CSC 442: Data Communications
LEC_1
Chapter 1
Introduction
OUTLINES
1. Data Communication
- Component
- Data Flow
- Data Representation
Data Communications
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Data communications are the exchange of data between two devices via some form
of transmission medium such as a wire cable .
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The term telecommunication means communication at a distance including telephone,
telegraph, and TV
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The word data refers to information presented in whatever form is agreed upon by
the parties creating and using the data.
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Data Communication System: Made up of a combination of hardware (physical
equipment) and software (programs) to facilitate for effective communication of data.
Five components of data communication system
1. Message. The message is the information (data) to be communicated. Popular
forms of information include text, numbers, pictures, audio, and video.
2. Sender. The sender is the device that sends the data message. It can be a computer,
workstation, telephone handset, video camera, and so on.
3. Receiver. The receiver is the device that receives the message. It can be a computer,
workstation, telephone handset, television, and so on.
4.
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.
5. 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.
• Protocol : What? How? and When?
It defines what is communicated, how it is communicated, and when it is communicated
The key elements of a protocol are syntax, semantics, and timing.
Syntax: the structure or format of the data (e.g. how many bits for each section)
For example, a simple protocol might expect the first 8 bits of data to be the address of the
sender, the second 8 bits to be the address of the receiver, and the rest of the stream to be the
message itself.
Semantics: the interpretation of the data bit patterns
The word semantics refers to the meaning of each section of bits. How is a particular pattern
to be interpreted, and what action is to be taken based on that interpretation?
For example, does an address identify the route to be taken or the final destination of the
message?
Timing: when and how fast the data should be sent For example, if a sender produces
data at 100 Mbps but the receiver can process data at only 1 Mbps, the transmission will
overload the receiver and some data will be lost.
Data Flow
Path taken by data within a device, network, or organization, as it
moves from its source to its destination (a data repository or a
data user).
Categorized by direction of flow:
❖Simplex
❖Half-duplex
❖Full-duplex
Simplex
• Communication is unidirectional, one of the two devices
on a link can transmit; the other can only receive (oneway street).
• Ex: keyboard (input), monitors (output)
Half-duplex
Each station can both transmit and receive , but not at the
same time. When one device is sending the other can
receive and vice versa. (one-lane road with two direction).
Full-duplex (also called duplex)
• Both stations can transmit and receive simultaneously. (two way
street with traffic flowing in both directions at the same time).
• Ex: telephone network.
Signals going in either direction share the capacity of the link in two ways:
➢Either the link must contain two physically separate transmission paths one for
sending and other for receiving.
➢Capacity of the channel is divided between signals traveling in both direction
Data Representation
Information today comes in different forms such as text, numbers, images, audio, and
video.
In data communications,
Text: represented using bit patterns , a sequence of bits (0s or 1s). 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
• Unicode: which uses 32 bits to represent a symbol or character used in any
language in the world
• ASCII) The American Standard Code for Information Interchange (:
7-bit (128 symbols)
• Extended ASCII: 8-bit (256 symbols)
• ISO: 32-bit (4B symbols)
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.
Images: bit patterns for pixels and colors
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
After an image is divided into pixels, each pixel is assigned a bit pattern.
Audio: analog or digitized form
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. we learn how to change sound or music
to a digital or an analog signal
Video: analog or digitized form
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
Characteristics of a Data Communication System
❖Delivery : System must deliver data to correct destination. Data must be
received by only intended device or user.
❖Accuracy: The system must deliver the data accurately
❖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: delay variation.
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 30 ms. If some of the packets arrive with 30-ms delay and others with 40ms delay, an uneven quality in the video is the result
NETWORKS
• A Network is a set of devices (nodes) connected by communication
links.
• A node can be a computer, printer, or any other device capable of
sending and/or receiving data generated by other nodes on the
network.
• A link is a communication pathway that transfer data from one
device to another.
• The connections can be cables or wireless.
Advantages of Computer Networks
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Enables users to share hardware like scanners and printers.
This reduces costs by reducing the number of hardware items
bought.
• Allows users access to data stored on others' computers. This
keeps everyone up-to-date on the latest data
• Can share access to the Internet.
• Can even let users run programs that are not installed on their
own computers but are installed elsewhere in the network.
