Iii Data Transmission Fundamentals


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Iii Data Transmission Fundamentals

  1. 1. DATA TRANSMISSION FUNDAMENTALS TRANSMISSION MODES F Simplex Transmission Allows data to flow in one direction only (unidirectional). F Half-duplex Transmission Allows data to flow in both directions but only one at a time. There is a problem with turnaround time (the time it takes for the transmission circuits to change direction). F Full-duplex Transmission Allows data to flow in both directions simultaneously. This usually requires one set of transmission circuits each for transmission and reception. Data Transmission Fundamentals 1
  2. 2. PARALLEL VS. SERIAL TRANSMISSION F Parallel transmission is the sending of several bits at the same time. One line or wire is needed for each bit (plus one line or wire for the signal ground and another for the timing or strobe). b0 b0 b1 b1 b2 b2 b3 b3 b4 b4 b5 b5 b6 b6 b7 b7 strobe strobe ground ground transmitter receiver Data Transmission Fundamentals 2
  3. 3. F Serial transmission is when bits are transmitted one at a time. Two lines are needed in the implementation of serial transmission, one for the signal and one for the signal ground. 1 1 0 0 0 1 1 0 data data ground ground transmitter receiver F All communication between chips and components inside a computer system (internal computer data transfer) unit takes place in parallel through the system unit bus. F The type of communication between a computer and an external device (external computer data transfer) depends on the distance between them. F Parallel transmission is common for distances less than 10 feet. Serial transmission is ideal for distances greater than 10 feet. Data Transmission Fundamentals 3
  4. 4. F The reasons why parallel transmission is not suitable for long distance communication are: 1. cost (parallel transmission uses more lines) 2. varying delays among the different bits or signals (bus skew). In other words, bits may arrive at the receiver at different times F For long distance communication, it would be more cost-effective to transmit data using serial transmission. The telephone lines can be readily used for serial transmission. F Since data inside a computer system move in parallel, it is necessary to convert them to serial before external communication can take place. Data Transmission Fundamentals 4
  5. 5. PARALLEL-TO-SERIAL AND SERIAL-TO-PARALLEL CONVERSION F Transmitter Part (Parallel-to-Serial) From CPU b7 b6 b5 b4 b3 b2 b1 b0 Transmit Buffer Transmit Transmitted Register Data F Receiver Part (Serial-to-Parallel) To CPU b7 b6 b5 b4 b3 b2 b1 b0 Receive Buffer Received Receive Data Register F The transmit and receive registers are simply shift registers. Data Transmission Fundamentals 5
  6. 6. SIGNAL PROPAGATION DELAY F The transmission delay (Tx ) of a signal is the time taken to transmit binary data at a given data rate. It is computed as: Tx = N / R where: N = number of bits to be transmitted R = data rate (bps) F There is always a short but finite time delay for a signal to propagate or travel from one end of a transmission medium to the other. This is the propagation delay (Tp) of the channel and is computed as: Tp = S / V where: S = distance to be travelled V = velocity of propagation Data Transmission Fundamentals 6
  7. 7. Example: A 1 Mbyte file is to be transmitted between two machines. Determine the propagation and transmission delays if the distance between the two is 10 Km and the data rate is 19.2 Kbps. Assume that the velocity of propagation is 200,000 Km/second. S = 10,000 m V = 200,000 x 103 m/s R = 19,200 bps N = 1 x (1,048,576) x 8 = 8,388,608 bits Tp = 10000 / 200,000 x 103 = 0.00005 sec Tx = 8388608 / 19200 = 436.91 sec Total Transmission Time = Tp + Tx = 0.00005 + 436.91 = 436.91005 sec. Data Transmission Fundamentals 7
  8. 8. SIGNAL MODULATION F When moving a voice or data signal through a communications channel, it is necessary to vary electrical energy in the channel so that the information moves from one point in the media to another. F Modulation of the process of varying the electrical energy in the channel. F A signal carrier is the electrical energy that flows in the channel (the one that is varied to transmit information). F A modulator is an electronic device that varies the signal carrier to reflect or represent the information in the original signal. Data Transmission Fundamentals 8
