Digital Encoding

UCCN2043 – Lecture Notes

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6.0 Digital Encoding

In a digital communication system, the first step is to convert the information into a bit

stream of ones and zeros. This involved the following processes:

a) Analog to digital conversion (for analog information source)

b) Source coding

Before transmission, the bit stream has to be represented as:

• electrical signal for copper-wire based medium

• optical signal for optical fiber medium

• electromagnetic wave for wireless

Depending on transmission type, the signal to be transmitted can be digital or analog. We

thus have the following scenario for digital communications:

• Digital data, digital transmission signal

• Digital data, analog transmission signal

When the term “Digital communication” is used, we actually mean the transmission of digital

data either through digital transmission signal or analog transmission signal.

Analog communication is beyond the scope of UCCN2043 and will not be discussed.

Transmission of digital data via analog transmission signal will be discussed in lesson 9 and

is usually called “Digital modulation”.

The following figures aid in the understanding of digital encoding and digital modulation.

Lesson 6 will focus in the transmission of digital data via digital transmission signal. In this

lesson, we will study the various representations of the bit stream as an electrical signal.


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6.1 Requirements for Digital Encoding

Once the information is converted into a bit stream of ones and zeros, the next step is to

convert the bit stream into its electrical representation using digital encoding scheme.

• Interpreting signals at receiver needs to know

� Timing of bits - when they start and end

� Signal levels

• Factors affecting successful interpreting of incoming signals:

� Signal to noise ratio: increased SNR decreases error rate

� Data rate: increased data rate increases error rate

� Bandwidth: increased bandwidth allows increasing data rate

� Encoding schema

The electrical signal representation has to be chosen carefully. The following serves as the

evaluation factors for Data Encoding / Modulation schemes:

• Signal Spectrum

� Lack of high frequencies reduces required bandwidth

� Lack of dc component is desirable, i.e. it should be avoided

� Concentrate power in the middle of the bandwidth

• Clocking, i.e. synchronizing transmitter and receiver

� External clock

� Sync mechanism based on signal

o Some electrical representation helps in clocking — to determine the

beginning and ending of each bit

• Error detection

� Can be built into signal encoding

• Signal interference and noise immunity

� Some codes (electrical representation ) are better than others

• Cost and complexity

� Higher signal rate (& thus data rate) lead to higher costs

� Some codes require signal rate greater than data rate

A variety of encoding schemes have been proposed that address all these issues. In all

communication systems, the standards specify which encoding technique has to be used.

In this lesson, we discuss the most widely used encoding schemes. A good understanding of

these is very important as encoding/modulation is a one of the very crucial process in digital

communications.


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6.2 Digital (Transmission) Signals & Encoding Schemes

• Digital signal characteristics:

� Discrete, discontinuous voltage pulses

� Each pulse is a signal element

� Binary data encoded into signal elements

• Numerous digital encoding schemas have been employed in practical communication

systems:

� Nonreturn to Zero-Level (NRZ-L)

� Nonreturn to Zero Inverted (NRZI)

� Bipolar -AMI

� Pseudoternary

� Manchester

� Differential Manchester

� B8ZS

� HDB3

Categories of Encoding Schemes

Encoding schemes can be divided into the following categories:

• Unipolar encoding

• Polar encoding

• Bipolar encoding

Unipolar encoding: In the unipolar encoding scheme, only one voltage level is used. Binary

1 is represented by positive voltage and binary 0 by an idle line. Because the signal will have

a DC component, this scheme cannot be used if the transmission medium is radio. This

encoding scheme does not work well in noisy conditions.

Polar encoding: In polar encoding, two voltage levels are used: a positive voltage level and a

negative voltage level. NRZ-I, NRZ-L, and Manchester encoding schemes, which we discuss

in the following sections, are examples of this encoding scheme.

Bipolar encoding: In bipolar encoding, three levels are used: a positive voltage, a negative

voltage, and 0 voltage. AMI and HDB3 encoding schemes are examples of this encoding

scheme.

Note: The encoding scheme to be used in a particular communication system is generally

standardized. You need to follow these standards when designing your system to

achieve interoperability with the systems designed by other manufacturers.

NRZ techniques

Multilevel binary techniques

Biphase techniques

Scrambling


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6.3 Non-Return To Zero Codes (NRZ)

NRZ are the simplest form of digital transmission, where the signal level remains constant for

the whole duration of one bit.

• Bits are represented with constant levels, for the whole bit duration

• No voltage for “0”, constant positive voltage for “1” (unipolar)

• In practice, negative voltage is assigned to “1”

Various NRZ versions exist

• Unipolar and bipolar

• Absolute and differential

More often, bipolar version used (+ and – pulses, zero voltage avoided).

