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Point-to-Point Communication Point-to-Point Communication

2 Terminal-Host Communication n Components n n Terminal Host (locus of processing) Transmission line 2 Terminal-Host Communication n Components n n Terminal Host (locus of processing) Transmission line (here, phone line and modem) Telephone line acts as a point-to-point link Host Terminal Phone Line Modem

Terminal Emulation n People Already Have PCs n Host operating systems only work with Terminal Emulation n People Already Have PCs n Host operating systems only work with terminals n Do not want to buy a terminal as well n PCs can emulate (act like) terminals n Only requires software (a communication program) n Turns an expensive PC into a cheap terminal 3

VT 100 Terminals n VT 100 Emulation Only Needs Software n n Communications program VT 100 Terminals n VT 100 Emulation Only Needs Software n n Communications program Terminal emulation software n Most Hosts Support VT 100 Terminals n Lowest Common Denominator n n Slow: maximum speed of 19 kbps, usually slower Uses inefficient asynchronous ASCII transmission, discussed later No graphics or even multiple fonts: plain text only No color 4

Terminal Emulation Software n File Transfer n n Terminal emulation program and host file Terminal Emulation Software n File Transfer n n Terminal emulation program and host file transfer program must support the same file transfer protocol standard n n n Transfer whole files with error correction Upload: from PC to host Download: from host to PC Kermit XMODEM, YMODEM IBM 3270 Terminals 5

Analog and Digital Transmission Analog Signal Digital Signal (1101) Modem Modulation Analog Signal Digital Analog and Digital Transmission Analog Signal Digital Signal (1101) Modem Modulation Analog Signal Digital Signal (1101) Modem Demodulation 6

7 Digital and Binary n Digital Transmission n Can have multiple voltage levels, say 7 Digital and Binary n Digital Transmission n Can have multiple voltage levels, say 4 n Change to one at start of each bit cycle n If 4, changes can represent 2 bits each: 00, 01, 10, 11 Voltage Level n 00 11 10 01 Time

8 Baud Rate and Bit Rate n Baud Rate n n n Number of 8 Baud Rate and Bit Rate n Baud Rate n n n Number of times line changes per second Let baud rate be 4 (4 changes per second) Let bits per line change be 2 Bit rate = 8 bits per second Bit rate = x 2 Baud rate in this example 11 10 00 01 One Second

Wave Characteristics n n n Amplitude (power) Frequency (cycles per second, Hertz) Wavelength (meters) Wave Characteristics n n n Amplitude (power) Frequency (cycles per second, Hertz) Wavelength (meters) Amplitude (power) Frequency (Hz) One Second 9

Wave Characteristics n Phase n n n 0 and rising Highest 0 and falling Wave Characteristics n Phase n n n 0 and rising Highest 0 and falling Lowest Fully cycle is 360 degrees Phase is degrees different from reference wave Human ears cannot hear. Equipment can Reference Wave 0 o 180 degrees out of phase 0 o 90 o 180 o 270 o 180 o 10

Wave Characteristics n Amplitude n Frequency and Wavelength n n As frequency rises, wavelength Wave Characteristics n Amplitude n Frequency and Wavelength n n As frequency rises, wavelength falls (Shorter guitar strings produce higher notes) n n Not independent Their product is constant--the speed of light, sound, etc. Phase 11

12 Frequency Modulation Vary the frequency (wavelength) to represent 1 and 0 Low Frequency 12 Frequency Modulation Vary the frequency (wavelength) to represent 1 and 0 Low Frequency (0) High Frequency (1) Frequency Modulation (1011) Wavelength 0 1 1 1

Amplitude Modulation Low Amplitude (0) High Amplitude (1) Amplitude Modulation (1011) Amplitude (low) Amplitude Amplitude Modulation Low Amplitude (0) High Amplitude (1) Amplitude Modulation (1011) Amplitude (low) Amplitude (high) 13

Phase Modulation In Phase (0) 180 degrees out of phase (1) Frequency Modulation (1011) Phase Modulation In Phase (0) 180 degrees out of phase (1) Frequency Modulation (1011) 14

