Download Introduction to Data Communications

Introduction to
Data Communications
NETE0510
Transmission media and data
communications
Dr.Apichan Kanjanavapastit
How to contact your lecturer?
• Office: F405 (Telecommunication System
Laboratory)
• Telephone: 029883655 Ext. 113 (office)
• Email: [email protected]
• Consultation:
– 9.30AM-11.30AM Saturday
– prefer contact via email first in another day
What is Data Communication?
• Data communication is the exchange of data (in
the form of 0s and 1s) between two devices via
some form of transmission medium (such as a
wire cable)
• Data communication is considered local if the
communicating devices are in the same building
or a similarly restricted geographical area
• It is considered remote if the devices are farther
apart
Fundamental characteristics of
effectiveness data communication
•
The effectiveness of a data
communication system depends on 3
fundamental characteristics:
1. Delivery. The system must deliver data to
the correct destination
2. Accuracy. The system must deliver data
accurately
3. Timeliness. The system must deliver data in
a timely manner
Components of a data
communication system
Step 1:
Step 2:
Step 3:
……
…...
Step 1:
Step 2:
Step 3:
……
…...
Protocol
Protocol
Message
Transmission medium
Sender
Receiver
Components of a data
communication system (cont.)
• Message. The message is the information to be
communicated. It can consist of text, number, pictures,
sound, or video-or any combination of these
(multimedia).
• 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 receive the
message
• Medium. The transmission medium is the physical path
by which a message travels from sender to receiver. It
can consist of twisted par wire, coaxial cable, laser, or
radio wares (terrestrial or satellite microwave).
• Protocol. A protocol is a set of rules that govern data
communication. It represents an agreement between the
communicating devices.
What are Networks?
• A network is a set of devices (often
referred to as node) connected by media
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
• The links connecting the devices are often
called communication channels
Network Criteria
• To be considered effective and efficient, a
network must meet a number of criteria. The
most important of these are performance,
reliability, and security
Data communication
network criteria
Performance
Reliability
Security
Performance
• The performance of a network depends on a
number of factors:
– Number of users. Having a large number of
concurrent users can slow response time in a network
not designed to coordinate heavy traffic loads
– Type of transmission medium. The medium defines
the speed at which data can travel through a
connection (the data rate)
– Hardware. A higher-speed computer with greater
storage capacity provides better performance
– Software. Well-designed software can speed the
process and make transmission more effective and
efficient
Reliability
• In addition to accuracy of delivery, network
reliability is measured by:
– Frequency of failure. A network that fails
often, however, is of little value of a user
– Recovery time of a network after a failure. A
network that recovers quickly is more useful
than one that does not
– Catastrophe. Networks must be protected
from catastrophe events such as fire,
earthquake.
Security
• Network security issues include protecting data
from unauthorized access and viruses
– Unauthorized access. Protection can be
accomplished at a number of levels. At the lowest
level are user identification codes and passwords. At
a higher level are encryption techniques
– Viruses. A virus is an illicitly introduced code that
damages the system. A good network is protected
from viruses by hardware and software designed
specifically fro that purpose
Basic concepts of relationship
between communicating devices
• There are five general concept provide the
basis for this relationship:
– Line configuration
– Topology
– Transmission mode
– Categories of networks
– Internetworks
Line Configuration
• 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 possible line configuration:
point-to-point and multipoint
Point-to-Point
• A point-to-point line configuration provides a
dedicated link between two device
• The entire capacity of the channel is received for
transmission between those two devices
Infrared link
Remote Controller
Television
Serial communication link
Modem
Personal Computer
Microwave link
Satellite dish
Workstation
Satellite dish
Workstation
Multipoint
• A multipoint (also called multidrop) line
configuration is one which more than two
specific devices share a single link
File Server
Personal Computer
Printer
Topology
• The term topology refers to the way a
network is laid out, either physically or
logically
• The topology of a network is the geometric
representation of the relationship of all the
links and linking devices (usually called
nodes) to each other
• There are five basic topologies possible:
mesh, star, tree, bus, and ring
Mesh
• In a mesh topology, every device has a dedicated pointto-point link to every other node
• A fully connected mesh network therefore has n(n-1)/2
physical channels to link n devices
• To accommodate that many link, every device on the
network must have n-1 input/output ports
Star
• In a star topology, each device has a dedicated
point-to-point link only to a central controller,
usually called a hub