Absolute NRZ is termed as NRZ-L whereas differential NRZ is termed NRZ-I.

NRZ encoding is used in slow speed communication interfaces (e.g. RS232) and storage

media (e.g. magnetic tapes)

Advantages

• The simplest codes

Disadvantages

• In its unipolar version a DC component occurs.

• Impossible to distinguish between a long sequence of 0s and the absence of a

signal.

• Difficult to keep the clocks of the source and receiver synchronized if there happen

to be long sequences of 1s or 0s. The receiver uses transitions in level to determine

clock cycle boundaries.

• A Long series of 0s and 1s causes the average signal value, which is used to

distinguish between high and low values, to drift.

Thus for many reasons, it is desirable to have frequent transitions between the high and low

values.

Non-Return To Zero Level (NRZ-L)

In NRZ-L, binary 1 is represented by negative voltage and 0 by positive voltage. This scheme,

though simple, creates problems: if there is a synchronization problem, it is difficult for the

receiver to synchronize, and many bits are lost.

NRZ-L is used for short distances between terminal and modem or terminal and computer


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Non-Return To Zero Invertive (NRZ-I)

NRZ-I is the differential version of NRZ. In the differential version, signal is encoded by

transitions rather than by levels (e.g. the occurrence of a bit of “1” causes a transition from

the previous voltage level).

Just as In NRZ-L, the voltage is constant during the bit interval.

• Binary 1 is represented by the existence of a signal transition at the beginning of the bit

time (either a low-to-high or a high-to-low transition)

• Binary 0 is represented by the maintenance of signal level at the beginning of the bit

time.

NRZ-I gets rid of the problems associated with long strings of 1s, but does nothing

about ;long strings of 0s.

Illustration:

NRZ-L encoding

NRZ-I encoding

Illustration: NRZ-L vs NRZ-I


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6.4 Multilevel Binary Techniques

• Multilevel Binary Techniques use three signal levels (0, +V and -V).

• Two implementations of multilevel binary techniques will be discussed

� Bipolar Alternate Mark Inversion (Bipolar-AMI)

� Pseudo-ternary

Bipolar-AMI Encoding

� ‘0’ represented by no line signal

� ‘1’ represented by alternating positive or negative pulse

� No loss of synchronization if a long string of 1s (0s still a problem)

Pseudo-ternary Encoding

� ‘1’ represented by absence of line signal

� ‘0’ represented by alternating positive and negative pulse

� No loss of sync if a long string of 0s (1s still a problem)

• No advantage or disadvantage if comparison is made between Bipolar-AMI and Pseudo-

ternary encoding.

Illustration:

Multilevel Binary Techniques: Pros & Cons

• Pros

� No loss of sync in case a long string of 1s (in Bipolar-AMI)

� Zero DC component by the nature of these codes

� Simple mechanism for error detection (two consecutive pulses of the same sign show

an error)

• Cons:

� Synchronization issues for long runs of ‘0’ (Bipolar-AMI) or of ‘1’ (pseudo-ternary)

� Introduce some kind of redundancy: three levels to encode two bits

� Receiver must distinguish between three levels (+A, -A, 0)


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6.5 Biphase Technique

• Bi-phase codes address the main disadvantage of NRZ, i.e. the lack of transitions in case

of long runs of identical bits.

• It is oftentimes sad that “Manchester is made of a data signal and a clock signal”, because

in a Manchester code a transition always occurs in the middle of the bit period (similar to

the transition exhibited by a clock signal).

• Two implementations of Biphase techniques will be discussed

� Manchester

� Differential Manchester

Manchester Encoding

� Transition in middle of each bit period

� Low to high represents ‘1’

� High to low represents ‘0’

� Transition serves as clock and data

� Used by IEEE 802.3 (10Mbps Ethernet)

Illustration:

Differential Manchester Encoding

� Mid-bit transition is clocking only

� Transition at start of a bit period represents ‘0’

� No transition at start of a bit period represents ‘1’

� Note: this is a differential encoding scheme

� Used by IEEE 802.5 (Token ring)

Illustration:


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Biphase Techniques: Pros & Cons

• Pros

� Predictable transition at the middle of each bit, which can be used for synchronization

(“self-clocking” codes)

� Error detection possible due to the same feature

� No DC component

• Cons:

� Bandwidth efficiency issues (half the bandwidth efficiency of NRZ)

Clearly, Manchester encoded signal has a larger number of transitions than NRZ or AMI

signals, thus line synchronization is easier.

