Politécnica University of Puerto Rico - Hato Rey

Computer Engineering Department

Assignment 2

Christian O. Pérez Pérez – 89536

CECS6130-DATA COMMUNICATION AND COMPUTER)
WI-20
Prof. Lisabel Rodriguez Espinosa;
February 20, 2021
Assignment 2:

The Assignment covers from the first day of class through Chapter 16. All topics that will be covered in
the Final test. Chapters 20 and 21 will also be part of the test although not part of the assignment.

1. Name the differences between a digital signal and an analog signal.

An analog signal is a continuous trough time signal where digital signals are discrete value levels
that are characterized by instantaneous changes between them. Digital signal are only
generated by humans not in nature.
2. How do we represent audio waves in digital signals?
By encoding de signal using sampling technics like Pulse code modulation and Delta modulation.
Using Nyquist theorem, the sampling ratio must be the double of the frequency of acceptable
audio quality (4000Hz) is 8000 samples per second.
3. Name the differences between Pulse Code Modulation and Delta Modulation
For pulse code modulation, signals are sampled, quantized, and encoded. For Delta modulation
it also takes samples but instead of sending a quantization for each sample, delta modulation as
the name implies, ends one quantization value followed by a string of values that give the
difference between the previous value and the current value. Delta modulation if an error
occurs all successive values will be misinterpreted.
4. List the different types of wired transmission media and their characteristics.
a. Unshielded
a. Twisted Pair (UTP) They are twisted pair of wiring without shielding. They are
twisted to reduce electrical noise.
b. Shielded
a. Coaxial cable – Is a cable that contains a metal shielding (from breaded wires to
maintain certain flexibility) that surrounds the center cable that carries the signal.
This provides a barrier to electromagnetic radiation from all direction.
b. Twisted Pair (STP)It’s a metal shield surrounding one or more twisted pairs of wires.
STP cable is more flexible than a coaxial cable and less susceptible to electrical
interference than unshielded twisted pair (UTP).
c. Fiber Optics Immune to electrical noise d Less signal attenuation d Higher bandwidth is a
thin strand of glass or transparent plastic encased in a plastic cover for light transmition.
5. List the main differences between wired and wireless media.

Differences
It is transported via a physical medium. There is no physical medium
It is most appropriate for point to point It is most suitable for broadcasting.
communication_
Signal is transmitted using electrical waves Signal is transmitted using electromagnetic
waves.

