This document provides an overview of distributed systems. It discusses different types of distributed systems like client-server, n-tier, and peer-to-peer systems. It outlines advantages like performance, reliability, and scalability. Challenges include networking and security issues. Key features are discussed such as concurrency, lack of a global clock, and independent failures. Characteristics of distributed systems include heterogeneity, openness, security, scalability, failure handling, concurrency, and transparency. Examples provided are the Internet, intranets, and mobile/ubiquitous computing systems.

1. UNIT-1
Distributed Systems
III-I

3. How to working Distributed Systems
• h/w,s/w architectures are used to maintain a distributed
systems
• Everything must be interconnected cpu via the communication
system
• Types of Distributed Systems
• i)client-server
• Client contact the server for data then format it and display it
to the end and user
• ii) 3-tier:
• Information about the client is stored in a middle tier rather
than on the client to simplify application deployment

4. iii)N-tier:
• Generally used when an application or server needs to forward
requests to additional enterprise services
• iv)peer to peer:
• There are no additional machines used to provide services or
manage resources
• Responsibility are uniformly distributed among machines
distributed system known as peers which can server

5. Advantages of distributed systems
• i)performance
• ii) distribution
• iii) Reliability(fault tolerance)
• iv) Incremental growth: are requirements on processing power
grow new machines can be added incrementally
• v) sharing of data/resources:
• Shared data is essential to many applications
• vi) communication

6. Disadvantages of distributed systems
• s/w
• Networking problems
• Security problems
• Performance
• Reliability and fault tolerance

9. Features of distributed systems:
• Concurrency
Concurrent program execution
• No global clock
Coordination of actions
• Independent failures
Failure in a machine would not known by other machines

10. Properties of distributed systems
• Heterogeneity
• Systems consist of heterogeneous h/w and s/w components
• Concurrency
• Multiple programs run together
• Shared data
• Data is accessed simultaneously by multiple entities
• No global clock
• No global clock is also one of the important properties and
inter dependencies are there

11. Distributed systems has the following significant consequences:
i)Concurrency:
• In a network of computers, concurrent program execution is
the norm.
• Components access and update shared resources (e.g.databases,
device drivers).
• Integrity of the system may be violated if concurrent updates
are not coordinated.
– Lost updates
– Inconsistent analysis

12. ii)No global clock:
• The only communication is by sending messages through a network
• Not possible to synchronize many computers on a network and guarantee
synchronization over time.
• Thus events are logically ordered not possible to have a process that can be
aware of a single global state.
• iii) Independent failures:
• The programs may not be able to detect whether the network has failed or
has become unusually slow
• Running processes may be unaware of other failure with context
• Failed processes may go undetected both are due to processes running in
isolation

13. Characterization of Distributed systems
• What are we trying to achieve when we construct a distributed
system?
• Certain common characteristics can be used to assess
distributed systems
i) Heterogeneity
ii)Openness
iii) Security
iv) Scalability
v)Failure Handling
vi)Concurrency
vii) Transparency

14. i)Heterogeneity
 The internet enables users to access services and run applications over a
heterogeneity(mixed)
 Variety and differences in
 Networks
 Computer hardware
 Operating systems
 Programming languages
 Implementations by different developers
 Middleware as software layers to provide a programming abstraction as
well as masking the heterogeneity of the underlying networks, hardware,
OS, and programming languages (e.g., CORBA).
 Mobile Code to refer to code that can be sent from one computer to another
and run at the destination (e.g., Java applets and Java virtual machine).

19. v) Failure Handling (Fault Tolerance)
• Hardware, software and networks fail!
• Distributed systems must maintain availability even at low
levels of hardware/software/network reliability.
• Fault tolerance is achieved by
– recovery
– redundancy

20. vi) Concurrency
• Components in distributed systems are executed in concurrent
processes.
• Components access and update shared resources (e.g. variables,
databases, device drivers).
• Integrity of the system may be violated if concurrent updates
are not coordinated.
– Lost updates
– Inconsistent analysis

21. vii)Transparency
• A system is transparent for a feature if the feature is
unobservable for the user
• Distributed systems should be perceived by users and
application programmers as a whole rather than as a collection
of cooperating components.
• Transparency has different aspects.
• These represent various properties that distributed systems
should have.