  9. 9. CASE STUDY : MODEMS F Digital signals cannot be transmitted directly over telephone lines which are basically analog lines. Limited bandwidth of telephone lines (300 to 3,400 Hz) Internal capacitance of telephone lines (sudden changes in voltages are not allowed) F Modems (modulator-demodulator) convert digital signals (1’s and 0’s) to analog signals (tones) having frequencies within the 300 to 3,400 Hz range. Modulation F At the receiving end, the tones are converted back to digital signals or pulses. Demodulation F The frequency used is approximately 1,700 to 1,800 Hz since transmission is best at frequencies at the center of the 300 to 3,400 Hz passband. Data Transmission Fundamentals 9
  10. 10. Example of a typical computer-to-computer communication using modems and the public telephone system: Modem Computer Telephone System Modem Computer Data Transmission Fundamentals 10
  11. 11. F In modems, a sine wave is used as a carrier. amplitude one cycle τ = period or length of one cycle in terms of time (seconds). f = frequency of signal in cycles per sec or Hz. = 1/τ A = amplitude or magnitude of the signal in volts (signal strength). Data Transmission Fundamentals 11
  12. 12. The phase angle of a signal is the number of degrees in which the signal or sine wave differs a reference sine wave. 90o 360o The phase angle of this signal is 90 degrees. Take note that one complete cycle is equivalent to 360 degrees. Data Transmission Fundamentals 12
  13. 13. F Modulation is therefore the process of changing the amplitude, frequency, or phase of a carrier sine wave signal to represent information. carrier signal 0 1 0 0 1 1 information signal amplitude modulation frequency modulation phase modulation Amplitude, frequency, and phase modulation are also known as amplitude shift keying (ASK), frequency shift keying (FSK), and phase shift keying (PSK). Data Transmission Fundamentals 13
  14. 14. DIGITAL SIGNAL MODULATION F Analog modulation techniques do not apply to digital communications. Digital modulation does not require the presence of an analog carrier. F The digital signal remains at a given voltage for a specified period to signal a binary or digital value. The signal modulates from one discrete value to another only when the information changes value. F Several factors combine to limit the channel length a digital signal can traverse without revitalization: 1. Electronic Noise 2. Signal Attenuation 3. Signal Reflection F The farther the signal travels through a medium, the more the signal becomes distorted because of the three factors. F A wire channel requires a proper termination to prevent signal reflection from further distorting the signal. Data Transmission Fundamentals 14
  15. 15. time original digital signal time digital signal after travelling 100 feet time digital signal after travelling 500 feet Data Transmission Fundamentals 15
  16. 16. F A digital signal cannot be amplified to increase its distance range in a channel. If a digital signal is amplified, the noise that has contaminated the signal is also amplified. F In the case of signal distortion, repeaters are placed along the digital channel to regenerate a digital signal. Regenerating a signal means that the signal is received and rebuilt to its original strength and shape. Distorted Regenerated Digital Digital Signal Signal Regenerative Repeater F Repeaters remove the noise from a signal while it is regenerating the signal. Data Transmission Fundamentals 16
  17. 17. SYNCHRONIZATION OF DIGITAL MODULATION F Digital Communications depend upon exact timing of signal generation and reception to be successful. F If the transmitter sends a signal and the receiver starts to examine the signal at the wrong time, the receiver will get meaningless information. F Synchronization is the process in which the receiver looks at the digital signal at the appropriate times to detect the proper transition from one energy level to another. F For the receiving device to decode and interpret the incoming bit pattern correctly, it must be able to determine: 1. the start of each bit cell. This is known as bit or clock synchronization. 2. the start and end of each character or byte. This is known as character or byte synchronization. 3. the start and end of each complete message block or frame. This is known as block or frame synchronization. Data Transmission Fundamentals 17
  18. 18. F Synchronization between a sending and receiving device requires an agreement on bit period or bit time between the two devices. F There are two types of synchronization techniques: 1. Asynchronous. The transmitter and receiver work independently of each other and exchange a specified signal pattern at the start of each signal exchange. In asynchronous communication, each character or byte is treated independently for clock (bit) and character (byte) synchronization purposes. 2. Synchronous. The transmitter and the receiver exchange initial synchronizing information, then continuously exchanges a digital stream that keeps them in lock step. In synchronous transmission, the complete frame (block) of characters is transmitted as a contiguous string of bits and the receiver endeavors to keep in synchronism with the incoming bit stream for the duration of the complete frame (block). Data Transmission Fundamentals 18