6. Types of Satellites and their differences

a. Low Earth Orbit (Leo)- Low Has the advantage of low delay, but the disadvantage Earth
Orbit that from an observer’s point of view on the earth, (LEO) the satellite appears to
 move across the sky . A cluster ok LEO satellites works between them to send and
receive messages.
b. Medium An elliptical (rather than circular) orbit used to Earth Orbit provide
communication at the North and South (MEO) Poles†
c. GEO – Rotates at the same rate as the earth, so it has the same distance to earth all the
time. Having a stationary satellite position means that once a ground station has been
aligned with the satellite, the equipment never needs to move. The disadvantage is that
it requires a lot of distance to work and this mean that dir is delay more delay in the
communication.
7. List the types of errors in the Data Transmission and their characteristics;
a. Interference. It happens when electromagnetic radiation affects the signal the
electromagnetic radiation can come from devices such as electric motors and
background cosmic radiation cause noise that can disturb radio transmissions and
signals traveling across wires.
b. Distortion. All physical systems distort signals. As a pulse travels along an optical fiber,
the pulse disperses. Wires have properties of capacitance and inductance that block
signals at some frequencies while admitting signals at other frequencies. Simply placing
a wire near a large metal object can change the set of frequencies that can pass through
the wire. Similarly, metal objects can block some frequencies of radio waves, while
passing others.
c. Attenuation. This happens because signal become weaker the higher the distance it
need to travel to reach the source.
8. List the types of serial transmissions and explain what is each one of them.
a. Asynchronous – It allows the transmission to occur at any time (physical medium can be
idle) with an arbitrary delay between 2 data terms. This is useful when data is not
constantly generated (random). Since a sender to remains idle an arbitrarily long time
between transmissions, additional information is sent before each transmission to
synchronize with the signal. Standard RS-232 sates that serial communication over short
distances must contain a start bit and a stop bit.
b. Synchronous – For this one, transmission has to occurred without delay, meaning
continuously between data items. To do this, sender and receiver are always
synchronized. This means that no start or stop bit is required.
c. Isochronous- Ins this one, the transmission occurs with a gap between the 2 data terms.
It is an important way to use synchronous transmission.
9. What are the differences between simple, Half-Duplex and Full-Duplex?
a. Symplex- It can send data in a single direction.
b. Full-Duplex – It sends data in two directions.
c. Half-Duplex – It has a shared medium that allows to send information in 2 direction but
can not happened simultaneously. For this, thy must be coordinated.
10. Define what is Modulation and Demodulation. Provide examples.
a. Modulation are the changes made to the carrier by the information that is being sent.
There are 3 main types of modulation for electromagnetic carries
i. Amplitude modulation
ii. Frequency modulation
 iii. Phase shift modulation
b. Demodulation is the process of extracting the information form the modulated signal.
Radios contains demodulators in order to understand de audio.
c. A modem is a hardware device that includes circuitry to perform both modulation and
demodulation; a pair of modems is used for full-duplex communication.
11. List the different types of Multiplexing. Their advantages and disadvantages.
a. Frequency division multiplexing
b. Wavelength division multiplexing
c. Time division multiplexing
d. Code division multiplexing
12. What are the main differences between Narrowband and Broadband technologies?
a. Narrow band are technologies that have low band. They can offer data up to 128 Kbps
i. Dialup telephone connections
ii. Leased circuit using modems
iii. Fractional T1 data circuits
iv. ISDN and other telco data services
b. Broadband are technologies that offer high data rates. The exact boundary is not well
defined between narrowband and broad band. Some say that broad bands are 1Mbps
or higher other states that higher than 128 Kbps is broad bands.
i. DSL technologies
ii. Cable modem technologies
iii. Wireless access technologies
iv. Data circuits at T1 speed or higher
c.
13. Explain in your own words why the downstream and upload stream are not (typically) the same
speeds provided by your ISP.
a. They are not the same because the end user needs date but requesting for this
information those not demand a lot of data. In contrast the provider need to send a lot
of data to the end user for the request.
14. Explain in your own words what is Circuit Switching and what is Packet Switching.
a. Circuit switching is the communication mechanisms that connects the sender and the
receiver.
b. Packet switching – is when information is sent via packets (dividing the message in
multiple block) from the sender to the receiver. It uses statistical multiplexing for this.
15. List at least 3 topologies covered in class and their characteristics.
a. Bus topology- is when the transmission occurs in the same cable or a device that share
that connects them to the same cable medium.
b. Start Topology- When a all computer are attached to a central point.
c. Ring topology – When the computer are link in a loop.
16. Define the following Random Access Protocols and list some of their characteristics: ALOHA,
CSMA/CD and CSMA/CA.
a. ALOHA _ was de pioneer of random access. It contains a central transmitter (also
located geographically centra) with surrounding substations. When a station sends a
packet inbound frequencies, the central transmitter repeats the transmitter repeats the
 message in the outbound frequency receiving the information all the stations. It
confirms that the transmission was successful if the transmitting station gets the
message in the outbound frequencies. With the confirmation it can send a new package.
b. CSMA/CD- (Cable) Carrier Sense Multiple Access With Collision Detection Each station is
monitoring the cable if to see if another transmission is in process. In de case of a
collision, it will use a Binary exponential back off to solve the issue, this is to double the
range of the random delay after each collision.
c. CSMA/CA.-(Wierless) Carrier Sense Multiple Access With Collision Avoidance. Since
transmitter might be far apart they might not know if a transmission is underway,
therefore CSMA/CD does not work well for wireless. The idea is to send a short message
to announce that is going to send and the receiving also send a short message to
confirm that is ready to receive the message. This allows that computer withing range
know that a transmission is going to take place avoiding collision
17. List the differences between PAN and WAN networks. Provide examples for each type of
network.
a. PAN (Personal Area Network) has communication over a short distance for application
of a single user.
b. Wan (wide Area Network)are used for wide ranges cellular and satellite communications
systems
18. List the Wireless LAN standards from the IEEE. Verify what is the most recent standard for
wireless.
a. 802.11a
b. 802.11e
c. 802.11h
d. 802.11i
e. 802.11k
f. 80211.p
g. 802.11r
h. 802.11s
i. The most recent standard is 802.11ay
19. What is the difference between Access Points (AP) and Ad hoc. Provide the characteristics of
each one.
a. Ad hoc is wireless communication among themselves without a base station.
b. Access point a host communicates to a central unit and it then relays the packets.