22. 1)Access Transparency
• Enables local and remote information objects to be accessed
using identical operations.
• Example: File system operations in NFS, hyper link web page,
sql queries
2) Location Transparency
• Enables information objects to be accessed without knowledge
of their location.
• Example: URL ,Tables in distributed databases

23. 3.Concurrency Transparency
• Enables several processes to operate concurrently using shared
information objects without interference between them.
• Example: Automatic teller machine network, Database
management system
4.Replication Transparency
• Enables multiple instances of information objects to be used to
increase reliability and performance without knowledge of the
replication by users or application programs
• Example: Web cache, Mirroring Web Pages.

24. 5.Failure Transparency
• Enables the concealment of faults
• Allows users and applications to complete their tasks despite
the failure of h/w, s/w components.
• Example: email
6.Mobility Transparency
• Allows the movement of information objects within a system
without affecting the operations of users or application
programs
• Example: mobile phone

25. 7.Performance Transparency
• Allows the system to be reconfigured to improve performance
as loads vary.
• Example: Distributed make.
8.Scaling Transparency
• Allows the system and applications to expand in scale without
change to the system structure or the application algorithms.
• Example: World-Wide-Web
• Example: Distributed Database

26. EXAMPLES OF DISTRIBUTED SYSTEMS
• Typical examples of Distributed systems 3 types are,
• i)The Internet
• ii) Intranets
• iii) Mobile and Ubiquitous computing.
i) The Internet:
• It is a very large distributed system that allows users
throughout the world to make use of its services.
• For example, World Wide Web, web search, online gaming,
email, social networks, ecommerce, etc. – WWW, email, FTP,
etc
• Internet protocols is a major technical achievement. - TCP/IP:
connect different type computer networks.

28. i) internet
• The figure illustrates a typical portion of the Internet.
Programs running on the computers connected to it interact by
passing messages in communication.
• the Internet protocols is a major technical achievement,
enabling a program running anywhere to address messages to
programs anywhere else.
• The figure shows a collection of intranets that sub networks
operated by compaines and other organizations.

29. internet
• Internet Service Providers (ISPs) are companies that provide
modem links and other types of connection to individual users
and small organizations, enabling them to access services
anywhere in the Intranets are linked together by backbones.
• A backbone is a network link with a high transmission
capacity, employing satellite connections, fiber optic cables
and other high-bandwidth circuits.

30. ii) Intranet
• An intranet is a portion of the internet that is separately
administered and has a boundary that can be configured to
enforce local security policies.
• It may be composed of several LANs linked by backbone
connections.
• The n/w configuration of a particular intranet is the
responsibility of the organization that administers it.
• An intranet is connected to the Internet via router, which
allows the users to use the services available in the Internet.

31. Intranet
• Firewall is used to protect intranet by preventing
unauthorized messages leaving or entering.
• The main issues arising in the design of components
for use in intranets are, File services are needed to
enable users to share data Firewalls should ensure
legitimate access to services.
• Cost of installation and support should be minimum.

32. the rest of
email server
Web server
Desktop
computers
File server
router/firewall
print and other servers
other servers
print
Local area
netw ork
email server
the Internet

33. Mobile and Ubiquitous computing:
• Due to the technological advances in device efficiency and
wireless networking have been increased to the integration of
small and portable computing into distributed systems.
These devices include:
• Laptop computers.
• Handheld devices, including mobile phones, smart phones,
GPS-enabled devices, pages, personal digital assistants
(PDAs), video cameras and digital cameras.
• Wearable devices, such as smart watches with functionality
similar to a PDA.
• Devices embedded in appliances such as washing machines,
hi-fi systems, cars and refrigerators.

35. Mobile and Ubiquitous computing:
• The user has access to three forms of wireless connection.
Their laptop has a means of connecting to the host’s wireless
LAN.
• This network provides coverage of a few hundred metres( a
floor of a building). It connects to the rest of the host intranet
via a gateway or access point.
• The user also has a mobile telephone, which is connected to
the internet.
• The phone gives access to the web and other internet services.
• Finally, the user carries a digital camera, which can
communicate over a personal area wireless network with a
device such as a printer.

36. Mobile and Ubiquitous computing:
• Mobile computing: moving computing devices in
and out intranets
– Transparent access to home intranet
– Access to local resources at remote site
 Location-aware computing
• Ubiquitous computing
– Small computing devices everywhere
– Communication between devices

37. Mobile computing:
• Mobile computing is the performance of computing tasks
while the user is on the move, or visiting places other than
their usual environment.
• In mobile computing, users who are away from their ‘home’
intranet are still provided with access to resources via the
devices they carry with them.
• The mobile computing increasing the service for users to
utilize resources such as printers or even sales points that are
conveniently nearby as they move around.
• This mobility service is also known as location-aware or
context-aware computing.
• Mobility introduces a number of challenges for distributed
systems including to maintain operation in the face of device
mobility.

