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Basics of Telecommunications

Basics of Telecommunications. Information is generated by a source (e.g., a telephone handset, computer terminal or fax machine) The information is converted into an electronic signal . This is done by coding the information into machine readable form . The output is known as a signal .

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Basics of Telecommunications

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  1. Basics of Telecommunications • Information is generated by a source (e.g., a telephone handset, computer terminal or fax machine) • The information is converted into an electronic signal. This is done by coding the information into machine readable form. The output is known as a signal. • The signal is applied to a medium. The medium carries this signal via electromagnetic (or optical) waves and delivers it to the receiver. • The receiver accepts the signal and either exactly or approximately recovers the original information.

  2. Overview of Communication Communications is the transmission of a signal by way of a medium from a sender to a receiver. (TP,Coax,Fiber, Wireless)

  3. Transmission Types • Digital • Analog (Telephone Network) DCE RS 232 DSU CSU DTE DCE (Digital Service Unit - Channel Service Unit) RS 232 DTE Modem Common Carrier Digital Circuit POTS Telephone Line DTE DCE DTE Modem DSU CSU RS 232 RS 232 DCE

  4. Terminology Sender Channel Receiver Encoder Modulate Decoder De- Modulate • Channel: • Type: Directionality, Speed, Medium • Capacity: Bandwidth, Mode Coding Scheme: e.g.:ASCII, EBCDIC • Modulation Techniques • Simple: Amplitude, Frequency, Phase • Complex: Quadrature (QAM), Trellis with QAM • Comm. Device: • Peripheral • Computer • Telephone

  5. Signals

  6. Analog and Digital Signals • Analog • Continuous fluctuations over time between high and low voltage • Digital • Discrete states (voltages, levels …) • Binary if limited to two

  7. Analog and Digital Signals • Analog Signal (Sine Wave) • Digital Signals (Binary) Time

  8. Analog Signals • Examples of analog signals are human voice and broadcast radio and television. • Intensity varies in smooth or continuous fashion over time. • It is also known as continuous signal. • It is represented as a sinusoidal (or sine) wave.

  9. Analog Signal Features • Frequency • The number of times per second a continuous wave completes one cycle • Amplitude • The difference of the extremes from the midpoint in a cycle • Phase • The angular position of a wave

  10. Digital signals • Digital -- Intensity maintains a constant level for a period of time and then changes to another constant level. • The intensity is discrete i.e., the intensity takes only specific values or levels & can have many levels. • In most communication systems, this value is limited to two; 0 represented by one level and 1 represented by another level. • This is known as BINARY, and the basic unit to represent digital information is a BINARY DIGIT (BIT). • Examples of digital (binary) signals are computer produced data (e.g., keyboard produced).

  11. Basic Telecommunication and Computer Storage Units • Telecommunication units (measured in bits) • kilobit - 1,000 bits • megabit - 1,000,000 bits • gigabit - 1,000,000,000 bits • Computer storage units (bytes - 8 bits) • kilobyte 210 = 1,024 bytes KB • megabtye 220 = 1,048,576 bytes MB • gigabyte 230 = 1,073,741,824 bytes GB

  12. Spectrum and Bandwidth • Spectrum - the range of frequencies a signal contains • Bandwidth - the width of the spectrum bandwidth = F(upper) - F(lower) • Telephone system uses 300 - 3400 Hz • spectrum = 300 - 3400 • bandwidth = 3100 Hz ( standardized at4 kHz ) • Capacity of analog transmission lines is measured by its bandwidth; the greater the bandwidth, the greater the capacity.

  13. Business Information • Data • Voice • Image • Video Increasing Requirements (Bandwidth, BitRate)

  14. Business Information: Data • Supports all applications that require numeric and text information, such as accounting. • Text data: Each letter of the alphabet is represented by one byte (character). The value of the byte depends on the coding scheme used (e.g., ASCII). It is estimated that a page of text contains 10,000 bits of data. Compression can reduce actual number of bits transmitted to about 40% of this. • Numeric data: Decimal data is transformed into binary form, and transmitted.

  15. Number of Bits 1 2 3 4 7 8 10 16 Number of Outcomes 2 4 8 16 128 256 1,024 65,536 Bits and Outcomes

  16. Encoding Text and Numbers ASCIIwith EvenParity( bit 8) Character bit8 bit 1 A 01000001 B 01000010 C 11000011 D 01000100 E 11000101 1 01110001 2 01110010 3 11110011

  17. Transmitting Data • How long to transmit a page of text? • a letter is represented as 8 bits - a byte • characters are typed in 61/2 x 9 inch space • 10 characters per inch (65 characters) - 27 lines per page • 8 * 27 * 65 = 14,040 bits • given a line speed of 2400 bps • 14,040 bits / 2400bps or 5.9 secs to transmit

  18. Business Information: Voice • Supports applications based on sound (telephone, voice mail, radio, teleconferencing) • The quality of sound is determined by bandwidth used. Voice on telephone has a bandwidth of about 3.1kHz, however: • Telephone transmission facilities use 4 KHz (International ( ITU ) Standard ) • Telecommunication support in organization is provided by PBX or Centrex • Voice can be analog or digital.