15 Complex Modulation Vary both amplitude and phase Several values (not just two) on 15 Complex Modulation Vary both amplitude and phase Several values (not just two) on each dimension In Phase High Amplitude 90 Degrees Out of Phase, High Amplitude Low Amplitude 180 Degrees Out of Phase

Modem Standards n Modems at Two Ends Must Communicate n Must follow same standards Modem Standards n Modems at Two Ends Must Communicate n Must follow same standards n Most modem standards set by ITU-T n Multiple category of standards: n Modem speed (modulation) n Error correction and compression n Facsimile n Etc. n When buying a modem, must check for standard(s) followed in each category 16

Modem Speed Standards n Set by the ITU-T n n Govern how modulation is Modem Speed Standards n Set by the ITU-T n n Govern how modulation is done Standards for speed governs modulation for data transmission n V. 92 56. 6 kbps plus quick connect, modem on hold, PCM upstream n V. 90 V. 34 V. 32 bis 56. 6 kbps 28. 8 kbps/33. 6 kbps 14. 4 kbps n n 17

Modem Speed Standards n Most data modems are also fax modems n n V. Modem Speed Standards n Most data modems are also fax modems n n V. 14 V. 29 14. 4 kbps 9, 600 bps 18

Error Correction and Compression n ITU-T Standards n n n V. 42 bis V. Error Correction and Compression n ITU-T Standards n n n V. 42 bis V. 44 Error detection and correction Data compression (up to 4: 1) Data compression (20 to 120% more than V. 42 bis) Independent of speed standards (but V. 44 only with V. 92) Microcom Standards n n Microcom Network Protocol (MNP) Both error correction and compression Several levels Independent of speed standards 19

Modem Intelligence n Computer Can Send Commands to Modem n n n Hayes Developed Modem Intelligence n Computer Can Send Commands to Modem n n n Hayes Developed the first Command Set n n Dial a number, including how long to wait, etc. Called intelligent modems Most modems follow the same command set We call them “Hayes compatible” Commands start with “AT” Other Standards for Fax Modems n Class 1 and Class 2: extensions to Hayes 20

Telephone Bandwidth is Limited n Telephone Transmission n Cuts off sounds below 300 Hertz Telephone Bandwidth is Limited n Telephone Transmission n Cuts off sounds below 300 Hertz Cuts off sounds above about 3, 400 Hz Bandwidth is the difference between the highest and lowest frequencies (3400 -300): about 3, 100 Hz Sound Loudness Bandwidth 3, 100 Hz 0 300 3400 Frequency (Hz) 20, 000 21

Telephone Bandwidth is Limited n Speed is Limited n Maximum speed is related to Telephone Bandwidth is Limited n Speed is Limited n Maximum speed is related to bandwidth (Shannon’s Law) n Maximum speed for phone lines for transmission is a little over 30 kbps n So modems can’t get much faster 22

23 Another Look at Compression n With 4: 1 Compression, a V. 34 Modem 23 Another Look at Compression n With 4: 1 Compression, a V. 34 Modem Can Receive Data at 115. 2 kbps from the PC n However the ~30 kbps limit of the phone system is not exceeded. Still transmit at 33. 6 kbps. ~35 kbps Maximum Compression in Modem 115. 2 kbps 33. 6 kbps

24 56 kbps Analog Modems n From home, you transmit n Analog-to-Digital Converter (ADC) 24 56 kbps Analog Modems n From home, you transmit n Analog-to-Digital Converter (ADC) n n Filters your signal to a bandwidth of ~3. 1 k. Hz This limits you to 33. 6 kbps PC V. 34 modem 33. 6 kbps ADC Telephone Network

25 56 kbps Analog Modems n But ISP Can Connect Digitally n n PC 25 56 kbps Analog Modems n But ISP Can Connect Digitally n n PC Signal travels through phone system at 56 kbps At user end, digital-to-analog converter (DAC) Sends signal to analog modem at wide bandwidth Modem can receive at 56 kbps modem DAC Telephone Network 56 kbps Digital Link ISP