38. Ubiquitous computing
• Ubiquitous computing is small, cheap computational device
that present in users’ physical environments, including the
home, office and even natural settings.
• The term ‘ubiquitous’ is intended to suggest that small
computing devices will eventually become so pervasive
(persistent) in everyday objects that they are hardly noticed.
• It means, its computational behavior will be transparently and
tied up with their physical function.

39. Ubiquitous computing:
• Ubiquitous computing is small, cheap computational device
that present in users’ physical environments, including the
home, office and even natural settings.
• The term ‘ubiquitous’ is intended to suggest that small
computing devices will eventually become so pervasive
(persistent) in everyday objects that they are hardly noticed.
• It means, its computational behavior will be transparently and
tied up with their physical function

40. Ubiquitous computing:
• The presence of computers everywhere only becomes useful
when they can communicate with one another.
• For example, it may be convenient for users to control their
washing machine or their entertainment system from their
phone or a ‘universal remote control’ device in the home.
• Equally, the washing machine could notify the user via a smart
badge or phone when the washing is done.
• The following figure shows a user who is visiting a host
organization.
• The figure shows the user’s home intranet and host intranet at
the site that the user is visiting. Both intranets are connected to
the rest of the Internet.

41. Ubiquitous computing:
• The user has access to three forms of wireless connection. Their
laptop has a means of connecting to the host’s wireless LAN.
• This network provides coverage of a few hundred metres( a floor of
a building).
• It connects to the rest of the host intranet via a gateway or access
point.
• The user also has a mobile telephone, which is connected to the
internet.
• The phone gives access to the web and other internet services.
Finally, the user carries a digital camera, which can communicate
over a personal area wireless network with a device such as a printer

42. Resource sharing and web
• The term “resource sharing” means the range of things that can
usefully be shared in a networked computer system
• Resource in a distributed systems are encapsulated within one
computer and can only be accessed from other computer by
communication
• Equipments are shared to reduce cost
• Types of resources
• i) h/w resource: hard disk, printer, camera, scanner
• ii) Data: File, database, web page
• iii)search engine: users need no contact between users

43. Resource sharing and web
Service – manage a collection of related resources and present
their functionalities to users and applications
Server – a process on networked computer that accepts requests
from processes on other computers to perform a service and
responds appropriately
• Client – the requesting process
• Remote invocation: is an interaction between client and server
from the point when the client sends its request to when it
receives the server response

44. Hardware and software resource sharing
• Example of h/w resource that can be usefully shared
• 1) cpu:
• a) computer server: it executes processor intensive applications
for clients
• b) remote object server: executes methods on behalf of clients
• c) worm program: it shares cpu capacity of desktop machine
with the local user
• 2)memory: cache server holds recently accessed web pages in
its RAM for faster access by other local computers
• 3)Disk: file server, virtual disk server, video on demand server
• 4) screen: Network window systems allow processes in remote
computers to update the content of windows

45. 5.printer: Networked printers accept print jobs from many
computers and managing them with a queuing system
• s/w resources:
1.web page: web servers enable multiple clients to share read only page
content
2.file: file servers enable multiple clients to share read write files
3.database: database are intended to record the definitive state of some related
sets of data. multi user computers to manage concurrent updates
4.News group content: it makes read only copies of the recently posted news
items available to clients throughout the internet
5.video/audio stream: servers can store entire videos on disk and deliver them
at plat back speed to multiple clients simultaneously.

46. www(world wide web)
• www is an evolving the system for publishing and accessing
resources and services across the internet
• Web is an open system its operation are based on
communication standards and documents standards.
• www is collection of millions of files stored on thousands of
servers all over the world
• These files represent documents, pictures, video, sounds,
programs, interactive environments
• A web server is a computer connected to the internet that runs
a program that takes responsibility for storing the web files
• Web client is a computer that requests files from the web.

