Object model components part 1

Object:
An object is a concept, abstraction, or thing with crisp boundaries and meaning for the
problem at hand. All objects have identity and are distinguishable
Objects are the physical and conceptual things we find in the universe around us. Hardware, software, documents, human beings, and even concepts are all examples of objects. For purposes of modeling his or her company, a chief executive officer could view employees, buildings, divisions, documents, and benefits packages as objects. An automotive engineer would see tires, doors, engines, top speed, and the current fuel level as objects. Atoms, molecules, volumes, and temperatures would all be objects a chemist might consider in creating an object-oriented simulation of a chemical reaction. Finally, a software engineer would consider stacks, queues, windows, and check boxes as objects.
Objects are thought of as having state. The state of an object is the condition of the object, or a set of circumstances describing the object. It is not uncommon to hear people talk about the "state information" associated with a particular object. For example, the state of a bank account object would include the current balance, the state of a clock object would be the current time, the state of an electric light bulb would be "on" or "off." For complex objects like a human being or an automobile, a complete description of the state might be very complex. Fortunately, when we use objects to model real world or imagined situations, we typically restrict the possible states of the objects to only those that are relevant to our models.
Objects are thought of as having state. The state of an object is the condition of the object, or a set of circumstances describing the object. It is not uncommon to hear people talk about the "state information" associated with a particular object. For example, the state of a bank account object would include the current balance, the state of a clock object would be the current time, the state of an electric light bulb would be "on" or "off." For complex objects like a human being or an automobile, a complete description of the state might be very complex. Fortunately, when we use objects to model real world or imagined situations, we typically restrict the possible states of the objects to only those that are relevant to our models.
We also think of the state of an object as something that is internal to an object. For example, if we place a message in a mailbox, the (internal) state of the mailbox object is changed, whereas the (internal) state of the message object remains unchanged.
An object
has the following four main characteristics:

• Unique identification
• Set of attributes
• Set of states
• Set of operations (behavior)

Unique identification, we mean every object has a unique name by which it is 

identified in the system. Set of attributes, we mean every object has a set of 

properties in which we are interested in. Set of states we mean values of 

attributes of an object constitute the state of the object. Every object will have 

a number of states but at a given time it can be in one of those states. Set of 

operations we mean externally visible actions an object can perform. When an 

operation is performed, the state of the object may change. 

The box may/may not be divided in
particular regions. Object instances can be used in instance diagrams, which
are useful for documenting test cases and discussing examples.

Class:
A class describes a group of objects with similar properties (attributes), common behavior
(operations), common relationships to other objects, and common semantics
A class describes a collection of similar objects. It is a template where certain
basic characteristics of a set of objects are defined. A class defines the basic
attributes and the operations of the objects of that type. Defining a class does
not define any object, but it only creates a template. For objects to be actually
created, instances of the class are to be created as per the requirement of the
case.
Classes are built on the basis of abstraction, where a set of similar objects is
observed and their common characteristics are listed. Of all these, the
characteristics of concern to the system under observation are taken and the
class definition is made. The attributes of no concern to the system are left
out. This is known as abstraction. So, the abstraction is the process of hiding
superfluous details and highlighting pertinent details in respect to the system
under development.

It should be noted that the abstraction of an object varies according to its 

application. For instance, while defining a pen class for a stationery shop, the 

attributes of concern might be the pen color, ink color, pen type etc., whereas 

a pen class for a manufacturing firm would be containing the other dimensions 

of the pen like its diameter, its shape and size etc. 

Each application-domain concept from the real world that is important to the 

application should be modeled as an object class. Classes are arranged into 

hierarchies sharing common structure and behavior and are associated with 

other classes. This gives rise to the concept of inheritance. 

Through inheritance, a new type of class can be defined using a similar 

existing class with a few new features. For instance, a class vehicle can be 

defined with the basic functionality of any vehicle and a new class called car 

can be derived out of it with a few modifications. This would save the 

developers time and effort as the classes already existing are reused without 

much change. 

ISDN

ISDN (Integrated Services Digital Network) is a set of CCITT/ITU standards for digital transmission over ordinary telephone copper wire as well as over other media. Home and business users who install an ISDN adapter (in place of a telephone modem) receive Web pages at up to 128 Kbps compared with the maximum 56 Kbps rate of a modem connection. ISDN requires adapters at both ends of the transmission so your access provider also needs an ISDN adapter. ISDN is generally available from your phone company in most urban areas in the United States and Europe. In many areas where DSL and cable modem service are now offered, ISDN is no longer as popular an option as it was formerly.