But, in the same time, the larger number of transitions leads to high frequency components of

important energy, meaning that the spectral efficiency of the Manchester encoded signal is

lower.

Practically, Manchester coding is a good choice in those applications where the physical

available bandwidth guaranteed that the signal is not distorted by the transmission

environment.

• In the 10Mbps l0BaseT Ethernet networks (let’it be on coaxial or twisted pairs cable),

the bandwidth of the physical medium is large enough such as to allow Manchester

coding.

• For 100 Mbps and higher Ethernet networks, the spectrum of a Manchester encoded

signal extends past the high frequency limit for unshielded twisted pair Ethernet cables.

Thus, other encoding schemes are used for high speed Ethernet.

6.6 Scrambling Techniques

• Based on bipolar-AMI, but use scrambling to replace sequences that would produce

constant voltage

• These filling sequences must:

� produce enough transitions to sync

� be recognized by receiver & replaced with original

� be same length as original

• Design goals

� have no dc component

� have no long sequences of zero level line signal

� have no reduction in data rate

� give error detection capability


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• Two implementations of Scrambling techniques will be discussed

� B8ZS: Bipolar With 8 Zeros Substitution

� HDB3: High Density Bipolar 3 Zeros

Both B8ZS and HDB-3 may be seen as improved versions of AMI. They are generally used

in long-distance communications.

� B8ZS employed in 1.544 Mbps-T1,

� HDB-3 in 2.048 Mbps-E1

B8ZS

• Bipolar With 8 Zeros Substitution

• Rule: 8 consecutive zeros are NOT encoded with no signal for eight bit periods, a signal

which has 4 transitions being used instead

• Whenever an all-zero octet occurs:

� encode as 0 0 0 + - 0 - + if last non-zero voltage pulse was positive

� encode as 0 0 0 - + 0 + - if the last non zero pulse was a negative

• AMI code rules are broken twice by “polarity violation”: once inside of the eight-zeros

group, once between the first non-zero pulse preceding the group and its correspondent

within the group

• Unlikely to occur as a result of noise

• Receiver detects and interprets as octet of all zeros

Illustration:


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HDB3

• High-density Bipolar, order 3

• Rule: 4 consecutive zeros are NOT encoded with no signal for four bit periods, but with a

signal which has at least one transition

• When a group of four ‘0’ (‘0000’) occurs:

� encode as 000V if the number of bits of ‘1’ after the last polarity violation is odd or if

this is the first occurrence of four ‘0’

� encode as B00V if the number of bits of ‘1’ after the last polarity violation is even or

zero

� V stands for violation bit and has to be the same polarity as the previous “mark”

� B is a stuffing bit and is of opposite polarity to the previous “mark”

* In other words: String of four zeros replaced according to the following:

• Special notes:

� If the violation bit and the immediate AMI bits’ polarity is similar then change the

polarity of all AMI bits at the right hand side of the violation bit to their opposite

polarity, keep this repeating for violation bits which satisfy the rule.

� As soon a 4 consecutive zeros is found convert it and proceed, because you have to

count + and – for the next 4 consecutive zeros including the + and – on the last 4

consecutive zeros which you finished converting.

• This rule targets the elimination of the DC component (the polarity violation always

changes its sign)

Example 1of HDB3 encoding:

The pattern of bits “ 1 0 0 0 0 1 1 0 ”

encoding using AMI is “ + 0 0 0 0 - + 0 ”

encoded in HDB3 is “ + 0 0 0 V - + 0 ”

which is “ + 0 0 0 + - + 0 ”

Example 2 of HDB3 encoding:

The pattern of bits “ 1 0 1 0 0 0 0 0 1 1 0 0 0 0 1 1 0 0 0 0 0 0 ”

encoding using AMI is “ + 0 - 0 0 0 0 0 + - 0 0 0 0 + - 0 0 0 0 0 0 ”

encoded in HDB3 is “ + 0 - 0 0 0 V 0 + - B 0 0 V + - B 0 0 V 0 0 ”

which is “ + 0 - 0 0 0 - 0 + - + 0 0 + - + - 0 0 - 0 0 ”

(Violation)

(000V) (B00V)


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Illustration:

Scrambling techniques pros and cons

• Pros:

� Classical drawbacks of AMI are eliminated

� No redundancy, therefore the same rate is maintained

� Error detection capabilities

o E.g.: when in HDB3 two consecutive polarity violations have the same sign,

this could be only caused by a transmission error.

� Good spectral efficiency

• Cons:

� Extra-processing needed at transmitter and, especially at receiver side

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Digital Encoding