Data Flow

Data Flow
Communication between two devices can be simplex, half-duplex, or full-duplex as shown in Figure 1.2.
Simplex.In simplex mode, the communication is unidirectional, as on a one-way street. Only one
of the two devices on a link can transmit; the other can only receive (see Figure 1.2a).
Keyboards and traditional monitors are examples of simplex devices. The keyboard
can only introduce input; the monitor can only accept output. The simplex mode
can use the entire capacity of the channel to send data in one direction.
Half-Duplex.In half-duplex mode, each station can both transmit and receive, but not at the same time. :
When one device is sending, the other can only receive, and vice versa (see Figure 1.2b).
The half-duplex mode is like a one-lane road with traffic allowed in both directions.
When cars are traveling in one direction, cars going the other way must wait. In a
half-duplex transmission, the entire capacity of a channel is taken over by whichever of
the two devices is transmitting at the time. Walkie-talkies and CB (citizens band) radios
are both half-duplex systems.
The half-duplex mode is used in cases where there is no need for communication
in both directions at the same time; the entire capacity of the channel can be utilized for
each direction.
Full-Duplex In full-duplex m.,lle (als@ called duplex), both stations can transmit and receive simultaneously
(see Figure 1.2c).
The full-duplex mode is like a tW<D-way street with traffic flowing in both directions
at the same time. In full-duplex mode, si~nals going in one direction share the
capacity of the link: with signals going in the other din~c~on. This sharing can occur in
two ways: Either the link must contain two physically separate t:nmsmissiIDn paths, one
for sending and the other for receiving; or the capacity of the ch:arillilel is divided
between signals traveling in both directions.
One common example of full-duplex communication is the telephone network.
When two people are communicating by a telephone line, both can talk and listen at the
same time.
The full-duplex mode is used when communication in both directions is required
all the time. The capacity of the channel, however, must be divided between the two
directions.

Data Representation

Data Representation
Information today comes in different forms such as text, numbers, images, audio, andvideo.

• Text

In data communications, text is represented as a bit pattern, a sequence of bits (Os or
Is). Different sets of bit patterns have been designed to represent text symbols. Each set
is called a code, and the process of representing symbols is called coding. Today, the
prevalent coding system is called Unicode, which uses 32 bits to represent a symbol or
character used in any language in the world. The American Standard Code for Information
Interchange (ASCII), developed some decades ago in the United States, now
constitutes the first 127 characters in Unicode and is also referred to as Basic Latin.
Appendix A includes part of the Unicode.

• Numbers

Numbers are also represented by bit patterns. However, a code such as ASCII is not used
to represent numbers; the number is directly converted to a binary number to simplify
mathematical operations. Appendix B discusses several different numbering systems.

• Images

Images are also represented by bit patterns. In its simplest form, an image is composed
of a matrix of pixels (picture elements), where each pixel is a small dot. The size of the
pixel depends on the resolution. For example, an image can be divided into 1000 pixels
or 10,000 pixels. In the second case, there is a better representation of the image (better
resolution), but more memory is needed to store the image.
After an image is divided into pixels, each pixel is assigned a bit pattern. The size
and the value of the pattern depend on the image. For an image made of only blackand-
white dots (e.g., a chessboard), a I-bit pattern is enough to represent a pixel.
If an image is not made of pure white and pure black pixels, you can increase the
size of the bit pattern to include gray scale. For example, to show four levels of gray
scale, you can use 2-bit patterns. A black pixel can be represented by 00, a dark gray
pixel by 01, a light gray pixel by 10, and a white pixel by 11.
There are several methods to represent color images. One method is called RGB,
so called because each color is made of a combination of three primary colors: red,
green, and blue. The intensity of each color is measured, and a bit pattern is assigned to
it. Another method is called YCM, in which a color is made of a combination of three
other primary colors: yellow, cyan, and magenta

• Audio

Audio refers to the recording or broadcasting of sound or music. Audio is by nature
different from text, numbers, or images. It is continuous, not discrete. Even when we.
use a microphone to change voice or music to an electric signal, we create a continuous
signal. In Chapters 4 and 5, we learn how to change sound or music to a digital or an
analog signal.

• Video

Video refers to the recording or broadcasting of a picture or movie. Video can either be
produced as a continuous entity (e.g., by a TV camera), or it can be a combination of
images, each a discrete entity, arranged to convey the idea of motion. Again we can
change video to a digital or an analog signal, as we will see in Chapters 4 and 5.

Components of Data Communication

Components of Data Communication
A data communications system has five components (see Figure 1.1).
Message.The message is the information (data) to be communicated. Popular forms of information include text, numbers, pictures, audio, and video.
Sender.The sender is the device that sends the data message. It can be a computer, workstation, telephone handset, video camera, and so on.
Receiver.The receiver is the device that receives the message. It can be a computer, workstation, telephone handset, television, and so on.

Transmission medium.The transmission medium is the physical path by which a message travels from sender to receiver. Some examples of transmission media include twisted-pair wire, coaxial cable, fiber-optic cable, and radio waves.

Protocol.A protocol is a set of rules that govern data communications. It represents

an agreement between the communicating devices. Without a protocol, two

devices may be connected but not communicating, just as a person speaking French

cannot be understood by a person who speaks only Japanese.

DATA COMMUNICATIONS

1.1 DATA COMMUNICATIONS
When we communicate, we are sharing information. This sharing can be local or
remote. Between individuals, local communication usually occurs face to face, while
remote communication takes place over distance. The term telecommunication, which
includes telephony, telegraphy, and television, means communication at a distance (tele
is Greek for "far").
The word data refers to information presented in whatever form is agreed upon by
the parties creating and using the data.
Data communications are the exchange of data between two devices via some
form of transmission medium such as a wire cable. For data communications to occur,
the communicating devices must be part of a communication system made up of a combination
of hardware (physical equipment) and software (programs). The effectiveness
of a data communications system depends on four fundamental characteristics: delivery,
accuracy, timeliness, and jitter.

• Delivery. 

The system must deliver data to the correct destination. Data must be received by the intended device or user and only by that device or user.

• Accuracy. 

The system must deliver the data accurately. Data that have been altered in transmission and left uncorrected are unusable.

• Timeliness.

The system must deliver data in a timely manner. Data delivered late are useless. In the case of video and audio, timely delivery means delivering data as they are produced, in the same order that they are produced, and without significant delay. This kind of delivery is called real-time transmission.

• Jitter

 Jitter refers to the variation in the packet arrival time. It is the uneven delay in the delivery of audio or video packets. For example, let us assume that video packets are sent every 3D ms. If some of the packets arrive with 3D-ms delay and others with 4D-ms delay, an uneven quality in the video is the result.

Shoaib Akhtar Bouncer to Brian Lara - Cricket by Khabri

Wasim Akram 5 Best Yorkers Ever The Great Bowling In Cricket

Wasim Akram 5 Best Yorkers Ever The Great Bowling In Cricket by Cricket World