ELET 4315 Homework #2 (Chapters 4 – 7)
Why is it necessary to convert the analog signal into a digital signal?
In the real world, most data is characterized by analog signals. In order to manipulate the data
using a microprocessor, we need to convert the analog signals to the digital signals, so that the
microprocessor will be able to read, understand and manipulate the data.
List and explain each of the steps involved in converting the digital signal into an analog signal.
Analog-to-digital conversion (A/D): An analog signal is sampled at the sampling frequency and
the sample values are then represented as numerical values by the encoder. These values,
presented as binary words, are then transmitted within regular time periods
through the digital channel.
2. Digital-to-analog conversion (D/A): At the other end of the channel, the decoder receives
numerical values of the samples that indicate the values of the analog signal at sampling instants.
The sample pulses that have amplitudes corresponding to the values of the original signal at
sampling instants are reconstructed and the series they form is filtered to produce an analog
signal close to the original one. The methods for these A/D and D/A conversions have to be
specified in detail so that the reproduction of the analog signal is compatible with the production
of the digital signal that may have occurred on the other side of the world.
Explain what is meant by the term PCM.
Pulse code modulation digital word, Pulse Code Modulation (PCM) is the simplest form of
waveform coding. Waveform coding is used to encode analogue signals (for example speech)
into a digital signal. The digital signal is subsequently used to reconstruct the analogue signal.
The accuracy with which the analogue signal can be reproduced depends in part on the number
of bits used to encode the original signal. Pulse code modulation is an extension of Pulse
Amplitude Modulation (PAM), in which a sampled signal consists of a train of pulses where
each pulse corresponds to the amplitude of the signal at the corresponding sampling time (the
signal is modulated in amplitude). Each analogue sample value is quantized into a discrete value
for representation as a digital code word. Pulse code modulation is the most frequently used
analogue-to-digital conversion technique. The main parts of a conversion system are the encoder
(the analogue-to-digital converter) and the decoder (the digital-to-analogue converter). The
combined encoder/decoder is known as a codec. A PCM encoder performs three functions
sampling, quantizing, and encoding
The human voice uses frequencies between 100Hz and 10,000Hz, but it has been found that most
of the energy in speech is between 300 Hertz and 3400 Hertz - a bandwidth of approximately
3100 Hertz. Before converting the signal from analog to digital, the unwanted frequency
components of the signal are filtered out. This makes the task of converting the signal to digital
form much easier, and results in an acceptable quality of signal reproduction for voice
communication. From an equipment point of viev, because the manufacture of very precise
filters would be expensive, a bandwidth of 4000 Hertz is generally used. This bandwidth
limitation also helps to reduce aliasing - aliasing happens when the number of samples is
insufficient to adequately represent the analog waveform (the same effect you can see on a
computer screen when diagonal and curved lines are displayed as a series of zigzag horizontal
and vertical lines).
What portion of the telephone network still depends on analog signals?
However analog two-wire circuits are still used to connect the last mile from the exchange to the
telephone in the home (also called the local loop).
What is the purpose of placing a multiplexer at the end of a circuit?
One use for multiplexers is cost saving by connecting a multiplexer and a demultiplexer (or
demux) together over a single channel (by connecting the multiplexer's single output to the
demultiplexer's single input). At the receiving end of the data link a complementary
demultiplexer is normally required to break single data stream back down into the original
streams. In some cases, the far end system may have more functionality than a simple
demultiplexer and so, while the demultiplexing still exists logically, it may never actually happen
physically. This would be typical where a multiplexer serves a number of IP network users and
then feeds directly into a router which immediately reads the content of the entire link into its
routing processor and then does the demultiplexing in memory from where it will be converted
directly into IP sections.
Often, a multiplexer and demultiplexer are combined together into a single piece of equipment,
which is usually referred to simply as a "multiplexer". Both pieces of equipment are needed at
both ends of a transmission link because most communications systems transmit in both
The introduction of multiplexing technology helped eliminate the need to add new facilities of
force customers to compete for space every time a customer was added. In analog circuit design,
a multiplexer is a special type of analog switch that connects one signal selected from several
inputs to a single output.
Define the term modulation.
Super imposing the information signal into a steady state signal
The term asynchronous data indicates the bandwidth in one direction, (bandwidth from the
Internet to the customer versus bandwidth from the customer to the Internet) is greater than the
bandwidth in the opposite direction.
a. True b. False
What purpose does the OSI stack perform?
The seven-layer architecture defines how end-to-end communication process function starting at
the physical medium and ending with the presentation of the application.
