Data Communication and networks note ppt

McGraw-Hill ©The McGraw-Hill Companies, Inc., 2000
Data Communications and
Networks
Alidu Abubakari
Street Sign
STOP
Line of Sight
Reflection
Diffraction
Scattering
Transmitter
Receive
r
Buildings
Earth surface

Course Instructor
ALIDU ABUBAKARI
M.Eng, Electronics and Control
Engineering (wireless communications)
BSc Telecommunication Engineering
0550672434
wimaxofdm@gmail.com

1.5
1-1 DATA COMMUNICATIONS
The term telecommunication means communication at a
distance. 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.
Components
Data Representation
Data Flow
Topics discussed in this section:

The effectiveness of a data communications system depends on four
fundamental characteristics: delivery, accuracy, timeliness, and jitter.
1. 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.
2. Accuracy. The system must deliver the data accurately. Data that
have been altered in transmission and left uncorrected are
unusable.
3. 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.
4. 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 30 ms. If
some of the packets arrive with 30-ms delay and others with 40-ms
delay, an uneven quality in the video is the result.

1.7
Figure 1.1 Five components of data communication
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.

1.8
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.

1.9
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.

1.10
Figure 1.2 Data flow (simplex, half-duplex, and full-duplex)

1.11
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.

1.12
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.

1.13
In full-duplex mode (also called duplex), both stations can transmit and receive
simultaneously (see Figure 1.2c).
The full-duplex mode is like a two-way street with traffic flowing in both directions
at the same time. In full-duplex mode, signals going in one direction share the
capacity of the link with signals going in the other direction. This sharing can occur in
two ways: Either the link must contain two physically separate transmission paths,
one for sending and the other for receiving; or the capacity of the channel 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.

1.14
1-2 NETWORKS
A network is a set of devices (often referred to as 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.
Distributed Processing
Network Criteria
Physical Structures
Network Models
Categories of Networks
Interconnection of Networks: Internetwork

1.15
Network Criteria
A network must be able to meet a certain number of criteria. The most important of these are
performance, reliability, and security.
Performance
Performance can be measured in many ways, including transit time and response time.
Transit time is the amount of time required for a message to travel from one device to another.
Response time is the elapsed time between an inquiry and a response. The performance
of a network depends on a number of factors, including the number of users, the type of
transmission medium, the capabilities of the connected hardware, and the efficiency of the
software.
Performance is often evaluated by two networking metrics: throughput and delay. We often need
more throughput and less delay. However, these two criteria are often contradictory. If we try to
send more data to the network, we may increase throughput but we increase the delay because
of traffic congestion in the network.
Reliability
In addition to accuracy of delivery, network reliability is measured by the frequency of failure, the
time it takes a link to recover from a failure, and the network’s robustness in a catastrophe.
Security
Network security issues include protecting data from unauthorized access, protecting data from
damage and development, and implementing policies and procedures for recovery from breaches
and data losses.

1.16
Figure 1.3 Types of connections: point-to-point and multipoint

1.17
A point-to-point connection provides a dedicated link between two
devices. The entire capacity of the link is reserved for transmission
between those two devices. Most point-to-point connections use an
actual length of wire or cable to connect the two ends, but other options,
such as microwave or satellite links, are also possible (see Figure 1.3a).
When we change television channels by infrared remote control, we are
establishing a point-to-point connection between the remote control and
the television’s control system.

1.18
A multipoint (also called multidrop) connection is one in which more
than two specific devices share a single link.
In a multipoint environment, the capacity of the channel is shared, either
spatially or temporally. If several devices can use the link
simultaneously, it is a spatially shared connection. If users must take
turns, it is a timeshared connection.

1.19
Figure 1.4 Categories of topology

1.20
Figure 1.5 A fully connected mesh topology (five devices)

1.21
In a mesh topology, every device has a dedicated point-to-point link to
every other device. The term dedicated means that the link carries traffic
only between the two devices it connects. To find the number of physical
links in a fully connected mesh network with n nodes, we first consider
that each node must be connected to every other node. Node 1 must be
connected to n – 1 nodes, node 2 must be connected to n – 1
nodes, and finally node n must be connected to n – 1 nodes. We need n
(n – 1) physical links. However, if each physical link allows
communication in both directions (duplex mode), we can divide the
number of links by 2. In other words, we can say that in a mesh
topology, we need n (n – 1) / 2 duplex-mode links. To accommodate that
many links, every device on the network must have n – 1 input/output
(I/O) ports (see Figure 1.4) to be connected to the other n – 1 stations.

1.22
A mesh offers several advantages over other network topologies.
• First, the use of dedicated links guarantees that each connection can
carry its own data load, thus eliminating the traffic problems that can
occur when links must be shared by multiple devices.
• Second, a mesh topology is robust. If one link becomes unusable, it
does not incapacitate the entire system.
• Third, there is the advantage of privacy or security. When every
message travels along a dedicated line, only the intended recipient
sees it. Physical boundaries prevent other users from gaining access
to messages.
• Finally, point-to-point links make fault identification and fault isolation
easy. Traffic can be routed to avoid links with suspected problems.
This facility enables the network manager to discover the precise
location of the fault and aids in finding its cause and solution.

1.23
Figure 1.6 A star topology connecting four stations

1.24
In a star topology, each device has a dedicated point-to-point link only to
a central controller, usually called a hub. The devices are not directly
linked to one another. Unlike a mesh topology, a star topology does not
allow direct traffic between devices. The controller acts as an exchange:
If one device wants to send data to another, it sends the data to the
controller, which then relays the data to the other connected device. star
topology is less expensive than a mesh topology. In a star, each device
needs only one link and one I/O port to connect it to any number of
others. This factor also makes it easy to install and reconfigure.