47. The Web is based on three main standard technological
components:
• i) Hyper Text Markup Language (HTML),
• ii) Uniform Resource Locators (URLs),
• iii) HTTP(The Hyper Text Transfer Protocol)
• i) Hyper Text Markup Language (HTML),
• Html is the universal language of the web html specifies the
contents and layout of web pages
• The content contains text, table, form, image,link, information
for search engine, etc
• The layout is in the form of text format, background and frame
• Html is also used to specify links and which resources are
associated with them

48. ii) Uniform Resource Locator(URL)
URLs : The purpose of a Uniform Resource Locator [www.w3.org III] is to
identify a resource. It means, the term used in web architecture documents
is Uniform Resource Identifier (URI)
The ‘scheme’, declares which type of URL this is. URLs are required to
identify a variety of resources.
For example,
mailto:joe@anISP.net identifies a user’s email address;
ftp://ftp.downloadIt.com/software/aProg.exe identifies a file that is to be
retrieved using the File Transfer Protocol (FTP) rather than the more
commonly used protocol HTTP.
HTTP URLs are the most widely used, for accessing resources using the
standard HTTP protocol. An HTTP URL has two main jobs: to identify
which web server maintains the resource, and to identify which of the
resources at that server is required.
HTTP URLs are of the following form:
http:// servername [:port] [/pathName] [?query] [ #fragment]

50. iii) HTTP:(HyperText Transfer Protocol)
• HTTP:The HyperText Transfer Protocol defines the ways in which browsers
and other types of client interact with web servers.
Main features
• –Request-replay interaction : HTTP is a ‘request-reply’ protocol. The client
sends a request message to the server containing the URL of the required
resource.
• The server looks up the path name and, if it exists, sends back the resource’s
content in a reply message to the client. Otherwise, it sends back an error
response such as the familiar ‘404 Not Found’.
• Content types : The strings that denote the type of content are called MIME
(RFC2045,2046)
• – One resource per request: Clients specify one resource per HTTP request.
If a web page contains nine images, say, then the browser will issue a total of
ten separate requests to obtain the entire contents of the page.
• – Simple access control: Any user with network connectivity to a web server
can access any of its published resources. for example, by typing in a password.

51. Challenges of Distributed systems
Design issues and challenges of distributed systems are as
follows
i) Heterogeneity
ii)Openness
iii) Security
iv) Scalability
v)Failure Handling
vi)Concurrency
vii) Transparency

52. Systems model

55. architectural model
• The architecture of a system is its structure in terms of separately specified
components and their interrelationships.
• The overall goal is to ensure that the structure will meet present and likely
future demands on it.
• Software Layers in DS
i) Platform:
• The bottom layers (i.e., operating system, computer and network hardware
layers) provide a platform for distributed applications.
• The services provided by these bottom layers are utilized by higher layers.
• In every computer, operating system layer and computer network layer are
implemented separately.
• This implementation facilitates communication and coordination b/w
processes by transferring the system’s programming interface to that level
• examples of platform are: Intel x86/Windows, Intel x86/Linux Intel
x86/Solaris , SPARC/SunOS, PowePC/MacOS

56. Architectural model
i)Software Layers
Applications, services
Computer and network hardw are
Platform
Operating system
Middlew are

57. ii. Middleware:
• A piece of software placed b/w the application and operating
system is called “middleware”.
• It is the middle most level which acts as mediator in between
the platform and application services it is used for
heterogeneity
2 .System Architecture:
The most evident aspect of distributed system design is the
• division of responsibilities between system components
(applications, servers, and other processes) and
• the placement of the components on computers in the
network.
• We have 9 basic architectural models in distributed systems.

58. i) Client server model
• The system is structured as a set of processes called server that
offer services to the users called clients
• The client server model is usually based on a simple request or
reply protocol, implemented with send/receive primitives or
using RPC or RMI
• The client send a request message to the server asking for
some service
• The server does the work and returns a result or an error code
if the work could not be performed

59. Client server model

60. ii)Peer to peer model
• All processes (object) play similar role do not require a server
process
• There is no distinction between clients and servers all are same
• The pattern of communication depends on the particular
application

61. iii) Multiple server model:
Services may be provided by multiple servers the server may be
partitions the set of objects on which the service is based and
distribute the work in between themselves

62. iv) Proxy server model
• A proxy server provides copies of resources which are
managed by other servers
• Proxy servers are typically used as caches for web resources
they maintain a cache of recently visited web pages or other
resources
• When a request is issued by a client the proxy server is first
checked if the requested object is available there
• Proxy servers can be located at each client or can be shared
several clients
• The purpose is to increase performance and availability by
avoiding frequent accesses to remote servers

63. Proxy server model

64. v) Mobile code
• A code that is sent from one computer to another computer and
run at the destination computer is called as mobile code
• Java applets are a well known and widely used example of
mobile code

65. Vi) mobile Agent
• A running program (code and data) that travels from one computer
to another in a network carrying out of a task, usually on behalf of
some other process.
• A complete program which carrier code plus data than can work
independently.
• Examples of the tasks that can be done by mobile agents are:
• To collecting information.
• To install and maintain software maintain on the computers within
an organization.

