinguishing where data start and where they end  start and stop bits • Governing protocol – RS-232

  21. Asynchronous transmission • Each character is individually synchronized. Bit 7 6 5 4 3 2 1 0 Stop Direction of Travel Start Start and stop bits are used to separate characters and to synchronize the receiving station with the transmitting station.

  22. Asynchronous transmission Bit 7 6 5 4 3 2 1 0 Stop Start bit – 1 bit (change of voltage from negative to positive) , 1.e. -12V to +12V (Seenext slide) Data bit – 7 bits + parity bit -12V for 1 and +12V for 0. Stop bit(s) – 1, 1.5, or 2 bits. (signified by negative voltage) • Typically 2 bits for 10cps (or 110 baud) • 1 bit for 30 cps (300 baud) or higher Direction of Travel Start

  23. RS-232 • Spec calls for 5 to 25V, but 12V is common • Use inverted logic (-12V for 1 and +12 for 0) • The transmit pin is held at -12 V (mark) at idle – nothing is being sent • To start a byte it jumps to +12 V (space or 0) for the start bit and remains at +12 V for the duration (period) of the start bit • Then comes the low-order bit of the data byte. • If the data stream is 11011100, the sequence of transmission is 00111011 (least significant to most significant) • If it's a 0-bit nothing changes and the line remains at +12 V for another bit-period • If it's a 1-bit, the voltage jumps from +12 to -12 V. • After that comes the next bit, and so forth • After the last data bit a parity bit may be sent and then a -12 V (mark) stop bit • Then the line remains at -12 V (idle) until the next start bit. • 0V is never used unless the unit is turned off.

  24. Modulation • Process of impressing a signal on a carrier wave. The result of this process is called a modulated carrier wave. • The reverse process, in which the signal is retrieved from the modulated carrier wave, is called demodulation. • The information signal impressed upon the carrier wave is called the baseband signal. Modem (Modulator-Demodulator)– a device that modulates and demodulates signal

  25. Amplitude Modulation (AM) The amplitude of the carrier wave is varied in accordance with the variations of the intelligence signal. A: Carrier Wave B: Digital Signal C: AM signal The modulated carrier C is simply a sequence of tone bursts with the tone representing one bits and absence of tone representing zero bits.

  26. Frequency Modulation (FM) • The frequency of the carrier wave is varied in accordance with the variations of the baseband or modulating signal. The amplitude of the carrier wave does not change - only its frequency changes. A: Carrier Wave B: Digital Signal C: FM signal

  27. Remote Communication DTE DCE DCE DTE where DTE (Data Terminal Equipment): e.g.: Computer, PC, workstation, Terminal DCE (Data Circuit Terminating Equipment) e.g.: modems

  28. Dialup Lines (typical) CPU FEP Front End Proc. modem modem PSTN Public Switched Telephone Network DTE modem modem modem DTE modem RS232 DTE

  29. Modem Modulator-Demodulator Digital data 1011101 1011101 Modem Analog wave

  30. More on bandwidth • Bandwidth – The amount of data a cable can carry • Bits per second for digital signals (bps) • Hertz (Hz) for analog signals

  31. Analog • Bandwidth = (Highest frequency – Lowest frequency) • For human voice, it is 2700 Hz (3000 – 300) • AM radio: 535 ~ 1,605 • FM radio: 88 ~ 108 MHz • VHF TV: 174 ~ 216 MHz • UHF TV: 470 ~ 890 MHz

  32. Digital • Digital phones <100 kbps • T1: 1.54 Mbps • Ethernet: 10 Mbps (10BaseTS) • Fast Ethernet: 100 Mbps (100BaseTX) • Gigabit Ethernet: 1Gbps (1000BaseTX) • 100 GPS Ethernet: 100Gbps (in lab) • Token Ring networks: 1, 4, 16 Mbps • Fast Token Ring networks: 100 Mbps • Fiber-optic (FDDI): 100 Mbps or higher • ATM: about 655 Mbps. Expect to be expanded to 1.488 Gbps in the future

  33. Twisted Pair • Two insulated copper wires, 1mm thick • Wires are twisted to reduce electrical interference from similar pairs close by. • Two parallel wires may constitute an antenna. • Can run several Km w/o amplification. • Can achieve several Mbps for several Km. • One of the oldest transportation medium and still popular • Inexpensive • UTP (Unshielded Twisted Pair) • STP (Shielded Twisted Pair)

  34. Twisted Pair • ANSI/EIA 568 governs twisted pair

  35. Baseband Versus Broadband Transmission Baseband Transmission Signal Transmitted Signal (Same) Source Transmission Medium Signal is injected directly into the transmission medium (wire, optical fiber) Inexpensive, so dominates wired LAN transmission technology BASE in standard names means baseband

  36. Broadband Transmission Modulated Signal Radio Channel Source Radio Tuner The radio tuner modulates the signal to a higher frequency. The transceiver then sends the signal in a radio channel. Expensive but needed for radio-based networks. Not used in Ethernet, but is used in wireless LANs (discussed in Chapter 5).

  37. Transmission Wire Categories

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