(1) Most signals found in nature are analog signals. Analog signals are continuous time
signals, so the signals contain a value for all values of time t (H.Baher 1990). Digital
signals are discrete time signals and are defined for discrete values of time t.
Figure 1: Analog signal examples (source:
Digital signal(source: (H.Baher 1990) )
(H.Baher 1990) )
Figure 2:
Analog and digital communication system
Analog communication:
Analog communication system uses analog signals to transfer and receive data. The source
generates the information/message (such as voice, image or text) to be communicated. The
transducer converts the output of the source (the generated information signal) to an electrical
signal for transmission. The transmitter modulates the signal from the transducer using amplitude
modulation, frequency modulation or phase modulation. The signal is also filtered, and the
modulated signal amplified to be transmitted. The channel is the physical medium through which
the signal is transmitted through from the transmitter to the receiver. The receiver’s main
function is to demodulate the modulated carrier signal to obtain the original message signal that
was transmitted. This received signal also contains noise/distortion. The receiver also performs
filtering of the signal and suppression of noise from the received signal. (John G. Proakis,
Masoud Salehi 2002). The output transducer converts the electrical signal back to the original
form of the signal. The output signal is the analog signal output of the transducer.
Figure 4: Analog communication system diagram
Digital communication
Figure 3: Analog communication system diagram
In digital communication system, the information signal is a discrete/ digital signal (the signal
may be of analog form, but it is converted to digital first before transmission. The input
transducer converts the signal into an appropriate form for transmittion.The source encoder then
converts the digital signal into a sequence of binary digits (the digital signal is represented by a
series of binary digits). The channel encoder makes the binary digital signal redundant (by
repeating the bits of the signal), this is done to overcome noise and interference during
transmission of the digital signal. The digital modulator maps the digital information signal to
signal waveforms (e.g. 1 and 0 may be represented by sin (t) and cos (t) respectively). The
channel is the physical medium used to transmit the signal from the transmitter to the receiver
(danyal 2013). The digital demodulator represents each wave form from the modulated signal by
a single binary number (1 or 0) that is an estimate of the signal that was transmitted. The channel
decoder tries to retrieve the original message signal from the encoded signal at the output of
demodulator. It recreates an approximate signal of the original message signal from the
noisy/distorted signal it receives from the demodulator. The output transducer converts the signal
back to its original form.
Figure 5: Block diagram of elements of a digital communication system.
Table 1; Comparison between analog and digital communication
Analog Communication
Advantage

Lower bandwidth requirement

Easier synchronization
Disadvantage
 Less noise immunity
Digital communication
Advantage
 Better noise immunity
 More secure
 Less redundancy than analog signal
(redundancy can be removed)
 Cheaper to implement