  19. 19. ASYNCHRONOUS SIGNAL SYNCHRONIZATION F The clocks of the transmitter and the receiver are not continually synchronized. But the receiver needs to know when the character begins and ends. F Each transmitted character is encapsulated or framed between an additional start bit and one or more stop bits. Start Bit - logic 0 Stop Bit - logic 1 line idle line idle 1 0 0 1 0 0 1 0 8-bit character start bit 1, 1.5, or 2 stop bits to ensure a negative transition at the start of each new character F The start bit resets the receiver’s clock so that it matches the transmitter’s. The clock needs to be accurate enough to stay in synch for the next 8 to 11 bits. Data Transmission Fundamentals 19
  20. 20. F The receiving device can determine the state of each transmitted bit in the character by sampling or reading the received signal approximately at the center of each bit cell period. F In order to receive the incomiong bits correctly, the receiving device performs the following operations: 1. Wait for the line to become a logic 0 (start bit of the incoming character). 2. Once the line becomes a logic 0, the receiving device should wait for ½ of the bit period. At this point the receiving device is approximately at the center of the start bit. 3. The receiving device should then sample or read the bit (which is still the start bit) to ensure that it is not a false start bit (voltage fluctuation). If the bit read is a logic 1, then it is assumed that it was a false start bit (go back to step 1). 4. The receiving device should then wait for a period of time equal to 1 bit period. This would take the receiving device to the center of the first data bit. Then the device should sample this bit. This step is repeated 8 times (since there are 8 data bits per character). Data Transmission Fundamentals 20
  21. 21. no Input bit = 1 ? yes no Input bit = 0 ? yes Flowchart of the Process Required Wait 1/2 Bit Delay to Recover Asynchronous no Serial Data Input bit = 0 ? yes Bit Counter = 8 Wait 1 Bit Delay Read Incoming Bit Wait 1 Bit Delay Decrement Bit no Counter Input Bit = 1 ? Framing Error yes no yes Counter = 0 ? Store the Byte Data Transmission Fundamentals 21
  22. 22. 1 2 3 4 5 6 7 8 received start stop data sample strobe output 0 1 1 0 1 0 0 1 0 1 Ideal Sampling at Midpoint of Each Bit 1 2 3 4 5 6 7 8 received start stop data sample strobe output 0 1 1 0 1 0 0 1 0 0 Sampling When Receiver Clock is Slightly Fast 1 2 3 4 5 6 7 8 received start data stop sample strobe output 0 1 1 0 1 0 1 0 1 Sampling When Receiver Clock is Too Slow Data Transmission Fundamentals 22
  23. 23. F Asynchronous transmission is often used in situations when characters may be generated at random intervals, such as when a user types at a terminal. F The main problem with asynchronous transmission is its high overhead primarily due to the additional start and stop bits for every byte. Example: 1 start bit and 2 stop bits To transmit 1 byte (8 bits), a total of 11 bits are needed. 8 bits for data plus 3 bits for control % Overhead = 3 x 100 = 27.27% 11 72.73% of what is transmitted actually contain data. The remaining 27.27% contain control bits. Data Transmission Fundamentals 23
  24. 24. If the data rate of the transmission is 9,600 bps, then the effective data rate will be: Effective = 0.7273 x 9600 Data rate = 6,982.08 bps F The overhead problem becomes more apparent for data transmission involving large quantities of data. Example: 1 MB file 1 MB = 1,048,576 bytes Total Data Bits = 8 x 1,048,576 = 8,388,608 bits Total Control Bits = 3 x 1,048,576 = 3,145,728 bits 11,534,336 bits Data Transmission Fundamentals 24
  25. 25. SYNCHRONOUS SIGNAL SYNCHRONIZATION F Synchronous signal modulation and demodulation require precise clocks at both ends of the communications link. F The sender provides the clock signal to generate the transmission frames. The receiver provides a clock to decipher the transmission when it arrives. F There are two techniques in implementing synchronous transmission: 1. Clock Encoding and Extraction The clock (timing) information is embedded into the transmitted signal and subsequently extracted by the receiver. 2. Data Encoding and Clock Synchronization This technique utilizes a stable clock source at the receiver which is kept in synchronism with the incoming bit stream. However, as there are no start and stop bits with a synchronous transmission scheme, it is necessary to encode the information in such a way that are always sufficient bit transitions (1→0 or 0→1) in the transmitted waveform to enable the receiver clock to be resynchronized at frequent intervals. Data Transmission Fundamentals 25
