Digital Transmission 4

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Kashif Bashir

Chapter 4

DIGITAL

TRANSMISSION



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1 DIGITAL TRANSMISSION

A computer network is designed to sendinformation from one point to another. Thisinformation needs to be converted to either adigital signal or an analog signal forTransmission.

Line Coding

Line Coding Schemes

Block Coding

Topics discussed in this section:



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Figure 4.1 Line coding and decoding

4.3



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The data rate defines the number of dataelements (bits) sent in 1 s. The unit isbits per second (bps). The data rate issometimes called the bit rate;

Note

4.4



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The signal rate is the number of signalelements sent in Is. The unit is the baud.The signal rate is sometimes called thepulse rate, the modulation rate, or thebaud rate.

Note



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One goal in data communications is toincrease the data rate while decreasingthe signal rate. Increasing the data rateincreases the speed of transmission;decreasing the signal rate decreases thebandwidth requirement.

Note


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Self-synchronization To correctly interpret the signalsreceived from the sender, the receiver's bit intervals mustcorrespond exactly to the sender's bit intervals. If thereceiver clock is faster or slower, the bit intervals are notmatched and the receiver might misinterpret the signals.

Note



Figure 4.3 Effect of lack of synchronization



In a digital transmission, the receiver clock is 0.1 percent

faster than the sender clock. How many extra bits per second does the receiver receive if the data rate is

1 kbps? How many if the data rate is 1 Mbps?

Solution

At 1 kbps, the receiver receives 1001 bps instead of 1000

bps.

Example 4.2

At 1 Mbps, the receiver receives 1,001,000 bps instead of

1,000,000 bps.


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Different Conversion Schemes

4.10



Digital to Digital Encoding

4.11



Types of Digital to Digital Encoding

4.12



Unipolar encoding uses only one

voltage level.

Note:

4.13



Unipolar Encoding

4.14



Polar encoding uses two voltage levels

(positive and negative).

Note:

4.15



Types of Polar Encoding

4.16



In NRZ-L the level of the signal is

dependent upon the state of the bit.

Note:

4.17



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In NRZ-I the signal is inverted if a 1 is

encountered.

Note:

4.18



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NRZ-L and NRZ-I Encoding

4.19



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RZ Encoding

4.20



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A good encoded digital signal must

contain a provision for synchronization.

Note:

4.21



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In Manchester encoding, the

transition at the middle of the bit isused for both synchronization and bit

representation.

Note:

4.22



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In differential Manchester encoding,

the transition at the middle of the bit isused only for synchronization.

The bit representation is defined by the

inversion or noninversion at the beginning of the bit.

Note:

4.23



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Manchester and Diff. Manchester Encoding

4.24



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In Manchester Scheme the only

drawback is the signal rate. The signal rate for Manchester and differential

Manchester is double that for NRZ.

Note:

4.25



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A long sequence of Os upsets the synchronization.

If we can find a way to avoid a long sequence of

Os in the original stream, we can use bipolar AMI for long distances.

Note:

4.26



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In bipolar encoding, we use three

levels: positive, zero, and negative.

Note:

4.27



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Types of Bipolar Encoding

4.28



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Bipolar AMI Encoding

4.29



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Bipolar with 8-zero substitution (B8ZS) is commonly used

in North America. In this technique, eight consecutive

zero-level voltages are replaced by the sequence000VBOVB. The V in the sequence denotes violation; this

is a nonzero voltage that breaks an AMI rule of encoding.

The B in the sequence denotes bipolar, which means a

nonzero level voltage in accordance with the AMI rule

Note:

4.30



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B8ZS Encoding

4.31



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Figure 4.19 Two cases of B8ZS scrambling technique



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Solution to Example 3

4.33



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High density bipolar 3 zero (HDB3) is commonly used

outside of north America. In this technique, four

consecutive zero-level voltages are replaced with a sequenced with of 000V or B000V.

Note:

4.34



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HDB3 Encoding

4.35



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The two rules can be stated as follows:1. If the number of nonzero pulses after the last substitution is

odd, the substitution pattern will be 000V, which makes the

total number of nonzero pulses even.

