1. Computer network architecture and models Week-8

2. Networking
The generic term node or host refers to any device on a network
Data transfer rate The speed with which data is moved from one place on a network to another
Data transfer rate is a key issue in computer networks

3. Networking
Computer networks have opened up an entire frontier in the world of computing called the client/server model
Figure Client/Server interaction

4. Networking
File server A computer that stores and manages files for multiple users on a network
Web server A computer dedicated to responding to requests (from the browser client) for web pages

5. Types of Networks
Local-area network (LAN) A network that connects a relatively small number of machines in a relatively close geographical area

6. Types of Networks
Various configurations, called topologies, have been used to administer LANs
Ring topology A configuration that connects all nodes in a closed loop on which messages travel in one direction
Star topology A configuration that centers around one node to which all others are connected and through which all messages are sent
Bus topology All nodes are connected to a single communication line that carries messages in both directions

7. Types of Networks
Figure Various network topologies
A bus technology called Ethernet has become the industry standard for local-area networks
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8. Types of Networks
Wide-area network (WAN) A network that connects two or more local-area networks over a potentially large geographic distance
Often one particular node on a LAN is set up to serve as a gateway to handle all communication going between that LAN and other networks
Communication between networks is called internetworking
The Internet, as we know it today, is essentially the ultimate wide-area network, spanning the entire globe

9. Types of Networks
Metropolitan-area network (MAN) The communication infrastructures that have been developed in and around large cities

10. So, who owns the Internet?
Well, nobody does. No single person or company owns the Internet or even controls it entirely. As a wide-area network, it is made up of many smaller networks. These smaller networks are often owned and managed by a person or organization. The Internet, then, is really defined by how connections can be made between these networks.

11. Types of Networks
Figure Local-area networks connected across a distance to create a wide-area network

12. Internet Connections
Internet backbone A set of high-speed networks that carry Internet traffic
These networks are provided by companies such as AT&T, GTE, and IBM
Internet service provider (ISP) A company that provides other companies or individuals with access to the Internet

13. Internet Connections
There are various technologies available that you can use to connect a home computer to the Internet
A phone modem converts computer data into an analog audio signal for transfer over a telephone line, and then a modem at the destination converts it back again into data
A digital subscriber line (DSL) uses regular copper phone lines to transfer digital data to and from the phone company’s central office
A cable modem uses the same line that your cable TV signals come in on to transfer the data back and forth

14. Internet Connections
Broadband A connection in which transfer speeds are faster than 128 bits per second
DSL connections and cable modems are broadband connections
The speed for downloads (getting data from the Internet to your home computer) may not be the same as uploads (sending data from your home computer to the Internet)

15. Packet Switching
To improve the efficiency of transferring information over a shared communication line, messages are divided into fixed-sized, numbered packets
Network devices called routers are used to direct packets between networks
Figure Messages sent by packet switching
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16. Open Systems
Proprietary system A system that uses technologies kept private by a particular commercial vendor
One system couldn’t communicate with another, leading to the need for
Interoperability The ability of software and hardware on multiple machines and from multiple commercial vendors to communicate
Leading to
Open systems Systems based on a common model of network architecture and a suite of protocols used in its implementation

17. Open Systems
The International Organization for Standardization (ISO) established the Open Systems Interconnection (OSI) Reference Model
Each layer deals with a particular aspect of network communication
Figure The layers of the OSI Reference Model

18. Network Protocols
Network protocols are layered such that each one relies on the protocols that underlie it
Sometimes referred to as a protocol stack
Figure Layering of key network protocols

19. TCP/IP TCP stands for Transmission Control Protocol
TCP software breaks messages into packets, hands them off to the IP software for delivery, and then orders and reassembles the packets at their destination
IP stands for Internet Protocol
IP software deals with the routing of packets through the maze of interconnected networks to their final destination

20. TCP/IP (cont.) UDP stands for User Datagram Protocol
It is an alternative to TCP
The main difference is that TCP is highly reliable, at the cost of decreased performance, while UDP is less reliable, but generally faster