  19. Encoding Voice • Analog • sound is transduced to electricity for transmission • Ordinary telephone (standard, electronic) • Digital • analog signal is sampled • each sample represented as alevel (number) • (number) represented as a groupof bits

  20. Encoding Voice – PCM( Pulse Code Modulation ) • Digital Telephone Voice Coding (ITU Standard) • sample at twice the maximum frequency • maximum frequency for voice on the telephone - 4,000 Hz • 2 x 4,000 Hz (cycles/sec)  8,000 samples/sec • each sample is represented by 8 bits for Standardized Digital Voice 256 levels used • 8,000 samples/sec x 8 bits/sample = 64,000 bits/sec • Digital Voice is 64,000 bps or 64 Kb/s

  21. Business Information: Image • Bit maps: an image is divided into rectangular grids. Each grid is called a picture element (pixels, or pels). Each pixel is then coded in bits. The more bits are used to represent a pixel, the better the quality. The quality is also enhanced by using more pixels per sq. in. Super VGA resolution is defined as 1024 x 768 pixels, 8 bits/pixel per image. • Instructions approach: basically a computer language is used to give instructions on how to draw figures.

  22. Business Information: Video • The broadcast TV standard format in North America (determined by National TV Standards Committee) is 640 pixels x 525 lines in each frame and a refresher rate of 30 frames per second. • The “rest” of the world uses 640 x 625 frame at a refresher rate of 25 frames/sec. • HDTV’s requirement is even higher (1280 x 720 x 60) • Both TV standards listed above are known as “full motion video”. The network bandwidth requirement is extremely high without compression

  23. Business Information: Video • To reduce the bandwidth requirement, techniques are used: • compression: for example, blanks are not sent • reduced resolution • reduced refresh rate: instead of 30 f/sec use movie level (24 f/sec). • motion compensation: instead of sending the whole image, send instructions on how the image moves – and/or send the difference between the old and the new image.

  24. Digital InterfacesandDigital Interface Standards • The point at which one device connects to another • Standards define what signals are sent, and how (functional, procedural) • Standards also define physical connector to be used (mechanical)

  25. EIA / RS Standards (EIA-232 & RS-232) • Standardized approach to connecting devices • EIA / RS Standards cover: • mechanical • electrical • functional • procedural

  26. Mechanical SpecificationsRS 232 C • 25-pin connector with a specific arrangement of leads • DTE devices usually have male DB25 connectors while DCE devices have female • In practice, fewer than 25 wires are generally used by devices in applications

  27. Functional Specifications • Specifies the role of the individual circuits, e.g.: Data circuits in both directions Timing signals for synchronous transmission

  28. Procedural Specifications • Protocals are specified (how data is exchanged) • Simple example: exchange of data on private line • sequence of attachment between computer and modem • sequence of transmitting data between devices • method of cooperation for the exchange of data between devices

  29. RS-232 DB-25 Pinouts

  30. Transmission Basics • There are two types of information source (analog and digital signals) and two forms of transmission facility (analog and digital) • The transmission facility (or line) is represented by a neutral wave called a carrier wave. • For analog transmission, the carrier wave is analog; for a digital line, it is digital.

  31. Digital Signal Transmission DSU: data service unit DSU digital line modulated data analog line modem

  32. Digital Signal - Analog Transmission • Amplitude shift keying (ASK) • Converts digital data to analog signals using a single frequency carrier signal • High-amplitude wave denotes a binary 1 • Low-amplitude wave denotes a binary 0 • Frequency shift keying (FSK) • Uses a constant amplitude carrier signal and two frequencies to distinguish between 1 and 0 • Phase shift keying (PSK) • Uses a phase shift at transition points in the carrier frequency to represent 1 or 0

  33. Digital Signal - Analog Transmission • Combinations • QAM (Quadrature Amplitude Modulation) combines PSK and ASK (different amplitudes and different phases) to achieve high data rates. • By using combinations (or other than binary), a data rate higher than the Symbol Rate (Baud Rate) can be achieved. • Bits per second (bps): number of bits that can be transferred in one second • Baud rate: number of symbols transmitted by the modem • A symbol is a level, a frequency, a phase – or a combination

  34. QAM Illustration 90 135 45 amplitude 1 180 0 amplitude 2 225 315 270

  35. Modem Specifications • Modem specifications cover: • Transmission rates • Error-correction techniques • MNP (Microcom Network Protocol) 2, 3, 4, 5 • LAPM (Link Access Procedure Method) (aka V.42) • Internal (built-in) compression methods • MNP 5 (also does 2:1 compression) • V.42 bis (4:1 compression)