1.25
Far less cabling needs to be housed, and additions, moves, and
deletions involve only one connection: between that device and the hub.
Other advantages include robustness. If one link fails, only that link is
affected. All other links remain active. This factor also lends itself to
easy fault identification and fault isolation. As long as the hub is working,
it can be used to monitor link problems and bypass defective links. One
big disadvantage of a star topology is the dependency of the whole
topology on one single point, the hub. If the hub goes down, the whole
system is dead. Although a star requires far less cable than a mesh,
each node must be linked to a central hub. For this reason, often more
cabling is required in a star than in some other topologies (such as ring
or bus).

1.26
Figure 1.7 A bus topology connecting three stations
The preceding examples all describe point-to-point
connections. A bus topology, on the other hand, is
multipoint.
One long cable acts as a backbone to link all the
devices in a network

1.27
Nodes are connected to the bus cable by drop lines and taps. A drop
line is a connection running between the device and the main cable. A
tap is a connector that either splices into the main cable or punctures
the sheathing of a cable to create a contact with the metallic core. As a
signal travels along the backbone, some of its energy is transformed
into heat. Therefore, it becomes weaker and weaker as it travels farther
and farther.
For this reason there is a limit on the number of taps a bus can support
and on the distance between those taps.

1.
Advantages of a bus topology include ease of installation. Backbone
cable can be laid along the most efficient path, then connected to the
nodes by drop lines of various lengths. In this way, a bus uses less
cabling than mesh or star topologies. In a star, for example, four
network devices in the same room require four lengths of cable
reaching all the way to the hub. In a bus, this redundancy is eliminated.
Only the backbone cable stretches through the entire facility. Each
drop line has to reach only as far as the nearest point on the
backbone.
Disadvantages include difficult reconnection and fault isolation. A bus
is usually designed to be optimally efficient at installation. It can
therefore be difficult to add new devices. Signal reflection at the taps
can cause degradation in quality. This degradation can be controlled
by limiting the number and spacing of devices connected to a given
length of cable. Adding new devices may therefore require modification
or replacement of the backbone.
In addition, a fault or break in the bus cable stops all transmission,
even between devices on the same side of the problem. The damaged
area reflects signals back in the direction of origin, creating noise in
both directions. Bus topology was the one of the first topologies

1.29
Figure 1.8 A ring topology connecting six stations
In a ring topology, each device has a dedicated point-to-point connection with
only the two devices on either side of it. A signal is passed along the ring in
one direction, from device to device, until it reaches its destination. Each
device in the ring incorporates a repeater. When a device receives a signal
intended for another device, its repeater regenerates the bits and passes
them along

1.30
Figure 1.9 A hybrid topology: a star backbone with three bus networks

1.31
Figure 1.10 An isolated LAN connecting 12 computers to a hub in a closet

1.32
Figure 1.11 WANs: a switched WAN and a point-to-point WAN

1.33
Figure 1.12 A heterogeneous network made of four WANs and two LANs

1.34
1-3 NETWORK TYPES
After defining networks in the previous section and
discussing their physical structures, we need to discuss
different types of networks we encounter in the world
today. The criteria of distinguishing one type of network
from another is difficult and sometimes confusing.
We use a few criteria such as size, geographical coverage,
and ownership to make this distinction. After discussing
two types of networks, LANs and WANs, we define
switching, which is used to connect networks to form an
internetwork (a network of networks).

1.35
A local area network (LAN) is usually privately owned and connects
some hosts in a single office, building, or campus. Depending on the
needs of an organization, a LAN can be as simple as two PCs and a
printer in someone’s home office, or it can extend throughout a
company and include audio and video devices. Each host in a LAN has
an identifier, an address, that uniquely defines the host in the LAN. A
packet sent by a host to another host carries both the source host’s and
the destination host’s addresses. In the past, all hosts in a network were
connected through a common cable, which meant that a packet sent
from one host to another was received by all hosts. The intended
recipient kept the packet; the others dropped the packet. Today, most
LANs use a smart connecting switch, which is able to recognize the
destination address of the packet and guide the packet to its destination
without sending it to all other hosts. The switch alleviates the traffic in
the LAN and allows more than one pair to communicate with each other
at the same time if there is no common source and destination among
them. Note that the above definition of a LAN does not define the
minimum or maximum number of hosts in a LAN. Figure 1.8 shows a
LAN using either a common cable or a switch.
Local Area Network

1.36
Wide Area Network
A wide area network (WAN) is also an interconnection of
devices capable of communication. However, there are some
differences between a LAN and a WAN. A LAN is normally
limited in size, spanning an office, a building, or a campus; a
WAN has a wider geographical span, spanning a town, a state, a
country, or even the world. A LAN interconnects hosts;
a WAN interconnects connecting devices such as switches,
routers, or modems. A LAN is normally privately owned by the
organization that uses it; a WAN is normally created and run by
communication companies and leased by an organization that
uses it. We see two distinct examples of WANs today: point-to-
point WANs and switched WANs.

1.37
1-3 THE INTERNET
The Internet has revolutionized many aspects of our daily
lives. It has affected the way we do business as well as the
way we spend our leisure time. The Internet is a
communication system that has brought a wealth of
information to our fingertips and organized it for our use.
A Brief History
The Internet Today (ISPs)

1.38
Figure 1.13 Hierarchical organization of the Internet

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