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Data Comm Concepts

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Data Comm Concepts

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    1. Chapter3-Data Comm Concepts 1 Data Comm Concepts Concept outlined are not specific to any PC or modem Concepts on coming slide divided into three sections Computer to Modem Within modem Modem to phone service

    2. Basic Data Comm. Concepts

    3. Chapter3-Data Comm Concepts 3 Computer to Modem Data from PC source to PC destination must be transformed to machine usable format States represented as discrete voltage or levels of light Commonly used standards – ASCII, EBCDIC, UNICODE/ISO 10646

    4. Chapter3-Data Comm Concepts 4 Commonly Used Standards ASCII ASCII 7 represents 128 characters (27) ASCII 8 represents 256 characters (28) EBCDIC Represents 256 characters UNICODE/ISO 10646 16 bit code representing 65,536 characters (216) Backward compatible with ASCII

    5. Serial vs. Parallel

    6. Serial and Parallel Transmission

    7. Chapter3-Data Comm Concepts 7 Universal Service Bus (USB) USB threatens to replace RS-232 USB advantages High speed Multipoint serial communication USB v1.1 speed of 1.5 or 12 Mbps USB v2.0 speeds up to 480 Mbps

    8. USB Hub Implementation

    9. Chapter3-Data Comm Concepts 9 USB continued Designated to be “plug and play” USB considered low to medium speed serial solution to data transfer USB ports and cables include power to the device USB A connector connects to computer USB B connector connects to device

    10. Chapter3-Data Comm Concepts 10 Synchronous-Asynchronous Asynchronous transmission – transmission in which each information character is individually synchronized (usually by the use of start elements and stop elements). Beginning character is start bit. Value is 0. Ending character is stop bit. Value is 1. Also called mark bit. Asynch is also termed start-stop transmission.

    11. Chapter3-Data Comm Concepts 11 Synch-Asynch contd’ Universal Asynchronous Receiver/Transmitter (UART) uses Asynch transmission. Communication between serial port of PC and external modem is asynchronous.

    12. Chapter3-Data Comm Concepts 12 Synch-Asynch contd’ Synchronous transmission – Data transmission in which the time of occurrence of each signal representing a bit is related to a fixed time frame. Data is sent a block at a time. Typical block contains 128, 256, 512, or 1024 characters.

    13. Chapter3-Data Comm Concepts 13 Synch-Asynch contd’ Start and stop bit not used. Header proceeds data. Trailer follows data. Header contains Destination address Synchronization signal Control information

    14. Chapter3-Data Comm Concepts 14 Synch-Asynch contd’ Trailer contains parity checking information. Header contains 32 bits. Trailer contains 8 to 16 bits. Asynch requires 10 bits for each character (start, character (8 bits), stop). If 256 character are sent, using asynch, then 2560 bits are required.

    15. Chapter3-Data Comm Concepts 15 Synch-Asynch contd’ Using synchronous to transmit the 256 characters, fewer bits are needed. 256 characters * 8 bits = 2048 64 bits of overhead for header and trailer Total of 2112 bits is transmitted

    16. Chapter3-Data Comm Concepts 16 Synch-Asynch contd’ Synchronous is faster than Asynch Synchronous is more complex, and provides for sophisticated error detection and error checking. Synch is used as a dedicated circuit between two high speed computers. Asynch is used between PC (or DTE) and modem (DCE) on dial up connection through PSTN.

    17. Chapter3-Data Comm Concepts 17 Synch-Asynch contd’ Modem converts binary signal from PC to analog signal for local loop. Modem also converts asynch signal from the PC into a synch signal signal for the PSTN. When discussing whether signal between two DTE devices is asynch or synch, we are talking about signal leaving the DTE device.

    18. Asynchronous vs. Synchronous Transmission

    19. Chapter3-Data Comm Concepts 19 Synch-Asynch contd’ Signal over the PSTN between two modems (DCE devices) is synch signal when high speed modems are used. PC sends bits in serial asynch transmission to external modem. Modem contains memory and software to arrange incoming bits from PC into blocks of data for synch transmission over PSTN.

    20. Chapter3-Data Comm Concepts 20 Synch-Asynch contd’ 2400 baud rate modem has 0.416 msec (1/2400) to detect and interpret incoming data Synchronization (detectable event timing) is reestablished with transmission of each character in asynch transmission by start and stop bits. Synchronization for synch transmission is provided by clocking signal supplied by modem or carrier

    21. Chapter3-Data Comm Concepts 21 Synch-Asynch contd’ In idle time activity, synchronization is maintained in synch transmission by clocking signal In idle time activity, synchronization is dropped for asynchronous transmission because no characters are being transmitted

    22. Chapter3-Data Comm Concepts 22 Digital vs. Analog via Modem Switch in Central Office connects your phone to phone at desired location Circuit chosen represents best path Modem must be able to dial and answer phone calls to other modems Modem must change 1s and 0s from input side to analog wave with limited bandwidth for dial up network

    23. I-P-O Analysis: Modems and PSTN

    24. Chapter3-Data Comm Concepts 24 Modulation/Demodulation Modem’s job is to convert digital data into analog data and back to digital Analog voice wave must be changed to two states Means that a normal (neutral) wave must exist to start. This changes to 1 or 0.

