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Lecture. 3 Data Transmission and Computer Communications.

Lecture. 3 Data Transmission and Computer Communications. DIGITAL-TO-DIGITAL CONVERSION. A computer network is designed to send information from one point to another. This information needs to be converted to either a digital signal or an analog signal for transmission.

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Lecture. 3 Data Transmission and Computer Communications.

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  1. Lecture. 3Data Transmission and Computer Communications.

  2. DIGITAL-TO-DIGITAL CONVERSION • A computer network is designed to send information from one point to another. • This information needs to be converted to either a digital signal or an analog signal for transmission. • First we discuss conversion to digital signals; next we conversion to analog signals.

  3. Line Coding • Converting a string of 1’s and 0’s (digital data) into a sequence of signals that denote the 1’s and 0’s. • For example a high voltage level (+V) could represent a “1” and a low voltage level (0 or -V) could represent a “0”.

  4. Characteristics

  5. Data rate versus signal rate 1 0 1 0 1 0 1 0 Signal rate =data rate 0 0 0 0 1 1 1 1 Signal rate =data rate/4

  6. Data Rate vs. Signal Rate • Data rate is the number of bits per second • Signal rate is the number of signal sent in 1 second. • Pulse rate, modulation rate, baud where S is the average value of the signal rate, N is data rate c is the case factor. r is the ratio between data element & signal element

  7. Example A signal is carrying data in which one data element is encoded as one signal element ( r = 1). If the bit rate is 100 kbps, what is the average value of the baud rate if c is 1/2? Solution

  8. DC Components • Voltage level is constant • Problems • System that cannot pass low frequencies • Long-distance link may use transformers to isolate different parts of the line electrically • This will require the removal of the dc component of a transmitted signal.

  9. Self-Synchronization • The clocks at the sender and the receiver must have the same bit interval. • If the receiver clock is faster or slower it will misinterpret the incoming bit stream.

  10. Self-Synchronization

  11. Line coding schemes

  12. Unipolar • All signal levels are on one side of the time axis - either above or below • NRZ - Non Return to Zero scheme is an example of this code. The signal level does not return to zero during a symbol transmission. • Scheme is prone to DC components. • Uses two signal levels: 0 and +V (typically 5 volts) • Long streams 0f 1s and 0s causes synchronization problem

  13. Unipolar NRZ scheme Unipolar NRZ scheme

  14. Polar - Non Return to Zero (NRZ) • The voltages are on both sides of the time axis. • Polar NRZ scheme can be implemented with two voltages. E.g. +V for 1 and -V for 0. • There are two versions: • NZR - Level (NRZ-L) - positive voltage for one symbol and negative for the other • NRZ - Inversion (NRZ-I) - the change or lack of change in polarity determines the value of a symbol. E.g. a “1” symbol inverts the polarity a “0” does not.

  15. Polar - Non Return to Zero (NRZ) Polar NRZ-L and NRZ-I schemes

  16. Polar – Non Return to Zero (NRZ) • Advantages • Easy to engineer. • Make good use of bandwidth. • Disadvantages • DC component. • Lack of synchronization capability.

  17. Polar - Return to Zero (RZ) • The Return to Zero (RZ) scheme uses three voltage values. +, 0, -. • Each symbol has a transition in the middle. Either from high to zero or from low to zero. • This scheme has more signal transitions (two per symbol) and therefore requires a wider bandwidth. • No DC components if there is no consecutive 1’s or 0’s . • Self synchronization - transition indicates symbol value. • More complex as it uses three voltage level. • It has no error detection capability.

  18. Polar – Return to Zero (RZ) Polar RZ scheme

  19. Biphase: Manchester and Differential Manchester • Manchester • Every symbol has a level transition in the middle: from high to low or low to high. Uses only two voltage levels. • Differential Manchester • Every symbol has a level transition in the middle. But the level at the beginning of the symbol is determined by the symbol value. One symbol causes a level change the other does not.

