chap4.pptx

Chapter 4. Digital Transmission

Line Coding

•

The process of converting binary data into digital

signal

Line Coding Schemes

Line Cod-ing

Uni-polar Polar Bipolar

Uses only one

voltage level Uses only

4-4

Digital-to-Digital Conversion

• _{Involves three techniques:}

– _{Line coding (always needed), block coding, and}

scram-bling

• _{Line coding: the process of converting digital data to}

Signal Element and Data Element

• _{Data elements are what we need to send; signal elements are what we}

4-6

Data Rate Versus Signal Rate

• _{Data rate defines the number of data elements (bits) sent in 1s: bps} • _{Signal rate is the number of signal elements sent in 1s: baud}

• _{Data rate = bit rate, signal rate = pulse rate, modulation rate, baud rate} • _{S = c x N x 1/r, where N is the date rate; c is the case factor, S is the}

number of signal elements; r is the number of data elements carried by each signal element

• _{Although the actual bandwidth of a digital signal is infinite, the}

effec-tive bandwidth is finite

• _{The bandwidth is proportional to the signal rate (baud rate)} • The minimum bandwidth: B_min = c x N x 1/r

Design Consideration for Line Coding

Scheme

•

Baseline wandering

– _{Long string of 0s and 1s can cause a drift in the}

base-line

•

DC components

– _{DC or low frequencies cannot pass a transformer or}

telephone line (below 200 Hz)

•

Self-synchronization

•

Built-in error detection

4-8

4-10

Unipolar Scheme

• _{One polarity: one level of signal voltage}

• _{Unipolar NRZ (None-Return-to-Zero) is simple, but}

– _{DC component : Cannot travel through microwave or transformer}

– _{Synchronization : Consecutive 0’s and 1’s are hard to be synchronized} 

Separate line for a clock pulse

Polar Scheme

•

Two polarity: two levels of voltage

•

Problem of DC component is alleviated (NRZ,RZ)

4-12

Polar NRZ

• _{NRZ-L (Non Return to Zero-Level)}

– _{Level of the voltage determines the value of the bit}

• _{NRZ-I (Non Return to Zero-Invert)}

Polar NRZ: NRZ-L and NRZ-I

•

Baseline wandering problem

– _{Both, but NRZ-L is twice severe}

•

Synchronization Problem

– _{Both, but NRZ-L is more serious}

•

NRZ-L and NRZ-I both have an average signal

rate of N/2 Bd

4-14

RZ

• _{Provides synchronization for consecutive 0s/1s} • _{Signal changes during each bit}

• _{Three values (+, -, 0) are used}

Biphase

• _{Combination of RZ and NRZ-L ideas}

• _{Signal transition at the middle of the bit is used for}

syn-chronization

• _Manchester

– _{Used for Ethernet LAN}

– _{Bit 1: negative-to-positive transition} – _{Bit 0: positive-to-negative transition}

• _{Differential Manchester}

– _{Used for Token-ring LAN}

4-16

Polar Biphase

Bipolar Scheme

• _{Three levels of voltage, called “multilevel binary”} • _{Bit 0: zero voltage, bit 1: alternating +1/-1}

– _{(Note) In RZ, zero voltage has no meaning}

• _{AMI (Alternate Mark Inversion) and pseudoternary}

– _{Alternative to NRZ with the same signal rate and no DC}

4-18

Multilevel Scheme

• _{To increase the number of bits per baud by encoding a}

pattern of m data elements into a pattern of n signal elements

• _{In mBnL schemes, a pattern of m data elements is encoded}

as a pattern of n signal elements in which 2m ≤ Ln

• _{2B1Q (two binary, one quaternary)} • _{8B6T (eight binary, six ternary)}

• _{4D-PAM5 (four-dimensional five-level pulse amplitude}

4-20

8B6T

• _{Used with 100Base-4T cable}

• _{Encode a pattern of 8 bits as a pattern of 6 (three-levels) signal}

ele-ments

• _{222 redundant signal element = 3}6(478 among 729) - 28(256)

• _{The average signal rate is theoretically, S}

ave = 1/2 x N x 6/8; in practice

4-22

Multiline Transmission: MLT-3

• _{The signal rate for MLT-3 is one-fourth the bit rate}

• _{MLT-3 when we need to send 100Mbps on a copper wire that cannot}

4-24

Block Coding

• _{Block coding is normally referred to as mB/nB coding; it}

4B/5B

• _{Solve the synchronization problem of NRZ-I}

• _{20% increase the signal rate of NRZ-I (Biphase scheme has the signal}

rate of 2 times that of NRZ-I

4-26

8B/10B

•

2

– 2

= 768 redundant groups used for disparity

4-28

Scrambling

• _{Biphase : not suitable for long distance communication due}

to its wide bandwidth requirement

• _{Combination of block coding and NRZ: not suitable for}

long distance encoding due to its DC component problem

B8ZS

• _{Commonly used in North America}

• _{Updated version of AMI with synchronization}

4-30

HDB3

• _{High-density bipolar 3-zero}

• _{Commonly used outside of North America}

• _{HDB3 substitutes four consecutive zeros with 000V or B00V depending}

Sampling: Analog-to-Digital Conversion

•

Analog information (e.g., voice)



digital signal

(e.g., 10001011…)

4-32

PCM

•

Pulse Code Modulation

•

Three processes

– _{The analog signal is sampled} – _{The sampled signal is quantized}

– _{The quantized values are encoded as streams of bits}

•

Sampling: PAM (Pulse amplitude Modulation)

– _{According to the Nyquist theorem, the sampling rate}

4-34

4-36

4-38

Quantization

•

Quantization level (L)

•

Quantization error : depending on L (or n

)

– SNR_dB = 6.02n_b + 1.76 dB

•

Nonuniform quantization:

– _{Companding and expanding}

4-40

PCM Bandwidth

•

The min. bandwidth of a line-encoded signal

– _B

min = c x N x 1/r = c x nb x fs x 1/r

= c x n

x 2 x B

x 1/r

= n

x B

where 1/r = 1, c = 1/2

•

Max. data rate of a channel

– _N

max = 2 x B x log2L bps

•

Min. required bandwidth

Delta Modulation

4-42

4-44

Parallel Transmission

• _{Use n wires to send n bits at one time synchronously} • _{Advantage: speed}

4-46

Serial Transmission

• _{On communication channel} • _{Advantage: reduced cost}

• _{Parallel/serial converter is required}

Asynchronous Transmission

• _{Use start bit (0) and stop bits (1s)}

• _{A gap between two bytes: idle state or stop bits} • _{It means asynchronous at byte level}

• _{Must still be synchronized at bit level}

4-48

Synchronous Transmission

• _{Bit stream is combined into “frames”}

• _{Special sequence of 1/0 between frames: No gap} • _{Timing is important in midstream}