2. If the number of nonzero pulses after the last substitution iseven, the substitution pattern will be B00V, which makes the

total number of nonzero pulses even.

High density bipolar 3 zero (HDB3)



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Figure 4.20 Different situations in HDB3 scrambling technique



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Solution to Example 4

4.38



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2 ANALOG-TO-DIGITAL CONVERSION

We have seen in Chapter 3 that a digital signal is

superior to an analog signal. The tendency today is to

change an analog signal to digital data. In this section

we describe two techniques, pulse code modulation

and delta modulation .

Pulse Code Modulation (PCM)

Delta Modulation (DM)




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Figure 4.21 Components of PCM encoder



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PAM



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Quantized PAM Signal



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Quantizing Using

Sign and Magnitude



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PCM



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From Analog to PCM



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Figure 4.22 Three different sampling methods for PCM



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According to the Nyquist theorem, thesampling rate must be

at least 2 times the highest frequencycontained in the signal.

Note



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Figure 4.23 Nyquist sampling rate for low-pass and bandpass signals

E l



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Example

What sampling rate is needed for a signal with a

bandwidth of 10,000 Hz (1000 to 11,000 Hz)?

Solution

The sampling rate must be twice the highest frequency inthe signal:

Sampling rate = 2 x (11,000) = 22,000 samples/s

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Example

A signal is sampled. Each sample requires at least 12

levels of precision (+0 to +5 and -0 to -5). How many bitsshould be sent for each sample?

Solution

We need 4 bits; 1 bit for the sign and 3 bits for the value.

A 3-bit value can represent 23 = 8 levels (000 to 111),

which is more than what we need. A 2-bit value is notenough since 22 = 4. A 4-bit value is too much because 24

= 16.

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Example

We want to digitize the human voice. What is the bit rate,

assuming 8 bits per sample?

Solution

The human voice normally contains frequencies from 0to 4000 Hz.

Sampling rate = 4000 x 2 = 8000 samples/s

Bit rate = sampling rate x number of bits per sample

= 8000 x 8 = 64,000 bps = 64 Kbps



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PCM is a very complex technique. Othertechniques have been developed to reducethe complexity of PCM. The simplest isdelta modulation . DM finds the change

from the previous sample, used fortransmission of voice information wherequality is not of primary importance.

Note



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Figure 4.28 The process of delta modulation



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Figure 4.29 Delta modulation components



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Figure 4.30 Delta demodulation components



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3 TRANSMISSION MODES

The transmission of binary data across a link can be accomplished in either parallel or serial mode. In

parallel mode, multiple bits are sent with each clock

tick. In serial mode, 1 bit is sent with each clock tick.

While there is only one way to send parallel data, there

are three subclasses of serial transmission:

asynchronous, synchronous, and isochronous.

Parallel Transmission

Serial Transmission




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Figure 4.31 Data transmission and modes



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Figure 4.32 Parallel transmission



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Figure 4.33 Serial transmission



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In asynchronous transmission, we send1 start bit (0) at the beginning and 1 ormore stop bits (1s) at the end of each

byte. There may be a gap between

each byte.

Note



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Asynchronous here means“asynchronous at the byte level,” but the bits are still synchronized;

their durations are the same.

Note



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Figure 4.34 Asynchronous transmission



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In synchronous transmission, we sendbits one after another without start or

stop bits or gaps. It is the responsibilityof the receiver to group the bits.

Note



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Figure 4.35 Synchronous transmission

Isochronous



In real-time audio and video, in which uneven delays betweenframes are not acceptable, synchronous transmission fails. For

example, TV images are broadcast at the rate of 30 images per

second; they must be viewed at the same rate. If each image is

sent by using one or more flames, there should be no delays

between frames. For this type of application, synchronization

between characters is not enough; the entire stream of bits must

be synchronized. The isochronous transmission guarantees that

the data arrive at a fixed rate.

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Digital Transmission 4