Attenuation of Digital Signals

Signal strength falls off with distance
• Depends on medium
• Received signal strength:
— must be enough to be detected
— must be sufficiently higher than noise to be received
without error
• Attenuation is an increasing function of
frequency
Delay Distortion

• Only in guided media
• Propagation velocity varies with frequency
Noise

• Additional signals inserted between transmitter
and receiver

• Thermal

— Due to thermal agitation of electrons
— Uniformly distributed

— White noise

• Intermodulation

— Signals that are the sum and difference of original
frequencies sharing a medium

• Crosstalk
— A signal from one line is picked up by another
• Impulse
— Irregular pulses or spikes
— e.g. External electromagnetic interference
— Short duration
— High amplitude
Channel Capacity
Data rate

In bits per second, bps (not Bps)

— Rate at which data can be communicated

Bandwidth

— In cycles per second of Hertz, Hz

— Constrained by transmitter and medium

• Convention: not all k’s are equal

— data rates are given as power of 10

• e.g., kHz is 1000Hz

— data is given in terms of power of 2

• e.g., KByte is 1024 Bytes

Nyquist Bandwidth

• If rate of signal transmission is 2B
then a signal with frequencies no
greater than B is sufficient to carry
the signal rate.
— Why? Assume we have a square wave
of repeating 101010. If a positive pulse
is a 1 and a negative pulse is 0, then
each pulse lasts 1/2 T1 (T1 = 1/f1) and
the data rate is 2f1 bits per second.

• If we limit the components to a maximum
frequency (restrict the bandwidth) we need to
make sure the signal is accurately represented.
• Based on the accuracy we require, the
bandwidth can carry a particular data rate. The
theoretical maximum communication limit is
given by the Nyquist formula:
C= 2B log2M
C = capacity or data transfer rate in bps
B = bandwidth (in hertz)
M = number of possible signaling levels
Signal Strength



— An important parameter in communication is the strength
of the signal transmitted. Even more important is the
strength being received.
— As signal propagates it will be attenuated (decreased)
— Amplifiers are inserted to increase signal strength
— Gains, losses and relative levels of signals are expressed in
decibels
• This is a logarithmic scale, but strength usually falls logarithmically
• Calculation of gains and losses involves simple addition and
subtraction
Delay Distortion

— Different frequency components of a signal
• are attenuated differently, and
• travel at different speeds through guided
media
— This may lead to delay distortion
Shannon capacity


— A transmission line may experience interference
from a number of sources, called noise. Noise is
measured in terms of signal to noise power ratio,

expressed in decibels:

Cross Talk



— near-end crosstalk (NEXT), cross talk of strong
transmit (output) signal to weak receive (input)
signal.
— adaptive NEXT canceling using op-amp



Noise

• Impulse Noise
— impulse caused by switching, lightning etc.
• Thermal Noise
— present irrespective of any external effects
— caused by thermal agitation of electrons

• White Noise

— random noise – entire spectrum
— listen:
• http://www.burninwave.com/download/whitenoise.wav
• Pink Noise
— “realistic spectrum”
— the power spectral density is inversely proportional
to the frequency
— listen:
• http://www.burninwave.com/download/pinknoise.wav

REFERENCE:-)

CS 420/520

DATA COMMUNICATION Terminology

Simplex

— One direction
• e.g. Television

Half duplex
— Either direction, but only one way at a time
• e.g. police radio
• Full duplex
— Both directions at the same time
• e.g. telephone

Frequency, Spectrum and
Bandwidth
Time domain concepts

Analog signal
• Varies in a smooth way over time
— Digital signal
• Maintains a constant level then changes to another constant
level
— Periodic signal
• Pattern repeated over time
— Aperiodic signal
• Pattern not repeated over time
Analogue & Digital Signals

Periodic

Signals

Frequency Domain Concepts

Signal is usually made up of many frequencies
• Components are sine waves
• It can be shown (Fourier analysis) that any
signal is made up of component sine waves
• One can plot frequency domain functions
Data Rate and Bandwidth

• Any transmission system has a limited band of
frequencies
• This limits the data rate that can be carried


Reference :-

Data Communication

Simplified File Transfer
Architecture:-
A Three Layer Model

1• Network Access Layer

2• Transport Layer

3• Application Layer

Network Access Layer

• Exchange of data between the computer and
the network
• Sending computer provides address of
destination
• May invoke levels of service
• Dependent on type of network used (LAN,
packet switched etc.)
Transport Layer

• Reliable data exchange
• Independent of network being used
• Independent of application
Application Layer

• Support for different user applications
• e.g. e-mail, file transfer

Protocol Architectures and
Networks
Addressing Requirements

• Two levels of addressing required
• Each computer needs unique network address
• Each application on a (multi-tasking) computer
needs a unique address within the computer
— The service access point or SAP
— The port on TCP/IP stacks





OSI Layers



• Network
— Transport of information
— Higher layers do not need to know about underlying technology
— Not needed on direct links
• Transport
— Exchange of data between end systems
— Error free
— In sequence
— No losses
— No duplicates
— Quality of service

• Session

— Control of dialogues between applications
— Dialogue discipline
— Grouping
— Recovery

• Presentation

— Data formats and coding
— Data compression
— Encryption
• Application
— Means for applications to access OSI environment


• Physical
— Physical interface between devices
• Mechanical
• Electrical
• Functional
• Procedural
• Data Link
— Means of activating, maintaining and deactivating a
reliable link
— Error detection and control
— Higher layers may assume error free transmission

data communication : IPv4 ADDRESSES

An IPv4 address is a 32-bit address that uniquely and universally defines the connection
of a device (for example, a computer or a router) to the Internet.