Better control of signal fidelity

Lower error rate and probability

Less power requirement
Disadvantage

Less secure

Higher bandwidth requirement

More expensive to implement

More difficult to synchronize

More reduncy

lower signal fidelity

higher error rate and probability

Higher power requirement
RS232 is a serial asynchronous communication method. The two signal levels of the RS232 are
the mark state represented by 1 (high state) and space state, represented by 0. The voltage level
of the transmitter in the high state (logic 1) is 15V to -15V. IN the low state (logic 0) the voltage
level of the transmitter is +5V to +15V. For the Receiver, the voltage levels are as follows, -3V
to -25V in the high state and +3V to +25V in the low state with undefined state between -3V to
+3V.
Figure 6: Figure of table showing voltage levels of states using RS232
Serial peripheral interface
Figure 7: DC characteristics of SPI interface
For the input terminal MOSI (Master out slave in) ,SCK (serial clock) the electrical parameters
are as followed, the input low voltage has a minimum low voltage of -0.3V and input high
voltage of +0.3V. Pull down current min =9 micro amps and max= 29 micro amps, while typical
pull down current is 17 micro amps. Output characteristics shows output low voltage of 0.5V and
output high voltage of Vdd-0.5V and since Vdd is typically 5V the output high voltage would be
4.5V.
(3)
Zigbee is a wireless technology used in low power and low cost, wireless IoT networks.
It is an open global standard that operates on the IEEE 802.15.4 radio specification. Zigbee
operates in the industrial, scientific and medical (ISM) radio bands. It operates at 2.4 GHz band
for global operation, 900 MHz band in the Americas and 868 MHz bands in Europe. Since
Trinidad and Tobago is not located in Europe or the Americas the band that is applicable to
Trinidad and Tobago is the 2.4GHz band. Zigbee operates on mesh technology and can be used
to connect sensors on a network, for monitoring and control applications. One disadvantage is
that zigbee has a low data rate of 250kb/s. on 2.4GHz.
Justification for operation of zigbee in Trinidad and Tobago being 2.4GHz.
Telecommunications Authority of Trinidad and Tobago provides the following information:
Trinidad and Tobago Frequency Allocation Table (8.3 kHz to 3000 GHz)
Figure 8: ITU-R Region 2 Table of Frequency Allocation Footnotes (Trinidad and Tobago allocation)
Figure 9:Trinidad and Tobago Footnotes
Figure 10:Trinidad and Tobago Footnotes
(Trinidad and Tobago Frequency Allocation Table (8.3 kHz to 3000 GHz) n.d.)
2.4GHz is the assigned spectrum for accommodation of wireless network and zigbee global
spectrum is 2.4GHz which will be the spectrum operated by zigbee in Trinidad.
5)
The main spectrum used by wifi products are:802.11a, 802.11b, 802.11g, 802.11n, 802.11ac,
established by the IEEE. The two frequencies used by wifi devices are 2.4GHz and 5 GHz.
2.4GHz is less expensive and unregulated while 5GHz is regulated. 2.4GHz is more stable and
can penetrate objects better than 5GHz wifi. The channel width tells how much bandwidth is
used within the spectrum. The 2.4GHz access point uses a 20MHz wide channel while 5GHz
uses 40MHz wide channel thus it is twice as wide. 2.4GHz is referred to as narrow channel and
5GHz wide channel. The speed and throughput are increased by increasing the channel width
therefore faster transfer rates/ data rate results. The wider the bandwidth the more information
that can be transmitted so there would be higher data speeds. The wider bandwidth is however
more prone to noise and interference.
(Elliot 2018)
Figure 11: Speeds and wireless spectrum
References
danyal, babak. 2013. Digital communication system. August 31. Accessed March 2019.
https://www.slideshare.net/lineking/digital-communication-system.
n.d. Data Speed vs Channel Bandwidth. Accessed March 2019. http://www.rfwirelessworld.com/Terminology/Data-speed-vs-Channel-Bandwidth.html.
n.d. Difference Between Analog and Digital Communication. Accessed March 2019.
https://electronicscoach.com/difference-between-analog-and-digital-communication.html.
Elliot. 2018. How does channel width affect my network? June 27. Accessed March 2019.
https://help.cloudtrax.com/hc/en-us/articles/115000257084-How-does-channel-width-affectmy-network-.
H.Baher. 1990. ANALOG & DIGITAL SIGNAL PROCESSING. New York : John WIley & Sons.
John G. Proakis, Masoud Salehi. 2002. COMMUNICATION SYSTEMS ENGINEERING. New Jersey: Prentice
Hall.
Munir, Asad. 2017. Zigbee Technology. July 29. Accessed March 2019.
https://spectrum.co.ae/blogs/technology/zigbee-technology.
Ramnath. n.d. Question: Explain the block diagram of analog and digital communication system? If
information rate is maximum which type of modulation techniques can be used? Accessed
March 2019. https://www.ques10.com/p/3315/explain-the-block-diagram-of-analog-anddigital--1/.
n.d. RS232 Specifications and standard. Accessed March 2019.
https://www.lammertbies.nl/comm/info/RS-232_specs.html.
Tomato, Learn. 2014. WiFi Frequency, WiFi Channel Width, and Wireless Spectrum. May 6. Accessed
March 2019. https://learntomato.flashrouters.com/wifi-frequency-wifi-channel-width-wirelessspectrum/.
n.d. Trinidad and Tobago Frequency Allocation Table (8.3 kHz to 3000 GHz). Accessed March 2019.
https://tatt.org.tt/DesktopModules/Bring2mind/DMX/Download.aspx?Command=Core_Downlo
ad&EntryId=456&PortalId=0&TabId=222.
n.d. Zigbee PRO offers low-power wireless mesh networking. It is the first multi-band mesh supporting
cross-band communication across 2.4GHz and sub-GHz devices. Accessed March 2019.
https://www.zigbee.org/zigbee-for-developers/zigbee-pro/.
n.d. Zigbee Wireless Mesh Networking. Accessed March 2019.
https://www.digi.com/resources/standards-and-technologies/zigbee-wireless-standard.