DIGITAL COMMUNICATION AND NETWORKS
Data transmission - is the physical transfer of data over a point-to-point
or point-to-multipoint transmission medium either wired or wireless.
Data – is a distinct piece of information. Data can exist in a variety of
forms such as sound, text, numbers or images. The data is often
represented as an electro-magnetic signal, such as an electrical voltage
signal, a radio wave or an infra-red signal.
Types of Data Transmission
Serial Transmission - is a method of data transmission in which the bits
of a data character are transmitted sequentially over a single channel.
Parallel Transmission - is a method of data transmission in which the bits
of a data character are transmitted simultaneously over a number of
channels/ports.
Transmission Media – refers to any modes or materials which can propagate
waves or electrical energy for telecommunications purposes.
TRANSMISSION MEDIA
The physical path through which computers send and receive these signal is
called the transmission medium.
Transmission media are divided into two categories:
1. Cable Media (Guided) – transmits signal along a solid medium such as
a transmission line.
2. Wireless Media (Unguided) - transmits signal over large distance by
using antenna. It employs the higher electromagnetic frequencies, such
as radio waves, microwaves and infrared.
Types of material use in guided transmission:
1. Twisted pair cable
3. Fiber optics
2. Coaxial cable
Three main types of wireless media:



Radio wave
Microwave
Infrared
Radio wave - Radio frequencies can be transmitted across electrical cables
(twisted pair or coaxial) or by using radio broadcast transmission.
Microwave – microwave transmissions are use to communicate between earth
stations and satellites.
Infrared Media – infrared media use infrared light to transmit signals.
Characteristics of Transmission Media:
1.
2.
3.
4.
5.
6.
Cost
Installation Requirements
Bandwidth
Band Usage (Baseband or Broadband)
Attenuation
Immunity from electromagnetic interferences
Bandwidth
- In computer networking, the term bandwidth refers to the capacity of
a medium to transmit data.
Bandwidth is measured in:




Bps - bits per second
Kbps - kilo-bits per second
Mbps - mega bits per second
Gbps - gigabits per second
NOTE: A medium that has a high capacity has a high bandwidth, whereas a
medium that has a limited capacity has a low bandwidth.
The greater the bandwidth, the more data can be transferred in a given
time.
Attenuation – is a measure of how much a signal weakness as it travels
through a medium. Electromagnetic signals tend to weaken during the
transmission.
Electromagnetic Interference (EMI) – consist of outside electromagnetic
noise that distorts the signal in a medium. It affects the signal that is
sent through the transmission media making it difficult for the receiving
computer to accept or decode the signal.
Signaling is the way data is transmitted across the medium.
Two forms of signaling:
1. Analog Signaling
2. Digital Signaling
Analog Signal - The signal propagated through the medium is continuously
varying electromagnetic waves.
Digital Signal - Digital signals vary between two discrete values of some
physical quantity, one value representing the binary number 0 and the
other representing 1.
Baseband and Broadband Transmission
Baseband – A signaling technology that sends digital signals over a single
frequency as discrete electrical pulses. The signal uses the entire
bandwidth of a transmission medium. The baseband signal is bidirectional
so that a baseband system can transmit and receive signals simultaneously.
Broadband – A signaling technology that sends signal over a range of
different frequencies as electromagnetic waves. A single transmission
medium is divided and shared simultaneously.
Broadband signals are unidirectional-traveling in only one direction at a
time so a broadband system can generally either transmit or receive but
cannot do both simultaneously.
TRANSMISSION MEDIUMS
Unshielded Twisted
Pair Cable
Shielded Twisted Pair
Cable
Coaxial Cable
Fiber Optics Cable