Lecture-8 Transmission of Signals

Transcription

1 Lecture-8 Transmission of Signals The signals are transmitted as electromagnetic waveforms. As the signal may be analog or digital, there four case of signal transmission. Analog data Analog Signal:- The simplest example is the telephone system. The analog voice data of human is converted into analog signal by the microphone in the telephone system. At the other end the telephone converts the analog signal back to voice data. Digital Data Analog Signal: This is the case when you transmit the digital data from your computer to the internet. You use a modem to convert your digital data into an analog signal that travels on the telephone lines. At the destination system another modem converts the analog signal back to digital data. Analog data Digital Signal: For transmitting an analog data as digital signal we require a device like codec (Coder-decoder) that converts an analog signal (like voice data) into digital signal. At the receiving end the bit stream is converted back by a codec into the analog voice data. Digital Data Digital Signal: transceivers automatically carry out the function of digital data to digital signals. One such function of transceiver is the line encoding which accepts the digital bit patter as the dat and converts them to digital signals. Transmission of Digital Signals: There are basically two approaches for transmission of digital signals, these are: Baseband transmission:- Digital data transmitted as digital signal Broadband transmission: Digital data transmitted as analog signal (use modulation) Channels are often shared by multiple signals; after all it is much more convenient to use a single wire to carry several signals than to have wire for every signals. This is done by the techniques called Multiplexing. Modulation techniques are used for converting bits to signals and vice versa. Baseband transmission: The scheme that directly convert bits into signals is called baseband transmission. The baseband transmission require a low pass channel ( A channel with a bandwidth starting from 0 to maximum). That means a dedicated channel for connecting the two devices. This type of transmission is for digital data as digital signals. Even if there are multiple devices connected on a medium, baseband transmission do not allow more than two devices to communicate at any time. Line coding:

2 It is the process of converting digital data to digital signals. At the transmitter end the digital data are encoded into the digital signals and at the receiving end the converse that is the digital signals are converted back to digital data. Signal Vs data elements: Data element:- It is the smallest entity called a bit to represent a digital information. Signal element:- It is the smallest unit (timewise) of a digital signal. Various combinations are: i. One signal element may carry one bit of digital information ii. Two signal element to carry one data element iii. One signal element may carry two digital data elements iv. Multiple signal element carry multiple bits (eg. 3 signal;s 4 data elements) Here once again it is emphasized that data rate is defifined in terms of bps and signal rate is defined in baud, so accordingly we have two terms bit rate and baud rate. One of the goal in data communication is to increase the data rate while decreasing the signal rate. The relationship between the data rat and the signal rate is defined as: S = N / r or r = N /S where N is the data rate, and S is signal rate There are three case of data rate as worst case, best case and average case. The relationship for these case is: Saverage = c x N x (1/r) baud Example: One data element is encoded in one signal element (r=1). If the data rate is 100 kbps, what is the average value of the baud rate if c is taken as between 0 and 1. Ans: Saverage = c x N x (1/r) = ½ x 100,000 x (1/1) = 50,000 = 50 kbaud Line Coding Scheme: I. Unipolar NRZ II. Polar - NRZ, RZ, and biphase (Manchaster and Differential Manchaster III. Bipolar - AMI and pseudoternary IV. Multilevel - 2B/1Q, 8B /6T and 4D-PAM5 V. Multi-transition MLT-3

3 VI. VII. LAN Coding Scheme (Block Coding Scheme)- nb/mb: 4B/5B NRZI, 8B/10B; kbnt Codes: 2B1Q, 8B6T, 4D-PAM5 WAN Coding Scheme (Scrambling)- B8ZS(North America), HDB3 (Europe, Japan High Density Bipolar with 3 zeros) Unipolar NRZ (Non-Return-to-Zero) The most simple and straight forward method is to use +ve voltage to represent 1 and a zero voltage to represent a 0, or for fiber this could be presence of light to mean 1 and absence of light to mean a 0. It is called NRZ because the signal does not return to zero at the middle of the bit. That the signal simply follows the data As the signal gets attenuated by the channel, it may not look the same at the receiving end. Advantage: Simple, to decode the bits the receiver maps the signal samples to the closest symbos. Disadvantage: The normalized power (Power needed to send 1 bit per unit line resistance) is double that of polar NRZ Polar Scheme: i. Non-Return-to-zero:There are two variants of polar NRZ, these are NRZ-L (level) and NRZ-I(invert) Problems: The baseline wandering is a problem in polar NRZ. If there is a long sequence of 0s and 1s in NRZ-L, the average signal power becomed skewed. The receiver might have difficulty discerning the bit value. NRZ-I : Non-return-to-zero (Invert) Inverting NRZ (NRZ-I) : Whenever the data is 1, this scheme inverts the voltage level to mark the next bit as 1. For 0 there is no change in voltage

