Questions

1.Think of some general techniques by which you can identify the digital & Analog Signals

Transmission Impairments

Transmission Impairments

_ Attenuation

1 To reduce the amplitude of an electrical signal with little or no distortion
2 Logarithmic in nature for guided media; expressed as a constant number of decibels per unit distance
3For unguided media, complex function of distance and atmospheric conditions

Three considerations for transmission engineer
1. Received signal must have su_cient strength to enable detection
2. Signal must maintain a level su_ciently higher than noise to be received without error
3. Attenuation is an increasing function of frequency

Signal strength must be strong but not too strong to overload the circuitry of transmitter or receiver, which will cause distortion
Data Transmission 21
Beyond a certain distance, attenuation becomes large to require the use of repeaters or ampli_ers to boost the signal

Attenuation distorts the received signal, reducing intelligibility
_ Attenuation can be equalized over a band of frequencies
_ Use ampli_ers than can amplify higher frequencies more than low frequencies
_ Delay distortion
Peculiar to guided transmission media Caused by the fact that the velocity of signal propagation through a guided medium varies with frequency
In bandlimited signal, velocity tends to be highest near the center frequency and falls o_ towards the two edges of band
_ Varying frequency components arrive at the receiver at di_erent times, resulting in phase shifts between di_erent frequencies
In digital data transmission, some signal components of one bit position will spill over into other bit positions, causing intersymbol interference
May be reduced by using equalization techniques
_ Noise
Undesired signals that are inserted into the real signal during transmission
Four types of noise
1. Thermal noise
_ Also called white noise
_ Occurs due to thermal agitation of electrons
_ Function of temperature and present in all electronic devices
_ Uniformly distributed across frequency spectrum
_ Cannot be eliminated and places an upper bound on system performance
_ Thermal noise in a bandwidth of 1 Hz in any device or conductor is

_ Noise is assumed to be independent of frequency
_ Thermal noise in a bandwidth of B Hz can be expressed as

2. Intermodulation noise
_ Signals at di_erent frequencies share the same transmission medium
_ May result in signals that are sum or di_erence or multiples of original frequencies
_ Occurs when there is some nonlinearity in the transmitter, receiver, or intervening transmission
system
_ Nonlinearity may be caused by component malfunction or excessive signal strength
3. Crosstalk
_ Unwanted coupling between signal paths
_ Occurs due to electric coupling between nearby twisted pairs, multiple signals on a coaxial cable,
or unwanted signals picked up by microwave antennas
_ Typically same order of magnitude or less than thermal noise
4. Impulse noise
_ Noncontinuous noise, consisting of irregular pulses or noise spikes of short duration and high
amplitudes
_ May be caused by lightning, or aws in communications system
_ Not a major problem for analog data but can be signi_cant for digital data
_ A spike of 0.01 s will not destroy any voice data but will destroy 560 bits being transmitted at
56 kbps
_ Channel capacity

Maximum rate at which data can be transmitted over a communication path or channel
Depends on four factors
1. Data rate { in bps
2. Bandwidth { constrained by transmitter and nature of transmission medium, expressed in cycles per second, or Hz
3. Noise { Average noise level over channel
4. Error rate { Percentage of time when bits are ipped
{ Bandwidth is proportional to cost
_ For digital data, we'll like to get as high a data rate as possible within a limit of error rate for a given
bandwidth
{ Nyquist bandwidth }

Basic Concepts