66. mobile Agent
• Fig:
code+data
Result
USERS
Web
server

67. vii)Network computers
• Network computer do not store os or application code locally
all code is loaded from the servers and run locally on the
network computer
• All files related to management of application is stored and
managed remotely
• As data and files are stored remotely so user can migrate from
one network computer to another network computer

68. Viii) thin client
• It is a software layer that supports a window-based user
interface on a computer that is local to the user while
executing application programs on a remote computer.
• This architecture has the same low management and hardware
costs as the network computer scheme.
• Instead of downloading the code of applications into the user’s
computer, it runs them on a compute server.
• Compute server is a powerful computer that has the capacity to
run large numbers of application simultaneously.

69. ix) Mobile devices
• Mobile devices are hardware computing components that
move between physical locations and thus networks, carrying
software component with them.
• Many of these devices are capable of wireless networking
ranges of hundreds of meters such as WiFi (IEEE 802.11), or
about 10 meters such as Bluetooth.
• Mobile devices include:
• Laptops
• Personal digital assistants (PDAs)
• Mobile phones
• Digital cameras
• Wearable computers such as smart watches

71. Fundamental model
• This models are based on some fundamental properties such as
characteristics, failures and security there are types of
fundamental models and they are
• 1) interaction model
• 2) failure model
• 3) security model
• 1) interaction model
this model, computation occurs within the processes
The processes interact by passing messages which result in
• a) Communication(information flow)
• b) Coordination (synchronizing and ordering of activities)
between processes.

72. c) Interaction model reflects the facts that communication takes
place with delay
• d) performance of communication channel depends on latency,
bandwidth,jitter
(which means flow between two systems order may get changed,
so jitter will put them in order and send to destination)
Two variance of the interaction model
• Synchronous distributed system
• Asynchronous distributed system

75. Clock drift rate
• The difference of clock rate is called clock drift no two clocks
would have the same clock rate of oscillations i.e, clock rate
would be different the drift rate varies from clock to clock.

76. 2.Failure model
• This model defines and classifies the faults that occur and it is
important to understand the kinds of failures that may occur in
a system
• Types of failures we come across in distributed system are:
• a) Fail stop: a process stops and remains halted other processes
can detect that the processes has failed
• b) crash: A process stops and remains halted but other
processes may not be able to detect this state
• C) Omission:
• A message is inserted in an outgoing message buffer never
arrives at the other end i.e, incoming message buffer

77. d) Arbitrary:
• If a process/channels exhibits arbitrary behavior it may
send/transmit arbitrary messages at arbitrary times, commit
missions where a process may stop or take an incorrect step
• e) Timing failure :
• If clock drift gets exceeded than allowable bound we get this
kind of failure
3) security model:
• There are several potential threats a system designer need to be
aware of and they are
• i) threats to processes
• ii) threat to communication channels
• iii) Denial of service

78. i) threats to processes
• If an attacker send a request or response using false identify
• ii) threat to communication channels
• If an attacker may listen to message over communication
channel and save it to misuse
iii) Denial of service: if an attacker overload the server by making
excessive requests
• We use cryptography and authentication are often used to
provide security

79. Networking and Internetworking
• The networks used in distributed systems are
built from a variety of
• Transmission media(wire,cable,fiber and
wireless channel)
• Hardware devices(routers, switches,
bridges,hubs)

80. Some Terminology
• Communication subsystem:
• The collection of h/w and s/w components that
provide the communication facilities for a
distributed system.
• Host: computer or other device that use
network for communication purposes
• Node: any computer or switching device
attached to a network

81. Network Performance
Latency:
• the delay that occurs after a send operation is executed
before data starts to arrive at the destination computer
• Measured as the time required to transfer an empty
message
Data Transfer Rate
- speed of transfer of data between two machines
- measured in bits per second
• Message Transfer Time = latency + length/data transfer
rate
Total system bandwidth - volume of traffic on a network

82. Network performance
km

83. Types of network
• A computer network is mainly of four types:
• LAN(Local Area Network)
• PAN(Personal Area Network)
• MAN(Metropolitan Area Network)
• WAN(Wide Area Network)

84. LAN(Local Area Network)
• Local Area Network is a group of computers connected to each
other in a small area such as building, office.
• LAN is used for connecting two or more personal computers
through a communication medium such as twisted pair, coaxial
cable, etc.
• It is less costly as it is built with inexpensive hardware such as hubs,
network adapters, and Ethernet cables.
• The data is transferred at an extremely faster rate in Local Area
Network.
• Local Area Network provides higher security.

