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4122102 :
Computer Network
& Distributed
Computer Network and Distributed
Rachphat Sriprom
Rajabhat Institute Chandrakasem
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Computer Network
• A network is a set of devices (nodes)
connected by media links
• A node can be a computer, printer,
etc.
• The links are called communication
channels
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Simplified Network Model
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LAN : Local Area Network
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WAN : Wide Area Network
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WAN : Dial On Demand-PSTN
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WAN : Leased line - PSTN
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WAN : PSDN
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WAN : ISDN
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Enterprise Network
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Network Criteria
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Performance
The performance of a network depends on a
number of factors ;
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Number of users
Type of transmission medium
Hardware
Software
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Reliability
• Frequency of failure
• Recovery time of a network after
failure
• Catastrophe
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Security
• Unauthorized access
– For a network to be useful, sensitive
data must be protected from
unauthorized access. Protection can
be accomplished at a number of levels
• Viruses
– A good network is protected from
viruses by hardware and software
designed specifically for that purpose.
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Computer Networks &
Computer Communications
• Computer Networks: two or more
computer interconnected via a
communication network
• Computer Communications: the
exchange of information between two
computers for purpose of cooperative
action
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Communications Tasks
• Transmission
system utilization
• Interfacing
• Signal generation
• Synchronization
• Exchange
Management
• Error detection and
correction
• Flow control
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Addressing
Routing
Recovery
Message
formatting
• Security
• Network
management
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Protocols
• Protocol - a set of rules governing
the exchange of data between two
entities in a system
• Must speak the same language
• Entities
• Systems
– User applications
-Computer
-Terminal
– e-mail facilities
-Remote sensor
– terminals
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Key Elements of a Protocol
• Syntax
– Data formats
– Signal levels
• Semantics
– Control information
– Error handling
• Timing
– Speed matching
– Sequencing
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Protocol Data Unit (PDU)
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Protocol Data Unit (PDU)
• The sending application generates
a block of data and passes this to
the transport layer
• Each of these blocks the transport
layer appends a transport header,
containing protocol information
• The combination of data from the
next higher layer and control
information is known as a protocol
data unit (PDU)
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Encapsulation in PDU
• Addition of control information to data
– Address information
– Error-detecting code
– Protocol control
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Protocol Data Unit (PDU)
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Standard or Nonstandard
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OSI Model
• The Open Systems Interconnection
model is a layered framework for the
design of network systems that allows
for communication across all types of
computer systems
• It consists of seven separate but
related layers
• An ISO standard
ISO is the organization. OSI is the model.
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OSI seven layers
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Application layer
Presentation layer
Session layer
Transport layer
Network layer
Data Link layer
Physical layer
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OSI layers
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The OSI Environment
Headers are added to the data at layers 6, 5, 4, 3, and 2. Trailers are usually added only at layer 2.
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Functions Of The Layers
• Physical Layer
– Physical interface between devices
• Information Transmission
• Medium & Signal
• Mechanical (Connector)
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Data Link Layer
• Control the information transfer over
Physical link
• Physical addressing
• Framing
• Flow control
• Error control
• Synchronization
• Access control
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Data Link Layer
Figure shows Framing
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Data Link Layer Example
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Network Layer
• The network layer is responsible for the
source-to-destination delivery of a
packet possibly across multiple
networks (links)
• Switching & Routing
• Logical Addressing
• Higher layers do not need to know
about underlying technology
• Not needed on direct links
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Network Layer Example
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Network Layer Example
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Transport Layer
• Control for information transfer
between end user system
• Segmentation & Reassembly
• In sequence
• Connection control
• Flow control
• Error control
• Quality of service (QoS)
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Transport Layer
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Transport Layer Example
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Session Layer
• Control of dialogues between
applications in end systems
• Dialogue discipline (full duplex or
half duplex)
• Grouping
• Recovery
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Presentation Layer
• The presentation layer is
concerned with the syntax and
semantics of the information
exchanged between two systems
– Data formats and Encoding
– Data compression
– Encryption (senders transforms the
original information to another from
and sends it out over the network)
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Presentation Layer
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Application Layer
• The application layer provides user
interface and support for services such as
electronic mail, remote file access and
transfer, shared database management, etc.
– Network virtual terminal (allows a user to log on
to a remote host)
– File transfer, access, and management (FTAM)
– Mail services
– Directory services
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Application Layer
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Summary of Layer Functions
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