4 level. Therefore, it is the change in the voltage level rather than the actual polarity that signifies a binary 1. This type of NRZ can offer some synchronisation or timing because a series of binary ones will result in a series of inversions that can be used by the receiving end to resynchronise its timer. Problem The NRZ scheme has most of its energy concentrated near zero frequency, which make it unsuitable for transmission over a channel having poor performance around this frequency. For ll schemes that encode bits into symbols, the receiver must know when a bit is ending and when it is starting to correctly decode the bits. With NRZ-L in which the symbols are simply voltage levels, a long run of 0s and 1s is leaves the signal unchanged. After a while it is hard to tell the bit apart as 15 zeros look like 16 zeros unless we have a very accurate clock. The popular USB(Universal Synchronous Bus) uses NRZI, with it long run of 1s do not cause a problem. Of course we need to fix the long stream of 0 s. Old digital telephone line called T1 lines did in fact required that no more than 15 consecutive 0 s be sent for proper working. Return to Zero (RZ) RZ uses three voltage levels, positive voltage to represent 1, negative voltage to represent 0 and zero voltage for none. Signals change during bits, not between bits. Signal goes to zero in the middle of each bit and remains there until the beginning of next bit. Advantage: There is no DC component Problem : A long sequence of 1s and 0s still cause a problem as seen here

5 Manchester Encoding RZ and NRZ-L are combined in to the Manchester scheme. Manchester coding states that there will always be a transition of the message signal at the mid-point of the data bit frame. What occurs at the bit edges depends on the state of the previous bit frame and does not always produce a transition. A logical 1 is defined as a mid-point transition from low to high and a 0 is a mid-point transition from high to low. In this scheme, the clock signal is mixed (by XORing) with the data signals so that no extra line is needed. Down Side of Manchester encoding is that it requires twice as much bandwidth as NRZ because of the clock, and we know that bandwidth often matters. However this scheme overcome the problem of NRZ-L Differential Manchester coding It combines the idea of RZ and NRZ-I. There is always a transition at the middle of the bit; but the bit value is determined at the beginning of the bit instead of the middle of bit as in Manchester coding. If the next bit is 0, there is a transition; if the next bit is 1 there is no transition. This coding scheme overcome many problems of NRZ-I. That is there is no bit wandering, second there is no DC components.

6 Now compare the Manchester and Differential Manchester coding. In Manchester coding, there is always at the middle of the bit. If bit is 0 there is low-to-high transition. If next bit is 1, then transition is high-to-low. Down Side of Manchester encoding is that it requires twice as much bandwidth as NRZ because of the clock, and we know that bandwidth often matters. Bipolar Scheme There are two types of bipolar encoding: i. Alternate Mark Inversion (AMI) ii. Pseudoternary Alternate Mark Inversion(AMI):

7 AMI is used for long distance communication, but has synchronization problem for long sequence of zeros, which can be solved by scrambling. In AMI there are three levels 0, +ve, -ve. In AMI A neutral zero voltage represents a binary 0 Alternating +ve and ve voltages represent binary 1. Pseudoternary encoding: In Pseudoternary encoding also there are three levels 0, +ve, -ve. In this scheme: A neutral low voltage represents a binary 1 Alternating +ve and ve voltages represent binary 0. Advantage of Bipolar scheme is that it is alternate to NRZ having same rate as NRZ but there is no DC component in it. LAN Coding Scheme (Block Coding Scheme)- nb/mb: 4B/5B NRZI, 8B/10B; kbnt Codes: 2B1Q, 8B6T, 4D-PAM5 Although a code like Manchester introduces systematic transitions in any bit sequence, it is not effective because it requires a signalling rate twice as high as the bit rate (2 pulses per bit). What is often used instead is nb/mb coding. With nb/mb coding schemes, n data bits will be represented by a sequence of m pulses, the efficiency increases from 50 % in Manchester to n/m % 4B/5B NRZI: This coding scheme is used for Ethernet 100 Mbps and FDDI (fiber distributed data interface) over optical fiber. With 4B/5B, 4 data bits will be represented by a sequence of 5 pulses (efficiency increases from 50 % in Manchester to 80 %), which will be themselves encoded using NRZI (nonreturn to zero inverted) coding. Note: In order to ensure enough transitions in the data WAN Coding Scheme (Scrambling)- B8ZS(North America), HDB3 (Europe, Japan High Density Bipolar with 3 zeros) mbnl encoding Scheme Questions: 1. Define Line Coding. Give the following line encoding for the digital word a. Unipolar RZ, NRZ b. Polar RZ, Manchester, differential Manchester 2. What are different types of line coding scheme. Explain each with example.