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McGraw-Hill ©The McGraw-Hill Companies, Inc., 2004

Chapter 2

NETWORK MODELS

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Layered Tasks Ex. A person sends a letter

Components: Sender, Receiver, carrier

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2-1 PROTOCOL LAYERS

In Chapter 1, we discussed that a protocol is required when two entities need to communicate. When communication is not simple, we may divide the complex task of communication into several layers. In this case, we may need several protocols, one for each layer. Let us use a scenario in communication in which the role of protocol layering may be better understood. We use two examples. In the first example, communication is so simple that it can occur in only one layer.

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Assume Maria and Ann are neighbors with a lot of common ideas. However, Maria speaks only Spanish, and Ann speaks only English. Since both have learned the sign language in their childhood, they enjoy meeting in a cafe a couple of days per week and exchange their ideas using signs. Occasionally, they also use a bilingual dictionary. Communication is face to face and Happens in one layer as shown in Figure 2.1.

ExampleExample 2.1

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Figure 2.1 Example 2.1

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Now assume that Ann has to move to another town because of her job. Before she moves, the two meet for the last time in the same cafe. Although both are sad, Maria surprises Ann when she opens a packet that contains two small machines. The first machine can scan and transform a letter in English to a secret code or vice versa. The other machine can scan and translate a letter in Spanish to the same secret code or vice versa. Ann takes the first machine; Maria keeps the second one. The two friends can still communicate using the secret code, as shown in Figure 2.2.

ExampleExample 2.2

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Figure 2.2 Example 2.2

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2-2 THE OSI MODEL

Established in 1947, the International Standards Organization (ISO) is a multinational body dedicated to worldwide agreement on international standards. Almost three-fourths of countries in the world are represented in the ISO. An ISO standard that covers all aspects of network communications is the Open Systems Interconnection (OSI) model. It was first introduced in the late 1970s.

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Figure 2.3 The OSI model

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Figure 2.6 Summary of OSI Layers

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7 Layers of the OSI ModelLayer Responsible For:

7.) Application Provides Services to User Apps

6.) Presentation Data Representation

5.) Session Communication Between Hosts, session management

4.) Transport Multiplexing, Segmenting the data

3.) Network Routing

2.) Data Link Synchronization, Error Detection/Correction

1.) Physical Medium, Interfaces, Puts Bits on Med.

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Examples

Layer Example

7.) Application HTTP, FTP, SMTP

6.) Presentation ASCII, JPEG, PGP

5.) Session BOOTP, NetBIOS, DHCP, DNS

4.) Transport TCP, UDP, SPX

3.) Network IP, IPX, ICMP

2.) Data Link Ethernet, Token Ring, Frame Relay

1.) Physical Bits, Interfaces, Hubs

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Mnemonics

(A)ll 7.) (A)pplication (A)way

(P)eople 6.) (P)resentation (P)izza

(S)eem 5.) (S)ession (S)ausage

(T)o 4.) (T)ransport (T)hrow

(N)eed 3.) (N)etwork (N)ot

(D)ata 2.) (D)ata Link (D)o

(P)rocessing 1.) (P)hysical (P)lease

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Layer 1: The Physical Layer

Defines physical medium and interfaces Determines how bits are represented Controls transmission rate & bit

synchronization Controls transmission mode: simplex, half-

duplex, & full duplex Protocol Data Unit (PDU): Bits Devices: hubs, cables, connectors, etc…

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Layer 2: The Data Link Layer

PDU: Frames Keeps Link alive & provides connection for

upper layer protocols Based on physical address space Flow control and error detection/correction at

the frame level is done. Ex: Ethernet, Token Ring, ISDN Devices: switches, bridges, NIC’s

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Layer 3: The Network Layer

PDU: Packet End to end delivery of packets Creates logical paths Path determination (routing) Hides the lower layers making things

hardware independent Devices: routers, firewalls

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Layer 4: The Transport Layer

PDU: Segment It reassemble segments into data using

sequence numbers Can use either connectionless or connection

oriented sessions Uses acknowledgements & retransmission

for error correction Example: TCP (used by things like telnet,

http)

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The Transport Layer (cont.)

