Lecture. 3 Data Transmission and Computer Communications

Lecture. 3Data Transmission and Computer Communications.

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DIGITAL-TO-DIGITAL CONVERSIONDIGITAL-TO-DIGITAL CONVERSION

A computer network is designed to send information from one point to another.

This information needs to be converted to either a digital signal or an analog signal for transmission.

First we discuss conversion to digital signals;

next we conversion to analog signals.

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Line CodingLine Coding

Converting a string of 1’s and 0’s (digital data) into a sequence of signals that denote the 1’s and 0’s.

For example a high voltage level (+V) could represent a “1” and a low voltage level (0 or -V) could represent a “0”.

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CharacteristicsCharacteristics

Data rate versus signal rate

1 0 1 0 1 0 1 0

0 0 1 10 0 1 1

Signal rate =data rate

Signal rate =data rate/4

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Data Rate vs. Signal RateData Rate vs. Signal Rate

Data rate is the number of bits per second

Signal rate is the number of signal sent in 1 second. Pulse rate, modulation rate, baud

where S is the average value of the signal rate, N is data rate

c is the case factor.

r is the ratio between data element & signal element

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ExampleExample

A signal is carrying data in which one data element is encoded as one signal element ( r = 1). If the bit rate is 100 kbps, what is the average value of the baud rate if c is 1/2?

Solution

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DC ComponentsDC Components

Voltage level is constant Problems

System that cannot pass low frequencies Long-distance link may use transformers to

isolate different parts of the line electrically

This will require the removal of the dc component of a transmitted signal.

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Self-SynchronizationSelf-Synchronization

The clocks at the sender and the receiver must have the same bit interval.

If the receiver clock is faster or slower it will misinterpret the incoming bit stream.

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Self-SynchronizationSelf-Synchronization

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Line coding schemesLine coding schemes

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UnipolarUnipolar

All signal levels are on one side of the time axis - either above or below

NRZ - Non Return to Zero scheme is an example of this code. The signal level does not return to zero during a symbol transmission.

Scheme is prone to DC components. Uses two signal levels: 0 and +V (typically 5

volts)

Long streams 0f 1s and 0s causes synchronization problem

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Unipolar NRZ schemeUnipolar NRZ scheme

Unipolar NRZ scheme

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Polar - Non Return to Zero (NRZ)Polar - Non Return to Zero (NRZ)

The voltages are on both sides of the time axis.

Polar NRZ scheme can be implemented with two voltages. E.g. +V for 1 and -V for 0.

There are two versions: NZR - Level (NRZ-L) - positive voltage for one

symbol and negative for the other NRZ - Inversion (NRZ-I) - the change or lack of

change in polarity determines the value of a symbol. E.g. a “1” symbol inverts the polarity a “0” does not.

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Polar - Non Return to Zero (NRZ)Polar - Non Return to Zero (NRZ)

Polar NRZ-L and NRZ-I schemes

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Polar – Non Return to Zero (NRZ)Polar – Non Return to Zero (NRZ)

Advantages Easy to engineer. Make good use of bandwidth.

Disadvantages DC component. Lack of synchronization capability.

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Polar - Return to Zero (RZ)Polar - Return to Zero (RZ)

The Return to Zero (RZ) scheme uses three voltage values. +, 0, -.

Each symbol has a transition in the middle. Either from high to zero or from low to zero.

This scheme has more signal transitions (two per symbol) and therefore requires a wider bandwidth.

No DC components if there is no consecutive 1’s or 0’s .

Self synchronization - transition indicates symbol value.

More complex as it uses three voltage level. It has no error detection capability.

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Polar – Return to Zero (RZ)Polar – Return to Zero (RZ)

Polar RZ scheme

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Biphase: Manchester and Differential Biphase: Manchester and Differential ManchesterManchester

Manchester Every symbol has a level transition in the

middle: from high to low or low to high. Uses only two voltage levels.

Differential Manchester Every symbol has a level transition in the

middle. But the level at the beginning of the symbol is determined by the symbol value. One symbol causes a level change the other does not.

