QSpiders - Day1 Network Basics

© 1999, Cisco Systems, Inc. www.cisco.com

Networking Basics

1-2CSE: Networking Fundamentals—Internetworking © 1999, Cisco Systems, Inc. www.cisco.com

The OSI Model

• OSI Layer is meant for Networking manufacturers and developers to provide them a standard based on which they can make their products.

• All OSI Layers are independent from each other, which makes introducing changes easier as no other layers are effected.

• Ease of Troubleshooting.


The Layered Model



Why a Layered Network Model?

7 Application

6 Presentation

5 Session

4 Transport

3 Network

2 Data Link

1 Physical

• Reduces complexity (one big problem to seven smaller ones)

• Standardizes interfaces• Facilitates modular

engineering• Assures interoperable

technology• Accelerates evolution• Simplifies teaching and

learning


Devices Function at Layers

7 Application

6 Presentation

5 Session

4 Transport

3 Network

2 Data Link

1 Physical

NIC Card

Hub


Host Layers

7 Application

6 Presentation

5 Session

4 Transport

Network

3 Data Link

1

Host layers: Provide accurate data delivery between computers}

Physical


Media Layers

7 Application

6 Presentation

5 Session

4Transport

3 Network

2 Data Link

1 Physical

Host layers: Provide accurate data delivery between computers

Media layers: Controlphysical delivery of messages over the network}

}


Layer Functions

Provides network services to application processes (such as electronic mail, file transfer, and terminal emulation)

7 Application


Layer Functions

Network services to applications

• Ensures data is readable by receiving system

• Format of data• Data structures• Negotiates data transfer

syntax for application layer

7 Application

6 Presentation Data representation


Layer Functions

Inter-host communication• Establishes, manages, and

terminates sessions between applications

7 Application

6 Presentation

5 Session


Data representation


Layer Functions

7 Application

6 Presentation

5 Session

Transport4

Inter-host communication


Data representation

End-to-end connection reliability• Concerned with data transport

issues between hosts• Data transport reliability• Establishes, maintains, and

terminates virtual circuits• Fault detection and recovery• Information flow control


Layer Functions

7 Application

6 Presentation

5 Session

Transport4

Network3



Data representation

End-to-end connection reliability

Addresses and best path• Provides connectivity and path

selection between two end systems

• Domain of routing


Layer Functions

7 Application

6 Presentation

5 Session

Transport4

Network3

Data Link2



Data representation


Addresses and best path

Access to media• Provides reliable transfer of data

across media• Physical addressing, network

topology, error notification, flow control


Layer Functions

7 Application

6 Presentation

5 Session

Transport4

Network3

Data Link2

Physical1



Data representation


Addresses and best path

Access to media

Binary transmission• Wires, connectors, voltages,

data rates


7 Application

6 Presentation

5 Session

4 Transport

3 Network

2 Data Link

1 Physical

Host A

Peer-to-Peer Communications

Application

Presentation

Session

Transport

Network

Data Link

PhysicalBits

Frames

Packets

Segments

Host B


Application Layer

• This is where users communicate to the computer.

• This is where communication between two users are established.

• This is a point where user or application interfaces with the protocols to gain access to the network.

• Examples are WWW, Telnet, FTP, TFTP, E-mail, SNMP, DNS


Presentation Layer

• Tasks like Translation, Encryption, decryption, compression, decompression are associated with this layer.

• It receives the data in native format & converts in standard format or receives data in standard format and converts in native format, ie. EBCDIC(Extended Binary code Decimal Interchange Code) to ASCII.

• It is mainly responsible for how the data is to be presented to the Application Layer.

• Examples are PICT, TIFF, JPEG, MIDI, MPEG, GIFF etc.


Presentation Layer

ASCIIEBCDICEncrypted

• Text• Data

login:

PICTTIFFJPEGGIF

• Graphics• Visual images

• Sound

• VideoMPEGQuickTime

MIDI

• Provides code formatting and conversion for applications


Session Layer

– Session Establishment•Establishes a session between two devices before actual transmission of data.

– Dialog Control•Simplex•Half Duplex•Full Duplex


Session Layer

• Network File System (NFS)• Structured Query Language (SQL)• Remote-Procedure Call (RPC)• X Window System• AppleTalk Session Protocol (ASP)• DEC Session Control Protocol (SCP)

Service Request

Service Reply

• Coordinates applications as they interact on different hosts


Transport Layer

• Segments upper-layer applications• Establishes an end-to-end connection• Sends segments from one end host to another• Optionally, ensures data reliability


Transport Layer• Transport Layer never actually transports the data

but only prepares for transporting.• Uses Socket to define the services running on a

particular node, the data is associated with.• Responsible for the following :

– Segmentation– End-to-end Communication– Flow Control– Error Control– Multiplexing of Applications

• TCP, UDP and SPX work at this layer


Socket

• Socket is a software component and points to a particular service running on a particular node.

• Structure of a socket– IP Address + Port Address

• Each service has a unique Port address• Max. Port Addresses can be 65,536 • Port address 1-1023 is reserved for specific Services like

– WWW - 80– FTP - 21– SMTP - 25

• Port Addresses are reserved for standardization purpose.


