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Computer NetworkComputer Network& Distributed& Distributed

Computer Network and DistributedComputer Network and DistributedRachphat SripromRachphat SripromRajabhat InstituteRajabhat Institute ChandrakasemChandrakasem

4122102 :4122102 :

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Computer Network 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 LAN : Local Area Network

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WAN : Wide Area Network WAN : Wide Area Network

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-WAN : Dial On Demand PSTN -WAN : Dial On Demand PSTN

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- WAN : Leased line PSTN - WAN : Leased line PSTN

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WAN : PSDN WAN : PSDN

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WAN : ISDN WAN : ISDN

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Enterprise Network Enterprise Network

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Network Criteria Network Criteria

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Performance

• Number of users• Type of transmission medium• Hardware• Software

The performance of a network depends on a number of factors ;

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Reliability

• Frequency of failure• Recovery time of a network afte

r failure• Catastrophe

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SecuritySecurity

• Unauthorized access– For a network to be useful, sensitive

data must be protected from unauth orized access. Protection can be acc

omplished at a number of levels• Viruses

– A good network is protected from vir uses by hardware and software desi gned specifically for that purpose.

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Computer Networks & Computer Networks & Computer Communications Computer Communications

• Computer Networks: two or more computer interconnected via a communication network

• Computer Communications: the exch ange of information between two co

mputers for purpose of cooperativeaction

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Communications Tasks Communications Tasks

• Transmission sys tem utilization

• Interfacing• Signal generation• Synchronization• Exchange Manag

ement• Error detection a

nd correction• Flow control

• Addressing• Routing• Recovery• Message formatti

ng• Security• Network manage

ment

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ProtocolsProtocols• Protocol - a set of rules governi

ng the exchange of data betwee n two entities in a system

• Must speak the same language• Entities

– User applications– e-mail facilities– terminals

• Systems-Computer-Terminal- Remote sensor

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Key Elements of a ProtocolKey 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 generat es a block of data and passes thi

s to the transport layer• Each of these blocks the transp

ort layer appends a transport he ader, containing protocol infor

mation• The combination of data from th

e next higher layer and control i nformation is known as a protoc

ol 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 Standard or Nonstandard

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OSI Model OSI Model

• The Open Systems Interconnection model is a layered framework for t

he design of network systems that allows for communication across al

l types of computer systems• It consists of seven separate but re

lated layers• An ISO standard

ISO is the organization. OSI is the model.

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OSI seven layers OSI seven layers

• Application layer• Presentation lay

er• Session layer• Transport layer• Network layer• Data Link layer• Physical layer

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OSI layers

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The OSI Environment

6 5 4 3 2Headers are added to the data at layers , , , , and . Trailers are usually added 2only at layer .

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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 Ph ysical 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 ne

tworks (links)• Switching & Routing• Logical Addressing• Higher layers do not need to know a

bout 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 bet ween 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 concern ed with the syntax and semantics

of the information exchanged be tween 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 interf

ace and support for services such as elect ronic 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