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Manchester and Differential Manchester and Differential ManchesterManchester

Polar biphase: Manchester and differential Manchester schemes

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Manchester and Differential Manchester and Differential ManchesterManchester

Advantages: Synchronization on mid bit transition (self

clocking). No DC component.

Disadvantages: At least one transition per bit time and possibly

two. Maximum modulation rate is twice NRZ. Requires more bandwidth (2 times that of

NRZ.)

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Multilevel Binary -BipolarMultilevel Binary -Bipolar

Code uses 3 voltage levels: - +, 0, -, to represent the symbols (note not transitions to zero as in RZ).

Voltage level for one symbol is at “0” and the other alternates between + & -.

Bipolar Alternate Mark Inversion (AMI) - the “0” symbol is represented by zero voltage and the “1” symbol alternates between +V and -V.

Pseudoternary is the reverse of AMI

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Bipolar schemes: AMI and pseudoternary

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It is a better alternative to NRZ. Has no DC component. Has no self synchronization because long

runs of “0”s results in no signal transitions.

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Block CodingBlock Coding

For a code to be capable of error detection, we need to add redundancy, i.e., extra bits to the data bits.

Block coding is done in three steps: division, substitution and combination.

The resulting bit stream prevents certain bit combinations that when used with line encoding would result in DC components or poor sync. Quality.

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Block coding is normally referred to as mB/nB coding; it replaces each m-bit group with an n-bit group.

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ANALOG-TO-DIGITAL CONVERSIONANALOG-TO-DIGITAL CONVERSION

A digital signal is superior to an analog signal because it is more robust to noise and can easily be recovered.

For this reason, the tendency today is to change an analog signal to digital data.

Pulse Code Modulation (PCM) Delta Modulation (DM)

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Pulse Code Modulation (PCM)Pulse Code Modulation (PCM)

• PCM consists of three steps to digitize an analog signal: Sampling Quantization Binary encoding

Before we sample, we have to filter the signal to limit the maximum frequency of the signal as it affects the sampling rate.

Filtering should ensure that we do not distort the signal, ie remove high frequency components that affect the signal shape.

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Pulse Code Modulation (PCM)Pulse Code Modulation (PCM)

Components of PCM encoder

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Delta ModulationDelta Modulation

• This scheme sends only the difference between pulses, if the pulse at time tn+1 is higher in amplitude value than the pulse at time tn, then a single bit, say a “1”, is used to indicate the positive value.

• If the pulse is lower in value, resulting in a negative value, a “0” is used.

• This scheme works well for small changes in signal values between samples.

• If changes in amplitude are large, this will result in large errors.

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Delta ModulationDelta Modulation

The process of delta modulation

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TRANSMISSION MODESTRANSMISSION MODES

Data transmission and modes

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Parallel transmission

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Serial transmission

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Serial TransmissionSerial Transmission

• Received messages are a string of bits which need to be interpreted correctly by determining the:– Start of each bit signaling element– Start of each character– Start and end of each message

• These three tasks are known as bit or clock synchronization, character or byte synchronization, and block or frame synchronization.

• These can be performed by two ways depending on whether the transmitter and receiver clocks are synchronized or independent (asynchronous)

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Synchronous TransmissionSynchronous Transmission

• There is no delay or gap between each 8 bit element

• Each frame of characters is transmitted as a contiguous bit strings and the receiver must stay synchronized for the entire frame

• The clock (timing) information must be embedded into the signal for it to be extracted by the receiver by using self-clocking codes

• By synchronization performed by placing special characters at the start of each frame

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Synchronous TransmissionSynchronous Transmission

Synchronous transmission

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Asynchronous TransmissionAsynchronous Transmission

• Each character (7 or 8 bits) is sent independently

• Mainly used with data transmitted at irregular intervals (eg. Keyboard) such that the line will be idle for long period

• Each character is encapsulated between an additional start bit and one or more stop bits, which have different polarity

• The additional start and stop bits required for transmission mean that the useful data rate is less than transmission time

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Asynchronous TransmissionAsynchronous Transmission

Asynchronous transmission

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Lecture. 3 Data Transmission and Computer Communications