56 kbps Modems n What they can do n n n Problems n n 56 kbps Modems n What they can do n n n Problems n n n Send at 33. 6 kbps (V. 92 with PCM upstream can go up to 48 kbps) Receive at 56 kbps (V 92 with V. 44 compression can go up to 120 kbps) past: competing standards from Rockwell, U. S. Robotics (V. 90 ended them) present: ISPs must support V. 92 (all support V. 90) Users and ISPs n n Users V. 90 analog modem or V. 92 ISPs V. 90 digital modem or V. 92 26

56 Kbps Modems n Telephone company n No changes needed, although. . . n 56 Kbps Modems n Telephone company n No changes needed, although. . . n Many not have an internal ADC conversion between ISP and customer (some do) n May not have long transmission line from last switch to the customer premises (local loop) n Not all phone lines to customer premises will support 56 kbps modems n Even when they do, speeds may only be 40 -50 kbps 27

28 Half-Duplex Transmission n Sender and receiver must take turns sending n n Like 28 Half-Duplex Transmission n Sender and receiver must take turns sending n n Like an old one-lane road No interruption for error handling or flow control A B Time 1 Only one side May communicate A does A B Time 2 Only one side May communicate B does

29 Full-Duplex Transmission n Both Sides May Transmit Simultaneously n n Needed for error 29 Full-Duplex Transmission n Both Sides May Transmit Simultaneously n n Needed for error correction, flow control Now almost universal in modem communication A B Time 1 Both sides may communicate Both do A B Time 2 Both sides may communicate A does

Asynchronous Transmission n ASCII Character Set n n 7 -bit is the standard 8 Asynchronous Transmission n ASCII Character Set n n 7 -bit is the standard 8 -bit extended ASCII is popular Bits transmitted backward Parity for Error Detection n Only for 7 -bit ASCII n Start/Stop Bits for Framing n Each frame is exactly 10 bits long 30

Asynchronous Transmission n ASCII Character Set n Created for sending printed American text n Asynchronous Transmission n ASCII Character Set n Created for sending printed American text n Each character is a 7 -bit code (e. g. , 1010101) n This allows 2^7 or 128 possible characters n Printing characters: A, a, !, <, %, etc. n Control codes: XOFF tells other side to pause 31

Asynchronous Transmission n 8 -bit ASCII n Used in PCs: 8 bits per character Asynchronous Transmission n 8 -bit ASCII n Used in PCs: 8 bits per character (1010) n Used for word processing format codes n Used in graphics that stores data in bytes 32

PC Serial Port n Bit Transmission of ASCII Characters n Transmits last bit first PC Serial Port n Bit Transmission of ASCII Characters n Transmits last bit first n If you wish to send 1111000, n The serial port transmits 0001111 33

Parity n For 7 -bit ASCII Only (No Parity = 8 -bit ASCII) n Parity n For 7 -bit ASCII Only (No Parity = 8 -bit ASCII) n Transmit an 8 th bit per character n Even parity: sum of data and parity bits is even n n To send 1110000 (odd), send 00001111 To send 1111000 (even), send 00011110 n Odd parity: sum is odd n If error is detected, the character is simply discarded. No way to ask for retransmission 34

Start and Stop Bits n When the Data Line is at Rest n n Start and Stop Bits n When the Data Line is at Rest n n n It is kept in the “ 1” state So “ 11110000” would look like 111111100001111 “ 00001111” would also look like 11111100001111 n How can you tell where a character begins? n Solution n Add a start bit (always 0) to change the line state n End with a stop bit (always 1) to guarantee at least a one-bit rest (1) against which to detect the next start bit (0) 35

36 The Final Asynchronous Frame n Always 10 bits n n Start, 7 data 36 The Final Asynchronous Frame n Always 10 bits n n Start, 7 data bits, parity, stop, or Start, 8 data bits, stop Start Parity 0 1 1 1 0 0 0 1 1 1 7 -bit ASCII Character Start 0 1 1 1 0 0 0 1 1 1 8 -bit ASCII Character Stop

