Solution in Software

Printing Code39 in Software Solution

At 1 Mbps, the receiver receives 1,001,000 bps instead of 1,000,000 bps 1,000,000 bits sent 1,001,000 bits received 1000 extra bps

Built-in Error Detection It is desirable to have a built-in error-detecting capability in the generated code to detect some of or all the errors that occurred during transmission Some encoding schemes that we will discuss have this capability to some extent Immunity to Noise and Interference Another desirable code characteristic is a code that is immune to noise and other interferences Some encoding schemes that we will discuss have this capability Complexity A complex scheme is more costly to implement than a simple one For example, a scheme that uses four signal levels is more difficult to interpret than one that uses only two levels

--NRZ NRZ, RZ, and biphase (Manchester and differential Manchester)

- - MLT-3

There are several schemes in each category We need to be familiar with all schemes discussed in this section to understand the rest of the book This section can be used as a reference for schemes encountered later Unipolar Scheme In a unipolar scheme, all the signal levels are on one side of the time axis, either above or below NRZ (Non-Return-to-Zero) Traditionally, a unipolar scheme was designed as a non-return-to-zero (NRZ) scheme in which the positive voltage defines bit I and the zero voltage defines bit O It is called NRZ because the signal does not return to zero at the middle of the bit Figure 45 show a unipolar NRZ scheme

o 1------'1---1---1--+-----_ _

Compared with its polar counterpart (see the next section), this scheme is very costly As we will see shortly, the normalized power (power needed to send 1 bit per unit line resistance) is double that for polar NRZ For this reason, this scheme is normally not used in data communications today

In polar schemes, the voltages are on the both sides of the time axis For example, the voltage level for 0 can be positive and the voltage level for I can be negative Non-Return-to-Zero (NRZ) In polar NRZ encoding, we use two levels of voltage amplitude We can have two versions of polar NRZ: NRZ-Land NRZ-I, as shown in Figure 46 The figure also shows the value of r, the average baud rate, and the bandwidth In the first variation, NRZ-L (NRZ-Level), the level of the voltage determines the value of the bit In the second variation, NRZ-I (NRZ-Invert), the change or lack of change in the level of the voltage determines the value of the bit If there is no change, the bit is 0; if there is a change, the bit is 1

1 : 1

T=:=

In NRZ-L the level of the voltage determines the value of the bit In NRZ-I the inversion or the lack of inversion determines the value of the bit

Let us compare these two schemes based on the criteria we previously defined Although baseline wandering is a problem for both variations, it is twice as severe in NRZ- L If there is a long sequence of Os or Is in NRZ-L, the average signal power

becomes skewed The receiver might have difficulty discerning the bit value In NRZ-I this problem occurs only for a long sequence of as If somehow we can eliminate the long sequence of as, we can avoid baseline wandering We will see shortly how this can be done The synchronization problem (sender and receiver clocks are not synchronized) also exists in both schemes Again, this problem is more serious in NRZ-L than in NRZ-I While a long sequence of as can cause a problem in both schemes, a long sequence of 1s affects only NRZ-L Another problem with NRZ-L occurs when there is a sudden change of polarity in the system For example, if twisted-pair cable is the medium, a change in the polarity of the wire results in all as interpreted as I s and all I s interpreted as as NRZ-I does not have this problem Both schemes have an average signal rate of NI2 Bd NRZ-L and NRZ-J both have an average signal rate of NI2 Bd Let us discuss the bandwidth Figure 46 also shows the normalized bandwidth for both variations The vertical axis shows the power density (the power for each I Hz of bandwidth); the horizontal axis shows the frequency The bandwidth reveals a very serious problem for this type of encoding The value of the power density is velY high around frequencies close to zero This means that there are DC components that carry a high level of energy As a matter of fact, most of the energy is concentrated in frequencies between a and NIl This means that although the average of the signal rate is N12, the energy is not distributed evenly between the two halves NRZ-L and NRZ-J both have a DC component problem