T305: DIGITAL COMMUNICATIONS Arab Open University-Lebanon Tutorial 31 T305: Digital Communications...

T305: DIGITAL COMMUNICATIONS

Arab Open University-Lebanon Tutorial 3 1

T305: Digital Communications

Block I – Representing Systems Modeling Activity


2Arab Open University-Lebanon

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Exercises 1

A piece of music is to be recorded in digital format:Frequency range: 100 Hz – 18KHzBit resolution 16 bitsWhat will be the bit rate in second

Answer:Sampling rate will be 18X2 KHz=36KHz.Each sample will take 16 bitsBit rate will be 16X36KHz=570 KBit/s



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Exercises 2

How many 64 Kbit/s telephone calls could be carried simultaneously on 622 Mbit/s link.

Answer:

622 Mbit/s = 622*1024 Kbit/s= 636928 Kbit/s the number of calls = 636928 Kbit/s divided on 64 Kbit/s =576 calls simultaneously



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Exercises 3What is the round tripe delay between two computers, each of which is connected by 80 m of cable to a repeater hub, and two repeaters are connected by 100 m of cable:Assume the following:Operation 10 Mbit/sVelocity of propagation on the cable v=1.77X10p8 m/sDelay through repeater is 1 Micros.

PC 2 repeater2 Repeater 1 pc1

80 m80 m100 m



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Answer 3The distance between two PC will be 80+100+80=260 m.The time to cross over 260 m with velocity v is then calculated using the formula d=v*t:T= d/v=260/1.77X10p8 =1.46 Micro s.There are two repeaters so the delay will be 2 micro second.the signal will need 2+1. 46 =3. 46 micro s to pass from one computer to the other and the double for a round tripe time will be 6.9 micro second.It can also be expressed in bits using the formula No of Bites= bitrate* timeNr of Bites =10 Mbit/s X 6.9 micro s= 69 bits



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Question 4

What is the quantization interval of a 10-bit converter covering range from +8 volts to -8 volts?

Answer:2 power 10 will give 1024 binary combinations.And the range of voltage is 8+8= 16, so dividing 16 over 1024 will give the quantization interval = 0.0156



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Exercises

Q1 (answer yourself) A piece of music is to be recorded in digital format in the manner explained in the sound section of the ICT CD_ROM. The frequency components within the source, to be sampled, range from 100 Hz to 20 kHz and each sample is represented using a 16-bit word. Which of the following options is closest to the minimum bit rate that could be used?Choose the closest option and pencil across the appropriate cell in rowA 1000 kbits/s E 320 kbit/sB 430 kbits/s F 270 kbit/sC 650 kBits/s G 720 kbits/sD 580 kbits/s H 680 kbits/s

souayed-adm



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Q2 (answer yourself)

Determine the quantisation interval of 21-bit converter having a dynamic range of –6V to +6V.Choose the closest option

A 15 mV E 1.2 mVB 5.75 μV F 3.45 μVC 10 μ V G 15 V VD 18 μV H 100 μV



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Q3 (answer yourself)

Determine the round trip delay between the two computers shown in Figure below. The velocity of propagation is 2.3*10 8 m/s and the delay introduced by a hub is 0.6 μs Choose the closest option and pencil across the appropriate cell in row 4A 3.3 μs E 2.9 μ sB 2 μs F 5 μsC 10 μ s G 6 μsD 4 μs H 100 μs



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Q4

Figure below shows three segments of a local area network which are separated by transparent bridges. The forwarding tables for the bridges are at present empty as they have only just been incorporated into the network. B1 and B2 are the bridges and PC1 –to- PC6 are terminals.

Note: definition of forwarding table is a table for each bridge which has two columns, first coulmn it will tell which terminal send the packet, second column tells which segment it comes from



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Q4

The first three packets to be sent are:Packet 1 from PC3 to PC6Packet 2 from PC4 to PC5Packet 3 from PC2 to PC4 Draw the forwarding tables after these three packets have been sent



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Table H


Lecture 3Representing Systems


Outline

Introduction Finite State Machines Object Oriented Models Specification and Description Language



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IntroductionModels or block-diagram are used throughout engineering to represent the behavior and structure of systems.

The structure of a system indicates the components of a system and the relationship between them. The behavior of the system indicates what the system does.



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Introduction (cont.)The modeling techniques can be used to describe communication systems; a system can consist entirely of software or of hardware or both.

Generally, systems are so complex it is necessary to ignore many of the details of the system. This process is called abstraction.



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Finite State Machines

The behavior of many communication systems can be expressed in terms of output signals generated in response to input signals.



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Finite State MachinesState transition tables

Input Output Current State Input Signal Next State Output Signal

State1 InSig1 State2 OutSig1, OutSig2 State1 InSig2 State1 OutSig2 State2 InSig2 State1 ------ State2 InSig1 State2 OutSig2

The behavior of a state machine can be represented by a state transition table as shown below.



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Finite State MachinesState transition diagrams (cont.)

InSig1/OutSig2

InSig1/ OutSig1, OutSig2 InSig2/

State1

State2

InSig2/OutSig2



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Finite State MachinesSignal sequence diagramsSignal sequence diagrams model exchange of signals between a finite state machine and its environment

Signals are represented by sloping lines. Vertical displacement of a sloping line gives a measure of time taken.



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Finite State MachinesSignal sequence diagrams (cont.)

Machine The finite state Machine is the

destination of the signal ‘InSig1’

State 1

State 2

State 1

State 1

Environment

InSig1

OutSig1

OutSig2

InSig2

InSig2

OutSig2

The environment is the source of

the signal ‘InSig1’



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Object Oriented ModelsA system can be viewed as a set of interacting components. In the object-oriented approach to modeling systems, these components are represented by objects.

A class of objects which is a specialization of an existing class is called a subclass and the more general class is called the parent class or super-class.



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Object Oriented Models (cont.)Relationships between objects are also described as associations. For example, some windows may be on top of others on a computer screen. This could be described by the relationship ‘is on top of’ between different window objects.One special type of relationship is when one object is part of another. This is called aggregation or some times containment and is often described as a ‘part of’ relationship. Fro example, a drop-down menu ‘is part of’ a window.



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Object Oriented Models (cont.)Relationships between objects have a multiplicity. This indicates how many instances of each of the classes are involved in the relationship.

In a one-to-one relationship only one instance of each class takes part in the relationship. For example, the aggregation relationship between ‘Window’ and ‘Title bar’ is one-to-one.In a one-to-many relationship there may be several instances of one of the classes but only one of the other. For example, the aggregation relationship between ‘Window’ and ‘Drop-down menu’ is one-to-many.



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Object Oriented Models (cont.)

In a many-to-many relationship each instance of one class may be related to many instances of the other class. For example, the relationship between windows described by ‘is on top of’ is many-to-many.



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Object Oriented Models (cont.) There are a number of different approaches or

methodologies which are used for building object-oriented models of systems. However, each include the following steps:

1. Identify the classes of objects which make up the system.

2. Define the relationships between the classes.3. Specify the attributes for the classes4. Identify the messages which need to pass

between the objects.5. Assign operations to the different classes.



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Object Oriented Models (cont.)Steps 1 to 3 : object model. Steps 4 and 5: dynamic model.one approach is to specify the flow of messages between objects in response to particular events. The diagram which results from this analysis is called an event flow diagram.

Mouse Window Title bar Clicked on window

Window activated

T305: DIGITAL COMMUNICATIONS Arab Open University-Lebanon Tutorial 31 T305: Digital Communications...

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