  20. Manchester and Differential Manchester Polar biphase: Manchester and differential Manchester schemes

  21. Manchester and Differential Manchester • Advantages: • Synchronization on mid bit transition (self clocking). • No DC component. • Disadvantages: • At least one transition per bit time and possibly two. • Maximum modulation rate is twice NRZ. • Requires more bandwidth (2 times that of NRZ.)

  22. Multilevel Binary -Bipolar • Code uses 3 voltage levels: - +, 0, -, to represent the symbols (note not transitions to zero as in RZ). • Voltage level for one symbol is at “0” and the other alternates between + & -. • Bipolar Alternate MarkInversion (AMI) - the “0” symbol is represented by zero voltage and the “1” symbol alternates between +V and -V. • Pseudoternary is the reverse of AMI

  23. Multilevel Binary -Bipolar Bipolar schemes: AMI and pseudoternary

  24. Multilevel Binary -Bipolar • It is a better alternative to NRZ. • Has no DC component. • Has no self synchronization because long runs of “0”s results in no signal transitions.

  25. Block Coding • For a code to be capable of error detection, we need to add redundancy, i.e., extra bits to the data bits. • Block coding is done in three steps: division, substitution and combination. • The resulting bit stream prevents certain bit combinations that when used with line encoding would result in DC components or poor sync. Quality.

  26. Block Coding • Block coding is normally referred to as mB/nB coding; it replaces each m-bit group with an n-bit group.

  27. Block Coding

  28. ANALOG-TO-DIGITAL CONVERSION • A digital signal is superior to an analog signal because it is more robust to noise and can easily be recovered. • For this reason, the tendency today is to change an analog signal to digital data. • Pulse Code Modulation (PCM) • Delta Modulation (DM)

  29. Pulse Code Modulation (PCM) • PCM consists of three steps to digitize an analog signal: • Sampling • Quantization • Binary encoding • Before we sample, we have to filter the signal to limit the maximum frequency of the signal as it affects the sampling rate. • Filtering should ensure that we do not distort the signal, ie remove high frequency components that affect the signal shape.

  30. Pulse Code Modulation (PCM) Components of PCM encoder

  31. Delta Modulation • This scheme sends only the difference between pulses, if the pulse at time tn+1 is higher in amplitude value than the pulse at time tn, then a single bit, say a “1”, is used to indicate the positive value. • If the pulse is lower in value, resulting in a negative value, a “0” is used. • This scheme works well for small changes in signal values between samples. • If changes in amplitude are large, this will result in large errors.

  32. Delta Modulation The process of delta modulation

  33. TRANSMISSION MODES Data transmission and modes

  34. TRANSMISSION MODES Parallel transmission

  35. TRANSMISSION MODES Serial transmission

  36. Serial Transmission • Received messages are a string of bits which need to be interpreted correctly by determining the: • Start of each bit signaling element • Start of each character • Start and end of each message • These three tasks are known as bit or clock synchronization, character or byte synchronization, and block or frame synchronization. • These can be performed by two ways depending on whether the transmitter and receiver clocks are synchronized or independent (asynchronous)

  37. Synchronous Transmission • There is no delay or gap between each 8 bit element • Each frame of characters is transmitted as a contiguous bit strings and the receiver must stay synchronized for the entire frame • The clock (timing) information must be embedded into the signal for it to be extracted by the receiver by using self-clocking codes • By synchronization performed by placing special characters at the start of each frame

  38. Synchronous Transmission Synchronous transmission

  39. Asynchronous Transmission • Each character (7 or 8 bits) is sent independently • Mainly used with data transmitted at irregular intervals (eg. Keyboard) such that the line will be idle for long period • Each character is encapsulated between an additional start bit and one or more stop bits, which have different polarity • The additional start and stop bits required for transmission mean that the useful data rate is less than transmission time

  40. Asynchronous Transmission Asynchronous transmission

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