IPv4 addresses are unique. They are unique in the sense that each address defines
one, and only one, connection to the Internet. Two devices on the Internet can never
have the same address at the same time. We will see later that, by using some strategies,
an address may be assigned to a device for a time period and then taken away and
assigned to another device.
On the other hand, if a device operating at the network layer has m connections to
the Internet, it needs to have m addresses

The IPv4 addresses are universal in the sense that the addressing system must be
accepted by any host that wants to be connected to the Internet.\

Address Space

A protocol such as IPv4 that defines addresses has an address space. An address space
is the total number of addresses used by the protocol. If a protocol uses N bits to define
an address, the address space is 2N because each bit can have two different values (0 or 1)
and N bits can have 2N values.

IPv4 uses 32-bit addresses, which means that the address space is 232 or
4,294,967,296 (more than 4 billion). This means that, theoretically, if there were no
restrictions, more than 4 billion devices could be connected to the Internet

Binary Notation

In binary notation, the IPv4 address is displayed as 32 bits. Each octet is often referred
to as a byte. So it is common to hear an IPv4 address referred to as a 32-bit address or a
4-byte address. The following is an example of an IPv4 address in binary notation:
          01110101 10010101 00011101 00000010


Dotted-Decimal Notation






To make the IPv4 address more compact and easier to read, Internet addresses are usu-
ally written in decimal form with a decimal point (dot) separating the bytes. The fol-
lowing is the dotted~decimal notation of the above address:
                  117.149.29.2




Reference :Data Communications and Networking By Behrouz A.Forouzan

ROUTERS

Routers Are networking devices used to extend or segment networks by forwarding packets from one logical network to another. Routers are most often used in large internetworks that use the TCP/IP protocol suite and for connectingTCP/IP hosts and local area networks (LANs) to the Internet using dedicated leased lines.

Routers work at the network layer (layer 3) of the Open Systems Interconnection (OSI) reference model for networking to move packets between networks using their logical addresses (which, in the case of TCP/IP, are the IP addresses ofdestination hosts on the network). Because routers operate at a higher OSI level than bridges do, they have better packet-routing and filtering capabilities and greater processing power, which results in routers costing more than bridges.

Switches

Switches are a special type of hub that offers an additional layer of intelligence to basic, physical-layer repeater hubs. A switch must be able to read the MAC address of each frame it receives. This information allows switches to repeat incoming data frames only to the computer or computers to which a frame is addressed. This speeds up the network and reduces congestion.

Bridges

A bridge is used to join two network segments together, it allows computers on either segment to access resources on the other. They can also be used to divide large networks into smaller segments. Bridges have all the features of repeaters, but can have more nodes, and since the network is divided, there is fewer computers competing for resources on each segment thus improving network performance.

Bridges can also connect networks that run at different speeds, different topologies, or different protocols. But they cannot, join an Ethernet segment with a Token Ring segment, because these use different networking standards. Bridges operate at both the Physical Layer and the MAC sublayer of the Data Link layer. Bridges read the MAC header of each frame to determine on which side of the bridgethe destination device is located, the bridge then repeats the transmission to the segment where the device is located.

proxy server

A server that sits between a client application, such as a Web browser, and a real server. It intercepts all requests to the real server to see if it can fulfill the requests itself. If not, it forwards the request to the real server.

Proxy servers have two main purposes:

Improve Performance: Proxy servers can dramatically improve performance for groups of users. This is because it saves the results of all requests for a certain amount of time. Consider the case where both user X and user Y access the World Wide Web through a proxy server. First user X requests a certain Web page, which we'll call Page 1. Sometime later, user Y requests the same page. Instead of forwarding the request to the Web server where Page 1 resides, which can be a time-consuming operation, the proxy server simply returns the Page 1 that it already fetched for user X. Since the proxy server is often on the same network as the user, this is a much faster operation. Real proxy servers support hundreds or thousands of users. The major online services such as America Online, MSN and Yahoo, for example, employ an array of proxy servers.

Filter Requests: Proxy servers can also be used to filter requests. For example, a company might use a proxy server to prevent its employees from accessing a specific set of Web sites.