Flow Control n Ask the Other Device to Pause (or Slow Down) n ASCII Flow Control n Ask the Other Device to Pause (or Slow Down) n ASCII n n XOFF tells other side to pause n n In asynch, usually done by sending ASCII control codes XON tells the other device to resume Serial Port n Signals on the pins control when PC, modem can transmit 37

Signal and Propagation 38 A signal is a disturbance in the media that propagates Signal and Propagation 38 A signal is a disturbance in the media that propagates (travels) down the transmission medium to the receiver If propagation effects are too large, the receiver will not be able to read the received signal 3 -38

39 Binary-Encoded Data n Computers store and process data in binary representations n n 39 Binary-Encoded Data n Computers store and process data in binary representations n n n Binary means “two” There are only ones and zeros Called bits 1101010110001110101100111 3 -39

40 Binary-Encoded Data n Non-Binary Data Must Be Encoded into Binary n n n 40 Binary-Encoded Data n Non-Binary Data Must Be Encoded into Binary n n n n Text Integers (whole numbers) Decimal numbers Alternatives (North, South, East, or West, etc. ) Graphics Human voice etc. Hello 11011001… 3 -40

Layering Perspective n Where is binary data encoding done? n n 41 It is Layering Perspective n Where is binary data encoding done? n n 41 It is done at the application layer, not at the physical layer. Where is signaling done n It is done at the physical layer 3 -41

42 Signaling 42 Signaling

On/Off Signaling On/off signaling is used in optical fiber The light is turned on On/Off Signaling On/off signaling is used in optical fiber The light is turned on during a clock cycle for a 1 The light is turned off during a clock cycle for a 0 There are two signaling states—on and off This is called binary signaling This is a simple type of signaling 43 3 -43

Binary Voltage Signaling in 232 Serial Ports 44 1 The high state (0) is Binary Voltage Signaling in 232 Serial Ports 44 1 The high state (0) is anything from +3 to +15 volts The low state (1) is anything from -3 to -15 volts 3 -44

45 Relative Immunity to Errors in Binary Signaling Binary signaling gives some immunity to 45 Relative Immunity to Errors in Binary Signaling Binary signaling gives some immunity to errors. This is one of its major attractions. 3 -45

46 UTP Propagation Unshielded Twisted Pair wiring 46 UTP Propagation Unshielded Twisted Pair wiring

Unshielded Twisted Pair (UTP) Wiring 47 n UTP Characteristics n Inexpensive and to purchase Unshielded Twisted Pair (UTP) Wiring 47 n UTP Characteristics n Inexpensive and to purchase and install n Dominates media for access links between computers and the nearest switch 3 -47

Unshielded Twisted Pair (UTP) Wiring 48 n Standards n The TIA/EIA-568 standard governs UTP Unshielded Twisted Pair (UTP) Wiring 48 n Standards n The TIA/EIA-568 standard governs UTP wiring in the United States n In Europe, the comparable standard is ISO/IEC 11801 3 -48

4 -Pair UTP Cord with RJ 45 Connector 49 3. 8 -pin RJ-45 Connector 4 -Pair UTP Cord with RJ 45 Connector 49 3. 8 -pin RJ-45 Connector 1. UTP cord Industry standard pen 2. 8 Wires organized as 4 twisted pairs UTP cord 3 -49

Unshielded Twisted Pair (UTP) Wiring n 50 Cord Organization n n A length of Unshielded Twisted Pair (UTP) Wiring n 50 Cord Organization n n A length of UTP wiring is a cord Each cord has eight copper wires n Each wire is covered with dielectric (nonconducting) insulation The wires are organized as four pairs n Each pair’s two wires are twisted around each other several times per inch There is an outer plastic jacket that encloses the four pairs 3 -50