    25. Modulation vs. Demodulation contd’

    26. Carrier Wave

    27. Chapter3-Data Comm Concepts 27 Physical characteristics Physical states that can be altered Amplitude Frequency Phase

    28. Chapter3-Data Comm Concepts 28 Amplitude Modulation Amplitude changes; phase and frequency remain constant Part of wave with increased height represent 0; lower wave amplitude represents 1 Proper name for one signaling event is baud Next figure illustrates amplitude modulation

    29. Amplitude Modulation contd’

    30. Chapter3-Data Comm Concepts 30 Frequency Modulation Frequency => how frequently the same spots on two subsequent waves pass a given point Waves with higher frequency take less time than waves with lower frequency

    31. Frequency Modulation contd’

    32. Chapter3-Data Comm Concepts 32 Phase Modulation Amplitude and frequency remain constant in coming diagram Phase modulation => shift or departure from the “normal” continuous patterns of the wave Coming figure shows phase shifts and wave goes in another direction

    33. Phase Modulation contd’

    34. Chapter3-Data Comm Concepts 34 Phase Modulation contd’ Phase shift is 180 degrees Phase shifts measured in degrees Stands to reason that we could shift the phase or carrier wave by varying degrees other than 180 degrees

    35. Chapter3-Data Comm Concepts 35 Phase Modulation contd’ By increasing the number of phase shifts, we increase the number of detectable events. 4 potential phase shifts (0, 90, 180, 270) means we can associate two bits with each potential detectable event

    36. Chapter3-Data Comm Concepts 36 Phase Modulation contd’ Next figure shows two potential detectable events (no phase shift), those two events are represented as 0 or 1 Introducing 4 potential phase shifts (0, 90, 180, 270), we are able to associate two bits with each potential detectable event Simpler to illustrate phase shifts with constellation points

    37. Chapter3-Data Comm Concepts 37 Phase Modulation contd’ A four quadrant representation of the 360 degrees of possible phase shift, individual points represent each different shifted wave Phase shift with four different phases is known as Quadrature Phase Shift Keying (QPSK)

    38. Phase shift & Potential Detectable Events

    39. Differential Quadrature Phase Shift Keying

    40. Chapter3-Data Comm Concepts 40 Quadrature Amplitude Modulation Limiting factor to increasing bits per baud in phase shift is quality of dial up lines. Phase is not the only wave characteristic that can be varied 16 different detectable events can be produced varying both phase and amplitude High speed modems vary both phase and amplitude

    41. Chapter3-Data Comm Concepts 41 Quadrature Amplitude Modulation contd’ Representative set of constellation points and associated quadbits and QAM modulation scheme follow Phase difference – represented in degrees around center Amplitude differences – represented by linear distance from center

    42. QAM Constellation Points and Quadbits

    43. Chapter3-Data Comm Concepts 43 Nyquist, and Shannon’s Law As number of constellation points increases, chances of modem making a mistake increase Interference in symbols can cause misinterpretations of data

    44. Chapter3-Data Comm Concepts 44 Simplex, Half-duplex, Full-Duplex Simplex => Data travels in one direction only, eg. point of sales terminals in which data is entered only. Half-duplex => One modem transmitting at a time while other modem is receiving Once two modems complete initial handshaking, one modem agrees to transmit while other agrees to receive Full-duplex => support simultaneous data signaling in both directions

    45. Half-Duplex vs. Full-Duplex Transmission

    46. Chapter3-Data Comm Concepts 46 Echo Cancellation Uses Digital Signal Processors (DSP); included in modems that offer echo cancellation DPSs are able to distinguish echoed transmission of local modem from intended transmission of remote modem DPS cancels echoed local transmission from total signal received

    47. Chapter3-Data Comm Concepts 47 Transmission Services Available Analog service Digital service – no modem required; uses a channel service unit/data service unit (CSU/DSU) Narrowband Digital Service – Digital service of less than 1.544 Mbps Broadband – T1 (1.544 Mbps) and T-3 (45 Mbps) leased lines

    48. Chapter3-Data Comm Concepts 48 Transmission Services Available contd’ Dial up Digital service – ISDN is a dial up digital service that can offer up to 144 Kbps Alternative Services – DSL services use frequency division multiplexing to analog encode a high speed data channel over a POTS local loop connection to central Office

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