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Data Transmission

EECE 542 Fall 2003


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Time/Frequency Relationships The relationship between time and

frequency domain representation of signalsis defined by Fourier analysis.

Unmodulated (non-sinusoidal) signals havetheir frequency domain spectra centeredabout 0 Hz. (i.e. baseband transmission)

General rule: A faster (shorter period) signal in the time

domain results in a wider (larger bandwidth)signal in the frequency domain


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Ex: A random sequence of 0s

and 1s


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Baseband Data Transmission

Most physical layer transmission systems rely on

baseband transmission.

Almost exclusively use a type of cable or fiber

Supports only one current transmission

No parallel transmissions on the same wire unless

multiple wires are used (both tx and rx)

Exception: some fiber optic systems

Transmission involves a mapping of binary data to

analog waveforms.


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Baseband Data Reception

Line components typically block the transmission

in the vicinity of 0 Hz (DC).

The received signal is first filtered and amplifiedto reduce the effects of noise and line attenuation.

Correct decisions on the data being a 0 or 1

requires knowledge of the bit transition edges or

boundaries.

Requires a bit clock which is not typically sent with the

data


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Bit Synchronization

A bit (data) clock must be generated at thereceiver for the data being received.

The generation of this clock and thealignment (phase adjustment) of its edgeswith the edges of the received data isperformed by a bit synchronizer.

A bit synch is basically a Phase Lock Loop(PLL)

PLLs work best if bit transitions occur atmost if not all data bit boundaries


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Line Coding Data embedded in a layer 2 frame may

easily contain long strings of 0s or 1s

Few bit transitions for the PLL to work well Line coding is the translation of the binary

data into a new digital stream

Good line coding schemes guarantee bit

transitions The spectral shape of the transmission is often

affected


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Types of Line Coding Unipolar

Polar

NRZ (Nonreturn to Zero)

RZ (Return to Zero)

Biphase

Bipolar


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Unipolar Line Coding Simple

Binary 1 = high voltage

Binary 0 = low (zero) voltage

Properties

No edge transitions when the original data

doesnt change

No change in the spectral shape (still has DC

component)


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Unipolar cont.


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NRZ (Nonreturn to Zero) Coding A type of polar (two non-zero voltage levels)

coding

Removes the DC component NRZ-I

NRZ-L 0 -> positive (or neg.) voltage

1 -> negative (or pos.) voltage NRZ-I

0 -> voltage remains the same

1 -> causes an inversion in the voltage

creates bit transitions in long strings of 1s (but not 0s)


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NRZ Cont.


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RZ (Return to Zero) Coding Another type of polar encoding

The first half of each bit is mapped as in

NRZ-L

The second half of each bit is set to 0 volts

Guarantees bit transitions

Removes the DC component The width of the transmitted pulse is cut in

half so the spectral bandwidth increases


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RZ Cont.


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Biphase Coding

Another type of polar

Like RZ, transitions are created in the middle of

the bit periods Most common methods used in LANs

Manchester

Middle transition = o if bit =1, q if bit = 0

Ethernet

Differential Manchester

Middle transition always present, but a transition at thebeginning of a bit only occurs if the bit = 0

Token Ring


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Manchester & Diff. Manchester


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Line Coding Spectra


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Block Coding

Enhances the performance of line codingwhile also introducing some error-detecting

capability Based on substituting a block of n bits for a

block of m bits, where n > m

A dictionary contains the mapping. Someof the n-bit blocks are not used in the one-to-one mapping


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Block Coding cont.


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Block coding subsitution(m=4, n=5)


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Block coding cont.

Errors can be detected if the received n-bitword is invalid

Also called mBnB coding

Used in some of the newer Ethernetstandards

100B

ase-TX (2-wire twisted pair) 100Base-FX (Fiber)

1000Base-T (2-wire Gigabit Ethernet)


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RF Transmission Not baseband

Requires modulation

The placement of data onto a cosinusoidal signal

Multiple bits may be mapped into one

modulation symbol

Baud rate = modulation symbol rate

Traditional schemes:ASK, FSK, PSK, QAM


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ASK Amplitude Shift Keying Susceptible to channel degradations


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FSK Frequency Shift Keying


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PSK Phase Shift Keying

BPSK: bit rate = baud rate, 0 or 180 deg. phase

QPSK: bit rate = 2 * baud rate, [45, 135, 225, 315] deg.


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QAM Quadrature Amplitude

Modulation Combined ASK and PSK

Higher-order modulation scheme that

lowers the symbol rate

More susceptible to noise and nonlinearities

Used in most modern phone modems


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8-QAM


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Multiplexing Transmission resources are usually limited

in either time, frequency, or both

Normally two separate signals cannot sharethe same time and frequency space

As multiple users or segments become

necessary, a method of sharing the theseresources is critical

Multiplexing allows this sharing


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FDM Frequency Division

Multiplexing The frequency channel is divided and each

user receives one portion of the spectrum

Requires at least one non-baseband signal

Guard bands are used to limit the effect of

adjacent channel interference (ACI)


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FDM cont.


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FDM cont.


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Time-division Multiplexing

(TDM) Dividing by time

Supports any combination of baseband and

modulated signals

Two types of TDM:

Synchronous TDM

Asynchronous TDM


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Synchronous TDM Each user (1, 2, n) is allocated a time slot

A frame consists of one full cycle of a time

slot from every user

Requires framing bits for time slot

synchronization

Inefficient if data is not always being sent

by ALL users


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Synch. TDM Cont.


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Asynchronous TDM m time slots for n users, m < n

Time slots are not reserved for each user

Scans user input lines for available data

Tries to fill all time slots during each frame

Requires addressing overhead for correct

de-multiplexing

Typically more efficient that synch. TDM


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Asynch. TDM cont.


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Limits on Data Throughput Nyquist Bit Rate

Noiseless, bandlimited channels

Bit Rate (bps) = 2 x B x log2(L) L = # of signal levels used to represent the data

B = frequency bandwidth available (Hz)

Shannons Capacity Theorem

Bandlimited channels with noise

C (bps) = B x log2(1 + SNR)

SNR = signal-to-noise ratio of the channel


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Nyquist Example A noiseless channel with a 5 kHz bandwidth

and binary transmission (2 levels) can

deliver:

Bit Rate = 2 x 5000 x log2(2) = 10,000 bit/sec.

If transmission using 4 bits/symbol is used

(16 levels) thenBit Rate = 2 x 5000 x log2(16) = 40,000 bit/sec.


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Shannon Capacity Example A modem operating over a telephone line

has a maximum useful bandwidth of about

3400 Hz (300 Hz to 3700 Hz). Themaximum SNR of the channel is 39 dB.

What is the maximum capacity?

First, un-dB the SNR: SNR = 10^(39/10) = 7943

C = 3400 x log2(1+7943) = 44 kbps

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