Unshielded Twisted Pair (UTP) Wiring n RJ-45 Jack 51 Connector n n RJ-45 connector Unshielded Twisted Pair (UTP) Wiring n RJ-45 Jack 51 Connector n n RJ-45 connector is the standard connector Plugs into an RJ-45 jack in a NIC, switch, or wall jack RJ-45 Jack 8 -pin RJ-45 connectors 3 -51

52 Attenuation and Noise Power 1. 4. Signal Noise Spike 5. Error 2. Signalto-Noise 52 Attenuation and Noise Power 1. 4. Signal Noise Spike 5. Error 2. Signalto-Noise Ratio (SNR) 1. 2. 3. 4. 5. 6. 3. Noise Floor (Average Noise level) 2. Noise Distance The signal attenuates (falls in power) as it propagates There is noise (random energy) in the wire that adds to the signal The average noise level is called the noise floor Noise is random. Occasionally, there will be large noise spikes Noise spikes as large as the signal cause errors You want to keep the signal-to-noise ratio high 3 -52

Limiting UTP Cord Length n 53 Limit UTP cord length to 100 meters n Limiting UTP Cord Length n 53 Limit UTP cord length to 100 meters n This keeps the signal-to-noise ration (SNR) high n This makes attenuation and noise problems negligible n Note that limiting cord lengths limits BOTH noise and attenuation problems 100 Meters Maximum Cord Length 3 -53

54 UTP Wiring n Electromagnetic Interference (EMI) n Electromagnetic interference is electromagnetic energy from 54 UTP Wiring n Electromagnetic Interference (EMI) n Electromagnetic interference is electromagnetic energy from outside sources that adds to the signal n n From fluorescent lights, electrical motors, microwave ovens, etc. The problem is that UTP cords are like long radio antennas n They pick up EMI energy nicely n When they carry signals, they also send EMI energy out from themselves 3 -54

Electromagnetic Interference (EMI) 55 and Twisting Electromagnetic Interference (EMI) UTP is twisted specifically to Electromagnetic Interference (EMI) 55 and Twisting Electromagnetic Interference (EMI) UTP is twisted specifically to reduce EMI Twisted Wire Interference on the Two Halves of a Twist Cancels Out 3 -55

56 Crosstalk Interference and Terminal Crosstalk Interference Untwisted at Ends Signal Crosstalk Interference Terminal 56 Crosstalk Interference and Terminal Crosstalk Interference Untwisted at Ends Signal Crosstalk Interference Terminal crosstalk interference Terminal Crosstalk normally is the biggest EMI problem for UTP Interference 3 -56

Interference Hierarchy n 57 EMI is any interference n Signals in adjacent pairs interfere Interference Hierarchy n 57 EMI is any interference n Signals in adjacent pairs interfere with one another (crosstalk interference). This is a specific type of EMI n Crosstalk interference is worst at the ends, where the wires are untwisted. This is terminal crosstalk interference—a specific type of crosstalk EMI Crosstalk Interference Terminal Crosstalk Interference 3 -57

Terminal Crosstalk Interference n 58 Terminal crosstalk interference dominates interference in UTP n Terminal Terminal Crosstalk Interference n 58 Terminal crosstalk interference dominates interference in UTP n Terminal crosstalk interference is limited to an acceptable level by not untwisting wires more than a half inch (1. 25 cm) at each end of the cord to fit into the RJ-45 connector n This reduces terminal crosstalk interference to a negligible level. 1. 25 cm or 0. 5 inches 3 -58

Shielded Twisted Pair Wiring (STP) 59 n We have been talking about unshielded twisted Shielded Twisted Pair Wiring (STP) 59 n We have been talking about unshielded twisted pair wiring. n Is there a shielded twisted pair wiring? n n Yes. It has a metal mesh shield around each pair to reduce cross-talk interference It also has a metal mesh shield around the four pairs to reduce external EMI It is no longer used extensively because UTP, which is much less expensive, was found to be good enough for normal environments However, we will see that Cat 7 wiring uses STP 3 -59