Tree
• A tree topology is a variation of a star. As in a star, nodes
in a tree are linked to a central hub that controls the
traffic to the network
• However, not every device plugs directly into the central
hub. The majority of devices connect to a secondary hub
that in turn is connected to the central hub
Bus
• A bus topology is multipoint. One long cable acts
as a backbone to link all the devices in the
network
Ring
• In a ring topology, each device has a dedicated point-topoint line configuration only with 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
Hybrid Topologies
• Often a network combines several topologies as
subnetworks 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 are three types of transmission mode:
simplex, half-duplex, and full-duplex
Transmission mode
Simplex
Half-duplex
Full-duplex
Simplex
• In simplex mode, the communication is unidirectional, as
on a one-way street
• Only one of the two stations on a link can transmit; the
other can only receive
• Keyboards and traditional monitors are both example 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
• Walkie-talkie and CB radios are both half-duplex
systems
Full-Duplex
• In full-duplex mode (also called duplex), both
stations can transmit and receive simultaneously
• One common example of full-duplex
communication is the telephone network
Categories of Networks
• When we speak of networks, we are generally
referring to three primary categories: local area
network (LAN), metropolitan area network
(MAN), and wide area network (WAN)
Network
Local Area Network
(LAN)
Metropolitan Area Network
(MAN)
Wide Area Network
(WAN)
Local Area Network (LAN)
• A local area network is usually privately owned and links the devices
in a single office, building, or campus
• LANs are designed to allow resources to be shared (i.e., printers or
applications programs )between PCs or workstations
• LANs are distinguished from other types of networks by their
transmission media and topology. In general, a given LAN will use
only one type of transmission medium and the most common
topologies are bus, ring, and star
Metropolitan Area Network (MAN)
• A metropolitan area network is designed to extend over an entire
city. It may be a single network such as a CATV network
• In addition, it may be a means of connecting a number of LANs into
a larger network
• A MAN may be wholly owned and operated by a private company, or
it may be a service provided by a public company (i.e., a local
telephone company)
Wide Area Network (WAN)
• 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
• WANs may utilize public, leased, or private communication devices,
usually in combinations, and can therefore span an unlimited
number of miles
• A WAN that is wholly owned and used by a single company is often
referred to as an enterprise network
Internetworks
• When two or more networks are connected, they
become an internetwork, or internet
• Individual networks are joined into internetworks
by the use of internetworking devices (i.e.,
routers and gateways)
• The term internet (lowercase i) should not be
confused with the Internet (uppercase I)
The OSI Model
• An ISO standard that covers all aspects of
network communications is the Open
Systems Interconnection (OSI) model
• An open system is a model that allows any
two different systems to communicate
regardless of their underlying architecture
• The OSI model is not a protocol; it is a
model for understanding and designing a
network architecture that is flexible, robust,
and interoperable
Fundamental of the OSI Model
• Understanding the following fundamentals
of the OSI model provides a solid basis for
exploration of data communication
– Layered Architecture
– Peer-to-Peer Processes
– Interfaces between Layers
– Organization of the Layers
Layered Architecture
• The OSI model is built of 7 ordered layers:
physical, datalink, network, transport, session,
presentation, and application
• In developing the model, the designers distilled
the process of transmitting data down to its most
fundamental elements
• They identified which networking functions had
related uses and collected those functions into
discrete groups that became the layers
Layered Architecture (cont.)
• By defining and localizing functionality in this fashion, the
designers created an architecture that is both
comprehensive and flexible
• Most important, the OSI model allows complete
transparency between otherwise incompatible systems
Peer-to-Peer Processes
• Within a single machine, each layer calls upon
the services of the layer just below it. For
example, layer 3 uses the services provided by
layer 2
• Between machines, a process of layer x on one
machine communicate with the same process of
layer x on another machine; this is called peerto-peer processes
• This communication is governed by an agreedupon series of rules and conventions called
protocols
Peer-to-Peer Processes (cont.)
• At the higher layers, communication must move
down through the layers on the sending machine
over to the destining machine, and then back up
through the layers
• Each layer in the sending machine adds its own
information (called headers or trailers) to the
message it receives from the layer just above it
and passes the whole package to the layer just
below it
• Headers are added to the message at layers 6,
5, 4, 3, and 2. A trailers is added at layer 2
Peer-to-Peer Processes (cont.)
