In the physical layer the physical and electrical characteristicts of wire and the description about capacitance,impedance and attenuation.

1. The Electrical characteristicts
The performance of the wired network is
greatly dependent on the electrical
characteristicts of the cable used.
1.Capacitance
2.Impedance
3.Attenuation

2. capacitance
• Capacitance is the property of a circuit that permits it to store an electrical
charge.
• The capacitance of a cable determines its ability to carry a signal without
distortion,which is rounding of the waveform due to stored charge
between the conductors of a cable.
• The more distorted the signal becomes the more likely a receiving node
will be unable to distinguish between 0’s and 1’s.
• High quality cable has low capacitance, the lower the capacitance the
longer the distance a signal can travel before signal distortion becomes
unacceptable.
• Network cables can have low characteristicts capacitance per meter,the
overall capacitance of a cable increases as the cable gets longer.
• Because of noise and other problems in the transmission, a maximum
cable length of about 100m exists for for unshielded twisted pairs network
cable.

4. Impedance
• Impedance is a measure of the opposition to the flow
of electrical current in an alternating current circuit.
• It is measured in ohms.
• Impedance is a function of capacitance ,resistance and
inductance.
• Impedance mismatches ,caused by mixing cables of
different types with different characteristicts
impedances,can result in signal distortion.
• For example token ring network cable requires 150Ωof
impedance .
• Ethernet and twisted pair networks want 85-111 Ω

5. Attenuation
• Attenuation is decrease in signal strength.whcich occurs as the signal
travels through a circuit or along a cable.
• The longer the cable the greater the attenuation.
• The higher the frequency the greater the attenuation .
• Different types of cables also subject to different amounts of
attenuation.
• In the twisted pairs the attenuation rises sharply ,as the signal
frequency increases.
• In the coaxial cables it rises less sharply as frequency increases.
• It is measured in decibels of signal loss.
• While selecting a cable,you should a type that has low mesure of
attenuation for the network speeds and distances involved.
• Signal quality is effected by the combination of the attenuation and
capacitance.

7. COPPER MEDIA
• It consists of
1.STP
2.UTP
3.IBM CABLE
4.COAXIAL CABLE

8. UNSHIELDED AND SHIELDED
TWISTED PAIR CABLE.
• Twisted pairs are the most popular type of cable used
in networks today.
• Twisted pair cable consists of 2 insulated copper wires
that have been twisted together.
• Data transmission requires 4 wires.
• One pair to transmit data and pair to receive data
• Two types of cables are there
• 1.unshielded
• 2.shielded
• Standards of the UTP ans STP are provided by EIA/TIA-
568 which is north american standard used world wide.

10. • Categories 3 and 5 mostly used for voice
transmission
• UTP poses two main problems in data
transmission at the higher frequencies
• 1.cross talk
• 2.attenuation.
• The combined effects of cross talk and distortion
results in the irregular variation in the shape or
timing of a signal.
• This irregular variation is called jitter.
• Jitter is mainly caused by shielded and unshieled
cable.

11. IBM CABLE
• Ibm has its own classifiaction cable,the IBM
cable system which specifies nine cable types.
• TYPE:
• It is a grouping of categories and fibre optic
cables in a bundle based up on which type is
being conducted.
• A category is an EIA specification for the
cables construction.