21. High-Level Protocols
Other protocols build on the foundation established by the TCP/IP protocol suite
Simple Mail Transfer Protocol (SMTP)
File Transfer Protocol (FTP)
Telnet
Hyper Text Transfer Protocol (http)

22. MIME Types
Related to the idea of network protocols and standardization is the concept of a file’s MIME type
MIME stands for Multipurpose Internet Mail Extension
Based on a document’s MIME type, an application program can decide how to deal with the data it is given

23. MIME Types
Figure Some protocols and the ports they use

24. Firewalls
Firewall A machine and its software that serve as a special gateway to a network, protecting it from inappropriate access
Filters the network traffic that comes in, checking the validity of the messages as much as possible and perhaps denying some messages altogether
Enforces an organization’s access control policy

25. Firewalls
Figure A firewall protecting a LAN

26. Network Addresses
Hostname A unique identification that specifies a particular computer on the Internet
For example
matisse.csc.villanova.edu
condor.develocorp.com

27. Network Addresses
Network software translates a hostname into its corresponding IP address
For example

28. Network Addresses An IP address can be split into
network address, which specifies a specific network
host number, which specifies a particular machine in that network
Figure An IP address is stored in four bytes

29. Domain Name System
A hostname consists of the computer name followed by the domain name
csc.villanova.edu is the domain name
A domain name is separated into two or more sections that specify the organization, and possibly a subset of an organization, of which the computer is a part
Two organizations can have a computer named the same thing because the domain name makes it clear which one is being referred to

30. Domain Name System
The very last section of the domain is called its top-level domain (TLD) name
Figure Top-level domains, including some relatively new ones

31. Domain Name System
Organizations based in countries other than the United States use a top-level domain that corresponds to their two-letter country codes
Figure Some of the top-level domain names based on country codes

32. Domain Name System
The domain name system (DNS) is chiefly used to translate hostnames into numeric IP addresses
DNS is an example of a distributed database
If that server can resolve the hostname, it does so
If not, that server asks another domain name server

33. Network Architecture The term architecture means the formation of a
structure, or an orderly, interconnected, complex
arrangement of parts.
An architecture encompasses hardware, software, data
link controls, standards, topologies, and protocols.

34. Protocols The term protocol defines how network components
establish communications, exchange data, and terminate
communications.
Protocols are sets of rules and agreements.
Modern networks are implemented using the concept of
layered protocols.
The OSI model was created in an attempt to unify
network development, but many protocols were already in use
(and quite popular) before the OSI model was developed.

35. Protocol Since these existing networks were already functional,
compliance with the model necessitated retrofitting.
Some vendors did this, some did not. Many are still working on
the problem.
Although the introduction of the layered protocol did have a
profound influence on newer protocols, the collection of
protocols available today includes some that conform well to
the OSI and some that do not.

36. OSI Model
OSI MODEL

37. Communication Architecture
OSI Model
Communication Architecture
Strategy for connecting host computers and other communicating equipment.
Defines necessary elements for data communication between devices.
A communication architecture, therefore, defines a standard for the communicating hosts.
A programmer formats data in a manner defined by the communication architecture and passes it on to the communication software.
Separating communication functions adds flexibility, for example, we do not need to modify the entire host software to include more communication devices.

38. Layer Architecture Layer architecture simplifies the network design.
OSI Model
Layer Architecture
Layer architecture simplifies the network design.
It is easy to debug network applications in a layered architecture network.
The network management is easier due to the layered architecture.
Network layers follow a set of rules, called protocol.
The protocol defines the format of the data being exchanged, and the control and timing for the handshake between layers.

39. Open Systems Interconnection (OSI) Model
International standard organization (ISO) established a committee in 1977 to develop an architecture for computer communication.
Open Systems Interconnection (OSI) reference model is the result of this effort.
In 1984, the Open Systems Interconnection (OSI) reference model was approved as an international standard for communications architecture.
Term “open” denotes the ability to connect any two systems which conform to the reference model and associated standards.

40. OSI Model
OSI Reference Model
The OSI model is now considered the primary Architectural model for inter-computer communications.
The OSI model describes how information or data makes its way from application programmes (such as spreadsheets) through a network medium (such as wire) to another application programme located on another network.
The OSI reference model divides the problem of moving information between computers over a network medium into SEVEN smaller and more manageable problems .
This separation into smaller more manageable functions is known as layering.