• At the receiving machine, the message is
unwrapped layer by layer, with each process
receiving and removing the data meant for it and
then passes the rest to the process in the upper
layer
Interfaces between Layers
• The passing of the data and network information
down through the layers of the sending machine and
back up through the layers of the receiving machine
is made by an interface between each pair of
adjacent layers
• Each interface defines what information and
services a layer must provide for the layer above it
• As long as a layer still provides the expected
services to the layer above it, the specific
implementation of its functions can be modified or
replaced without requiring changes to the
surrounding layers
Organization of the Layers
• The 7 layers can be thought of as belonging to 3
subgroup:
– Layers 1-3 are the network support layers, they deal with the
physical aspects of moving data from one device to another
– Layers 5-7 can be thought of as the user support layers; they
allow interoperability among unrelated software system
– Layer 4 ensures end-to-end reliable data transmission while
layer 2 ensures reliable transmission on a single link
• The upper OSI layers are almost always implemented in
software; lower layers are a combination of hardware
and software, except for the physical layer, which is
mostly hardware
Functions of the Physical Layers
• The physical layer coordinates the functions required to transmit a
bit stream over a physical medium
• It deals with the mechanical and electrical specifications of the
interface and transmission medium
• It also defines the procedures and functions that physical devices
and interfaces have to perform for transmission to occur
Functions of the Physical Layers
(cont.)
• The physical layer is concerned with the
following:
–
–
–
–
–
–
–
Physical characteristics of interface and media
Representation of bits
Data rate (transmission rate)
Synchronization of bits
Line configuration
Physical topology
Transmission mode
Functions of the Data Link Layer
• The data link layer transforms the physical layer
to a reliable link and is responsible for node-tonode delivery
• It makes the physical layer appear error free to
the upper layer (network layer)
Functions of the Data Link Layer
(cont.)
• Specific responsibilities of the data link
layer include the following:
– Framing
– Physical addressing
– Flow control
– Error control
– Access control
Example
• a node with physical address 10 sends a frame to a node with
physical address 87. The two nodes are connected by a link. At the
data link level this frame contains physical addresses in the header.
These are the only addresses needed. The rest of the header
contains other information needed at this level. The trailer usually
contains extra bits needed for error detection
Functions of the Network Layer
• The network layer is responsible for the source-to-destination
delivery of a packet possibly across multiple networks
• Whereas the data link layer oversees the delivery of the packet
between two systems on the same network, the network layer
ensures that each packet gets from its point of origin to its final
destination
• Specific responsibilities of the network layer include the following:
– Logical addressing
– Routing
Functions of the Transport Layer
• The transport layer is responsible for source-todestination (end-to-end) delivery of the entire message
• Whereas the network layer threats each one
independently, the transport layer ensures that the whole
message arrives intact and in order
Functions of the Transport Layer
(cont.)
• For added security, the transport layer may
create a connection between the two end
ports
• A connection is a single logical path
between the source and destination that is
associated with all packet in a message
• Creating a connection involves 3 steps:
connection establishment, data transfer,
and connection release
Functions of the Transport Layer
(cont.)
• Specific responsibilities of the transport
layer include the following:
– Service-point addressing (or port address)
– Segmentation and reassembly
– Connection control. There are 2 types of
connection control: connectionless or
connection oriented.
– Flow control
– Error control (damage, loss, or duplication)
Functions of the Session Layer
• The services provided by the lowest four layers of
the OSI model may be not sufficient
• For example, a remote terminal access application
might require a half-duplex dialogue. A transactionprocessing application might require checkpoints in
the data transfer stream to permit backup and
recovery
• Since these types of dialogue-structuring tools have
widespread applicability, it make sense to organize
them into a separate layer: the session layer which
establishes, maintains, and synchronizes the
interaction between communicating systems
Functions of the Session Layer
(cont.)
• Specific responsibilities of the session สฟำพ
include the following:
– Dialog control– half duplex or full duplex
– Synchronization. The session layer allows a process
to add checkpoints (synchronization points)
From presentation layer
Data
H
Session layer
Synchronizing point
Data
To transport layer
Functions of the Presentation Layer
• The presentation layer is concerned with the
syntax and semantics of the information
exchanged between two systems
• Specific responsibilities of the presentation layer
include the following:
– Translation. Since different computers use different
encoding systems, the presentation layer responsible
for interoperability between these different encoding
methods
– Encryption. To carry sensitive information, a system
must be able to assure privacy
– Compression. Data compression reduces the number
of bits to be transmitted
Functions of the Application Layer
• The application layer enables application
programs to access the network
• It provides interfaces and support for services
such as electronic mail, remote file access and
transfer
Summary of Layer Functions
Homework
1. Give your own example of how the type
of transmission medium can affect the
performance of a network
2. List the advantages and disadvantages
of each topology type
3. Draw a diagram to illustrate the
correlation between layers of the TCP/IP
protocol suite and layers of the OSI
model