Signal strength falls off with distance

• Depends on medium

• Received signal strength:

— must be enough to be detected

— must be sufficiently higher than noise to be received

without error

• Attenuation is an increasing function of

frequency

__Delay Distortion__• Only in guided media

• Propagation velocity varies with frequency

**Noise**

• Additional signals inserted between transmitter

and receiver

• Thermal

— Due to thermal agitation of electrons— Uniformly distributed

— White noise

• Intermodulation

— Signals that are the sum and difference of originalfrequencies sharing a medium

**• Crosstalk**

— A signal from one line is picked up by another

**• Impulse**

— Irregular pulses or spikes

— e.g. External electromagnetic interference

— Short duration

— High amplitude

**Channel Capacity**

**Data rate**

In bits per second, bps (not Bps)

— Rate at which data can be communicated

**Bandwidth**

— In cycles per second of Hertz, Hz

— Constrained by transmitter and medium

**• Convention: not all k’s are equal**

— data rates are given as power of 10

• e.g., kHz is 1000Hz

— data is given in terms of power of 2

• e.g., KByte is 1024 Bytes

Nyquist Bandwidth

• If rate of signal transmission is 2B

then a signal with frequencies no

greater than B is sufficient to carry

the signal rate.

— Why? Assume we have a square wave

of repeating 101010. If a positive pulse

is a 1 and a negative pulse is 0, then

each pulse lasts 1/2 T1 (T1 = 1/f1) and

the data rate is 2f1 bits per second.

• If we limit the components to a maximum

frequency (restrict the bandwidth) we need to

make sure the signal is accurately represented.

• Based on the accuracy we require, the

bandwidth can carry a particular data rate. The

theoretical maximum communication limit is

given by the Nyquist formula:

C= 2B log2M

C = capacity or data transfer rate in bps

B = bandwidth (in hertz)

M = number of possible signaling levels

**Signal Strength**

— An important parameter in communication is the strength

of the signal transmitted. Even more important is the

strength being received.

— As signal propagates it will be attenuated (decreased)

— Amplifiers are inserted to increase signal strength

— Gains, losses and relative levels of signals are expressed in

decibels

• This is a logarithmic scale, but strength usually falls logarithmically

• Calculation of gains and losses involves simple addition and

subtraction

__Delay Distortion__— Different frequency components of a signal

• are attenuated differently, and

• travel at different speeds through guided

media

— This may lead to delay distortion

**Shannon capacity**— A transmission line may experience interference

from a number of sources, called noise. Noise is

measured in terms of signal to noise power ratio,

expressed in decibels:

**Cross Talk**

— near-end crosstalk (NEXT), cross talk of strong

transmit (output) signal to weak receive (input)

signal.

— adaptive NEXT canceling using op-amp

**Noise**

**• Impulse Noise**

— impulse caused by switching, lightning etc.

**• Thermal Noise**

— present irrespective of any external effects

— caused by thermal agitation of electrons

• White Noise

**—**random noise – entire spectrum

— listen:

• http://www.burninwave.com/download/whitenoise.wav

**• Pink Noise**

— “realistic spectrum”

— the power spectral density is inversely proportional

to the frequency

— listen:

• http://www.burninwave.com/download/pinknoise.wav

**REFERENCE:-)**

# CS 420/520

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