41. OSI Reference Model: 7 Layers
OSI Model
OSI Reference Model: 7 Layers

42. OSI: A Layered Network Model
OSI Model
OSI: A Layered Network Model
The process of breaking up the functions or tasks of networking into layers reduces complexity.
Each layer provides a service to the layer above it in the protocol specification.
Each layer communicates with the same layer’s software or hardware on other computers.
The lower 4 layers (transport, network, data link and physical —Layers 4, 3, 2, and 1) are concerned with the flow of data from end to end through the network.
The upper four layers of the OSI model (application, presentation and session—Layers 7, 6 and 5) are orientated more toward services to the applications.
Data is Encapsulated with the necessary protocol information as it moves down the layers before network transit.

43. OSI Model
Physical Layer
Provides physical interface for transmission of information.
Defines rules by which bits are passed from one system to another on a physical communication medium.
Covers all - mechanical, electrical, functional and procedural - aspects for physical communication.
Such characteristics as voltage levels, timing of voltage changes, physical data rates, maximum transmission distances, physical connectors, and other similar attributes are defined by physical layer specifications.

44. OSI Model
Data Link Layer
Data link layer attempts to provide reliable communication over the physical layer interface.
Breaks the outgoing data into frames and reassemble the received frames.
Create and detect frame boundaries.
Handle errors by implementing an acknowledgement and retransmission scheme.
Implement flow control.
Supports points-to-point as well as broadcast communication.
Supports simplex, half-duplex or full-duplex communication.

45. OSI Model
Network Layer
Implements routing of frames (packets) through the network.
Defines the most optimum path the packet should take from the source to the destination
Defines logical addressing so that any endpoint can be identified.
Handles congestion in the network.
Facilitates interconnection between heterogeneous networks (Internetworking).
The network layer also defines how to fragment a packet into smaller packets to accommodate different media.

46. OSI Model
Transport Layer
Purpose of this layer is to provide a reliable mechanism for the exchange of data between two processes in different computers.
Ensures that the data units are delivered error free.
Ensures that data units are delivered in sequence.
Ensures that there is no loss or duplication of data units.
Provides connectionless or connection oriented service.
Provides for the connection management.
Multiplex multiple connection over a single channel.

47. OSI Model
Session Layer
Session layer provides mechanism for controlling the dialogue between the two end systems. It defines how to start, control and end conversations (called sessions) between applications.
This layer requests for a logical connection to be established on an end-user’s request.
Any necessary log-on or password validation is also handled by this layer.
Session layer is also responsible for terminating the connection.
This layer provides services like dialogue discipline which can be full duplex or half duplex.
Session layer can also provide check-pointing mechanism such that if a failure of some sort occurs between checkpoints, all data can be retransmitted from the last checkpoint.

48. OSI Model
Presentation Layer
Presentation layer defines the format in which the data is to be exchanged between the two communicating entities.
Also handles data compression and data encryption (cryptography).

49. OSI Model
Application Layer
Application layer interacts with application programs and is the highest level of OSI model.
Application layer contains management functions to support distributed applications.
Examples of application layer are applications such as file transfer, electronic mail, remote login etc.

50. OSI Model
OSI in Action
A message begins at the top application layer and moves down the OSI layers to the bottom physical layer.
As the message descends, each successive OSI model layer adds a header to it.
A header is layer-specific information that basically explains what functions the layer carried out.
Conversely, at the receiving end, headers are striped from the message as it travels up the corresponding layers.

51. TCP/IP Model
TCP/IP MODEL

52. TCP/IP Model
OSI & TCP/IP Models

53. Transport Layer (TCP/UDP)
TCP/IP Model
TCP/IP Model
Application Layer
Application programs using the network
Transport Layer (TCP/UDP)
Management of end-to-end message transmission,
error detection and error correction
Network Layer (IP)
Handling of datagrams : routing and congestion
Data Link Layer
Management of cost effective and reliable data delivery,
access to physical networks
Physical Layer
Physical Media

54. Summary