Explain baseband and broadband transmission. discuss attenuation, distortion and noise.Ripunjay Tiwari
ans. baseband transmission- when a digital signal is transmitted on the medium using one of the signal codes like NRZ, RZ Manchester biphase-M code etc. is called baseband transmission.baseband transmission is limited to low data rates because at high data rates, significant frequency components Vare spread over a wide frequency band over which the transmission characteristics of the medium do not remain uniform. for faithful and reproduction of a signal it is necessary that the relative amplitudes and phase relationships of the frequency components are maintained during transmission .for transmitting data at high bit rates we need to use modulation techniques.
broadband transmission- broadband system use modulation techniques to reduce the effect of noise in the environment. broadband transmission employs multiple channel unidirectional transmission using combination of phase and amplitude modulation.It uses conventional CATV components and a remodulator at the head end. Separate carriers are used for the transmit andreceive directions. There can be several transmit and receive carriers and each
carrier provides 5 or 10 Mbps data rate. Broadband LAN can cover a span ofseveral kilometres.
Attenuation –Attenuation means loss of energy. When a signal, simple
or complex, travels through a medium, it losses some of its energy so that it
can overcome the resistance of the medium. That is why a wire carrying
electrical signals gets warm, if not hot, after a while. Some of the electrical
energy in the signal is converted to heat. To compensate for this loss, amplifiers
are used to amplify the signal. Fig. 1.10 shows the effect of attenuation and
also shows the effect of amplification.
To indicate that a signal has lost or gained strength, use the concept of decibel. The decibel (dB) measures the relative strengths of two signals or a signal at two different points. Note that the dB is negative if a signal is attenuated and positive if a signal is amplified.
dB= 10 log10 ( P2/P1)
where P1 and P2 are the power of a signal at points 1 and 2.
Distortion – Distortion means that the signal changes its form or shape.
Distortion occurs in a composite signal, made of different frequencies. Each
signal component has its own propagation speed through a medium and, thecfore, its own delay in arriving at the final destination. Fig. 1.11 shows the effect of distortion on a composite signal.
Noise- Noise is unwanted signal. Several types of noise such as thermal
noise, induced noise, crosstalk, and impulse noise may corrupt the signal.
thermal noise is the random motion of electrons in a wire that creates an extra
signal not originally sent by the transmitter. Induced noise comes from sources
such as motors and appliances. These devices act as a sending antenna and
the transmission medium acts as the receiving antenna. Crosstalk is the effect
of one wire on the other. One wire acts as a sending antenna and the other as
the receiving antenna. Impulse noise is a spike (a signal with high energy in a
very short period of time) that comes from power lines, lightning, and so on.
Fig. 1.12 shows the effect of noise on a signal.