The Physical layer of the OSI model is concerned with the
transmission of the stream of data in bits. Data transmission between networks
takes place through the Physical layer. This transmission takes place using
various transmission media, such as:
Magnetic media: Physically transmits
data from one host to another. Magnetic transmission media provides maximum
bandwidth for data transmission and can be reused multiple times to transmit
data. But magnetic media are prone to destruction, both accidental or due to
natural calamity. Examples of magnetic media are floppy disks and cassette
tapes.
Twisted pair cables: Pairs of copper
wires that are twisted to reduce electrical interference from other twisted pair
cables placed alongside. Twisted pair can transmit data across long distances
with the help of repeaters. In addition, the bandwidth provided by twisted pair
depends upon the thickness of the cable and the distance it must travel. The
most common application of twisted pair cables is the telephone system.
Coaxial cable: Copper wire cable
protected from changes in temperature by an insulating material and a conducting
material. Coaxial cable provides high bandwidth and can facilitate high-speed
data transmission for longer distances than twisted pair cables.
Fiber optics: Uses light signals to
transmit data over long distances without using repeaters.
|
Note |
Coaxial cables are of two types: baseband and broadband.
Baseband coaxial cable is of 50 ohm and transmits digital data; broadband cable
is of 75 ohm and transmits analog data. (Ohm is the unit for
resistance.) |
Problems of the Physical layer are related to the choice of the
transmission media, which depends on the amount, distance, and rate of data
transmission.
If data needs to be transmitted in small blocks, over a short
distance, and frequency of data transmission is less, twisted pair or coaxial
cable can be used. However, if large blocks of data are to be transmitted over a
long distance and with high frequency, optical fiber should be used.
Apart from this, data can be transmitted across the networks using
wireless media such as radio, infrared, and microwave transmission. These
transmission media provide connectivity to mobile users. Unlike static
transmission media, wireless media provide long-distance communication and data
transmission, which spans continents across the globe.
The Physical layer also deals with the frequency and
bandwidth problems of the data being transmitted. The Physical layer applies
multiplexing, such as time-division and frequency-division multiplexing, to
overcome problems related to optimizing bandwidth of the transmission media.
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