Transcript Data Communications and Networking

Data Communications and Networking: Basic
Definitions.
• Data Communication is the exchange of data (in the
form of 0’s and 1’s) between two devices via some
form of transmission medium (such a cable)
• Data Communication: Local or Remote
• A data communication is considered local if the
communicating devices are in the same building or in
similarly restricted geographical area.
•
•
A data communication is considered remote if the
devices are farther apart.
A communication system is a set of devices
(hardware and software) whose purpose is sharing
data.
The fundamental characteristics of a communication
system are
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.
• Accuracy: The system must deliver 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, audio, voice data, 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 realtime transmission.
• The following figure shows a basic model of a
communication system
Data communication system components
Components of a Communication System
• Message: The message is the information (or data)
to be transmitted. For example, text, numbers,
pictures, sound, video, or any combination of these.
• Sender: The sender is the device that sends the data
message.It can be a computer, telephone, video
camera, and so on.
• Receiver: The receiver is the device that receives
the message.
• Medium (Channel): The transmission medium is the
physical path by which a message travels from the
sender to receiver. For example twisted pair wire,
coaxial cable, fiber-optic cable, radio waves
(terrestrial or satellite microwaves).
• The medium defines the speed (the data rate) at
which data can travel through a connection. For
example 100 megabits per second.
• Protocol: A protocol is a set of rules that govern
data communication. It represents an agreement
between the communicating devices.
• Network: A network is a set of devices (often referred
to as nodes) connected by 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.
Line Configuration
•
•
•
1.
2.
•
Line configuration refers to the way two or more
communication devices attach to a link.
A link is the physical communication pathway that
transfers data from one device to another.
There are two line configurations:
Point-to-Point
Multipoint
A point-to-point line configuration provides a
dedicated link between two devices. The entire
capacity of the channel is reserved for transmission
between those two devices.
Point-point connection
• A multipoint line configuration 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 line configuration.
• If users must take turns, it is a time-shared line
configuration.
Topology
• The topology of a network is the geometric
representation of the relationship of all links and
linking devices to one other.
• Two or more devices connect to a link; two or more
links form a topology.
Multipoint line configuration
•
1.
2.
3.
4.
5.
•
There are five basic topologies:
Mesh
Star
Tree
Bus
Ring
Note: A consideration when choosing a topology is
the relative status of the devices to be linked. Two
relations are possible: peer-to-peer, where the
devices share the link equally, and primarysecondary, where one device controls traffic and
the others must transmit through it.
Topology
Mesh
Ring
Bus
Star
Tree
Bus
Purpose
Peer-to-Peer
Primary-Secondary
Mesh Topology
In a mesh topology, every device has a dedicated point-topoint link to every other device.
The term dedicated means that the link carries traffic only
between the two devices it connects.
Mesh topology
Advantages
• The use of a dedicated links guarantees that each
connection can carry its own load, thus eliminating
the traffic problems that can occur when links must
be shared by multiple devices.
• A mesh topology is robust. If one link becomes
unusable, it does not incapacitate the entire system.
• Privacy. When every message sent travels along a
dedicated link, only the intended recipient sees it.
Physical boundaries prevent other users from
gaining access to messages.
• Point-to Point links make fault identification and fault
isolation easy.
Disadvantages
• A big amount of cabling and a big number of I/O
ports required, i.e, the hardware required can be
very expensive.
• Because every device must be connected to every
other device, installation and reconfiguration are
difficult.
• For these reasons a mesh topology is usually
implemented in a limited fashion.
• For example, as a backbone connecting the main
computers to a hybrid network that can include
several other topologies.
Star Topology
In a star topology, each device has a dedicated point-topoint link only to a central controller, usually called a
hub.
A star topology does not allow direct traffic between
devices. 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.
Advantages
• It is easy to install and reconfigure
• The star topology is robust. If one link fails, only that
link is affected. This factor also lends itself to easy
fault identification and fault isolation.
Star topology
Disadvantages
• A big amount of cabling and a big number of I/O
ports required, i.e, the hardware required can be
very expensive.
Note: Star topology requires far less cable than Mesh
topology.
Tree Topology
• A tree topology is a variation of a star. As in the star,
nodes in a tree topology are linked to a central hub
that controls the traffic to the network.
The majority of devices connect to a secondary hub that
in turn is connected to a central hub.
• The central hub in a tree is an active hub. An active
hub contains a repeater, which is a hardware device
that regenerates the received bit patterns before
sending out.
• Repeating improves the transmission quality, and
increases distance that a signal can travel.
• The advantages and disadvantages of a tree
topology are generally the same as those of a star.
• A good example of tree topology can be seen in
cable TV technology where the main cable from the
main office is divided into main branches and each
branch is divided into smaller branches and so on.
The hubs are used when a cable is divided.
Bus Topology
In a bus topology one long cable acts as a backbone to
link all the devices in the network.
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.
• When a signal travels along the backbone, some of
its energy is transformed into heat.Therefore, it
becomes weaker and weaker the farther it has to
travel. For this reason there is a limit on the number
of taps a bus can support and on the distance
between those taps.
Advantages
• Easy to install
• Bus topology uses less cabling than mesh, star, or
tree topologies.
Bus topology
Disadvantages
• Difficult reconfiguration and fault isolation.
A bus is usually designed to be optimally efficient at
installation. It can therefore be difficult to add a new
devices.
• 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.
Ring Topology
In a ring topology, each device has a dedicated point-topoint line configuration only with the two devices on
either side of it.
Advantages
• Easy to install and
reconfigure. To add
or delete a device
require only two
connections.
• Easy fault isolation
Disadvantages
• A break in the ring
(such as a disabled
station) can disable
the entire network.
Hybrid Topologies
Often a network combines several topologies as sub
networks linked together in a larger topology.
Transmission Mode
The term transmission mode is used to define the direction of
signal flow between two linked devices.
There three types of transmission modes: simplex, halfduplex and full-duplex
Simplex:In simplex mode, the communication is
unidirectional, as one-way street.
• Examples: keyboards and traditional monitors are both examples
of simplex devices.
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.
Examples: Walkie-talkies and CB (citizen’s band) radios are both
half-duplex systems.
Full-Duplex: In a full-duplex mode (also
called duplex), both stations can transmit
and receive simultaneously.
Examples: the telephone network
Categories of Networks
LANs
• A local area network (LAN) is usually privately
owned and links the devices in a single office,
building or campus. Currently, LAN size is
limited to a few kilometers.
• LANs are designed to allow resources be
shared between personal computers or
workstations. The resources to be shared can
include hardware (e.g. a printer), software (e.g.
an application program), or data.
• In general, LANs use only one type of transmission
medium.
• The most common LAN topologies are bus, ring and
star.
• LANs have data rates in the 4 to 16Mbs range.
Today, however, speeds are increasing and can
reach 100Mbps with gigabit systems in development.
Metropolitan Area Networks
• A metropolitan area network (MAN) is designed to
extend over an entire city.
• A MAN may be wholly owned and operated by a
private company, or it may be a service provided by a
public company, such as a local telephone company.
MAN
Wide Area Network
A Wide Area Network (WAN)
provides long-distance
transmission of data,
voice, image, and video
information over large
geographical areas that
may comprise a country, a
continent, or even the
whole world.