Connection oriented sessions require the sender to first request a connection, the receiver to acknowledge the connection, and that they negotiate how much data can be sent/received before its reception is acknowledged.

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Layer 5: The Session Layer PDU: Data The Session Layer provides the mechanism for

opening, closing and managing a session between end-user application processes, i.e. a semi-permanent dialogue.

Communication sessions consist of requests and responses that occur between applications.

Session Layer services are commonly used in application environments that make use of remote procedure calls (RPCs).

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The Session Layer (cont.)

In case of a connection loss this protocol may try to recover the connection.

If a connection is not used for a long period, the Session Layer Protocol may close it and re-open it.

It provides for either full duplex or half-duplex operation and provides synchronization points

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Layer 6: The Presentation Layer

The Presentation Layer is responsible for the delivery and formatting of information to the application layer for further processing or display.

An example of a presentation service would be the conversion of an EBCDIC-coded text file to an ASCII-coded file.

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Layer 7 : Application Layer

The Application Layer contains all protocols and methods that fall into the realm of process-to-process communications via an Internet Protocol (IP) network.

The common application layer services provide semantic conversion between associated application processes.

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Function of Each Layer

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Layer 1

The Physical Layer

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Transmission Media

Transmission Media and Physical Layer

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Transmission Media Transmission medium is the physical path between

the transmitter and receiver. It is the Transmission medium through which

information usually moves from one network device to another.

In some cases, a network will utilize only one type of cable, other networks will use a variety of cable types.

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Transmission media

Transmission media are located below the physical layer.

Computers use signals to represent data.Signals are transmitted in form of

electromagnetic energy.

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Factors to Select Transmission Media

Data Rate and Bandwidth (BPS and Hz) Distance and Attenuation (meters, dB/km) Interference Characteristics Number of receivers (broadcast vs. point to

point) Cost

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Transmission Impairments

Impairments exist in all forms of data transmission media Analog signal impairments result in random

modifications that impair signal quality Digital signal impairments result in bit errors (1s

and 0s transposed)

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Types of Media

Two major classes Conducted or guided media

use a conductor such as a wire or a fiber optic cable to move the signal from sender to receiver.

Energy is confined to the medium and guided by it Wireless or unguided media

use radio waves of different frequencies and do not need a wire or cable conductor to transmit signals

Energy spreads out and is not confined

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Classes of transmission media

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Layer 2

Data Link Layer

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Functions of the Data Link Layer (2)

Relationship between packets and frames.

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Data Link Layer

• Providing services to the Network Layer

• Framing• Error Control• Flow Control

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Functions of the Data Link Layer

• Provide service interface to the network layer

• Dealing with transmission errors• Regulating data flow

• Slow receivers not swamped by fast senders

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Services Provided to Network Layer

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Layer 3

Network Layer

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A Sample Network

fig 5-1

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Figure 19.3 Network layer in an internetwork`1

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Network layer duties

The challenge is interconnecting different networks (various LAN technologies, telephone network, satellite link, ATM networks etc.).

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Network layer duties

Unique addresses are required to define each host/machine/device/user in the network.

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Network layer duties

The Packets coming from the transport layer must be placed in network-layer packets and sent to the data-link layer.

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Network layer duties

Remember, the network layer must be able to operate on top of any data-link layer technology (Ethernet, Fast Ethernet, ATM etc.). All these technologies can handle a different packet length. The network layer must be able to fragment transport layer PDUs into smaller units so that they can be transferred over various data-link layer technologies.

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Network layer duties

This layer decides the route to be taken.

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Layer 4

Transport Layer

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Main Functions

Divides each message into segments and reassembles them at the destination.

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Functions that you can encounter in the Transport Layer are: Error Handling Flow Control Multiplexing Connection Set-up and Release Congestion Handling Segmentation and Reassembly Addressing

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Transport Layer Protocols: User Datagram Protocol (UDP)

Simple (asynchronous) multiplexing protocol

Transport Control Protocol (TCP)Reliable connection oriented byte-

stream protocol

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Layer 5

Session Layer

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5. Session Layer Example application program: Winsock, UNIX Socket

Appl.

Pres.

Sess.

Tran.