Although it does not actually perform any functions or do any of the actual work, the OSI model
defines the way things should be done by the software and hardware on a network so that
communications can take place between two computers or nodes
The Open Systems Interconnection (OSI) reference model was developed by the International
Standards Organization (ISO) as a model for computer communications architectures, and as a
framework for developing protocol standards. It was intended as a first step towards international
standardization of communications protocols. The model divides the communication process into
seven layers, as shown below. The diagram shows how communication takes place indirectly
between peer layers at each end of a communications channel (denoted by the bi-directional
horizontal arrows), and clearly identifies the concept of an interface between adjacent layers
(denoted by the bi-directional vertical arrows).
What are the advantages of organizing the functions of the OSI module into layers?
A major advantage of the OSI model is that it clearly distinguishes between the concepts of
services, interfaces and protocols. A strictly modular approach to the design of system
architecture is encouraged, allowing the protocols operating within each layer to be replaced
relatively easily. The purely theoretical basis for the model means that it is not biased towards a
particular technological approach, and makes it very useful as a reference model, although it also
means that the model does not benefit from practical experience, as a result of which some fairly
arbitrary decisions have been made about what functionality should go into each layer. The
session and presentation layers, for example, do not actually do a great deal, whereas the data-
link layer has had to be divided into two distinct sub-layers (LLC and MAC).
What functions reside under the Physical Layer?
Physical layer - concerned with the physical transmission of a bit stream. Issues include the
physical and electrical characteristics of the cables and connections, the encoding and signaling
schemes used, and the mechanical, electrical and procedural interfaces. Network devices that
operate at this layer include hubs and repeaters.
Manipulating the voltage on the line falls under the Data Link Layer.
a. True b. False
List 2 Layer 1 protocols.
Layer 1 protocol is responsible for:
a. Define the physical characteristics of the medium.
b. Defines the functional characteristics of the signal.
c. Technique used to modulate the signal.
d. Defines the frame format of the signal.
e. A, B and D only
f. A and B only
g. A, B and C only
h. None of the above
Why is statistical multiplexing a more efficient way to transport information across a network
than traditional circuit switched TDM networks?
Time-division multiplexing (TDM) is a method of transmitting and receiving independent
signals over a common signal path by means of synchronized switches at each end of the
transmission line so that each signal appears on the line only a fraction of time in an alternating
Statistical time division multiplexing (STDM) is an advanced version of TDM in which both the
address of the terminal and the data itself are transmitted together for better routing. Using
STDM allows bandwidth to be split over one line. Many college and corporate campuses use this
type of TDM to distribute bandwidth.
On a 10-Mbit line entering a network, STDM can be used to provide 178 terminals with a
dedicated 56k connection (178 * 56k = 9.96Mb). A more common use however is to only grant
the bandwidth when that much is needed. STDM does not reserve a time slot for each terminal,
rather it assigns a slot when the terminal is requiring data to be sent or received.
What does the term backbone refer to in telecommunications?
The portion of the network that connects main data of switch centers refers to the long haul
portion of the network.
Which part of the network is considered to be the access portion?
The access network is also perhaps the most valuable asset an operator owns, since this is what
physically allows them to offer a service. Access networks consist largely of pairs of copper
wires, each traveling in a direct path between the exchange and the customer.
Frame relay has a fixed length frame of fifty-three octets.
Frame relay is able to carry time-sensitive traffic such as voice and video as easily as it carries
a. True b. False
Why can a circuit carry multiple PVCs?
Two types of circuits exist: permanent virtual circuits (PVCs) which are used to form logical
end-to-end links mapped over a physical network, and switched virtual circuits (SVCs). The
latter are analogous to the circuit-switching concepts of the public switched telephone network
(PSTN), the global phone network. PVC‘s are usually for long term use ( months, years)
between two end points. Once establish, no further intervention by the network is required to
transfer end-user information between two locations. Each PVC is made up of a series of
connected virtual paths(VPs) or virtual channels (VCs). The user more VCs than LLC
encapsulation, but reduces overhead, because a header is not necessary. The user multiplexes
multiple protocols over a single ATM VC. The protocol of a carried protocol of a carried
protocol data unit (PDU) is identified by prefixing the PDU with an LLC hander.
What does ATM stand for?
Asynchronous transfer mode
ATM has a fixed length cell that can hold fifty-three octets of information.
What is the CBR standard used for?
An application wishing to send data across an ATM network should advise the network of the
type of data is to be sent, together with any Quality of Service (QoS) requirements. The ATM
Forum has defined five service categories in order to try and match traffic characteristics and
QoS requirements to network behavior, which are described below.
Constant Bit Rate (CBR) - used for traffic requiring a consistent and predictable bit rate for the
lifetime of the connection. Typical applications include video conferencing and telephony.
Ethernet protocol was designed as a logical star and physical bus topology.
a. True b. False
The Ethernet switch is used to:
A. Repeat the signal and extend the distance the signal can travel.
B. Reduce traffic on a segment by maintaining bridge tables that contain device
C. Connecting multiple stations together thus improving the efficiency of the physical wire
D. A and B
E. B and C
F. All of the above
F. All of the above