85. LAN

86. PAN(Personal Area Network)
• Personal Area Network is a network arranged within an
individual person, typically within a range of 10 meters.
• Personal Area Network is used for connecting the computer
devices of personal use is known as Personal Area Network.
• Thomas Zimmerman was the first research scientist to bring
the idea of the Personal Area Network.
• Personal Area Network covers an area of 30 feet.
• Personal computer devices that are used to develop the
personal area network are the laptop, mobile phones, media
player and play stations.
Example:
• Offline Network, Small Home Office:

87. PAN

88. There are two types of Personal Area Network:
i)Wireless Personal Area Network:
• Wireless Personal Area Network is developed by simply using
wireless technologies such as WiFi, Bluetooth. It is a low
range network.
ii) Wired Personal Area Network:
• Wired Personal Area Network is created by using the USB.

89. MAN(Metropolitan Area Network)
• A metropolitan area network is a network that covers a larger
geographic area by interconnecting a different LAN to form a
larger network.
• Government agencies use MAN to connect to the citizens and
private industries.
• In MAN, various LANs are connected to each other through a
telephone exchange line.
• The most widely used protocols in MAN are RS-232, Frame
Relay, ATM, etc.
• It has a higher range than Local Area Network(LAN).

91. Uses Of Metropolitan Area Network:
• MAN is used in communication between the banks in
a city.
• It can be used in an Airline Reservation.
• It can be used in a college within a city.
• It can also be used for communication in the military.

92. WAN(Wide Area Network)
• A Wide Area Network is a network that extends over a large
geographical area such as states or countries.
• A Wide Area Network is quite bigger network than the LAN.
• A Wide Area Network is not limited to a single location, but it
spans over a large geographical area through a telephone line,
fibre optic cable or satellite links.
• The internet is one of the biggest WAN in the world.
• A Wide Area Network is widely used in the field of Business,
government, and education.

94. Example of WAN
• Mobile Broadband: A 4G network is widely used across a
region or country.
• Private network: A bank provides a private network that
connects the 44 offices.
• This network is made by using the telephone leased line
provided by the telecom company.

95. Network principles
• Packet Transmission:
• A packet is a sequence of binary data with addressing
information to identify the so and destination computers.
• A network message with arbitary length is divided before
transmission into packets of restricted length.
• 4 types of switching are used in computer network:
• 1.Broadcast:
• In this type of switching all messages sent to all connected
computers.
• each computer is responsible of extracting messages addressed
to it. this approach is used in Ethernet and wireless networks

96. ii) Circuit switching
• This approach is used in the telephone system a physical link
is established between the sender and receiver
iii) packet switching:
• It is also known as store and forward, at each switching node a
computer manages the packets by reading each one into
memory, examining its destination and choosing an outgoing
circuit appropriately
iv) frame relay:
• Reading and storing the whole of each packet makes
performance overhead.
• so, instead of reading and storing overall packet it is better to
examine its header which contains the information stream of
bits

97. Protocol software/protocol suites
• Protocol is a well –known set of rules and format for
communication between processes to perform a given task
• It is implemented by a pair of software modules located in
sending and receiving computers
• Protocol software modules are arranged in a hierarchy of
layers
• Data is sent through different layers each layer of network
software communicates with the layers above and below it
• 7 layers reference model for open systems
interconnection(OSI) adopted by ISO to encourage the
development of protocol standards that would meet the
requirements for open systems

98. 1.Physical layer:
this layer coordinates the functions required to transmit a bit stream
over a physical medium
2.Data link layer:
• this layer transforms the raw transmission facility to a reliable
link and is responsible for node to node delivery
3.network layer:
• This layer is responsible for the source –destination delivery of
a packet
4.transport layer:
• This layer is responsible for process to process delivery of
entire message.

99. 5.session layer:
• It is a network dialog controllers it was designed to establish
,maintain and synchronize the interaction between
communicating devices
• 6.Presentation layer
• It was designed to handle the syntax and semantics of the
information exchanged between the two system
7.Application layer:
• It enables the user to access the network it provides user
interfaces and support for services such as electronic email,
remote file access, www etc