Lecture # 02Network Models

Course Instructor:Engr. Sana Ziafat

ProtocolSet of rules that govern data communicationProtocol Defines:1.What is communicated2.How it is communicated3.When it is communicated

Protocolprotocols define format, order of msgs sent

and received among network entities, and actions taken on msg transmission, receipt

a human protocol and a computer network protocol:

Hi

Hi

Got thetime?

2:00

TCP connection req.

TCP connectionreply.Get http://gaia.cs.umass.edu/index.htm

<file>

time

Key Elements of PROTOCOLSyntax – format or structure of dataSemantics – meaning of each section of bitsTiming – when and how fast data should be

sent

What’s a protocol?human protocols:“I have a question” introductions

… specific msgs sent… specific actions taken when msgs received, or other events

network protocols:machines rather than humansall communication activity in Internet governed by

protocols

Layering & Protocol Stacks

Layered TasksAn example from the everyday life

Hierarchy?

Services

Why layered communication?To reduce complexity of communication task

by splitting it into several layered small tasksFunctionality of the layers can be changed as

long as the service provided to the layer above stays unchanged makes easier maintenance & updating

Each layer has its own taskEach layer has its own protocol

Reference Models

OSI reference modelTCP/IP

OSI Reference modelOpen System Interconnection7 layers

1.Create a layer when different abstraction is needed

2.Each layer performs a well define function3.Functions of the layers chosen taking

internationally standardized protocols4.Number of layers – large enough to avoid

complexity

Seven layers of the OSI model

Peer-to-peer Processes Layer x on one machine communicates with layer x on

another machine - called Peer-to-Peer Processes. Interfaces between Layers

Each interface defines what information and services a layer must provide for the layer above it. Well defined interfaces and layer functions provide modularity to a network

Organizations of the layers Network support layers : Layers 1, 2, 3 User support layer : Layer 5, 6, 7

It allows interoperability among unrelated software systems

Transport layer (Layer 4) : links the two subgroups

The interaction between layers in the OSI model

Exchange using OSI Model

OSI Layers

Physical layer

Transporting bits from one end node to the next

physicalconnection

Duties performed by Physical layer are:1.Physical characteristics of interfaces and

media2.Representation of bits3.Data rate4.Synchronization of bits5.Topology6.Line configuration7.Transmission mode

The physical layer is responsible for movements ofindividual bits from one hop (node) to the next.

Note

Hardware equipment Network adapterRepeaterNetwork hubModem

Data Link layer

Transporting frames from one end node to the next one

logicalconnection

- framing - physical addressing

- flow control - error control

- access control

Data Link layer- hop-to-hop delivery-

The data link layer is responsible for moving frames from one hop (node) to the next.

Note

Network Devices: -Bridge, Switch, ISDN Router,

Intelligent Hub, NIC, Advanced Cable Tester

Network Layer The network layer is responsible for the delivery

of individual packets from the source host to the destination host.

Network layer End-to-End packet delivery

From the original source to a destination Needed when 2 devices are attached to

different networks What is the network definition here?

Main duties:1. Logical addressing2. Routing

Source to destination delivery

Data Link

Network layer

The network layer is responsible for the delivery of individual packets from

the source host to the destination host.

Note

Router works as the post office and network layer stamps the letters (data) for the specific destinations.

Protocols: These protocols work on the network layer IP, ICMP, ARP, RIP, OSI, IPX and OSPF.

Network Devices: Network devices including Router, Frame Relay device and ATM switch devices work on the network layer.

Transport layerProcess-to-Process delivery of the entire

message◦ From the original source to a destination

Needed when several processes (running programs) active at the same time

Note: What is a process

Main tasks:◦ Service Point addressing◦ Segmentation and reassembly◦ Connection control◦ Flow control◦ Error control

Transport Layer The transport layer is responsible for the delivery of a message from one process to another.

Transport layer -an example of a reliable delivery -

The transport layer is responsible for the delivery of a message from one process to another.

Note

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The session layer is responsible for dialog control and synchronization.

Note

Session LayerThe session layer allows a system to add check

points or synchronization points.This layer allow two systems to enter in to

dialog

Presentation Layer

Presentation layer is concerned with syntax and semantics of information exchanged between two systems.

Provides translation , encryption and compression to data

It is a best layer for cryptography.

The presentation layer is responsible for translation, compression, and encryption.

Note

Application Layer The application layer is responsible for

providing services to the user.

Services provided by Application layer:

- File transfer, Access - Mail services

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Application layerEnables user to access the networkProvides services to a user

E-mailRemote file access and transfer (Telnet,

FTP)Access to WWW (HTTP)

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The application layer is responsible for providing services to the user.

Note

Protocols: FTP, DNS, SNMP, SMTP, FINGER, TELNET, TFTP, BOOTP and SMB protocol are operated on the application layer.

OSI Reference ModelA convenient aid for remembering the

OSI layer names is to use the first letter of each word in the phrase:

All People Seem To Need Data Processing

Summary of layers and protocols

Low-level protocols define the electrical and physical standards to be observed, bit- and byte-ordering and the transmission and error detection and correction of the bit stream

High-level protocols deal with the data formatting, including the syntax of messages, the terminal to computer dialogue, character sets, sequencing of messages

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TCP/IP Protocol

TCP/IP Vs OSI Model

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Four Level of Addresses

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Relationship of Layers & Addresses in TCP/IP

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The physical addresses will change from hop to hop,but the logical addresses usually remain the same.

Note

In Figure on next slide a node with physical address 10 sends a frame to a node with physical address 87. The two nodes are connected by a link (bus topology LAN). As the figure shows, the computer with physical address 10 is the sender, and the computer with physical address 87 is the receiver.

Example 1

Figure Physical addresses

As we will see in later lectures, most local-area networks use a 48-bit (6-byte) physical address written as 12 hexadecimal digits; every byte (2 hexadecimal digits) is separated by a colon, as shown below:

Example 2

07:01:02:01:2C:4B

A 6-byte (12 hexadecimal digits) physical address.

Figure on next slide shows a part of an internet with two routers connecting three LANs. Each device (computer or router) has a pair of addresses (logical and physical) for each connection. In this case, each computer is connected to only one link and therefore has only one pair of addresses. Each router, however, is connected to three networks (only two are shown in the figure). So each router has three pairs of addresses, one for each connection.

Example 3

Figure IP addresses

Figure shows two computers communicating via the Internet. The sending computer is running three processes at this time with port addresses a, b, and c. The receiving computer is running two processes at this time with port addresses j and k. Process a in the sending computer needs to communicate with process j in the receiving computer. Note that although physical addresses change from hop to hop, logical and port addresses remain the same from the source to destination.

Example 4

Figure Port addresses

Example 5

As we will see in later chapters, a port address is a 16-bit address represented by one decimal number as shown.

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A 16-bit port address represented as one single number.

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ReadingsChapter 1 (B. A Forouzan)

Section 1.4

Chapter 2 (B. A Forouzan)Section 2.1, 2.2,2.3, 2.4, 2.5

Q & A