T305: Digital Communications

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T305: Digital Communications
Block I – Representing Systems Modeling Activity Arab Open University-Lebanon Tutorial 3

Arab Open University-Lebanon Tutorial 3
Exercises 1 A piece of music is to be recorded in digital format: Frequency range: 100 Hz – 18KHz Bit resolution 16 bits What will be the bit rate in second Answer: Sampling rate will be 18X2 KHz=36KHz. Each sample will take 16 bits Bit rate will be 16X36KHz=570 KBit/s Arab Open University-Lebanon Tutorial 3

Arab Open University-Lebanon Tutorial 3
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= Kbit/s the number of calls = Kbit/s divided on 64 Kbit/s =576 calls simultaneously Arab Open University-Lebanon Tutorial 3

Arab Open University-Lebanon Tutorial 3
Exercises 3 What 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/s Velocity of propagation on the cable v=1.77X10p8 m/s Delay through repeater is 1 Micros. PC 2 pc1 Repeater 1 repeater2 80 m 100 m 80 m Arab Open University-Lebanon Tutorial 3

Arab Open University-Lebanon Tutorial 3
Answer 3 The distance between two PC will be =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 =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* time Nr of Bites =10 Mbit/s X 6.9 micro s= 69 bits Arab Open University-Lebanon Tutorial 3

Arab Open University-Lebanon Tutorial 3
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 = Arab Open University-Lebanon Tutorial 3

Arab Open University-Lebanon Tutorial 3
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 row A 1000 kbits/s E 320 kbit/s B 430 kbits/s F 270 kbit/s C 650 kBits/s G 720 kbits/s D 580 kbits/s H 680 kbits/s Sampling rate=2x20KHZ Mimum bit rate = 16x2x20KHZ= 640 kb/s H is the closest Arab Open University-Lebanon Tutorial 3

Arab Open University-Lebanon Tutorial 3
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 mV B 5.75 μV F 3.45 μV C 10 μ V G 15 V V D 18 μV H 100 μV Quantization interval=12/221 = 5.75 μV Arab Open University-Lebanon Tutorial 3

Arab Open University-Lebanon Tutorial 3
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 4 A μs E 2.9 μ s B 2 μs F 5 μs C 10 μ s G 6 μs D 4 μs H 100 μs T=2.3x108 / 190 = 1.9 μs Slot time = 2x 1.9 μs = 3.8 μs D is correct Arab Open University-Lebanon Tutorial 3

Arab Open University-Lebanon Tutorial 3
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 Arab Open University-Lebanon Tutorial 3

Arab Open University-Lebanon Tutorial 3
Q4 The first three packets to be sent are: Packet 1 from PC3 to PC6 Packet 2 from PC4 to PC5 Packet 3 from PC2 to PC4 Draw the forwarding tables after these three packets have been sent Arab Open University-Lebanon Tutorial 3

Arab Open University-Lebanon Tutorial 3
Table H H is correct Arab Open University-Lebanon Tutorial 3

Lecture 3 Representing Systems

Outline Introduction Finite State Machines Object Oriented Models
Specification and Description Language

Arab Open University-Lebanon Tutorial 3
Introduction Models 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. Arab Open University-Lebanon Tutorial 3

Arab Open University-Lebanon Tutorial 3
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. Arab Open University-Lebanon Tutorial 3

Arab Open University-Lebanon Tutorial 3
Finite State Machines The behavior of many communication systems can be expressed in terms of output signals generated in response to input signals. Arab Open University-Lebanon Tutorial 3

Finite State Machines State transition tables
The behavior of a state machine can be represented by a state transition table as shown below. Arab Open University-Lebanon Tutorial 3

Finite State Machines State transition diagrams (cont.)
Arab Open University-Lebanon Tutorial 3

Finite State Machines Signal sequence diagrams
Signal 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. Arab Open University-Lebanon Tutorial 3

Finite State Machines Signal sequence diagrams (cont.)
Arab Open University-Lebanon Tutorial 3

Object Oriented Models
A 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. Arab Open University-Lebanon Tutorial 3

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. Arab Open University-Lebanon Tutorial 3

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. Arab Open University-Lebanon Tutorial 3

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. Arab Open University-Lebanon Tutorial 3

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: Identify the classes of objects which make up the system. Define the relationships between the classes. Specify the attributes for the classes Identify the messages which need to pass between the objects. Assign operations to the different classes. Arab Open University-Lebanon Tutorial 3

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. Arab Open University-Lebanon Tutorial 3

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