  36. Examples of Modems Speed (bps) Standard Modulation Max Baud Rate (Symbol Rate) 1200 Bell 212/A Phase (4) 600 2400 V.22bis QAM (16) 600 4800 V.32 Trellis/QAM 2400 9600 V.32 Trellis/QAM 2400 14.4Kbs V.32bis Trellis/QAM 2400 28.8Kbs V.34 (V.FC) Trellis/QAM 3429 33.6Kbs 56Kbs Flex56,X56 (V.90)

  37. Example: V.32 bis Modems • allows transport of asynchronous or synchronous data up to 14400 bps • the modulation rate is 2400 baud (Symbol) • 6 Bits per Symbol (Baud) (6 x 2400 = 14,400 ) • uses the Trellis coding with QAM • uses groupings of seven bits • only six of these bits contain actual user information, the remaining bit is the “convolutional code”, calculated from the user information bits • 7 x 2400 = 16,800 bps = transmission rate • 6 x 2400 = 14,400 bps = information rate

  38. 56kbps Modems • asymmetrical; can download at 56kbps but upload at 33.6kbps only • requires digital T-1 or equivalent connection at central site or ISP • originally two incompatible systems • U.S. Robotics (56K x2) • Rockwell (56K flex) • V.90 makes them compatible

  39. Cable Modems • To deliver data services, one TV channel dedicated to downstream traffic and one TV Channel used for upstream • Features • 1 - 5 Mbps is a realistic access speed • constant connection (no dial-up) • Cable Providers, essentially, have to build up an intranet: backbone connectivity, routers, servers, network management tools, etc.

  40. Digital Signal - Digital Transmission • The parameters of the transmission line (e.g., voltage levels representing 1 and 0) are different from those of the source signal. • Conversion performed by devices known as Channel Service Unit/Data service Unit (CSU/DSU) • Common methods include “NRZ” - Non-Return-to-Zero means that a one level (+) represents a 1 and the opposite level (-) represents a 0; the level never returns to zero ( NORMALLY BINARY).

  41. Binary TransmissionDigital Signal –Digital Transmission “1” “1” +5v. “0” “0” +0v. Transmission Terminal (DSU) “1” “1” +15v. -15v. “0” “0”

  42. Digital Signal - Digital Transmission • Receiver and transmitter must be synchronized otherwise a long stream of 1s or zeros may not be distinguished correctly at the receiving end. • Transmission coding methods such as bi-phase coding can address this problem.Examples are Manchester and Differential Manchester coding schemes

  43. DIGITAL ENCODING SCHEMESText Figure 8.7 Binary Data 0 0 1 1 0 1 NRZ-L NRZ-I Zero level Manchester Differential Manchester

  44. Analog Signal - Analog Transmission • Required because carrier wave has higher frequency for effective transmission. (e.g., radio broadcast and voice over phone) • Performed by transducers • The techniques used • Amplitude modulation - The amplitude of a carrier wave (of fixed frequency) varies with the pattern (amplitude) of the information(modulating signal). E.g., voice to telephone; AM radio. • Frequency/phase modulation - The frequency/phase of the carrier wave (of fixed amplitude) varies with the pattern of the modulating signal. E.g., FM radio.

  45. Communications Speed: Baseband vs. Broadband One Channel Baseband -- One data signal per line (channel) Bandwidth Low Speed Data ( < 19.2 Kbps) Medium Speed Data ( < 120 Kps) High Speed Data (to 155 Mps) Multiple Voice Channels Video Channel Video Channel Broadband -- Multiple data-carrying channels

  46. Synchronizing Transmission • Sender transmits a signal for a given period of time; bits are “clocked” out with specific timing • Sender and receiver’s clocks must be “synchronized” or “locked” together • If clocks drift apart, then the receiver will not detect bits correctly. • Synchronization depends on transmission mode • Asynchronous and Synchronous

  47. Asynchronous Transmission Unpredictable time delay between packets Start Bit Stop Bit(s) Data Bits

  48. Synchronous Transmission • block of bits transmitted in a steady stream without start and stop codes • efficient for sizable blocks of data • requires sender and receiver clocks to be synchronized

  49. Synchronous Transmission SYNC and Control Characters Sync and Control Characters . . . . . . . . . . . . . . Multiple data characters Continuous Stream of Bits

  50. Error Detection • Error detection ensures that the receiver receives the same bit stream as sent by the transmitter. • The principle is: • perform some sort of mathematical procedure on the transmitted data, • transmit the result along with the data • have the receiver apply the same procedure to the received data and compare the result to the received result. • If they are not the same, error!

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