SUBMITTED BYBHAWNA0081184408MCA4TH SEM

TERMS Data rate

Defines the number of data elements(bits) sent in 1s. Unit is bpsSignal rate

number of signal elements sent in 1s. Unit is baudBandwidth in bits per sec

speed of bit transmission in channel or linkBaseline wandering

The incoming signal power is evaluated against the baseline to determine the value of the data element. A long string of 0’s and 1’s can cause a drift in the baseline, and make it difficult for the receiver to decode correctly.

LINE CODING

• Unipolar encoding uses only one voltage level (either 0 or 1) 1 encoded as a positive value and the 0 encoded as zero value.– NRZ

• Polar encoding uses two voltage levels (one positive, one negative)– NRZ, RZ and biphase

• Bipolar encoding uses three voltage levels: positive, negative, zero– AMI and pseudoternary

Continued……

• There are numerous techniques available to convert digital data into digital signals.

• Let’s examine few:1. Nonreturn to Zero-Level (NRZ-L)2. Nonreturn to Zero Inverted (NRZI)3. Multilevel (Bipolar –AMI)4. Manchester5. Differential Manchester6. B8ZS7. HDB3

Unipolar encoding

Nonreturn to Zero-Level (NRZ-L)

• Two different voltages for 0 and 1 bits

• Negative voltage for one value and positive for the other

• Voltage constant during bit interval• No transition to 0V

For Example: • Negative Voltage (-5V) use to represent binary 1 and Positive

Voltage (+5v) use to represent binary 0

Nonreturn to Zero Inverted (NRZ-I)

• Nonreturn to zero and inverted on 1

• Constant voltage pulse for duration of bit

• Data encoded as presence or absence of signal transition at beginning of bit time

• Transition denotes a binary 1

• No transition denotes binary 0

NRZ

NRZ-L and NRZ-I both have an average signal rate of N/2 Bd.

Fundamental difference exists between NRZ-L and NRZIWith NRZ-L, the receiver has to check the voltage level for each bit to determine whether the bit is a 0 or a 1, With NRZI, the receiver has to check whether there is a change at the beginning of the bit to determine if it is a 0 or a 1

NRZ pros and cons• Pros

– Easy to engineer– Make good use of bandwidth

• Cons– String of 0’s or 1’s leads a constant voltage over a period of time– Loss of synchronization between transmitter & receiver, this

problem is more serious in NRZ-L– Baseline wandering is a problem for both schemes, but twice as

severe in NRZ-L

• Used for magnetic recording• Not often used for signal transmission

Return to Zero (RZ)

• The main problem with NRZ encoding occurs when the sender and reciever clocks are not synchronized, the solution is RZ scheme.

• Signals changes during the bit

Disadvantages• Requires two signal changes to encode a bit, therefore

occupies greater bandwidth• Complexity: RZ uses three levels of voltage, which is more

complex to create.• Scheme is not used today

Biphase Schemes

• Overcomes the limitations on NRZ codes

• Two biphase techniques are commonly used:– Manchester– Differential Manchester

• Heavily used in LAN applications

BIPHASE continued…1. Manchester

– Transition in middle of each bit period– Idea of RZ and NRZ-L are combined– Mid-Bit transition serves for clocking and data– Rule

• Low to high represents binary ‘1’• High to low represents binary ‘0’

– Enables effective clock signal recovery at receiver

• Poor bandwidth utilization– Effective sending rate is cut in half

• Used by IEEE 802.3 (ethernet) standard for baseband coaxial cable and twisted-pair CSMA/CD bus LANs

BIPHASE continued…

In Manchester encoding, the transition at the middle of the bit is used for both synchronization and bit representation.

BIPHASE continued…

1. Differential Manchester– Combines the ideas of RZ and NRZ-I– Mid-Bit transition serves only for clocking– Rule

• If there is a ‘0’, there will be a transition (start of a bit period)

• If there is a ‘1’, there will be no transition (start of a bit period)

– Used by IEEE 802.5 token ring LAN, using shielded twisted pair

Modulation Rate

Modulation rate for Manchester and Differential Manchester is twice the data rate inefficient encoding for long-distance applications

Biphase Pros and Cons• Cons

– At least one transition per bit time and possibly two– Maximum baud rate is twice NRZ– Requires more bandwidth– Signal rate is double that for NRZ

• Pros– Synchronization on mid bit transition (self clocking)– No baseline wandering – Error detection

• Big difference between NRZ and Manchester codes:– For long strings of 0-bits, NRZ codes generate signal that

does not change over long time period– Manchester codes always produce signal change during

every bit transmission.– Manchester codes are called self-clocking codes, because

they provide a guaranteed voltage change (a “clock signal”) in the middle of every bit received

• Why do we care about self-clocking codes?– Transmitter / receiver clocks are not perfectly

synchronized to tick at same rate (too expensive).– NRZ-L or NRZ-I cannot be used at high data rates or long

distances unless a separate clock signal is sent on another wire.

– Manchester codes can be used at high data rates or long distances, because receiver continuously gets feedback on sender clock rate.

BIPOLAR ENCODING

• It uses three voltage levels: positive, negative and zero.

• The zero level is used to represent binary 0, while the 1s are represented by alternating positive and negative voltages

• Same signal rate as NRZ

Bipolar-AMI Encoding Scheme• The bipolar-AMI (Alternate Mark Inversion) encoding scheme is

unique among all the encoding schemes because it uses three voltage levels– When a device transmits a binary 0, a zero voltage is transmitted– When the device transmits a binary 1, either a positive voltage or a

negative voltage is transmitted– Which of these is transmitted depends on the binary 1 value that was

last transmitted

• Used for long distance communication, No loss of sync if a long string of ones (zeros still a problem)

• Lower bandwidth• Easy error detection

Pseudoternary

• Variation of AMI encoding

• I bit is encoded as zero voltage

• 0 bit is encoded as alternating positive and negative voltages.

BIPOLAR ENCODING

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Scrambling Technique

• Used for long distance transmission (WAN)– Solve the problem of bipolar AMI code where

sequence of zero was problem.– Provide synchronization

• Used to replace sequences that would produce constant voltage

• Avoid long sequences of zero level line signal• No reduction in data rate• Error detection capability• Two commonly used techniques are: B8ZS, and HDB3

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Bipolar With 8 Zeros Substitution (B8ZS)

• Based on bipolar-AMI– Used in North America

• Eight consecutive zeros –level voltages are replaced by the sequence of 000VB0VB

• V denotes violation; this is a non zero voltage that breaks an AMI rule of encoding

• B denotes bipolar, which means a non zero level voltage in accordance with the AMI rule

• Scrambling in this case does not change bit rate

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High Density Bipolar 3 Zeros(HDB3)

• HDB3 – similar but based on 4 zeros

• Based on bipolar-AMI

• String of four consecutive zero level voltages are replaced with a sequence of 000V or B00V

• The reason for two different substitutions is to maintain the even number of non zero pulses after each substitution

if no. is odd- substitution pattern-000Vif no. is even- substitution pattern-B00V

B8ZS and HDB3

B8ZS

HDB3

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Recap of Digital Signal Encoding Formats

0 1

NRZL High level Low level

NRZI No transition at start of interval

transition

Bipolar-AMI No line signal +ve line signal

Manchester Transition from high to low in the middle of interval

Transition from low to high in the middle of interval

Diff Manchester (always a Transition in the middle of interval)

Tran at start of interval No transition at start of interval

HDB3 Same as bipolar-AMI, except that any string of four zeros is replaced by a string with one code violation

B8ZS Same as bipolar-AMI, except that any string of eight zeros are replaced by a string of two code violations