Transport Layer— Segments Upper-Layer Applications

Electronic Mail

File Transfer

Application

Presentation

Session

Segments

DataApplication

PortTransport Data

ApplicationPort

Terminal Session


Port Numbers

TCP

Port Numbers

FTP

TransportLayer

TELNET

DNS

SNMP

TFTP

SMTP

UDP

ApplicationLayer

21 23 25 53 69 161

RIP

520


Segmentation

• This is a mechanism wherein the data is divided into multiple segments and sent over the network.

• By doing this different segments can use different links for travelling across the network.

• If one segment is lost the only segment is required to be re-sent and not the entire data.

• Once all segments reach to the destination the received segments have to be sequenced back, which is also done at this layer.


Transport Layer— Sends Segments with Flow Control

Transmit

Buffer FullNot ReadyStop ProcessSegments

Buffer OKReadyGo

Resume Transmission

ReceiverSender


Flow Control

• Used while connection oriented communication• It helps to have a control on over flow of Buffer.• Advantages are:

– The segments delivered are acknowledged if received– Any segment not acknowledged are retransmitted– segments are sequenced back upon their arrival– Congestion, Overloading and data loss are avoided

• To achieve all this it uses the technique of Sliding window or Windowing


Transport Layer— Establishes Connection

Synchronize

Synchronize

Acknowledge

Negotiate Connection

Receiver

Data Transfer

Connection Established

(Send Segments)

Sender


End-to-End Communication

• Connection Less Transmission–UDP is used–Not reliable–Faster

• Connection Oriented Transmission–TCP or SPX is used–Reliable–Slower


Connection Oriented Protocol

• These protocols relies on Acknowledgement.• Positive acknowledgement means data has

been received.• Negative acknowledgement means data is

lost no further data is sent till positive acknowledgement is received.

• It is slow but Reliable.• Eg. TCP and SPX


Transport Layer— Reliability with Windowing

• Window Size = 1

Sender

Send 1 Receive 1

Receiver

Ack 2

Send 2 Receive 2Ack 3

Sender

Send 1Send 2

Receive 1Receive 2

Receiver

• Window Size = 3

Send 3 Receive 3Ack 4

Send 4


Transport Layer— An Acknowledgement Technique

Sender Receiver

Send 2Send 1

Send 3Ack 4

Send 5Send 4

Send 6Ack 5

Send 5Ack 7

1 2 3 4 5 6 71 2 3 4 5 6 7


Connection Less Protocol

• They do not provide acknowledgement neither sequence numbers.

• It is faster but not reliable

• Eg. UDP


Network Layer

• It is responsible for communicating Networks• It recognizes Networks with the help of Netwok Addresses

– Network Address is a logical address like IP Address or IPX Address

– It is common for a group of computers• It works only with Network IDs and has got nothing to do with host

Ids.• Path determination or Routing is performed at this layer.• Router works at this layer.


Network Layer: Path Determination

Which Path?Which Path?

• Layer 3 functions to find the best path through the internetwork


Network Layer: Communicate Path

1

2

3

4

5

6

7

89

10 11

• Addresses represent the path of media connections


Addressing—Network and Node

Network Node

1

2

123

1

3 1

1.1

2.1

3.1

1.2

1.3

• Network address—Path part used by the router• Node address—Specific port or device on the network


Protocol Addressing Variations

Network Node

1 1

GeneralExample

Network Host

10. 8.2.48

TCP/IPExample (Mask 255.0.0.0)

Network Node

1aceb0b. 0000.0c00.6e25

Novell IPXExample


Network Layer Protocol Operations

• Each router provides its services to support upper layer functions

X Y

AA

CC


Routed Versus Routing Protocol

• Routed protocolused betweenrouters to directuser traffic

Examples: IP, IPX, AppleTalk

• Routing protocolused only betweenrouters to maintainrouting tables

Examples: RIP, IGRP, OSPF


Static Versus Dynamic Routes

Uses a protocol route that a networkadministrator enters into the router

Static Route

Uses a route that a network protocoladjusts automatically for topology ortraffic changes

Dynamic Route


Static Route Example

Point-to-point orcircuit-switched

connection

“Stub” network

Only a single networkconnection with no need

for routing updates

AA

BB

• Fixed route to address reflects administrator’s knowledge


Adapting to Topology Change

AA BB

CCDD

• Can an alternate route substitute for a failed route?



AA BB

CCDD

XX



AA BB

CCDD

XX

• Can an alternate route substitute for a failed route? Yes—With dynamic routing enabled


Data Link Layer

• It uniquely identifies each device in the Network.• It translates data from Network Layer into bits for the

Physical layer to transmit.• It formats the messages into Data Frames• Adds a customized header containing Source and

Destination hardware address• This layer works with FramesThis layer is logically divided in two sub-layers:

LLC (Logical Link Control)MAC (Media Access Control)


Physical Layer

• Electrical and Mechanical settings are provided at this layer.

• Transmits data in the form of bits.• This layer communicates directly with actual

communication media.• At this layer DCE & DTE are identified• HUBs & REPEATERS works at this layer.

• Max. troubleshooting occurs at this layer.

Education

QSpiders - Day1 Network Basics