  26. 26. Option 1: Clock Encoding and Extraction This uses the Manchester encoding scheme (also known as Biphase-Level) in encoding the bit stream to be transmitted. 1 0 0 1 1 1 0 1 bit steam to be transmitted Manchester encoded waveform extracted clock decoded signal The presence of a positive or negative transition at the center of each bit cell period in the Machester encoded waveform is used by the clock extraction circuit at the receiving side to produce a clock pulse at approximately the center of the bit. The Manchester encoded waveform is then decoded into the conventional encoding form (Non-Return-to-Zero Level or NRZ-L). With the extracted clock and the decoded waveform, Data Transmission Fundamentals 26
  27. 27. the receiver can easily read the incoming bit stream. Data Transmission Fundamentals 27
  28. 28. Option 2: Data Encoding and Clock Synchronization This technique uses bit transitions (1→0 or 0→1) in the transmitted waveform to enable the receiver clock to be resynchronized at frequent intervals. However, there has to be sufficient bit transitions in order for this to be accomplished. A contiguous stream of 1s or 0s will prevent the resynchronization of the receiver clock. This technique therefore uses the Non- Return-to-Zero Space (NRZ-S) scheme in encoding the bit stream to be transmitted. With NRZ-S encoding, the signal level (1 or 0) does not change for the transmission of a binary 1 whereas a binary 0 does cause a change. bit steam to be 1 0 0 1 1 1 0 1 transmitted NRZI waveform Data Transmission Fundamentals 28
  29. 29. This means that there will be bit transitions in this incoming signal of the an NRZ-S waveform, provided there are no contiguous streams of binary 1’s. To solve the problem of continuous streams of 1’s, use the zero bit insertion or bit stuffing technique. In the zero-bit insertion technique, if there is a sequence of five contiguous binary 1 digits, a zero is automatically inserted after the fifth binary 1 bit. Example: 1011111110010111101011111001101111111 1011111011001011110101111100011011111011 stuffed zeros Consequently, the resulting waveform will contain a guaranteed number of transitions, since 0’s cause a transition in a bit cell, and this enables the receiver to adjust its clock so that it is in synchronism with the incoming bit stream. Data Transmission Fundamentals 29
  30. 30. F Sample Synchronous Frame Formats: 1. Binary Synchronous Control (BSC) SYN SYN STX ETX BCC BCC DATA BYTES SYN (00010110) - Synchronizing Character. It main function is to enable the receiver to achieve character synchronization (reading each character on the correct bit boundary). STX (00000010) - Start of Text Character. It indicates the start of a frame. ETX (00000011) - End of Text Character. It indicates the end of a frame. BCC - Block Check Character. This allows the receiver to identify errors in the frame and request a retransmission of the frame. BSC is a character-oriented synchronous transmission control scheme. Data Transmission Fundamentals 30
  31. 31. 2. Synchronous Data Link Control (SDLC) SF SSA C INFORMATION FCS EF SF (01111110) - Opening Flag. This signals the start of a frame. SSA - Secondary Station Address. This contains the unique address of the intended recipient of the frame. C - Control. This indicates if the frame is an information frame or supervisory frame. FCS - Frame Check Sequence. This is for error handling EF (01111110). Ending Flag. This signals the end of a frame. The SDLC is a bit-oriented protocol. The frame contents need not necessarily comprise multiples of eight bits. Data Transmission Fundamentals 31
  32. 32. F Comparison of Synchronous and Asynchronous Points Regarding Synchronous Transmission 1. Low overhead. 2. Ideal for high-volume, high-speed data transfer. 3. Very complicated to implement. Points Regarding Asynchronous Transmission 1. High overhead. 2. Ideal for low-volume, low-speed data transfer. 3. Very easy to implement. However, most networks use asynchronous transmission even for high-volume file transfer because of its simplicity. Data Transmission Fundamentals 32
  33. 33. DIGITAL SIGNAL ADVANTAGES F It takes more electrical noise to corrupt a digital signal than it does to contaminate an analog signal. If the voltage levels that represent each digital value are far apart, it will take a large amount of noise to get the signal to move from one digital value to another to cause an error. F Most digital communications systems also send specific and separate data, along with the information they convey, that allows the receiver to detect errors. The receiver can request a retransmission of the erroneous information. Data Transmission Fundamentals 33