• reduces the effort for networks administrators to keep programs
configured correctly and saves a lot of storage space.
Network Criteria
1. Performance
Can be measured in many ways!
Transmit time: the amount of time required for a message to
travel from one device to another(or propagation delay)
Response time: the elapsed time between an inquiry and a
response (round trip delay).
Bit rate vs. packet rate vs. throughput
❑ Often evaluated by two networking metrics:
throughput and delay.
We often need more throughput and less delay
Throughput
• Throughput is an important network metric which is also known as
bandwidth.
• The bandwidth of a network is given by the number of bits that can be
transmitted over the network in a certain period of time.
• Depends on the network technology (hardware capabilities) and
therefore is constant.
Throughput Example
A network might have a bandwidth of 10 million bits/second
(Mbps), meaning that it is able to deliver 10 million bits every
second.
Delay
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Also known as latency.
corresponds to how long it takes a message to travel from one
end of a network to the other.
Latency is measured strictly in terms of time.
Effected by number of users and hence may change from time
to time.
Delay Example
A transcontinental network might have a latency of 24 milliseconds
(ms); that is, it takes a message 24 ms to travel from one end of
North America to the other.
Factors Affecting Performance
➢Type of transmission media,
➢Capabilities of connected H.W
➢ the efficiency of software.
➢Number of user
2. Reliability
The network’s robustness in a catastrophe.
• Measured by:
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Bit error rate vs. packet error rate
Error detection and correction
Link failure recovery time
Frequency of failures
3. Security
Many issues at the different layers!
Examples:
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Protecting data from unauthorized access.
Protecting data from damage.
Protect against alteration
Validity of sender
Implementing policies and procedures for recovery
from breaches and data losses.
Network Configuration
There are a number of ways that computers can be
connected together to form networks.
Physical attributes of a network include:
❖Type of connection(Line configuration)
❖Physical topology
Type of Connection(Line configuration)
For communication to occure, two devices must be connected in someway
to the same link at the same time.
Two possible connections:
• Point-to-point
• Multipoint
1. Point –to-point
Dedicated link between two devices. Most of them uses an actual length
of wire or cable to connect the two ends but other options ,such as
microwave satellite are possible.
2. Multipoint
Is one in which more than two specific devices share a single link
Network Configuration
Physical attributes of a network include:
✓Type of connection
❖Physical topology
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The topology of a network refers to the geometric
representation of the relationship of all the links and
linking devices (nodes) to one another.
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Two or more links for a topology.
Physical Topology
The way in which a network is laid out physically.
Mesh Topology
• Every device has a dedicated point-to-point link to every other devices
• Fully connected mesh network has n(n-1)/2 physical connection to link n devices.
• Every device on the network must have n-1 input/output (I/O) ports
Star topology
Each device has a dedicated point-to-point link only to a central controller (hub)
Unlike a mesh , a star topology does not allow direct traffic between devices, if one
device want to send data to another , it send it to the hub, which send it to other device
Bus topology
•Multipoint connection. Acts as a backbone to link all the
devices in a network.
The bus pattern connects the computer to the same communications line.
Communications goes both directions along the line. All the computers can
communicate with each other without having to go through the server.
Ring Topology
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Each device has a dedicated point-to-point connection only with
the two devices on either side of it ( as circle)
A signal is passed along the ring in one direction from device
until it reaches its destination.
There is no central host computer that holds all the data.
➢Repeater :regenerates the signal, it receives a
weakened signal, creates a copy, bit for bit, at the
original strength (Repeater can amplify the signal
along the ring.
A hybrid topology: a star backbone with three bus networks
Q. For n devices in a network, what is the number of cable links
required for a mesh, ring, bus, and star topology?
Q. Categorize the four basic topologies in terms of line configuration.
Q. For each of the following four networks, discuss the consequences if a connection fails.
a. Five devices arranged in a mesh topology
b. Five devices arranged in a star topology (not counting the hub)
c. Five devices arranged in a bus topology
d. Five devices arranged in a ring topology
Network Categories
Network category is determined by its size, ownership, the
distance it cover and its physical architecture.
Q. What are some of the factors that determine whether a communication
system is a LAN or WAN?