2 60 UTP Limitations n Limit cords to 100 meters n n Do not 2 60 UTP Limitations n Limit cords to 100 meters n n Do not untwist wires more than 1. 25 cm (a half inch) when placing them in RJ-45 connectors n n Limits BOTH noise AND attenuation problems to an acceptable level Limits terminal crosstalk interference to an acceptable level Neither completely eliminates the problems but they usually reduce the problems to negligible levels 3 -60

3 -18: Serial Versus Parallel Transmission 61 3 -61 3 -18: Serial Versus Parallel Transmission 61 3 -61

Optical Fiber Transmission Light through Glass Spans Longer Distances than UTP 62 Optical Fiber Transmission Light through Glass Spans Longer Distances than UTP 62

Optical Fiber Transceiver and Strand 63 An optical fiber strand has a thin glass Optical Fiber Transceiver and Strand 63 An optical fiber strand has a thin glass core This core is 8. 3, 50, or 62. 5 microns in diameter This glass core is surrounded by a tubular glass cladding The outer diameter of the cladding is 125 microns, regardless of the core’s diameter The transceiver injects laser light into the core 3 -63

Optical Fiber Transceiver and Strand 64 When a light wave ray hits the core/cladding Optical Fiber Transceiver and Strand 64 When a light wave ray hits the core/cladding boundary, there is perfect internal reflection. There is no signal loss 3 -64

Roles of UTP and Optical Fiber in LANs 65 3 -65 Roles of UTP and Optical Fiber in LANs 65 3 -65

Two-Strand Full-Duplex Optical Fiber Cord 66 with SC and ST Connectors Cord Two Strands Two-Strand Full-Duplex Optical Fiber Cord 66 with SC and ST Connectors Cord Two Strands A fiber cord has two-fiber strands for full-duplex (two-way) transmission SC Connectors ST Connectors 3 -66

Full-Duplex Optical Fiber Cord with SC and ST 67 Connectors SC Connector (push and Full-Duplex Optical Fiber Cord with SC and ST 67 Connectors SC Connector (push and click) ST Connector (bayonet connectors: push and click) In contrast to UTP, which always uses RJ-45 connectors, there are several optical fiber connector types SC and ST are the most popular 3 -67

Optical Fiber Strand 68 In optical fiber transmission, light is expressed in nanometers. The Optical Fiber Strand 68 In optical fiber transmission, light is expressed in nanometers. The transceiver transmits at 850 nm, 1, 310 nm, or 1, 550 nm Shorter-wavelength (850 nm) transceivers are less expensive Longer-wavelength (1, 310 or 1, 550 nm) light travels farther for a given speed For LAN fiber, 850 nm provides sufficient distance and dominates 3 -68

Multimode Fiber and Single-Mode Fiber 69 Multimode fiber has a thick core (50 or Multimode Fiber and Single-Mode Fiber 69 Multimode fiber has a thick core (50 or 62. 5 microns in diameter) Light can only enter the core at certain angles, called modes Modes traveling straight through arrive faster than modes that bounce against the cladding several times 3 -69

70 Radio Propagation 70 Radio Propagation

Radio Propagation 71 Radio signals also propagate as waves. As noted earlier, radio waves Radio Propagation 71 Radio signals also propagate as waves. As noted earlier, radio waves are measured in hertz (Hz), which is a measure of frequency. Radio usually operates in the MHz and GHz range. 3 -71

Omnidirectional and Dish Antennas 72 3 -72 Omnidirectional and Dish Antennas 72 3 -72

Wireless Propagation Problems 73 UTP and optical fiber propagation are fairly predictable. However, radio Wireless Propagation Problems 73 UTP and optical fiber propagation are fairly predictable. However, radio suffers from many propagation effects. This makes radio transmission difficult to manage. 3 -73

74 Topology Network topology is the physical arrangement of a network’s computers, switches, routers, 74 Topology Network topology is the physical arrangement of a network’s computers, switches, routers, and transmission lines It is a physical layer concept: l l Point-to-point Star Bus Ring