Netw.

Data.

Phys.

Appl.

Pres.

Sess.

Tran.

Netw.

Data.

Phys.

• Establishment, maintaining and release of session

• Provide dialog management

• Regulate which side transmit, when, for how long (Sync.)

• Provide synchronization between user tasks

ENG224

INFORMATION TECHNOLOGY – Part I6. Network Model

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A. Consecutive sessions on a transport layer connection

1 2 3

B. Consecutive sessions on a multiple transport layer connections

1 2 3

What is a Session?

C. Both

1 2 3

ENG224

INFORMATION TECHNOLOGY – Part I6. Network Model

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Layer 6

Presentation Layer

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6. Presentation Layer Example application program: redirector (NT), SSL

Appl.

Pres.

Sess.

Tran.

Netw.

Data.

Phys.

Appl.

Pres.

Sess.

Tran.

Netw.

Data.

Phys.

• Translate data from Application Layer to the format suitable for session layer (the network)

• Provide data encryption, compression

• Changing or converting character set, graphic, and file format

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Layer 7

Application Layer

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7. Application Layer Example protocols: FTP, Telnet, HTTP

• Entry point for application to access network

• Directly support user applications

• E.g. File transfer, email

• General capabilities:

Network access, flow control, Error recovery

Appl.

Pres.

Sess.

Tran.

Netw.

Data.

Phys.

Appl.

Pres.

Sess.

Tran.

Netw.

Data.

Phys.

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Design Issues

Addressing Direction of Transmission Error Control Avoid loss of Sequencing Ability of receiving long messages Multiplexing

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Simplex, Half-Duplex, and

Full-Duplex Transmission

A communications channel is classified as one of three types:(depending on the direction of transfer) Simplex Full-Duplex Half-Duplex

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Simplex, Half-Duplex, and

Full-Duplex Transmission

Simplex: a simplex mechanism can only transfer data in a single direction It is analogous to broadcast radio or television

Full-Duplex: allows transmission in two directions simultaneously It is analogous to a voice telephone conversation

in which a participant can speak even if they are able to hear background music at the other end.

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Simplex, Half-Duplex, and

Full-Duplex Transmission

Half-Duplex: A half-duplex mechanism involves a shared transmission medium The shared medium can be used for communication in each

direction But the communication cannot proceed simultaneously It is analogous to using walkie-talkies where only one side can

transmit at a time

An additional mechanism at each end of a half-duplex communication coordinates transmission to insure that only one side transmits at a given time

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Simplex, Half-Duplex, and

Full-Duplex Transmission

60

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2-3 TCP/IP PROTOCOL SUITE

The TCP/IP protocol suite was developed prior to the OSI model. Therefore, the layers in the TCP/IP protocol suite do not match exactly with those in the OSI model. The original TCP/IP protocol suite was defined as four software layers built upon the hardware. Today, however, TCP/IP is thought of as a five-layer model with the layers named similarly to the ones in the OSI model.

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Figure 2.7 Layers in the TCP/IP Protocol Suite

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Figure 2.15 Addresses in the TCP/IP protocol suite

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Description of TCP/IP model:

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Layer 1: The Physical Layer Defines physical medium and interfaces Determines how bits are represented Controls transmission rate & bit

synchronization Controls transmission mode: simplex, half-

duplex, & full duplex

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Layer 2: The Data Link Layer PDU: Frames Keeps Link alive & provides connection for

upper layer protocols Based on physical address space Flow control and error detection/correction

at the frame level is done.

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Layer 3: The Network Layer PDU: Packet End to end delivery of packets Creates logical paths Path determination (routing) Hides the lower layers making things

hardware independent Devices: routers, firewalls

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Layer 4: The Transport Layer PDU: Segment It reassemble segments into data using

sequence numbers Can use either connectionless or

connection oriented sessions Uses acknowledgements & retransmission

for error correction Example: TCP (used by things like telnet,

http), UDP

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Layer 5 : Application Layer Session management and data

representation is done at this layer. The Application Layer contains all

protocols and methods that fall into the realm of process-to-process communications via an Internet Protocol (IP) network.

The common application layer services provide semantic conversion between associated application processes.

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